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Showing posts with label Data Communication. Show all posts
Showing posts with label Data Communication. Show all posts

23 May 2012

Basics Of Data Communication: Part 7

This article will explain, Introduction to LAN, MAN and MAC, IEEE Standards, LAN, Topologies in very detailed.



Introduction to LAN, MAN and MAC

There are two types of Network Technology: Switch Networks and Broadcast Networks. In switch network, systems are interconnected by means of point-to-point transmission lines, multiplexers and switches. In switching Networks, the transfer of packet across networks requires routing, to direct packets from source to destination, as the source and destination stations will not be connected by a single transmission link most of the cases. But in Broadcast Networks, all the system are connected to common transmission medium, which acts as a broadcast medium making routing unnecessary. Since the transmission medium is shared by all the connected systems, there is a need for additional layer, consisting of Medium Access Control (MAC) Protocol to orchestrate the transmission from various systems. The role of MAC protocol is to co-ordinate the access to the transmission medium so that packets transmitted from the systems do not interface with other packets. There are two networks based on broadcast medium that is LAN and MAN. The definition of LANs and MANs taken from IEEE 802 standards.

IEEE Standards

A set of network standards developed by the IEEE. They include:

  • IEEE 802.1: Standards related to network management.
  •  
  • IEEE 802.2: General standard for the data link layer in the OSI Reference Model. The IEEE divides this layer into two sublayers -- the logical link control (LLC) layer and the media access control (MAC) layer. The MAC layer varies for different network types and is defined by standards IEEE 802.3 through IEEE 802.5.
     
  • IEEE 802.3: Defines the MAC layer for bus networks that use CSMA/CD. This is the basis of the Ethernet standard.
     
  • IEEE 802.4: Defines the MAC layer for bus networks that use a token-passing mechanism (token bus networks).
     
  • IEEE 802.5: Defines the MAC layer for token-ring networks.
     
  • IEEE 802.6: Standard for Metropolitan Area Networks (MANs).
LAN (Local Area Network)

A LAN is a high-speed data network that covers a relatively small geographic area. It typically connects workstations, personal computers, printers, servers, and other devices. LANs offer computer users many advantages, including shared access to devices and applications, file exchange between connected users, and communication between users via electronic mail and other applications.

LAN data transmissions fall into three classifications: unicast, multicast, and broadcast. In each type of transmission, a single packet is sent to one or more nodes.

  1. In a unicast transmission, a single packet is sent from the source to a destination on a network. First, the source node addresses the packet by using the address of the destination node. The package is then sent onto the network, and finally, the network passes the packet to its destination.
  2.  
  3. A multicast transmission consists of a single data packet that is copied and sent to a specific subset of nodes on the network. First, the source node addresses the packet by using a multicast address. The packet is then sent into the network, which makes copies of the packet and sends a copy to each node that is part of the multicast address.
     
  4. A broadcast transmission consists of a single data packet that is copied and sent to all nodes on the network. In these types of transmissions, the source node addresses the packet by using the broadcast address. The packet is then sent on to the network, which makes copies of the packet and sends a copy to every node on the network.
LAN Architecture

The 3 most common types of LAN architectures are:
  • Ethernet
  •  
  • Token Ring
     
  • ArcNet
These architectures are sometimes referred to as "lower-level protocols" because they represent the specifications for the IEE802 model which encompasses the physical (1st) and data link (2nd) layers of the OSI model.
  1. Ethernet is a popular, relatively inexpensive, easy-to-install LAN architecture with the following characteristics:
     
    • Uses the CSMA/CD media access control.
    • Data transmission normally occurs at 10Mbps.
    • Typically implemented in a bus or star topology.
    • Ethernet LANs are normally distinguished by the type of cable they use (Thinnet, Thicknet, or Twisted Pair).
    The Ethernet architecture conforms to most but not all of the IEEE 802.3 specification (the physical layers are identical but the MAC layers are somewhat different).
  2. Token ring is a relatively expensive LAN architecture that is strongly influenced by IBM. It is very stable and can be expanded without a significant degradation in network performance. Token ring uses the token passing media access control. Data transmission normally occurs at 4 or 16 Mbps depending on the cable. Token ring is normally implemented in a logical ring/physical star topology with a MAU (Multistation Access Unit) as the hub. The maximum number of stations on one ring is 260 for shielded twisted pair and 72 for unshielded twisted pair (UTP). There can be up to 33 MAUs per ring. Token Ring LANs normally use shielded twisted pair (STP) but may also use unshielded twisted pair (UTP) or fiber-optic cable. The maximum distance to the MAU from the workstation depends on the cable and varies from 45 meters for UTP to 100 meters for STP. The Token Ring architecture conforms generally to the IEEE's 802.5 specification
     
  3. ArcNet (Attached Resource Computing Network) is a relatively inexpensive, reliable, and easy-to-install LAN architecture with the following characteristics:
     
    • Additional workstations are easily added.
    • ArcNet is a baseband, token passing media access control architecture
    • ArcNet is relatively slow. Data transmission occurs at 2.5 Mbps. (20 Mbps for ArcNet Plus)
    • ArcNet can be implemented in a bus or star topology.
    • ArcNet LANs normally use coaxial cable but can also use twisted pair or fiber-optic cable. Maximum cable segment depends on the type of cable and hub connection (120m -600m)

     The ArcNet architecture conforms very loosely to the IEEE's 802.4 specification. ArcNet is a baseband star or bus and 802.4 defines a broadband bus.
LAN Topologies

A topology refers to the manner in which the cable is run to individual workstations on the network. The dictionary defines topology as: the configurations formed by the connections between devices on a local area network (LAN) or between two or more LANs
There are three basic network topologies (not counting variations thereon): the bus, the star, and the ring.
It is important to make a distinction between a topology and architecture. A topology is concerned with the physical arrangement of the network components. In contrast, an architecture addresses the components themselves and how a system is structured (cable access methods, lower level protocols, topology, etc.).

Bus Topology:

A bus topology connects each computer (node) to a single segment trunk. A 'trunk' is a communication line, typically coax cable that is referred to as the 'bus.' The signal travels from one end of the bus to the other. A terminator is required at each end to absorb the signal so it does not reflect back across the bus.

In a bus topology, signals are broadcast to all stations. Each computer checks the address on the signal (data frame) as it passes along the bus. If the signal's address matches that of the computer, the computer processes the signal. If the address doesn't match, the computer takes no action and the signal travels on down the bus.

Only one computer can 'talk' on a network at a time. A media access method called CSMA/CD is used to handle the collisions that occur when two signals are placed on the wire at the same time. The bus topology is passive. In other words, the computers on the bus simply 'listen' for a signal;
they are not responsible for moving the signal along.

A bus topology is normally implemented with coaxial cable.

Advantages of bus topology:
  • Easy to implement and extend
  • Well suited for temporary networks that must be set up in a hurry
  • Typically the least cheapest topology to implement
  • Failure of one station does not affect others
Disadvantages of bus topology:
  • Difficult to administer/troubleshoot
  • Limited cable length and number of stations
  • A cable break can disable the entire network; no redundancy
  • Maintenance costs may be higher in the long run
  • Performance degrades as additional computers are added
Start Topology:

All of the stations in a star topology are connected to a central unit called a hub.

The hub offers a common connection for all stations on the network. Each station has its own direct cable connection to the hub. In most cases, this means more cable is required than for a bus topology. However, this makes adding or moving computers a relatively easy task; simply plug them into a cable outlet on the wall.

If a cable is cut, it only affects the computer that was attached to it. This eliminates the single point of failure problem associated with the bus topology. (unless, of course, the hub itself goes down.)

Star topologies are normally implemented using twisted pair cable, specifically unshielded twisted pair (UTP). The star topology is probably the most common form of network topology currently in use.

Advantages of star topology:
  • Easy to add new stations
  • Easy to monitor and troubleshoot
  • Can accommodate different wiring
Disadvantages of ring topology:
  • Failure of hub cripples attached stations
  • More cable required
Ring Topology:

A ring topology consists of a set of stations connected serially by cable. In other words, it's a circle or ring of computers. There are no terminated ends to the cable; the signal travels around the circle in a clockwise direction.

Note that while this topology functions logically as ring, it is physically wired as a star. The central connector is not called a hub but a Multi-station Access Unit or MAU. (Don't confuse a Token Ring MAU with a 'Media Adapter Unit' which is actually a transceiver.)
Under the ring concept, a signal is transferred sequentially via a "token" from one station to the next. When a station wants to transmit, it "grabs" the token, attaches data and an address to it, and then sends it around the ring. The token travels along the ring until it reaches the destination address. The receiving computer acknowledges receipt with a return message to the sender. The sender then releases the token for use by another computer.

Each station on the ring has equal access but only one station can talk at a time.

In contrast to the 'passive' topology of the bus, the ring employs an 'active' topology. Each station repeats or 'boosts' the signal before passing it on to the next station.

Rings are normally implemented using twisted pair or fiber-optic cable.

Advantages of ring topology:

  • Growth of system has minimal impact on performance
  • All stations have equal access
Disadvantages of ring topology:
  • Most expensive topology

  •  Failure of one computer may impact others
     Complex
Tree Topology:

Among all the Network Topologies we can derive that the Tree Topology is a combination of the bus and the Star Topology. The tree like structure allows you to have many servers on the network and you can branch out the network in many ways. This is particularly helpful for colleges, universities and schools so that each of the branches can identify the relevant systems in their own network and yet connect to the big network in some way. A Tree Structure suits best when the network is widely spread and vastly divided into many branches. Like any other topologies, the Tree Topology has its advantages and disadvantages. A Tree Network may not suit small networks and it may be a waste of cable to use it for small networks. Tree Topology has some limitations and the configuration should suit those limitations.

Advantages of Tree topology:
  • A Tree Topology is supported by many network vendors ad even hardware vendors.
  • A point to point connection is possible with Tree Networks.
  • All the computers have access to the larger and their immediate networks.
  • Best topology for branched out networks.
Disadvantage of Tree topology:
  • In a Network Topology the length of the network depends on the type of cable that is being used.
  • The Tree Topology network is entirely dependant on the trunk which is the main backbone of the network. If that has to fail then the entire network would fail.
  • Since the Tree Topology network is big it is difficult to configure and can get complicated after a certain point.
The Tree Topology follows a hierarchical pattern where each level is connected to the next higher level in a symmetrical pattern. Each level in the hierarchy follows a certain pattern in connecting the nodes. Like the top most level might have only one node or two nodes and the following level in the hierarchy might have few more nodes which work on the point to point connectivity and the third level also has asymmetrical node to node pattern and each of these levels are connected to the root level in the hierarchy. Think of a tree that branches out in various directions and all these branches need the roots and the tree trunk to survive. A Tree Structured network is very similar to this and that is why it is called the Tree Topology. 

Basics of Data Communication: Part 8

This article will explain, MAN, MAC, MAC Frame Format, LLC, LAN Systems, CSMA and CD, CSMA and CD Frame Format, Token Right, FDDI, Bridge etc.



MAN (Metropolitan Area Network)

MAN is a computer network usually spanning a campus or a city, which typically connect a few local area networks using high speed backbone technologies. A MAN often provides efficient connections to a wide area network (WAN). There are three important features which discriminate MANs from LANs or WANs:

  1. The network size falls intermediate between LANs and WANs. A MAN typically covers an area of between 5 and 50 km range. Many MANs cover an area the size of a city, although in some cases MANs may be as small as a group of buildings.
  2.  
  3. A MAN (like a WAN) is not generally owned by a single organization. The MAN, its communications links and equipment are generally owned by either a consortium of users or by a network service provider who sells the service to the users.
     
  4. A MAN often acts as a high speed network to allow sharing of regional resources. It is also frequently used to provide a shared connection to other networks using a link to a WAN.
MAN adopted technologies from both LAN and WAN to serve its purpose. Some legacy technologies used for MAN are ATM, FDDI, DQDB and SMDS. These older technologies are in the process of being displaced by Gigabit Ethernet and 10 Gigabit Ethernet. At the physical level, MAN links between LANs have been built on fiber optical cables or using wireless technologies such as microwave or radio.

Medium Access Control (MAC)

The Media Access Control is often said to be a sub-layer of the OSI data Link layer. On every network interface adaptor card there is a set of computer chips that handle communication with the physical media (copper wire, fiber optic cable or the air) by controlling the communication signal (electricity, light or radio frequencies) over the physical media. In plain english, the computer chips that control the electricity transmitted and received on a copper wire are MAC-related hardware. The MAC sublayer provides the means to access the the physical medium used for communication. The MAC sublayer also communicates with the Logical Link Control (LLC) sub-layer above it allowing it to access and speak to the upper layer network protocols such as IP.

In a centralized scheme, a controller is designated that has the authority to grant access to the network. A station wishing to transmit must wait until it receives permission from the controller. In a decentralized network, the stations collectively perform a medium access control function to dynamically determine the order in which stations transmit. A centralized scheme has certain advantages as listed below.
  • It may afford greater control over access for providing such things as priorities, overrides and guaranteed capacity.
  • It enables the use of relatively simple access logic at each station.
  • It avoids problems of distributed coordination among peer entities.
Centralized scheme has following disadvantages as listed below.
  • It creates a single point of failure; that is, there is a point in the network that, if it fails, causes the entire network to fail.
  • It may act as a bottleneck, reducing performance.
In general, we can categorize access control techniques as being either synchronous or asynchronous. With synchronous techniques, a specific capacity is dedicated to a connection; this is the same approach used in circuit switching, frequency division multiplexing (FDM) and synchronous time division multiplexing (TDM). The asynchronous approach can be further subdivided into three categories.
  1. Round Robin

    In round robin each station in turn is given the opportunity to transmit. During that opportunity, the station may decline to transmit or may transmit subject to a specific upper bound, usually expressed as a maximum amount of data transmitted or time of this opportunity. In any case, the station, when it is finished, relinquishes its runs, and the right to transmit passes to the next station in logical sequence. Control of sequence may be centralized or distributed. Polling is an example of a centralized technique.
  2.  
  3. Reservation

    In stream traffic reservation techniques are well suited. In general, for these techniques time on the medium is divided into slots, much as with synchronous TDM. A station wishing to transmit reserves future slots for an extended or even an indefinite period. Again, reservations may be made in a centralized or distributed fashion.
     
  4. Contention

    For busty traffic, contention techniques are usually appropriate. With these techniques, no control is exercised to determine whose turn it is; all stations contend for time in a way that can be, as we shall see, rather rough and tumble. These techniques are, of necessity, distributed by nature. Their principal advantage is that they are simple to implement and under light to moderate load of data traffic, they are efficient. For some of these techniques, however, performance tends to collapse under heavy load.
MAC Frame Format Structure

MAC layer receives a block of data from the LLC (Logical Link Control) layer and is responsible for performing functions related to medium access and for transmitting the data. As with other protocols layers, MAC implements there function, making use of a protocol data unit at its layer; in this case, the PDU (Protocol Data Unit) is referred to as a MAC frame. The exact format of the MAC frame differs somewhat for the various MAC protocols in use but in general we have the following format.

Here is list of fields in detailed.

  • MAC Control: This field contains any protocol control information needed for the functioning of the MAC protocol. For example, a priority level could be indicated here.
  •  
  • Destination MAC Address: The address of the destination device on the LAN for this frame.
     
  • Source MAC Address: The address of the source device on the LAN from which this frame is being transmitted.
     
  • LLC: The LLC data from the next higher layer.
     
  • CRC: The Cyclic Redundancy Check field also known as the Frame Check Sequence (FCS). This is an error-detecting technique.
Logical Link Control (LLC)

The LLC is part of the data link layer in a protocol stack. The data link layer controls access to the network medium and defines how upper-layer data in the form of packets or datagrams is inserted into frames for delivery on a particular network. The underlying physical layer then transmits the framed data as a stream of bits on the network medium.
The IEEE 802.2 standard defines LLC, which is positioned in the protocol stack. Note that LLC resides on the upper half of the data link layer. The MAC (Medium Access Control) sub-layer is where individual shared LAN technologies such as Ethernet are defined. Early on, the data link layer contained only LLC-like protocols; but when shared LANs came along, the IEEE positioned the MAC sub-layer into the lower half of the data link layer.

Basically, LLC provides a common interface, and provides reliability and flow-control features. It is a subclass of HDLC (High-level Data Link Control), which is used on wide area links. LLC can provide both connection-oriented and connectionless services.
The LLC acts like a software bus, allowing multiple higher-layer protocols to access one or more lower-layer networks. For example, a server may have multiple network interface cards (and an Ethernet and a token ring card). The LLC will forward packets from upper-layer protocols to the appropriate network interface. This scheme allows upper-layer protocols to operate without specific knowledge of the lower-layer network in use.

LLC Services

LLC specifies the mechanism for addressing stations across the medium and for controlling the exchange of data between two users. The operation and format of this standard is based on HDLC. Three services are provided as alternative for attached devices using LLC.
  • Unacknowledged connectionless service

    This service is a datagram style service. It is very simple service that does not involve any of the flow and error-control mechanisms. Thus, the delivery of data is not guaranteed. However, in most devices, there will be some higher layer of software that deals with reliability issues.
  •  
  • Connection-mode service

    This service is similar to that offered by HDLC. A logical connection is set up between two user exchanging data and flow control and error control are provided.
     
  • Acknowledged connectionless services

    This is a cross between the previous two services. It provides that datagrams are to be acknowledged, but no prior logical connection is set up.
Typically, a vendor provides these services as options that the customer can select when purchasing the equipment. Alternatively, the customer can purchase equipment that provides two or all three services and select a specific service based on application.

LAN Systems

The medium access control technique and topology are key characteristics used in the classification of LANs and in the development of standards. There are following system we will be discussing.
  1. Ethernet and Fast Ethernet (CSMA and CD)

    Ethernet is a standard communications protocol embedded in software and hardware devices, intended for building a local area network (LAN)Ethernet was designed by Bob Metcalfe in 1973, and through the efforts of Digital, Intel and Xerox (for which Metcalfe worked), "DIX" Ethernet became the standard model for LANs worldwide. The term Ethernet refers to the family of local-area network (LAN) products covered by the IEEE 802.3 standard that defines what is commonly known as the CSMA/CD protocol. Three data rates are currently defined for operation over optical fiber and twisted-pair cables:
  2.  
    • 10 Mbps-10Base-T Ethernet
    • 100 Mbps-Fast Ethernet
    • 1000 Mbps-Gigabit Ethernet
As mentioned earlier, Ethernet uses Carrier Sense Multiple Access with Collision Detection (CSMA/CD). When an Ethernet station is ready to transmit, it checks for the presence of a signal on the cable i.e. a voltage indicating that another station is transmitting. If no signal is present then the station begins transmission, however if a signal is already present then the station delays transmission until the cable is not in use.

History of CSMA/CD

The original Ethernet was developed as an experimental coaxial cable network in the 1970s by Xerox Corporation to operate with a data rate of 3 Mbps using a carrier sense multiple access collision detect (CSMA/CD) protocol for LANs with sporadic but occasionally heavy traffic requirements. Success with that project attracted early attention and led to the 1980 joint development of the 10-Mbps Ethernet Version 1.0 specification by the three-company consortium: Digital Equipment Corporation, Intel Corporation, and Xerox Corporation. The original IEEE 802.3 standard was based on, and was very similar to, the Ethernet Version 1.0 specification. The draft standard was approved by the 802.3 working group in 1983 and was subsequently published as an official standard in 1985 (ANSI/IEEE Std. 802.3-1985). Since then, a number of supplements to the standard have been defined to take advantage of improvements in the technologies and to support additional network media and higher data rate capabilities, plus several new optional network access control features.

Ethernet Network Elements

Ethernet LANs consist of network nodes and interconnecting media. The network nodes fall into two major classes:
  • Data terminal equipment (DTE) - Devices that are either the source or the destination of data frames. DTEs are typically devices such as PCs, workstations, file servers, or print servers that, as a group, are all often referred to as end stations.
  •  
  • Data communication equipment (DCE) - Intermediate network devices that receive and forward frames across the network. DCEs may be either standalone devices such as repeaters, network switches, and routers, or communications interface units such as interface cards and modems.
Ethernet Network Topologies and Structures

LANs take on many topological configurations, but regardless of their size or complexity, all will be a combination of only three basic interconnection structures or network building blocks.
  • The simplest structure is the point-to-point interconnection as shown in figure below. Only two network units are involved, and the connection may be DTE-to-DTE, DTE-to-DCE, or DCE-to-DCE. The cable in point-to-point interconnections is known as a network link. The maximum allowable length of the link depends on the type of cable and the transmission method that is used.
  •  
  • The original Ethernet networks were implemented with a coaxial bus structure, as shown in figure given below. Segment lengths were limited to 500 meters, and up to 100 stations could be connected to a single segment. Individual segments could be interconnected with repeaters, as long as multiple paths did not exist between any two stations on the network and the number of DTEs did not exceed 1024. The total path distance between the most-distant pair of stations was also not allowed to exceed a maximum prescribed value. Although new networks are no longer connected in a bus configuration, some older bus-connected networks do still exist and are still useful.
Since the early 1990s, the network configuration of choice has been the star-connected topology as shown in figure given below. The central network unit is either a multi-port repeater (also known as a hub) or a network switch. All connections in a star network are point-to-point links implemented with either twisted-pair or optical fiber cable.

CSMA/CD MAC Frame

The diagram given below describes the structure of the standard 802.3 Ethernet frames.
  • Preamble Field: A 7 octet pattern of alternating 0s and 1s used by the receiver to establish bit synchronization.
  •  
  • Start Frame Delimiter: Sequence 10101011 in a separate field, only in the 802.3 frame.
     
  • Destination Address: Hardware address (MAC address) of the destination station (usually 48 bits i.e. 6 bytes).
     
  • Source Address: Hardware address of the source station (must be of the same length as the destination address, the 802.3 standard allows for 2 or 6 byte addresses, although 2 byte addresses are never used).
     
  • Length: Specifies the length of the data segment, actually the number of LLC data bytes, (only applies to 802.3 frame and replaces the Type field).
     
  • LLC: Data unit supplied by LLC.
     
  • Data Unit: Actual data which is allowed anywhere between 46 to 1500 bytes within one frame.
     
  • Pad: Zeros added to the data field to 'Pad out' a short data field to 46 bytes (only applies to 802.3 frame).
     
  • FCS: Frame Check Sequence to detect errors that occur during transmission (802.3 version of CRC). This 32 bit code has an algorithm applied to it which will give the same result as the other end of the link, provided that the frame was transmitted successfully.
Token Ring

Unlike Ethernet, Token Ring uses a ring topology whereby the data is sent from one machine to the next and so on around the ring until it ends up back where it started. It also uses a token passing protocol which means that a machine can only use the network when it has control of the Token; this ensures that there are no collisions because only one machine can use the network at any given time.

Token Ring Operation

Token Ring and IEEE 802.5 are two principal examples of token-passing networks. Token-passing networks move a small frame, called a token, around the network. Possession of the token grants the right to transmit. If a node receiving the token has no information to send, it passes the token to the next end station. Each station can hold the token for a maximum period of time.
If a station possessing the token does have information to transmit, it seizes the token, alters 1 bit of the token (which turns the token into a start-of-frame sequence), appends the information that it wants to transmit, and sends this information to the next station on the ring. While the information frame is circling the ring, no token is on the network (unless the ring supports early token release), which means that other stations wanting to transmit must wait. Therefore, collisions cannot occur in Token Ring networks. If early token release is supported, a new token can be released when frame transmission is complete.

The information frame circulates the ring until it reaches the intended destination station, which copies the information for further processing. The information frame continues to circle the ring and is finally removed when it reaches the sending station. The sending station can check the returning frame to see whether the frame was seen and subsequently copied by the destination.
Unlike CSMA/CD networks (such as Ethernet), token-passing networks are deterministic, which means that it is possible to calculate the maximum time that will pass before any end station will be capable of transmitting. This feature and several reliability features, which are discussed in the section "Fault-Management Mechanisms," later in this chapter, make Token Ring networks ideal for applications in which delay must be predictable and robust network operation is important. Factory automation environments are examples of such applications.
A good gif example on internet here:

Simple Token Ring: http://www.datacottage.com/nch/anigifs/trani.gif
Hub Token Ring: http://www.datacottage.com/nch/anigifs/trhubani.gif

Token Ring MAC Frame Format

Token Ring and IEEE 802.5 support two basic frame types: tokens and data/command frames. Tokens are 3 bytes in length and consist of a start delimiter, an access control byte, and an end delimiter. Data/command frames vary in size, depending on the size of the Information field. Data frames carry information for upper-layer protocols, while command frames contain control information and have no data for upper-layer protocols. Both formats are shown figure given below.

It consist the following fields:
  • Start Deliminater (SD): Indicates start of the frame.
  •  
  • Access Control (AC): Indicates the frame's priority and whether it is a token or a data frame.
     
  • Frame Control (FC): Contains either Media Access Control information for all computers or "end station" information for only one computer.
     
  • Destination Address (DA): Indicates the address of the computer to receive the frame.
     
  • Source Address (SA): Indicates the computer that sent the frame.
     
  • Data Unit (DU): Contains the data being sent.
     
  • Frame Check Sequence (FCS): Contains CRC error-checking information.
     
  • End Deliminator (ED): Indicates the end of the frame.
     
  • Frame Status (FS): Tells whether the frame was recognized, copied, or whether the destination address was available.
Fiber Distributed Data Interface (FDDI)

FDDI (Fiber-Distributed Data Interface) is a standard for data transmission on fiber optic lines in that can extend in range up to 200 km (124 miles). The FDDI protocol is based on the token ring protocol. In addition to being large geographically, an FDDI local area network can support thousands of users.

An FDDI network contains two token rings, one for possible backup in case the primary ring fails. The primary ring offers up to 100 Mbps capacity. If the secondary ring is not needed for backup, it can also carry data, extending capacity to 200 Mbps. The single ring can extend the maximum distance; a dual ring can extend 100 km (62 miles).

FDDI is a product of American National Standards Committee X3-T9 and conforms to the open system interconnect (OSI) model of functional layering. It can be used to interconnect LANs using other protocols. FDDI-II is a version of FDDI that adds the capability to add circuit-switched service to the network so that voice signals can also be handled. Work is underway to connect FDDI networks to the developing Synchronous Optical Network.

Function of FDDI

The Fiber Distributed Data Interface (FDDI) specifies a 100-Mbps token-passing, dual-ring LAN using fiber-optic cable. FDDI is frequently used as high-speed backbone technology because of its support for high bandwidth and greater distances than copper. It should be noted that relatively recently, a related copper specification, called Copper Distributed Data Interface (CDDI) has emerged to provide 100-Mbps service over copper. CDDI is the implementation of FDDI protocols over twisted-pair copper wire. This chapter focuses mainly on FDDI specifications and operations, but it also provides a high-level overview of CDDI.

FDDI uses dual-ring architecture with traffic on each ring flowing in opposite directions (called counter-rotating). The dual-rings consist of a primary and a secondary ring. During normal operation, the primary ring is used for data transmission, and the secondary ring remains idle. The primary purpose of the dual rings, as will be discussed in detail later in this chapter, is to provide superior reliability and robustness. Figure shows the counter-rotating primary and secondary FDDI rings.

Bridges

A LAN bridge connects two or more LANs at layer two in the OSI network model. The LAN bridge receives packets from a LAN segment connected to one port and forwards them to another LAN segment connected to a different port. While a LAN bridge serves the purpose of extending network range, it also relieves the problem of congestion that multiple devices can cause on a single Ethernet segment. LAN bridges employ varying mechanisms to deliver their functionality. A simple LAN bridge regulates the transmission of frames to avoid congestion on the network. A learning LAN bridge remembers (learns) the Ethernet address of each frame it receives, in order to record which devices are connected to each port. The learning bridge can then examine the destination address of each received frame to determine whether or not it should be forwarded to another part of the network. This selective forwarding improves the efficiency of communications across the network. While bridges provide services similar to those offered by routers and repeaters, there are some significant differences. Routers, like LAN bridges, act as agents to receive and forward messages. Unlike a router, however, a LAN bridge has no network-layer address. The LAN bridge is transparent to both client and server workstations. Repeaters, are like LAN bridges in that they also transmit information across an Ethernet network. But having no memory, a repeater will retransmit all the data it receives, including any frames that cause collisions. Unlike a repeater, A LAN bridge has the memory and intelligence to alleviate collisions when forwarding Ethernet frames.

Bridges can be grouped into categories based on various product characteristics. Using one popular classification scheme, bridges are either local or remote. Local bridges provide a direct connection between multiple LAN segments in the same area. Remote bridges connect multiple LAN segments in different areas, usually over telecommunications lines. The figure illustrates these two configurations.

Remote bridging presents several unique internetworking challenges, one of which is the difference between LAN and WAN speeds. Although several fast WAN technologies now are establishing a presence in geographically dispersed internetworks, LAN speeds are often much faster than WAN speeds. Vast differences in LAN and WAN speeds can prevent users from running delay-sensitive LAN applications over the WAN. Remote bridges cannot improve WAN speeds, but they can compensate for speed discrepancies through a sufficient buffering capability. If a LAN device capable of a 3-Mbps transmission rate wants to communicate with a device on a remote LAN, the local bridge must regulate the 3-Mbps data stream so that it does not overwhelm the 64-kbps serial link. This is done by storing the incoming data in onboard buffers and sending it over the serial link at a rate that the serial link can accommodate. This buffering can be achieved only for short bursts of data that do not overwhelm the bridge's buffering capability.

There are several reasons for the use of multiple LAN's interconnected:
  • Geography
  • Performance
  • Reliability
  • Security

Basics Of Data Communication Part: 1

This article will explain, Data Communication Model, Data Communication System Tasks and Communication Network and Services.


Introduction and Basics of Data Communication Model

Data Communications is the transfer of data or information between a source and a receiver. The source transmits the data and the receiver receives it. Data communication involved the following like communication networks, different communication services required, the kind of networks available, protocol architectures, OSI models, TCP/IP protocol models etc. Data Communication is interested in the transfer of data, the method of transfer and the preservation of the data during the transfer process.

In Local Area Networks, we are interested in "connectivity", connecting computers together to share resources. Even though the computers can have different disk operating systems, languages, cabling and locations, they still can communicate to one another and share resources.

The purpose of Data Communications is to provide the rules and regulations that allow computers with different disk operating systems, languages, cabling and locations to share resources. The rules and regulations are called protocols and standards in Data Communications.

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Source

It is the generator of data that will pass on the destination using networks. Without any request source never passes the data to destination. So, if source is passing the data means any of the destinations is requesting for data using some query languages.

Transmitter

It is simply a device used to convert the data as per the destination requirement. For example a modem, converts the analog (telephonic signals) signal to digital (computer signals) signals and alternatively digital to analog also.

Transmission System

To transmit the data on different connected systems we use different transmission systems. Data transmission using transmission system means the physical transfer of data over point-to-point or point-to-multipoint communication channels. Example of such channels are copper wires, optical fibers even wireless communication channels etc.

Receiver

This receives the signals from the transmission system and converts it into a form that is suitable to the destination device. For example, a modem accepts analog signal from a transmission channel and transforms it into digital bit stream which is acceptable by computer system.

Destination

It is simply a device for which source device sends the data.

Data Communication System Tasks

There are some tasks performed by the communication systems are

Signal Generation

To transmit the data over the transmission system, communicating device must be able to generate and receive these signals. The generation of the signals should be in such a way that the resultant signal can be acceptable by the transmission mediums.

Interface

Device must interface with the transmission system to communicate or transfer the data over network.

Data Synchronization

It is the process of establishing consistency among data from a source to destination devices and vice versa and continuous harmonization of the data over time.

Exchange Management

For meaningful data transaction there should be some management of data being exchanged. Both the transmitter and receiver should adhere to some common convention about the format of data, amount of data, time required, data format etc.

Transmission System Utilization

Due to the importance of Data transmissions without interruptions or failures the transmission systems is usually well dimensioned and are being operated with margins that minimize the possibility of outages. Various techniques are available to allocate the total capacity of a transmission channel among connected devices like Digital, Analog, Multiplex, Simplex, Duplex, Half-Duplex etc.

Error Detection and Correction

In any communication system transmitted data is prone to error. Either it is because of transmitted signal getting distorted in the transmission medium leading to misinterpretation of signal or errors introduced by the intermediate devices. Error detection and correction is required in cases where there is no scope for error in the data transaction. We can think of file transfer between two computers or even on remote network computers where there is a need for this. But in some cases it may not be very important as in the case of telephonic conversation.

Flow Control

At the time of transmission of data, source computer is generating data faster than receiver device capable to receive it. To handle such problem, there is some kind of flow control mechanism used. Before getting started the transmission of data they have to agree upon between two communication devices.

Addressing

When more than two devices share a transmission facility, a source system must somehow indicate the identity or address of the destination. Addresses are in form of IP or we can say ftp address and there are used lots of credentials.

Routing

Routing means to send data to appropriate destinations. In this process the evolved computer ensures that the data is being sent on destination system only or any other hacking happening. To eliminate such problem developers uses SSL level security.

Communication Network and Services

Communication Network is set of equipment or say facilities that provide a communication services like to transfer of data between two or more nodes located in any of its geographical point. Example of such networks includes computer networks (LAN/WAN), intranet networks, telephone networks, television broadcasting networks, cellular networks etc.

Radio and Television Networks
These networks are very common network usage various stations to transmit an ensemble of signals simultaneously over network of cables. Aside from selecting the station of interest, the role of the user in these services is passive. Relatively high audio and video quality is expected here but a significant amount of delay (fraction of second) can be tolerated even in live broadcasting.

Telephone Networks

This service is real-time service provided by a network. Two persons are able to communicate by transmitting their voice across the network. These services is called connection-oriented service because to establish such communication users must first interact with the network.

Cellular Networks

These networks extends the normal telephone service to mobile users who are free to move within a regional area covered by an interconnected array of smaller geographical areas called cells. Each cell has a radio transmission system that allows it to communicate with users in its area. Cellular provides also support a roaming service where a subscriber is able to place calls while visiting regional area other than the home.

There are many other network services like Video on Demand, Streaming Audiovisual, and Audio-Video Conferencing etc available.

Basics Of Data Communication: Part 6

This article will explain, High-level Data Link Control (HDLC), HDLC Frame Structure, HDLC Operation etc.



High-level Data Link Control (HDLC)

It is the most important data link control protocol is HDLC. It is not only widely used, but also it is the basis for many other important data link control protocols, which use the same or similar formats and the same mechanisms as employed in HDLC.

Characteristics of HDLC

To satisfy a variety of applications, HDLC defines three types of stations, two link configurations and three data-transfer modes of operation.

The three station types are:

  1. Primary Station

    It has responsibility for controlling the operation of the link. Frames issued by the primary are called commands.
  2.  
  3. Secondary Station

    It has responsibility for controlling operations under the control of the primary station. Frames issued by a secondary are called responses. The primary maintains a separate logical link with each secondary station on the line.
     
  4. Combined Station

    It combines the features of primary and secondary stations. A combined station may issue both commands and responses.

The two link configurations are:
  1. Unbalanced Configuration

  2. It consists of one primary and one or more secondary stations and supports both full-duplex and half-duplex transmission.
     
  3. Balanced Configuration

    It consists of two combined stations and supports both full-duplex and half-duplex transmission.


The three data transfer modes are:
  1. Normal Response Mode (NRM)

    It is used with an unbalanced configuration. The primary may initiate data transfer to a secondary but a secondary may only transmit data in response to a command from the primary.
     
  2. Asynchronous Balanced Mode (ABM)

    It is used with an balanced configuration. Either combined station may initiate transmission without receiving permission from the other combined station.
     
  3. Asynchronous Response Mode (ARM)

    It is used with an unbalanced configuration. The secondary may initiate transmission without explicit permission of the primary. The primary still retains responsibility for the line, including initialization, error recovery and logical disconnection.

HDLC Frame Structure


HDLC is most important data link control protocol. It uses synchronous transmission. Any transmission is held in the form of frames and a single frame format suffices for all types of data and control exchange.

Here is the list of frame structure part:
  1. Flag Fields

    It delimits the frame at both ends with the unique pattern 01111110. A single flag may be used as the closing flag for one frame and the opening flag for the next. On both sides of the user-network interface, receivers are continuously hunting for the flag sequence to synchronize on the start of a frame. While receiving a frame, a station continues to hunt for that sequence to determine the end of the frame. However, it is possible that the pattern 01111110 will appear somewhere inside the frame, thus destroying frame-level synchronization. To avoid this, a procedure known as bit stuffing is used.
  2.  
  3. Address Field

    The address field identifies the primary or secondary stations involvement in the frame transmission or reception. Each station on the link has a unique address. In an unbalanced configuration, the A field in both commands and a response refers to the secondary station. In a balanced configuration, the command frame contains the destination station address and the response frame has the sending station's address.
     
  4. Control Field

    HDLC uses the control field to determine how to control the communications process. This field contains the commands, responses and sequences numbers used to maintain the data flow accountability of the link, defines the functions of the frame and initiates the logic to control the movement of traffic between sending and receiving stations. There three control field formats:
     
    • Information Transfer Format (I-Frame)

      The frame is used to transmit end-user data between two devices.
    •  
    • Supervisory Format (S-Frame)

      The control field performs control functions such as acknowledgment of frames, requests for re-transmission, and requests for temporary suspension of frames being transmitted. Its use depends on the operational mode being used.
       
    • Unnumbered Format (U-Frame)

      This control field format is also used for control purposes. It is used to perform link initialization, link disconnection and other link control functions.
       
  5. Information Field

    This field presents only Information Frame (I-Frame) and Unnumbered-Frame (U-Frame). The field can contain any sequence of bits but must consist of integral number octets. The length of the information field is variable up to some system-defined maximum.
     
  6. FCS (Frame Check Sequence) Field

    It is an error-detecting code, calculated from the remaining bits of the frame, exclusive of flags. The normal code is the 16-bit CRC (Cyclic Redundancy Check). An optional 32-bit FCS, using CRC-32, may be employed if the frame length or the line reliability dictates this choice.

HDLC Operation


HDLC operation consists of the exchange of I-Frame, S-Frame and U-Frame between two or more stations. The operation of HDLC involves three most important phases. First, one side or another initializes the data link so that frames may be exchanged in an orderly fashion. During this phase, the options that are to be used are agreed upon. After initialization, the two sides exchange user data and the control information to exercise flow and error control. Finally, one of the two sides signals the termination of the operation.

Initialization

This is done by either side by issuing one of the six set-mode commands as described in Frame Structure. This command serves three purposes:
  • It signals the other side that initialization is required.
  • It specifies which of the three modes (NRM, ABM, ARM) is required.
  • It specifies whether 3 or 7 bit sequence numbers are to be used.
If the other side accepts this request, then the HDLC module on that end transmits an unnumbered acknowledged (UA) frame back to the initiating side. If the request is rejected then a disconnected mode (DM) frame is sent.

Data Transfer

When the initialization has been requested and accepted, then a logical connection is established. Both sides may begin to send user data in I-frames. Flow control and error control are provided by using the N(s) and N(r) fields (send/receive sequence number). A station numbers the frames it sends sequentially modulo 8 or 128, depending on whether 3- or 7-bit sequence numbers are used, and places the sequence number in N(s). When a station receives a valid I-frame, it acknowledges that frame with its own I-frame by setting the N(r) field to the number of the next frame it expects to receive. This is known as piggi-backed acknowledgment, since the acknowledgment rides back on an I-frame. Acknowledgments can also be sent on a supervisory frame.

The use of sequence numbers accomplishes three important functions:
  • Flow Control

    A station is only allowed to send 7 frames (3-bit sequence number) or 127 frames (7-bit sequence number) without an acknowledgment. No more frames may be sent until some of the outstanding frames are acknowledged. Thus, if the receiver is slow to acknowledge, the sender's output is restricted.
  •  
  • Pipelining

    More than one frame may be in transit at a time; this allows more efficient use of links with high propagation delay, such as satellite links.
     
  • Error Control

    If a frame is received in error, a station can send a negative acknowledgment via a supervisory frame to specify which frame was received in error. This may be done in one of two ways. In the go-back-N() approach, the sending station retransmits the rejected frame plus all subsequent frames that have been transmitted since the rejected frame. In the selective-repeat approach, the sending station retransmits only the frame received in error.
Supervisory Frames:-

There are four types of S-frames. The receive-ready (RR) frame is used to acknowledge the last I-frame received by indicating the next I-frame expected. The RR is used when there is no reverse use data traffic (I-frames). Receive-not-ready (RNR) acknowledges an I-frame, as with RR, but also asks the peer entity to suspend transmission of I-frames. When the entity that issued the RNR is again ready, it sends an RR. REJ indicates that the last I-frame received has been rejected and that retransmission of all I-frames beginning with number N(r) is required. Selective reject (SREJ) is used to request retransmission of just a single frame.

Disconnect

Either HDLC module can initiate disconnect, either on its own initiative if there is some sort of fault, or at the request of its higher-layer user. HDLC issues disconnect by sending disconnect (DISC) frame. The other side must accept disconnect by replying with an acknowledgement. 

Basics of Data Communication: Part 4

This article will explain about, Transmission Media, Guided Transmission Media, Twisted Pair, Coaxial Cable, Optical fiber, Wireless Transmission media, Multiplexing, Circuit Switching etc.



Transmission Media

In data transmission system, transmission medium is used between transmitter and receiver. Transmission media can be classified as guided and unguided. In both cases, communication is in the form of electromagnetic waves. With guided media, the waves are guided along a solid medium, such as copper twisted pair, copper coaxial cable and optical fiber. The atmosphere and outer space are examples of unguided media that provide a means of transmitting electromagnetic signals but do not guide them, this form of transmission is usually referred as wireless transmission.


Guided Transmission Media

In guided transmission media, transmission depend on data rate, bandwidth, distance, medium is point-to-point or multipoint in LAN.
 

There are mainly three types of guided media used as discussed below.

Twisted Pair

A twisted pair consists of two insulated copper wires arranged in a regular spiral pattern. A wire pair acts as a single communication link. Typically, a number of these pairs are bundled together into cable by wrapping them in a tough (hard) protective sheath. It is one of the oldest and most common transmission medium.

TwistedPair.gif

Twisted pair may be used to transmit both analog and digital signals. For analog transmission, amplifiers are required about every 5 to 6km of distance. For digital signals, repeaters are required in about every 2 to 3km of distance. For point-to-point analog signaling, bandwidth of up to about 250 kHz is possible. For long-distance digital point-to-point signaling data-rates of up to a few Mbps are possible but for short distance data-rate of up to 100Mbps have been achieved.

Coaxial Cable

Coaxial cable consists of two conductors, an inner conductor supported by an insulating material. The insulator is encased in an outer cylindrical conductor and the outer conductor is covered with a protected plastic sheath. Due to its better construction and shielding, coaxial cable provides high bandwidth and frequency and also excellent noise immunity. It is most versatile transmission medium which finds its use in a variety of applications such as LAN, log distance telephone transmission, telephone distribution etc. A higher bandwidth can be achieved by a cable of short distance for example, for a cable of 1km, data rate can be 1 to 2Gbps but it goes down as long increases. The bandwidth can be 100 kHz to 500 kHz for the same cable.

CoaxialCable.gif

Coaxial cable is perhaps the most versatile transmission medium and is enjoying widespread use in a wide variety of applications, the most important of these are
  • Television distribution
  • Long-distance telephone transmission
  • Short-run computer system links
  • Local Area Networks

Coaxial cable has traditionally been an important part of the long-distance telephone network. Today, it is getting replaced by optical fiber, terrestrial microwave and satellite. Using frequency-division multiplexing, a coaxial cable can be carry over 10000 voice channels simultaneously. Coaxial cable is also commonly used for short-range connections between devices. Using signaling, coaxial cable can be used to provide high-speed I/O channels on computer systems.

Optical Fiber

Optical fiber transmission has three components; the light source, the transmission medium and detector. A pulse of light indicates a bit 1 and absence of light indicates bit 0. Transmission medium is an ultra thin fiber of glass. The transmitter generates the light pulses based on the input electrical signal. The detector regenerates the electrical signal based on the light signal it detects on the transmission medium, by attaching a light source to one end of an optical fiber and a detector to the other end.

OpticalFiber.gif

The major advantages offered by optical fiber over wire media (i.e., twisted pair and coaxial cable) are its immunity to noise, less signal distortion and higher bandwidth. Some of the advantages/disadvantages of optical fiber have been discuss below:

Advantages
  1. Immunity to Electrical Noise

    Fiber optics communication uses light signals in place of electrical signals. Instead of electrons that are found in metallic media fiber optics have electrically non-conducting photons, so the light signal is totally immune to the electrical noise.  
  2. Reduced Loss of Signal Strength

    Optical fibers have less loss of signal strength. The strength of a light signal is typically reduced only slightly after propagation through several kilometers of optical fiber cables. For regeneration of signals, repeaters can be placed as far as 50-60kms.
     
  3. Large Information Carrying Capacity

    Optical fiber cables can support much higher bandwidths than wire media and hence offer large information carrying capacity.
     
  4. Security

    Signals in the cable are secure from unauthorized listeners. It is difficult to tap into the cable as no light escapes out.

Disadvantages
  1. Cost Factor

    The major disadvantage of optical fiber is its high costs. It is very expensive transmission media because of its high manufacturing costs.  
  2. Installation and Maintenance of Cable

    Laying of the cable is a complex job and it needs a lot of time and ground work.
     
  3. Termination and Connection

    Any break or cracking in the core of an optical fiber diffuse light and distorts the signal. Termination and connection of fiber optic cable needs special tools. It is very difficult to support multiple users on the fiber, that is why, fiber optic cable is still used for point to point communication systems.
     
  4. Fragility

    Optical fiber cable is very fragile. Due to its glass or plastic construction, it can be more easily broken than wires.

Wireless Transmission

Wireless transmission comes under unguided transmission. The need of wireless transmission arises, when it is required to be on-line all the time. For wireless communication, there is no need of using twisted pair, coaxial or optical fiber cables. In this type of transmission, sending and receiving of signals are achieved by means of an antenna. In data communication, at sending end, the antenna radiates electromagnetic energy into the medium and at receiving end; the antenna receives the electromagnetic energy from the surrounding medium. There are basically two types of transmission: directional and omni-directional. In directional transmission, the transmitting antenna puts out a focused electromagnetic beam, so it is very important that the transmitting and receiving antenna must be carefully aligned to each other. In omni-directional transmission, the transmitting antenna spreads out the transmitting signals in all directions that are received by many antennas. Here are list of ways in which wireless transmission is possible:

Radio TransmissionRadio waves are easy to generate, can travel long distances and penetrate (passes) buildings easily. So they are widely used for communication. They are omni-directional, which means that can travel in all directions. The properties of radio waves are frequency dependent. At low frequencies, these waves pass very well through obstacles but he power falls off sharply as the distance increases from source. At high frequencies, radio waves tend to travel in straight lines and bounce off obstacles. They are also absorbed by rain. At all frequencies, radio waves are subject to interference from motor and other electrical equipments. In VLF, LF and MF bands, radio waves follow the ground. These waves can be detected at the lower frequencies for perhaps 1000km less than higher ones. At such frequencies, radio waves easily pass through buildings but the main problem is that under certain atmospheric conditions the signals may bounce several times.

Microwave TransmissionTransmission of waves, above 100 Mhz frequency is known as microwave transmission. Microwave transmission is a directional type of transmission in which multiple transmissions are lined up in a row to communicate with multiple receivers in a row without any interface. Microwave is also relatively in-expensive than the optical fiber. There are two types of microwave transmissions:
  1. Terrestrial Microwave

    Microwaves travel in a straight line and therefore required line of sight transmission. The distance that can be covered by a line of sight signal, depends on the height of the antenna. For 10m high towers, repeaters can be spaced 80km apart. Typically these antennas are mounted on towers.  
  2. Satellite Microwave

    Emerging mobile networks encompass a broad spectrum of technologies ranging from analog to digital cellular phones, satellite networks, etc. satellite communication uses microwave frequency antennas to receive radio signals from transmitting stations on earth and to relay the signals back to earth stations. The satellite serves as an electronic relay station. Earth station A transmits signals of a specific frequency (uplink) to the satellite. In turn the satellite receives the signals and retransmits them back to earth station B.

Multiplexing

Combining multiple signals (analog and digital) for transmission over a single line or media is known as multiplexing. A common type of multiplexing combines several low-speed signals for transmission over a single high-speed connection. Multiplexing is used to enable many users to share the system.

Multiplexing.gif

Three types of multiplexing approaches that can be employed are frequency, time and wavelength divisions.

Frequency-Division Multiplexing (FDM)
FDM is useful when the bandwidth of the transmission medium is more than the required bandwidth of all the signals to be transmitted. A number of signals can be carried simultaneously, if each signal is modulated onto a different carrier frequency and the carrier frequencies are sufficiently separated such that the bandwidths of the signals do not overlap. FDM was introduced in the telephone network in the 1930s. The basic analog multiplexer combines 12 voice channels in one line. Each voice signal occupies 4 kHz of bandwidth. The multiplexer modulates each voice signal so that it occupies a 4 kHz slot in the band between 60 and 108 kHz. The combined signal is called group.

Frequency-Division-Multiple.gif

Time-Division Multiplexing (TDM)
Time division multiplexing provides a user with full channel capacity but divides the channel usage into time slots. Here, the available time is divided into small slots and each of these slots is occupied by one of the signal streams that are to be sent. The slot is assigned to each signal one by one in a round robin manner. Only one signal occupies the channel at one instant. It is thus quite different from FDM, in which all signals are sent at the same time each occupying a different band.

TDM.gif

A TDM system is a serial system because the signal from each user follows in time the signal from another user. The overall bandwidth of TDM result exceeds any individual signal. This is because the TDM output carries much more information and information requires bandwidth. The final bandwidth is equivalent to sum of individual bandwidths of all signals.

Wavelength Division Multiplexing (WDM)
Wavelength Division Multiplexing (WDM) is a technique that multiple signals are carried together as separate wavelengths (color) of light in a multiplexed signal. WDM is used in optical fiber networks. WDM and FDM (Frequency Division multiplex) are both based on the same principles but WDM applies to wavelengths of light in optical fiber while FDM is used in electrical analog transmission. A WDM optical system using a diffraction grating is completely passive, unlike electrical FDM, and thus is highly reliable. Further, a carrier wave of each WDM optical channel is higher than that of an FDM channel by a million times in frequency (THz versus MHz).

WDM.gif

Circuit Switching

When we have multiple work stations then a problem is connecting them for one to one communication. One solution for this problem is to establish point to point connection between each individual pair of stations. To interconnect n stations, n(n-1)/2 individual connections are needed. For example, to connect 100 components then totally 4950 individual connections are required. This requirement increases drastically as the number of stations to be increases and for large number stations, it is almost impossible to maintain connections. Now to overcome this problem switching is better solution. The switches can be placed on the transmission path so that the stations do not need to be interconnected directly. Switches are hardware/software capable of creating connections temporally between two or more stations.

CircuitSwiching.gif

Circuit switching can use either of the following two technologies:

Space Division Switching

It was originally developed for transmitting analog data but it now carries digital data also. In space-division switching, users have a channel dedicated to themselves. Each connection requires the establishment of physical path through the switch that is dedicated solely to the transfer of data between two endpoints. The basic building block of the switch is a metallic cross-point that can be enabled or disabled by a control unit.

Space.gif

# Multistage Switches

In this different crossbar switches are combined in several stages. Here, devices are linked to switches, which in turn are linked to a hierarchy of other switches. The design of multistage switches depends on the number of stages and the desired number of switches in each stage.

Time Division Switching

Time division switching uses time-division multiplexing to implement switching function. Two methods are used in time-division multiplexing.

Basics Of Data Communication: Part 5

This article will explain, Data Link Control, Error Detection, Flow Control, Error Control Mechanisms etc.



Data Link Control

Data Link Control (DLC) is the service provided by the data link layer of function defined in the Open Systems Interconnection (OSI) model for network communication. The Data Link layer is responsible for providing reliable data transfer across one physical link within the network.

To see the need for data link control, we list some requirements and objectives for effective data communication between two directly connected transmitting-receiving stations. Here is the list of all

  1. Frame Synchronization
    Data are sent in blocks called frames. The beginning and end of each frame must be recognizable.

  2. Flow Control
    The sending station must not send frames at a rate faster than the receiving station can absorb them.

  3. Error Control
    Any bit errors introduced by the transmission system must be corrected.

  4. AddressingOn a multipoint line, such as a local area network (LAN), the identity of the two stations involved in a transmission must be specified.
  5. Control and data on same link
    It is usually not desirable to have a physically separate communications path for control information. Accordingly, the receiver must be able to distinguish control information from the data being transmitted.

  6. Link Management
    The initiation, maintenance and termination of a sustained data exchange require a fair amount of coordination and cooperation among stations. Procedures for the management of this exchange are required.
Error Detection

While transmitting data from one station to another, care should be taken to ensure that the receiver receives the same data as sent by the sender without any error. The purpose of error-detection methods is to detect whether the data is error free or not. If not, then ascertain what type of error exists. Two types of error need to be detected: single-bit errors and burst errors. In single-bit errors, only one bit of data is corrupted while in burst errors two or more bits in the data are corrupted. Error detection methods are based on the concept of redundancy. A shorter group of bits is appended to the end of each data unit which serves to detect errors. It contains no data information as such and hence can be discarded after the accuracy of the transmission has been checked. The most common error detection techniques are:

  1. Single Parity checking

    The simplest code is the single parity check code that takes k information bits and appends a single check bit to form a codeword, which will be transmitted over the channel. The parity check at the receiver ensures that the total number of 1s in the receiver codeword is even; that is, the codeword has even parity. The check bit in this case is called a parity bit. Here the receiver codeword is valid if it has even number of 1s, otherwise it is invalid and there is some error in the received codeword. This error-detection code is used in ASCII where characters are represented by seven bit and the eight bit consists of a parity bit

    If a codeword undergoes a single error during transmission, then the corresponding binary block at the output of the channel will contain an odd number of 1s and the error will be detected. More generally, if the codeword undergoes an odd number of errors, the corresponding output block will also contain an odd number of 1s. Therefore, the single parity bit allows us to detect all error patterns that introduce an odd number of errors. On the other hand, the single parity bit will fail to detect any error patterns that introduce an even number of errors, since the resulting codeword will have even parity which is valid codeword.

    At the transmitter a checksum is calculated from the information bits and transmitted along with the information. At the receiver, the checksum is recalculated, based on the received information. The received and recalculated checksums are compared and the error alarm is set if they disagree.

    The simple example can be used to present two fundamental observations about error detection. The first observation is that error detection requires redundancy in that the amount of information that is transmitted is over and above the required minimum. For a single parity check code of length k+1, k bits are information bits and one bit is the parity bit.
  2. Cyclic Redundancy Code (CRC)
    One of the most common and most powerful, error-detecting code is the CRC which can be described as follows. Given a k bit block of bits or message, the transmitter generates an n bit sequence known as a Frame Check Sequence (FCS), so that the resulting frame, consisting of k+n bits, is exactly divisible by some predetermined number called CRC polynomial. The receiver then divides the incoming frame of k+n bits by the same CRC polynomial number and if there is no remainder, assumes there was no error. The CRC polynomial number by which the information frame bits are divided are selected such that distance between two valid codeword is high and with proper selection of CRC polynomial it is possible to detect errors with very high probability i.e. more than 99.9% of the errors can be detected. Even it is possible to find out which bits are in error when only few bits are corrupted in the information bit stream. So with CRC it is also possible to correct the error though it is not possible for all possible errors.

Flow Control

One major problem that occurs in data link layer is of handling the situation when frames are sent at a rate faster than the receiving rate of the receiver. This situation occurs when the sender is running on a fast computer and receiver is on a slow machine. The sender keeps pumping the frames at a higher rate until the receiver is completely flooded. Suppose the transmission is error-free, but still at some point in time the receiver will simply not be able to handle the frames as they arrive and will start to lose them. Some measures have to be taken to prevent such a situation.

The solution to this is to introduce a flow control mechanism. Flow control mechanisms are employed to ensure that the data link layer at the sending end does not transmit more frames than what the data link layer at receiving end is capable of handling. In this situation, the receiver is provided with a control to regulate the flow of the incoming frames in the form of acknowledgment (ACK). After receiving the frame, the receiver sends some acknowledgment so as to make the sender aware whether the receiver received the frame or not. The acknowledgment solves the dual purpose - of clearing the sending end to transmit the next data frame and to acknowledge receipt of all previous frames.

Sliding Window Protocol

In contrast to stop-and-wait, which can have only one frame to transmit at a time? Sliding window can have more than one frame (the exact number depending on the size of the window) at a time, i.e. sender can send a certain number of frames without waiting for the acknowledgment.

This overcomes the inefficient use of bandwidth (as one frame takes the whole bandwidth) in stop-and-wait. Sliding window refers to imaginary slots (each slot capable of holding one frame) at both the sender and receiver end.
At the sender's end initially all these slots contain data frames and at the receiver's end, available memory spaces are used to receive these frames. The window is called sliding because it decreases from the left when sending window sends a frame and increases to the right when sender receives acknowledgment.

For the receiving end, window decreases from left when a frame is received and moves to the right when an ACK is sent. The window has a size n-1 where frames are numbered modulo-n, i.e. from 0 to n-1 for identification purpose. The size of window is n-1 and not n so that there is no ambiguity in knowing which frames have been acknowledged.

Error Control Mechanisms

Error control means methods of errors detection and transmission. Error control methods, when incorporated with flow-control protocols are called Automatic Repeat Request (ARQ), i.e. whenever the receiver detects an error in data, it sends back a negative acknowledgment (NAK) and the specified frame is retransmitted. ARQ also retransmits in case of lost frames, lost acknowledgment or lost NAK.

ARQ is implemented in the following three variations:

  1. Stop-and-Wait ARQ

    A few modification or capabilities are added to the basic stop-and-wait flow control to implement it as an ARQ. These are keeping a copy of the last frame transmitted at the sender end and giving sequence numbers 0 and 1 alternatively to each frame to help in identification of which frame is received correctly. The acknowledgment frame holds the number of the frame the received next accepts to receive thus, sending an implicit message to the sender that the last frame has been received correctly. A negative acknowledgment contains no seque3nce number; the sender just retransmits the last frame sent. This number also helps in receiver identifying the duplicate transmissions in case of lost acknowledgments.
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  2. Go-back-n ARQ
    The following additions are made to the basic sliding-window protocol to accommodate sliding-window ARQ. The sending end keeps the copies of all the frames it has sent but not received a positive acknowledgment as yet. In sliding-window, even the NAK is numbered to identify which frame(s) is to be resent. ACK frames carry the number of the next frame the receiver expects while NAK carries the number of the damaged frame.
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  3. Selective-Reject ARQ

    In this mechanism, if a damaged frame is received then unlike Go-back-n, only that frame is retransmitted. This is requires a more complex logic for implementation as the receiver will receive frames out of sequence and must have the capability to arrange frames in order. Thus, if a sender receivers a NAK, it must resend the frame out of sequence of frames. The sender thus must contain a searching mechanism.
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Basics of Data Communication Part: 2

This article will explain, Data Communication Networking, Protocols and Protocol Architecture.



Data Communication Networking

Data Communication takes place between two devices that are directly connected by some form of point-to-point transmission medium. The devices are very far apart. It would be inordinately expensive if string a dedicated link between two devices which is thousands of miles apart. There is set of devices used to establish such networks. There are following types of networking available:

Wide Area Network (WAN)
Wide area networks have traditionally been considered to be those that cover a large geographical area. Typically, a WAN consists of a number of interconnected switching nodes. Transmission from any one device is routed through these internal nodes to the specific destination device. WAN has been implemented using one of the following technologies:

Circuit Switching
In circuit-switching, this path is decided upon before the data transmission starts. The system decides on which route to follow, based on a resource-optimizing algorithm, and transmission goes according to the path. For the whole length of the communication session between the two communicating bodies, the route is dedicated and exclusive, and released only when the session terminates.

Packet Switching
In packet-switching, the packets are sent towards the destination irrespective of each other. Each packet has to find its own route to the destination. There is no predetermined path; the decision as to which node to hop to in the next step is taken only when a node is reached. Each packet finds its way using the information it carries, such as the source and destination IP addresses.

Frame Relay

Frame relay was developed to take advantage of these high data rates and low error rates. Whereas the original packet switching networks were designed with a data rate to the end user of about 64kbps, frame relay networks are designed to operate efficiently at user data rates of up to 2Mbps.

ATM

Asynchronous Transfer Mode (ATM) can be viewed as an evolution from frame relay. The most obvious difference between frame relay and ATM is that frame relay uses variable-length packets, called frames, and ATM uses fixed-length packets, called cells. As with frame relay, ATM provides little overhead for error control, depending on the inherent reliability of the transmission system and on higher layers of logic in the end systems to catch and correct errors. By using a fixed packet length, the processing overhead is reduced ever further for ATM compared to frame relay. The result is that ATM is designed to work in the range of 10s and 100s of Mbps, compared to the 2Mbps target of frame relay.

Local Area Network (LAN)
LAN is a communication network that interconnects a variety of devices and provides a means for information exchanged among those devises. The scope of the LAN is small, typically a single building or a cluster of buildings. It is usually the case that the LAN is owned by the same organization that owns the attached devices. The internal data transfer rates of the LANs are greater than WANs. There are various topologies are possible like Bus, Right etc.

Ethernet
In Bus topology, at any instance one machine is the master and is allowed to transmit. An arbitration mechanism is needed to resolve conflicts when two or more machine wants to transmit simultaneously. The arbitration mechanism may be centralized or distributed. IEEE 802.3 CSMA/CD popularly known as Ethernet, is a bus based broadcast network with decentralized control operating at 10 or 100 Mbps. Computers on an Ethernet can transmit whenever they want to. If two or more packets collide, each computer just waits a random time and tries again later to send.

Token Ring
Unlike Ethernet, Token Ring uses a ring topology whereby the data is sent from one machine to the next and so on around the ring until it ends up back where it started. It also uses a token passing protocol which means that a machine can only use the network when it has control of the Token; this ensures that there are no collisions because only one machine can use the network at any given time.

ISDN and Broadband ISDN
Through Integrated Service Digital Network (ISDN) user with a single access point to ISDN network can avail of different kinds of communication like his computer can access the internet, he can use the network for his telephone usages and also probably video communication. The ISDN is intended to be a worldwide public telecommunications network to replace existing public telecommunications networks and deliver a wide variety of services.
The second generation referred to as Broadband ISDN, supports vary high data rates (100s of Mbps) and has a packet switching oriented.

Protocols and Protocol Architecture

To transfer any file or say data between two or more computers there must be a path either directly or any communication network known as computer communication. Similarly when two or more computers are interconnected via a communication network referred as computer network. Computer communication and computer network has some protocols and communication architecture.

A protocol is set of rules for communicating between computers. Protocol includes the key like data format, timings, sequencing, error controls etc. Without these rules, the computer can not make sense of the stream of incoming bits.

Basically, protocol is software that resides either in a computer's memory or in the memory of a transmission device, like network interface card. When data is ready for transmission, this software is executed.

Two protocol architectures have served as the basis for the development of interoperable communications standards: the TCP/IP protocol suite and the OSI reference model. TCP/IP is the most widely used interoperable architecture, and OSI has become the standard model for classifying communication functions. Here is the brief introduction of them.

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The OSI Protocol Architecture

Open System Interconnection (OSI) includes set of protocols that attempt to define and standardize the data communication process defined by International Standardization for Organizations. The OSI model has seven layers as discussed below.

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  1. Physical Layer
    Physical Layer deals with the hardware level like, transmission media, connections and the voltage for digital signals. In other word physical layer provides the electrical and mechanical interface to the network medium (cables).

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  3. Data Link Layer

    Data Link Layer deals with physical transfer, framing (the assembly of data bits into single unit), flow control and error control functions. It is responsible for getting the data packaged for the physical layer. Data Link layer subdivided into two parts LLC (Logical Link Control) and MAC (Medium Access Control).
     
  4. Network Layer

    Network Layer is very important layer in OSI Protocol Architecture. Network layer deals with the transfer of data in the form of packets over the communication networks. A key aspect of this transfer is the routing of packets from the source to destination machine. Routing is the process by which a path is selected out of many available paths to the destination so that data packets reach the destination fast, efficiently, reliably as required. Network layer is also responsible for translating logical address or names into physical (or data link) addresses.
     
  5. Transport Layer
    Transport Layer ensures that data is successfully sent and received between two end nodes. If data is sent incorrectly, this layer has the responsibility to ask for re-transmission of the data. Also it ensures data are passes onto the upper layers in the same order in which they where sent. Specially, it provides a reliable, network-independent message interchange service to the top three application oriented layers.
     
  6. Session Layer
    Session Layers decides when to turn communication on or off between two computers. It also deals with the programs running in each machine to establish conversations between them.
     
  7. Presentation Layer
    Presentation Layer performs code conversation and data re-formatting (translating). It is translator of the network, making sure the data is in the correct form for the receiving application.
     
  8. Application Layer

    This layer provides the interface between the software running in a computer and the network. It provides functions to the user's software, including file transfer access and management and electronic mail services.
TCP/IP Protocol Architecture
TCP/IP has no any official protocol model as there is in case of ISI model. OSI model where defined by International Standardization for Organizations (ISO). But in case TCP/IP, has no any such. However, based on protocol standards TCP/IP have been developed and it has five layers.

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  1. Physical Layer
    Physical Layer covers the interface between a data transmission device (computer) and a transmission medium or network.

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  3. Network Layer
    This layer covers the exchange of data between end systems and the network to which it is attached. The transmitter computer provide the network path and destination address.
     
  4. Internet Layer

    Internet Protocol is used by this layer to provide the routing function across multiple networks.
     
  5. Transport Layer
    This layer ensures that data is sent and received successfully or not. If any error occurred then it sends the re-transmission request.
     
  6. Application Layer
    It provides user friendly interface between user and transmission devices. For example, file transfer, electronic mail services etc.