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. 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.
    
3. 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. 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.
    
3. 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:
 
• 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