Local Area Networks CHAPTER 12 12.2 BUS/TREE LANs 12.7 Problems 364 CHAPTER 1 2 / LAN TECHNOLOGY rt, we examine local area networks (LANs) and metropolitan area net- MANs). These networks share the characteristic of being packet broad- g networks. With a broadcast communications network, each station is ed to a transmission medium shared by other stations. In its simplest form, a transmission from any one station is broadcast to and received by all other stations.
As with packet-switched networks, transmission on a packet broadcasting network is in the form of packets.1 provides useful definitions of LANs and MANs, taken from one of the IEEE 802 standards documents. This chapter begins our discussion of LAN? with a description of the proto- col architecture that is in common use for implementing LANs. This architecture is also the basis of standardization efforts. Our overview covers the physical, medium access control (MAC), and logical link control (LLC) levels.
Following this overview, the chapter focuses on aspects of LAN technology. The key technology ingredients that determine the nature of a LAN or MAN are Topology Transmission medium * Medium access control technique This chapter surveys the topologies and transmission media that are most commonly used for LANs and MANs. The issue of access control is briefly raised, but is covered in more detail in Chapter 13. The concept of a bridge, which plays a critical role in extending LAN coverage, is discussed in Chapter 14.1 LAN ARCHITECTURE The architecture of a LAN is best described in terms of a layering of protocols that organize the basic functions of a LAN.
This section opens with a descriptioi of the standardized protocol architecture for LANs, which encompasses physical, medium access control, and logical link control layers. Each of these layers is then examined in turn. Protocol Architecture Protocols defined specifically for LAN and MAN transmission address issues relat- ing to the transmission of blocks of data over the network. In OSI terms, higher- layer protocols (layer 3 or 4 and above) are independent of network architecture and are applicable to LANs, MANs, and WANs.
Thus, a discussion of LAN proto- cols is concerned principally with lower layers of the OSI model.1 relates the LAN protocols to the OSI architecture (first intro- duced in Figure 1. This architecture was developed by the IEEE 802 committee and has been adopted by all organizations working on the specification of LAN standards. It is generally referred to as the IEEE 802 reference model. 'For the sake of brevity, the book often uses LAN when referring to LAN and MAN concerns.
The con- text should clarify when only LAN or both LAN and MAN is meant.1 / LAN ARCHlTECTUlU? 365 TABLE 12.1 Definitions of LANs and MANS.* - The LANs described herein are distinguished from other types of data networks in that they are optimized for a moderate size geographic area such as a single office building, a warehouse, or a campus. The IEEE 802 LAN is a shared medium peer-to-peer communications network that broadcasts infor- mation for all stations to receive. As a consequence, it does not inherently provide privacy. The LAN enables stations to communicate directly using a common physical medium on a point-to-point basis without any intermediate switching node being required.
There is always need for an access sublayer in order to arbitrate the access to the shared medium. The network is generally owned, used, and operated by a single organization. This is in contrast to Wide Area Networks (WANs) that interconnect commu- nication facilities in different parts of a country or are used as a public utility. These LANs are also dif- ferent from networks, such as backplane buses, that are optimized for the interconnection of devices on a desk top or components within a single piece of equipment.
A MAN is optimized for a larger geographical area than a LAN, ranging from several blocks of buildings to entire cities. As with local networks, MANs can also depend on communications channels of moderate-to-high data rates. Error rates and delay may be slightly higher than might be obtained on a LAN. A MAN might be owned and operated by a single organization, but usually will be used by many individuals and organizations.
MANs might also be owned and operated as public utilities. They will often provide means for internetworking of local networks. Although not a requirement for all LANs, the capability to perform local networking of integrated voice and data (IVD) devices is considered an optional function for a LAN. Likewise, such capabilities in a network covering a metropolitan area are optional functions of a MAN.
* From IEEE 802 Standard, Local and Metropolitan Area Networks: Overview and Architecture, 1990. Working from the bottom up, the lowest layer of the IEEE 802 reference model corresponds to the physical layer of the OSI model, and includes such func- tions as Encodingldecoding of signals Preamble generationlremoval (for synchronization) Bit transmissionlreception In addition, the physical layer of the 802 model includes a specification of the trans- mission medium and the topology. Generally, this is considered below the lowest layer of the OSI model. However, the choice of transmission medium and topology is critical in LAN design, and so a specification of the medium is included.
Above the physical layer are the functions associated with providing service to LAN users. These include On transmission, assemble data into a frame with address and error-detection fields. On reception, disassemble frame, perform address recognition and error detection. Govern access to the LAN transmission medium.
Provide an interface to higher layers and perform flow and error control. These are functions typically associated with OSI layer 2. The set of functions in the last bulleted item are grouped into a logical link control (LLC) layer. The 3 66 CHAPTER 12 / LAN TECHNOLOGY OSI Reference Model Application Presentation IEEE 802 Reference Model Session Transport LLC Service Access Point (LSAP) Network Data link 1 Scope of Physical IEEE 802 Standards FIGURE 12.1 IEEE 802 protocol layers compared to OSI model.
1 functions in the first three bullet items are treated as a separate layer, called medium access control (MAC). The separation is done for the following reasons: The logic required to manage access to a shared-access medium is not found in traditional layer-2 data link control. For the same LLC, several MAC options may be provided. The standards that have been issued are illustrated in Figure 12.
Most of the standards were developed by a committee known as IEEE 802, sponsored by the Institute for Electrical and Electronics Engineers. All of these standards have sub- sequently been adopted as international standards by the International Organiza- tion for Standardization (ISO).3 illustrates the relationship between the levels of the architecture (compare Figure 9. User data are passed down to LLC, which appends control IEEE 802.2 *Unacknowledged conneclionless service *Connection-mode service .Acknowledged connectionless service I ,I I I I I I I I 1 I I I I I I I 1 CSMAICD I Token bus Round robin I Token ring Token ring DQDB 1 CSMA; priority polling - I I 1I 1 I I I I I 7 I rf I N n 1 iI mi I .i 1 0 1 xi I 2' $ 1 J 8 -2 2- &- I 2 'Baseband 3- / 100 Mbps I Broadband Unshielded Q Shielded tw~sled Optieal fiber Q Optical fiber: Inlra~rd: I J /eoaxla~. lflMbps 8 1M)Mbps palr: u 1 pair: 100 Mbps 8 I1,ZMbps !110 Mbps 1 4.
I 6 Mbps ti! 1 I Unshirldrd 'twisted pair: I 1 Carrierband I Unsh~clded 1 Spread 1 Unshielded twisted pair: / spectrum: 110. 100 Mhps l~hiclded 1 coaxial: Il,S,lOMbps 1 twisted paw 100 M b ~ s I 1.2 Mbps 1 4 Mbps I twkted pair: I I I / l o 0 Mbps I Optical f~her: ;Broadband / ~ , I O , Z~Oh p s I I I coaxial: I I I I 110 Mbps I I I loptlcal fiber: I I I I 11OMhps I I I L Busltrrelslar ropologies Ring lopology Dual bus topology Wircless FIGURE 12. Application data Application layer TCP layrr IP layer LLC layer - - TCP segment IP datagram t + t LLC protocd data unit + --i MAC frame + FIGURE 12.3 LAN protocols in context. 368 CHAPTER 12 / LAN TECHNOLOGY information as a header, creating an LLC protocol data unit (PDU).
This control information is used in the operation of the LLC protocol. The entire LLC PDU is then passed down to the MAC layer, which appends control information at the front and back of the packet, forming a MAC frame. Again, the control information in the frame is needed for the operation of the MAC protocol. For context, the fig- ure also shows the use of TCPIIP and afi application layer above the LAN protocols.
Topologies For the physical layer, we confine our discussion for now to an introduction of the basic LAN topologies. The common topologies for LANs are bus, tree, ring, and star (Figure 12. The bus is a special case of the tree, with only one trunk and no branches; we shall use the term busltree when the distinction is unimportant. Bus and Tree Topologies Both bus and tree topologies are characterized by the use of a multipoint medium.
For the bus, all stations attach, through appropriate hardware interfacing known as a tap, directly to a linear transmission medium, or bus. Full-duplex operation between the station and the tap allows data to be transmitted onto the bus and received from the bus. A transmission from any station propagates the length of the medium in both directions and can be received by all other stations. At each end of the bus is a terminator, which absorbs any signal, removing it from the bus.
'l'ermmatmg p Flow of data /l a+-------------+resistance (a) Bus (c) Ring I Central hub, switch, I (b) Tree (d) Star FIGURE 12. The tree topology is a generalization of the bus topology. The transmission medium is a branching cable with no closed loops. The tree layout begins at a point known as the headend, where one or more cables start, and each of these may have branches.
The branches in turn may have additional branches to allow quite com- plex layouts. Again, a transmission from any station propagates throughout the medium and can be received by all other stations. Two problems present themselves in this arrangement. First, because a trans- mission from any one station can be received by all other stations, there needs to be some way of indicating for whom the transmission is intended.
Second, a mecha- nism is needed to regulate transmission. To see the reason for this, consider that if two stations on the bus attempt to transmit at the same time, their signals will over- lap and become garbled. Or, consider that one station decides to transmit continu- ously for a long period of time. To solve these problems, stations transmit data in small blocks, known as frames.
Each frame consists of a portion of the data that a station wishes to trans- mit, plus a frame header that contains control information. Each station on the bus is assigned a unique address, or identifier, and the destination address for a frame is included in its header.5 illustrates the scheme. In this example, station C wishes to trans- mit a frame of data to A. The frame header includes A's address.
As the frame propagates along the bus, it passes B, which observes the address and ignores the frame. A, on the other hand, sees that the frame is addressed to itself and therefore copies the data from the frame as it goes by. (a) C transmits frame addressed to A (b) Frame is not addressed to B; B ignores it (c) A copies frame as it goes by FIGURE 12.5 Frame transmission on a bus LAN.