LOCAL AREA NETWORK

1 DEFINITION AND APPLICATIONS

Local area networks (LANs) use a common transmission medium to interconnect workstations, servers, computers, and/or other related assets over a limited geographical area. Several LAN standards specify capability to serve up to a thousand or more devices on a single LAN. The geographical extension or “local area” may extend no more than several hundred feet (<100 m) to over 6 miles (>10 km) or more in other cases. The transmission media providing this connectivity may be wire pair, coaxial cable, or fiber-optic cable. Local area radio (wireless) schemes based on IEEE 802.11 standards are gaining wide acceptance.

Certain LAN schemes accommodate other devices as well such as digital telephones, facsimile, and video equipment. A basic rule on LANs was that only one user at a time may have access to the medium. Switching hubs and LAN routers have now changed this rule.

Data rates on current LANs vary from 1 Mbit/s to 10,000 Mbits/s. LAN data rates, the number of devices connected to a LAN, the spacing of those devices, and the network extension depend on:

• The transmission medium employed

• Transmission technique (i.e., baseband or broadband)

• Network access protocol

Many LANs operate without error correction with bit error rates (BERs) specified in the range of 1 × 10−8 to 1 × 10−12 or better.

The most common application of a LAN is to interconnect data terminals (workstations) with processing resources, where all the devices reside in a single building or complex of buildings, and usually these resources have a common owner. Cost containment is a driving force toward the implementation of LANs.

A LAN permits effective cost sharing of high-value data processing equipment, such as mass storage, mainframe or minicomputers, and high-speed printers.

There are other benefits as well. One, of course, is resource sharing. Another is e-mail and similar messaging services leading to a “paperless” environment. A

LAN may also be considered an aggregator of data for eventual transport of this data over a WAN.

LANs can be extended, up to a certain point, with repeaters or bridges. They can be segmented by means of switches, switching hubs, smart bridges, or routers.

Segmenting of a LANs can notably improve performance, especially on a LAN with many users.

The interconnection of LANs in the local area with a high-speed backbone is current practice. LAN interconnection with the outside world such as with distant

LANs via a wide area network is becoming prevalent. This is frame relay’s principal application. The interface is carried out with a router or gateway.

There are two generic transmission techniques utilized by LANs: baseband and broadband. Baseband transmission can be defined as the direct application of the baseband signal to the transmission medium. Broadband transmission, in this context, is where the baseband signal from the data device is translated in frequency to a particular frequency slot in the RF spectrum. Broadband transmission requires a modem to carry out the translation. Baseband transmission may require some sort of signal conditioning device. With broadband LAN transmission, we usually think of simultaneous multiple RF carriers that are separated in the frequency domain. Present broadband technology comes from the cable television (CATV) industry.

Use of the asynchronous transfer mode (ATM) is beginning to find favor in the LAN community and could radically change many of the concepts introduced in this chapter.

2 LAN TOPOLOGIES

There are three types of LAN topology: bus, ring, and star. These are shown in

Figure 13.1 along with the tree network, which is a subset of the conventional bus topology. All three topology configurations have been shown previously in. They are repeated here for convenience of the reader.

A bus is a stretch of transmission medium. Originally, the medium was coaxial cable. Today it can also be

UTP or STP (unshielded twisted pair or shielded twisted pair). For high data rate

(e.g., ≥100 Mbits/s) LANs, fiber-optic cable is now widely used.

A ring is simply a bus that is folded back onto itself.                                                                                                                             

 User traffic flows in one direction around the ring. In one approach, which we discuss in this chapter, a second ring is added where the traffic flow is in the opposite direction. Such a dual counter rotating ring concept improves reliability in case of a failed station or a cut of the ring. At the center of the star is a switching device. This could be a switching hub. Users can be paired, two at a time, three at a time, or all at a time, segmented into temporary families of users depending on the configuration of the switch at that moment in time. Such a concept lends itself particularly well to ATM. In this case each user is connected to the switch on a point-to-point basis for the period of connectivity.

THE TWO BROAD CATEGORIES OF LAN TRANSMISSION

TECHNIQUES

Baseband and broadband are the two basic transmission techniques employed by LANS, which were introduced above.

With both these types of LANs (i.e., baseband and broadband), we are dealing with multipoint operation. Two transmission problems arise as a result. The first deals with signal level and signal-to-noise ratio, and the second deals with standing waves. Each access on a common medium must have sufficient signal level and S/N such that copied signals have a BER in the range of 1 × 10−8 to 1 × 10−12. If the medium is fairly long in extension and there are many accesses, the signal level must be sufficiently high for a transmitting access to reach its most distant destination. The medium is flossy, particularly affecting the higher bit rates, and each access tap has an insertion loss. This leads to very high signal levels which may be rich in harmonics and spurious emissions, degrading bit error rate. On the other hand, with insufficient signal, the S/N degrades, which will degrade error performance. A good level balance must be achieved for all users. Each and every multipoint connectivity must be examined. The number of multipoint connectivities can be expressed by n(n − 1), where n is the number of accesses. If, on a particular LAN, 100 accesses are planned, there are

9900 possible connectivities to be analyzed to carry out signal level balance.

One way to simplify the job is to segment the network, placing a regenerative repeater or bridge at each boundary. This reduces the signal balance job to reasonable proportions and ensures that a clean signal of proper level is available at reach access tap. For baseband LANs, 50-_ coaxial cable is favored over the more common 75-_ cable. The lower impedance cable is less prone to signal reflections from access taps and provides better protection against low-frequency interference.

The effects of standing waves can be reduced by controlling the spacing between access taps. For example, the Ethernet technical summary [1] recommends spacing no less than 2.5 m, for the 10-Mbit/s standard Ethernet LAN.

The technical summary says that by following this placement rule, the chance the objectionable standing waves will result is reduced to a very low (but not zero) probability. Again for Ethernet, up to 100 devices may be placed on a cable segment and the maximum segment length is 500 m. The segments can be connected through regenerative repeaters, with a maximum total end-to-end length of 2.5 km [1, 4, 21].

The baseband technique incorporates single signal transmission of a digital waveform on a transmission medium. Broadband transmission has the capability of transmitting multiple signals simultaneously on a medium, typically coaxial cable. Each signal is assigned a frequency slot in a frequency division multiplex plan. Broadband technology derives from the CATV (cable television) industry.

Baseband lends itself to bus and ring topologies, and broadband to bus and tree topologies. Broadband systems require a modem at each access; baseband systems do not.∗ One extremely important consideration for baseband transmission is that only a single thread (transmission line) exists. It can accommodate only one user at a time; otherwise there is a high probability of data message collision. Collision is where the electrical signals bearing the traffic of two or more users interfere one with the other, corrupting the traffic of each. To help avoid collisions, segmenting has become an excellent alternative using bridges, hubs, or switches. These devices are discussed at the end of this chapter.

3.1 Broadband Transmission Considerations

Broadband transmission permits multiple users to access the medium without collision. Broadband means that we take advantage of the medium’s wide bandwidth.

This wide bandwidth is broken down into smaller bandwidth segments in an analogous fashion to FDM. Each of these segments is assigned to a family of users. The statement regarding collision is correct if there are no more than two accesses per frequency segment connected on a point-to-point basis, where one access receives while the other transmits. If, in this case, we assume contention as the access protocol, then as the family increases in number, the chances of collision start to increase. With a little imagination, we can see that, with the proper switching scheme implemented, a user can join any family by simply switching to the proper frequency band of that family. All that is required is a change in modem frequency and possibly modulation waveform. There may also be certain protocol considerations as well.

Unlike their baseband counterparts, broadband systems can be designed to accommodate digital or analog voice, data from kilobit to multimegabit rates, video, and facsimile. Thus broadband systems are versatile. They are also much more expensive than their baseband counterparts and require a higher level of design engineering effort.

As mentioned, much of our present broadband technology derives from cable television technology. Total system bandwidths are on the order of 300–500 MHz.

Each access requires a modem to modulate and demodulate the data or other user signal and to translate the modulated frequency to the assigned frequency slot on the cable.

This, then, is RF transmission and, by its very nature, must be one way or unidirectional. Thus a user can only access another user “downstream” from it.

If we assume a single medium, usually a 75-_ coaxial cable, then how does one access another user “upstream”? This is done using a similar approach to that of a two-way or interactive CATV (cable television) system, where two paths are provided on a single coaxial cable. This is accomplished by splitting the cable spectrum into two frequency segments, one segment for one direction and the other for the opposite direction. At a cable terminating point, which some even call a head end from CATV terminology, a frequency translator converts and amplifies signals from one direction (frequency segment) into signals for transmission in the opposite direction (frequency segment). Another term for “head end” for broadband LANs is central retransmission facility (CRF).

There are two choices of topology for broadband LANs: bus and tree. The head end or CRF is located at some termination point on the bus, and in the case of tree topology the CRF is located at its root, so to speak.

Another approach to achieve dual path operation is to use two cables; one provides the “go” path and the other the “return” path. For single-cable split-band operation, a rather large guard band is left in the center of the cable spectrum to ensure isolation between the two paths. Then we can see that with the provision of two-cable operation the usable bandwidth can be more than doubled. Modem operation is also simpler because, to access a particular net, only one frequency operation is required (i.e., the send and receive frequencies can be the same).With single-cable split-band operation, send and receive frequencies must necessarily be different [2].

Broadband services make up about 5% of the total LAN market. Baseband predominates.

Table 13.2 gives broadband channel allocations for CSMA/CD services and

Table 13.3 provides channel allocations for a broadband token bus configuration.

Recommended channelization for both single and dual cable operation is given.

3.2 Fiber-Optic LANs

Fiber-optic LANs may be considered broadband† in that a class of modem is required to place the digital signal on the fiber. The modem, of course, consists of a light source, detector, and the necessary driver and signal conditioning circuitry.

With wavelength-division multiplexing (WDM) we have a true broadband system.

At this time, single-wavelength operation prevails.

The type of fiber-optic cable selected for a LAN is a cost trade-off. The extension of a LAN is generally short such that multimode fiber can be used, and the mature short-wavelength technology permits other cost savings. Even plastic fiber may be considered. The losses of the fiber itself will generally be low due to the short-distance operation even at 820-nm operation. A LAN that is

1 km long might display a fiber loss from 2 dB to 5 dB. The major contributor to loss is the taps if a fully passive network is to be implemented.

Fiber lends itself to all three LAN topologies: bus, ring, and star. The use of passive couplers leads to a more reliable system, but the insertion loss of passive couplers is a consideration. With active coupling, the loss of one source or detector can cause the entire network to crash unless bypass switches are used.

4 OVERVIEW OF IEEE/ANSI LAN PROTOCOLS

4.1 General

Many of the widely used LAN protocols have been developed in North America through the offices of the Institute of Electrical and Electronic Engineers (IEEE)

The American National Standards Institute (ANSI) has subsequently accepted and incorporated these standards, and they now bear the ANSI imprimatur.

The IEEE develops LAN standards in the IEEE 802 committee, which is currently organized into the following subcommittees:

802.1∗ Bridging & Management

802.2∗ Logical Link Control

802.3∗ CSMA/CD Access Method

802.4∗ Token Passing Bus Access Method

802.5∗ Token Ring Networks

802.6∗ Metropolitan Area Networks (MANs)

802.7∗ Broadband Technical Advisory Group

802.10∗ LAN Security

802.11∗ Wireless LAN

802.12∗ Demand Priority Access Working Group

802.15∗ Wireless Personal Area Networks

802.16∗ Broadband Wireless Metropolitan Area Networks

802.17 Resilient Packet Ring

802.18 The Radio Regulatory TAG (Technical Advisory Group)

802.19 Coexistence Advisory Group

802.20 Mobile Broadband Wireless Access (Working Group)

The fiber distributed data interface (FDDI) standard, which is discussed in

Section 5.6, has been developed directly by ANSI.

4.2 How LAN Protocols Relate to OSI

LAN protocols utilize only OSI layers 1 and 2, the physical and data-link layers, respectively. The data-link layer is split into two sublayers: medium access control

(MAC) and logical link control. These relationships are shown in Figure 13.3.

Stallings [3] presents an interesting and rational argument on the reasoning for limiting the layering to the first two OSI layers. There is no question that the functions of OSI layers 1 and 2 must be incorporated in a LAN architecture. We now ask, Why not layer 3? Layer 3, the network layer, is concerned with routing.

There is no routing involved with LANs. There is a direct link involved between any two points. The other functions carried out by OSI layer 3—addressing, sequencing, and flow control—are carried out by layer 2 in LANs. The difference is that layer 2 performs these functions across a single link. OSI layer 3 carries out these functions across a sequence of links required to traverse a network. Of course, there is only one link required to traverse a LAN.

It would seem that layer 3 is required when viewed through an attached device.

The reason is that the device sees itself attached to a network connecting multiple devices. One would think that ensuring delivery of a message to one or more accesses would be a layer 3 function. the characteristics of the network allow these functions to be performed in the first two layers.

As shown in Figure 13.3, the OSI data-link layer is divided into two sub layers: logical link control (LLC) and medium access control (MAC). These sub layers carry out four functions:

1. Provide one or more service access points (SAPs). A SAP is a logical interface between two adjacent layers.

2. Before transmission, assemble data into a frame with address and error detection fields.

3. On reception, disassemble the frame and perform address recognition and error detection.

4. Manage communications over the link.

The first function and those related to it are performed by the LLC sub layer. The last three functions are handled by the MAC sub layer.

In the following subsections we will describe four common IEEE and ANSI standardized protocols. Logical link control (LLC) is common to all four. They differ in the medium access control (MAC) protocol.

A station on a LAN may have multiple users; oftentimes these are just processes, such as processes on a host computer. These processes may wish to pass traffic to another LAN station which may have more than one “user” in residence. We will find that LLC produces a PDU with its own source and destination address. The source address, in this case, is the address of the originating user. The destination address is the address of a user in residence at a LAN station. Such a user is connected through a service access point (SAP) at the upper boundary of the LLC layer. The resulting LLC PDU is then embedded in the information field of a MAC frame