LAN TECHNOLOGY

LAN TECHNOLOGY

We examine local area networks (LANs) and metropolitan area net-

Work (MANs). These networks share the characteristic of being packet broad

Casting networks. With a broadcast communications network, each station is

attached 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. Table 12.1 provides useful definitions of LANs and MANs,

taken from one of the IEEE 802 standards documents.

This lesson begins our discussion of LAN? with a description of the protocol

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 lesson 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

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 lesson 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 relating

to the transmission of blocks of data over the network. In OSI terms, higherlayer

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 protocols

is concerned principally with lower layers of the OSI model.

Figure 12.1 relates the LAN protocols to the OSI architecture (first introduced

in Figure 1.10). 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.

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 information

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 communication

facilities in different parts of a country or are used as a public utility. These LANs are also different

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 functions

as

Encodingldecoding of signals

Preamble generationlremoval (for synchronization)

Bit transmissionlreception

In addition, the physical layer of the 802 model includes a specification of the transmission

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

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 12.2. 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 subsequently

been adopted as international standards by the International Organization

for Standardization (ISO).

Figure 12.3 illustrates the relationship between the levels of the architecture

(compare Figure 9.17). User data are passed down to LLC, which appends control

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 figure

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 12.4). 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.

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 complex

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 transmission

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 mechanism

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 overlap

and become garbled. Or, consider that one station decides to transmit continuously

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 transmit,

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.

Figure 12.5 illustrates the scheme. In this example, station C wishes to transmit

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.

So the frame structure solves the first problem mentioned above: It provides

a mechanism for indicating the intended recipient of data. It also provides the basic

tool for solving the second problem, the regulation of access. In particular, the stations

take turns sending frames in some cooperative fashion; this involves putting

additional control information into the frame header.

With the bus or tree, no special action needs to be taken to remove frames

from the medium. When a signal reaches the end of the medium, it is absorbed by

the terminator.

Ring Topology

In the ring topology, the network consists of a set of repeaters joined by point-topoint

links in a closed loop. The repeater is a comparatively simple device, capable

of receiving data on one link and transmitting them, bit by bit, on the other link as

fast as they are received, with no buffering at the repeater. The links are unidirectional;

that is, data are transmitted in one direction only and all are oriented in

the same way. Thus, data circulate around the ring in one direction (clockwise or

counterclockwise).

Each station attaches to the network at a repeater and can transmit data onto

the network through that repeater.

As with the bus and tree, data are transmitted in frames. As a frame circulates

past all the other stations, the destination station recognizes its address and copies

the frame into a local buffer as it goes by. The frame continues to circulate until it

returns to the source station, where it is removed (Figure 12.6).

Because multiple stations share the ring, medium access control is needed to

determine at what time each station may insert frames.

Star Topology

In the star LAN topology, each station is directly connected to a common central

node. Typically, each station attaches to a central node, referred to as the star coupler,

via two point-to-point links, one for transmission and one for reception.

In general, there are two alternatives for the operation of the central node.

One approach is for the central node to operate in a broadcast fashion. A transmission

of a frame from one station to the node is retransmitted on all of the outgoing

links. In this case, although the arrangement is physically a star, it is logically a bus;

a transmission from any station is received by all other stations, and only one station

at a time may successfully transmit.

Another approach is for the central node to act as a frame switching device.

An incoming frame is buffered in the node and then retransmitted on an outgoing

link to the destination station.

Medium Access Control

All LANs and MANS consist of collections of devices that must share the network's

transmission capacity. Some means of controlling access to the transmission

medium is needed to provide for an orderly and efficient use of that capacity. This

is the function of a medium access control (MAC) protocol.

The key parameters in any medium access control technique are where and

how. Where refers to whether control is exercised in a centralized or distributed

fashion. 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, such as the

following:

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.

The principal disadvantages of centralized schemes are

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.

The pros and cons of distributed schemes are mirror images of the points

made above.

The second parameter, how, is constrained by the topology and is a trade-off

among competing factors, including cost, performance, and complexity. 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 mul

tiplexing (FDM), and synchronous time-division multiplexing (TDM). Such techniques

are generally not optimal in LANs and MANS because the needs of the stations

are unpredictable. It is preferable to be able to allocate capacity in an asynchronous

(dynamic) fashion, more or less in response to immediate demand. The

asynchronous approach can be further subdivided into three categories: round

robin, reservation, and contention.

Round Robin

With 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

specified upper bound, usually expressed as a maximum amount of data transmitted

or time for this opportunity. In any case, the station, when it is finished, relinquishes

its turn, 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.

When many stations have data to transmit over an extended period of time,

round robin techniques can be very efficient. If only a few stations have data to

transmit over an extended period of time, then there is a considerable overhead in

passing the turn from station to station, as most of the stations will not transmit but

simply pass their turns. Under such circumstances, other techniques may be preferable,

largely depending on whether the data traffic has a stream or bursty characteristic.

Stream traffic is characterized by lengthy and fairly continuous transmissions;

examples are voice communication, telemetry, and bulk file transfer. Bursty

traffic is characterized by short, sporadic transmissions; interactive terminal-host

traffic fits this description.

Reservation

For 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.

Contention

For bursty 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, efficient. For some of

these techniques, however, performance tends to collapse under heavy load.

Although both centralized and distributed reservation techniques have been

implemented in some LAN products, round robin and contention techniques are

the most common.

The discussion above has been somewhat abstract and should become clearer

as specific techniques are discussed in Lesson 13. For future reference, Table 12.2

lists the MAC protocols that are defined in LAN and MAN standards.