LAN Topologies and Transmission Media

LANs performanceDepends on:-

LAN topology.

Used transmission medium.

Used medium access control (MAC) protocol.

LANs performanceChoice of the transmission medium depends on the following:

The medium capacity.

The medium reliability.

The types of data needed to support.

Environment on which the LAN is used

Bus topology Bus topology use multipoint connection where; only one non-closed transmission

medium is used.

All stations are connected to this bus medium through interface devices called taps.

Data are transmitted through the tap in full duplex (bidirectional) way.

Both medium ends are provided with a terminator to absorb signals.

As in all LANs; messages are divided into frames each includes the source and

destination addresses.

Bus topology

Bus topology The same steps are used to send acknowledgement.

Multicasting and broadcasting are built easily since the frame passes through the whole

medium

Note that the number of stations, the length between two stations and the length of

the whole bus depends on the transmission medium used.

Bus topology Bus transmission media (Baseband)

Baseband Bus Topology

Broadband Bus Topology

Optical Fiber Bus Topology

Bus topology Bus transmission media (Baseband)The entire bus is only one channel.

Use digital signaling.

Transmission is bidirectional on the bus

Usually uses Manchester or differential Manchester encoding.

The used taps are passive devices (pass the signal without any amplification).

Bus topology Bus transmission media (Baseband)The bus length is limited by the following;

Data rate

Cable thickness

Transmission's impairments

Bus topology Bus transmission media (Baseband)These three factors are related due to the following:

1) Lower data rate signals last longer; so can be recovered easily from impairments

than higher data rate signals that last shorter time.

2) As the cable is thinner; the transmission impairments affects increases.

3) As the bus length increases; the transmission impairments affects increases.

Also the environment in which we install the LAN has an effect; places with lot of corners

needs thinner cable which is easier to bend.

Bus topology Bus transmission media (Baseband)

Signal balancing is needed to limit the signal over certain limit; not too weak to attenuate

across the medium before reaching far stations and not too strong to cause distortion, in

both cases cannot be recovered.

This can be done within certain limit and also this limit the bus length.

To increase the length of the LAN, the bus is split into segments joined by repeaters

(transparent device); to overcome the impairments effects; in a non closed way.

Bus topology Bus transmission media (Broadband )The bus spectrum is divided into multiple channels using frequency division

multiplexing; so has high data rate

Use analog signaling.

Transmission is unidirectional into the bus.

Since analog signals can propagate greater distances before impairments affect it, the

bus length is much longer than the baseband bus length.

Passive taps are also used to connect stations to the bus.

Bus topology Bus transmission media (Broadband )

Dual cable broadband

If a large amount of frequency is needed; dual cable is used.

Dual broadband cable is used for short bus LANs.

Bus topology Bus transmission media (Broadband )

Split broadband

The frequency converter may be an analog device known as a frequency translator, or a

digital device known as a remodulator.

Bus topology Bus transmission media (Optic fiber )

Use light rays for data transmission; so has very high data rate.

A fiber is only one channel.

Transmission is unidirectional on the fiber bus.

The bus length is much longer than coaxial cables

Bus topology Bus transmission media (Optic fiber )

Loop bus

The operation is similar to that used in dual cable broadband.

Bus topology Bus transmission media (Optic fiber )Dual bus

Bus topology Bus transmission media (Optic fiber )Dual bus

Bus topology Bus transmission media (Optic fiber )One copy of the frame propagates on the upper fiber towards the right side (to station

A), while the other propagates on the lower fiber towards the left side (to station C, D

and E).

Each station compares its address with the destination address included in the frame;

then lets the frame passes, except destination station D which takes a copy of it before

lets it passes.

The frame is finally absorbed by both terminators they reach.

Bus topology Bus transmission media (Optic fiber )

Optical taps configuration

Active tap

Passive tap

Bus topology Bus transmission media (Optic fiber )

Active tap

An active tap splits the fiber into parts; i.e. the fiber is really cut into several point to point

fibers. It is complex, increase the delay (due to encoding/ decoding) and has high cost.

Bus topology Bus transmission media (Optic fiber )

Passive tap

It doesn't need the splitting of the fiber, instead it extracts a portion from the optical

signal from the fiber to send to the station (to be processed) and injects the optical signal

directly into the medium. It has a lossy nature so limits the number of stations and

medium length.

Bus topology Advantages Easy to install.

The failure of any station doesn't disable the whole LAN.

Due to the open nature; adding a new station is easy.

Bus topology DisadvantagesMaintenance of the medium is not simple.

Locating any failure position may require checking every part of the LAN.

The number of station is limited to few hundreds.

Signal reflection at the taps degrades the network's performance.

Number of devices and cable length are limited according to the medium's type.

A bus's break disables the whole bus.

Fault isolation is difficult.

Tree topologyTree topology is a generalization of the bus topology;

where its transmission medium is multiple buses (called branches) connected at one

end using a headend and provided with terminators at the other end.

Each end may have additional branches. As in bus topology; no closed loop exists.

It has same advantages and disadvantages of bus topology.

References

Data Communications and Networking, Fourth Edition by Behrouz A.

Forouzan

Data And Computer Communications, Eighth Edition

Lecture notes of Prof Iman Saroit.