Extending LANs

• Fiber modems• Repeaters• Bridges• Switches

Distance limitation in LANs

• MAC protocols such as CSMA/CD require time proportional to the length of the cable

• Electrical signal weakens with distance• Network designers choose a combination of

capacity, delay and distance that can be achieved within a given cost

Fiber optic extensions

• Fiber modems extend connection between computer and transceiver

Repeaters

• Join Ethernet cables (segments) together• Amplified signal - no knowledge of frames• Deals with signal strength, but not delay

Multiple repeaters

• Ethernet standard says no more than four repeaters between two computers

• Fiber modems can be used between repeaters for long distance extensions

• Biggest problem with repeaters is that they transmit all signals including collisions and noise which limits scalability

Bridges• Connect two segments, but work at the

frame level• Use promiscuous mode and forward all

frames• Don’t forward erroneous frames (e.g.,

collisions and noise)

Frame filtering

• Only forward a frame if necessary– destination is on the other segment– broadcast address is used

• Bridge learns which segment a computer is on when that computer sends a frame

• When a frame arrives the bridge– extracts source address and updates knowledge– inspects destination address for forwarding

Example of learning in a bridge

Planning a bridged network• Propagation principle - a bridge will only

forward frames as far as is necessary• Bridges allow communication on separate

segments to occur at the same time• Plan the network so that computers that

communicate frequently are on the same segment

• May be possible to improve an existing LAN’s performance by adding a bridge

Bridging between buildings

Bridging longer distances

A cycle of bridges

• Compute a distributed spanning tree

Switching• Switch - a single electronic device that transfers

frames between computers• Whereas a hub simulates a shared medium, a

switch simulates a bridged LAN with one computer per segment

• Advantage is greater data rate due to parallelism• Some organisations combine hubs and switches

to reduce cost

The concept of a switched LAN

Summary

• Fiber optic extensions• Repeaters and multiple repeaters• Bridges– frame filtering– bridging distances

• Switches

Summary of LANs

• Locality of reference• LAN topologies• MAC protocols, especially CSMA/CD• Hardware addressing and frame types• LAN wiring• Extending LANs

Exam question

• http://www.cs.nott.ac.uk/~mvr/2-CCN.doc