Extending LANs

hellhollowreadingΔίκτυα και Επικοινωνίες

26 Οκτ 2013 (πριν από 3 χρόνια και 9 μήνες)

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Extending LANs
Fiber modems
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/hub

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
Repeaters cont.

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
Repeater photos

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)
Proxim wireless bridge

bridges a wireless Proxim local area network
with a standard Ethernet network
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
• 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

May be possible to improve an existing
LAN’s performance by adding a bridge
Bridging between buildings
Bridging longer distances
A cycle of bridges
Distributed spanning tree

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

Some organisations combine hubs and
switches to reduce cost
The concept of a switched LAN

Fiber optic extensions

Repeaters and multiple repeaters

• frame filtering
• bridging distances

Summary of LANs

Locality of reference

LAN topologies

MAC protocols, especially CSMA/CD

Hardware addressing and frame types

LAN wiring

Extending LANs