17-LAN extensions: Fiber Modems, Repeaters, Bridges and

SwitchesDr. John P. Abraham

Professor

UTPA

Distance Limitation

• Designing considerations– Capacity– Maximum delay– Distance: hardware emit a fixed amount of

energy, so only can travel a certain distance– Cost

Fiber Modem extensions

• Optical fiber extension require a pair of fiber modems.

• Each modem accepts packets from the ethernet interface and sends them over the fiber, And accepts packets that arrive over the optical fiber and send them over the Ethernet Interface.

Repeaters

• An analog device to propagate LAN signals over long distances. It does not inspect the packets, instead amplifies signals received and transmits over the outputs.

• Thick and thinnet used repeaters. An active hub is a repeater.

• Repeaters are available for infrared.

Bridges and Bridging

• Discussed in prior lectures.• Additional info is given here.

– Bridges listen in promiscuous mode (listen to all) on each segment. Connects both sides so that it acts like a single LAN.

– Learning bridges and frame filtering• Bridge uses MAC addresses to perform filtering to decide

either to forward or not.• Bridges learn which computers are connected to which

segment by examining source address and then creating a table of its own. As it learns forwarded only to the right one, until then sent to both segments. See fig. 17.4 p.297

Distributed Spanning Tree

• Views bridges as nodes in a graph and imposes a tree on the graph. 3 steps– Root election – bridge with the smallest ID is

chosen – Shortest path computation. Each bridge

calculates a shortest path to the root.– Forwarding. Interface that connects to the

shortest path is enabled for forwarding packets.

VLAN switches

• Virtual LAN switch.

• Configure one switch to emulate multiple switches. Choose which ports belong to witch virtual switch.

• When a computer on VLAN1 broadcasts only computers attached to that VLAN get the packets.