Extending LANs using Repeaters, Bridges, and Switches

LAN Distance Limitations Each LAN technology has a maximum length specification that is associated with the specific technology Signal looses its strength as it propagates along the wire, so medium can’t be too long CSMA/CD is designed to work correctly with a maximum length cable – recall collision must be detected while station is transmitting

How do we extend the length of the LAN? Repeaters Bridges Switches

Repeater Device used to overcome attenuation on the line Device amplifies the signal Repeaters fall into two categories: –Amplifiers –Signal-regenerating repeater Repeaters operate at the Physical Layer of the OSI model

Amplifiers Amplify the entire signal coming in The signal and the noise are both amplified

Signal-Regenerating Repeater Data from incoming signal is extracted and then data is placed on a new carrier signal This eliminates any noise found in the incoming signal

Repeaters Continued Maximum length of Thicknet Ethernet segment is 500 meters Adding a repeater can double the length of an Ethernet LAN A repeater propagates all signals without any attempt at filtering The Standard specifies that you can have no more than 4 repeaters in the LAN –see handout

Problems with Repeaters Repeaters operate at the Physical Layer of the OSI model They do not interpret frames They do not check for complete frames A collision signal on one segment is transmitted to the other segment by the repeater

Bridge Electronic device that connects two LAN segments together The bridge, however, knows about frame formats It listens to traffic on each segment and when it encounters a frame, it verifies that the frame is intact Unlike a repeater, a bridge filters frames between the two segments For an incoming frame, the bridge reads the addresses and determines if it should transmit the frame to the other segment or not

Bridges Continued A frame from segment A that is addressed to a client on segment A is discarded by the bridge A frame from segment A that is addressed to a client on segment B is retransmitted on segment B. This process causes frame traffic to be localized to the segments within the LAN instead of traveling all over the LAN. This reduces network traffic. NOTE: A bridge just has two connections – it can only connect two LAN segments

Frame Filtering by a Bridge To determine whether to forward a frame or not, a bridge uses the physical address found in the frame header The bridge must know the location of each computer connected to each of the LAN segments The bridge keeps a table of addresses in memory to determine where to send frames

How does Bridge Build Table? The bridge listens to all segment traffic As it receives frames from the two segments, it creates a table of what machines are attached to each segment. See Figure 11.5, p. 165 to see how table is built What happens when bridge is first connected and it doesn’t have a table built? Bridges usually learn location of each computer rather quickly

Important Property of Bridge A bridge allows communication on each of the segments at the same time. In planning a segmented LAN, group together computers that will interact a lot with each other. This is often done by grouping together computers that are physically close to each other (say in one office area)

Cycles Within a Bridges Network A bridged network can contain many bridges A cycle can exist in the network – See Figure 11.9 for an example The existence of a cycle can cause problems with forwarding of frames – a broadcast frame may continue to traverse the cycle Sometimes a cycle occurs in a LAN on accident and sometime it is done on purpose

How To Prevent Infinite Loops? The scheme used to prevent loops in a bridged network is automated The bridges communicate with each other and implement an algorithm called the Distributed Spanning Tree (DST) algorithm In this algorithm, a bridge does not forward frames if the bridge finds that each segment to which it is connected already contains a bridge that has agreed to forward frames. This results in the bridges that agree to forward frames form a binary tree structure (no cycles)

Switch A device that is used to segment LANs just like bridges do The switch differs from the bridge in that it has multiple ports A switch is sometimes called a multi-port bridge Each port of the switch is a segment where communication can be taking place irrespective of what is happening on the other segments Each segment may contain multiple computers or only one computer per segment If a switch has N ports with one computer per port, how many simultaneous transmissions can you have?