Now (switched network) LAN Wiring Wiring Scheme Generation Speed Distance Interference 10Base5 1st Gen (1970s) 10Mbps 500m More resistant 10Base2 2nd Gen 200m Somewhat resistant 10BaseT 3rd Gen 100m Susceptible 100BaseT/ 1000BaseT Now (switched network) 100Mbps/ 1000Mbps Highly susceptible As the Ethernet speed increases, the maximum distance for a LAN segment decreases because of the increased susceptibility to interference.

LAN Extensions

Distance Limitation and LAN Design Hardware is designed to emit a fixed amount of energy Long distance may cause weak signals and errors due to interference Can we use other transmission media that is insensitive to electrical noise? 3 3

Fiber Modems Communicate over an optical fiber Can span long distance Provide standard network interface (e.g., Ethernet) Expensive and fragile, seldom used

Other Technologies That Extend LANs Three key extension technologies Repeaters (early 90s) Bridges Switches

Repeaters A repeater is an analog device used to propagate LAN signals over long distances A layer 1 device A repeater does not understand packets or signal coding Instead, it merely amplifies the signal received 6 6

Connecting Two LAN Segments B C D E F R When there is a collision on segment 1, will NIC-F sense the collision? Can we use 99 repeaters to connect 100 LAN segments? No A super long hallway  too busy / too many collisions Repeater also amplifies noise and distortion  too many errors

Pros and Cons of Repeater Low cost Simple to implement Cons Limited number of repeaters that can be used in a network. General rule-of-thumb: 5 segments / 4 repeaters 8 8

Bridges A bridge is a mechanism that connects two LAN segments. A layer 2 device Receives all packets sent on the segment. Forwards an intact frame from one segment to the other. Two LAN segments connected by a bridge appear to behave like a single LAN. LAN Segment 2 LAN Segment 1 P-B A D E F NIC-L NIC-R 9 9

Bridges Steps for P-A to send “I like you” to P-F: P-A will compose the packet P-A forwards the packet to NIC-A NIC-A calculates the CRC, adds SOH, EOT NIC-A modulates the packet  send out using CSMA/CD After a while, NIC-L will receive the m-carrier NIC-L demodulates the m-carrier and put it into the cache NIC-L does length and CRC checkings If the packet passes all checkings, NIC-L will strip off SOH, EOT, CRC, and forward “|F|A|I like you|” to P-B P-B checks the destination address of this packet, if this address is for a computer on segment 1, the packet will be discarded; otherwise, the bridge forwards the packet NIC-R calculates the CRC, adds SOH, EOT NIC-R modulates the packet  send out using CSMA/CD After a while, NIC-F will receive the m-carrier… I like you F A CRC I like you SOH EOT F A

Bridges Q1: If the packet was sent from computer A to computer D, will the bridge P-B see the packet? Q2: If there is a collision on segment 1, will P-B know of this collision? Q3: When we use bridges, do we need to follow the 5-4 rule? 1. Yes, because NIC-L does not do address filtering 2. No, because NIC-L will do CRC checking and discard the packet 3. No. Signal distortion will not be propagated with bridges

Why Bridging Works Well? A bridged network can exhibit higher overall performance than a single LAN Each segment forms a separate collision domain A bridge permits simultaneous transmission on each segment 12 12

Review: Switches Steps for P-A to send “I like you” to P-C: P-A will compose the packet P-A forwards the packet to NIC-A NIC-A calculates the CRC, adds SOH, EOT NIC-A modulates the packet  send out After a while, NIC-1 will receive the m-carrier NIC-1 demodulates the m-carrier and put the packet into a queue P-S checks this packet and find destination is “C” NIC-3 modulates the packet  send out After a while, NIC-C will receive the m-carrier NIC-C demodulates the packet and put it into its cache NIC-C does two checkings; If failed, the packet will be discarded Length checking: the payload should be between 46 and 1500 bytes CRC checking If the packet passes all checkings, NIC-C will strip off SOH, EOT, CRC, and forward “|C|A|I like you|” to P-C I like you C A CRC I like you SOH EOT C A

Switching and Layer 2 Switches Logically, an Ethernet switch is similar to a set of bridges. Dealing with packets, not signals. A switch as simulating a bridged network that has one computer per LAN segment. 14 14

Switches vs. Bridges Switches can be thought as more efficient versions of bridges Completely eliminate collisions Perform forwarding via hardware Bridges use software Do NOT add/remove SOH, EOT, CRC No length or CRC checking