Lecture 8: Ethernet and Token Ring Networks

Ethernet Carrier Sense, Multiple Access and Collision Detect (CSMA/CD) LAN Ethernet Standard-DEC, Intel, Xerox 1978  IEEE Mbps—We are going to talk about this 100,1000 Mbps—These are used for point- to-point links in switched networks

50  cable Taps at least 2.5 meters apart Up to 500 m between hosts- < 4 repeaters between hosts- End-to-end length < 2500 m Propagation delay <8.7  s Not more than 1024 hosts

Network topology with at most 2 repeaters between hosts Repeaters have queuing delay is 4.2  s for 10 Mbps ethernet Max queuing delay=16.9  s/between any two hosts Max delay= 25.6  s between any two hosts

Network topology with two stars connected by a point-to-point link

Newer Technologies 10BaseT –Cat 5 copper 100 Mbps ethernet –Switched, point-to-point Gigabit ethernet –Switched, point-to-point technology

Access Protocol Frame Format Addresses Receiver Algorithm Transmitter Algorithm

Ethernet Frame MAX=1500 bytes Min=46 bytes Type—demux key to identify application CRC-32

Addresses Media Access Control (MAC) address –48 bits/6 bytes-X 1 X 2 :X 3 X 4 :X 5 X 6 :X 7 X 8 :X 9 X 10 :X 11 X 12 –Unique worldwide for all time –Belongs to adapter Broadcast address –FF:FF:FF:FF:FF:FF Multicast address –1xxxxxxx:XX:XX:XX:XX:XX

Receiver Algorithm Adaptor –Receives all frames on the ethernet –Accepts Frames Addressed to: Its unicast address the broadcast address A mult-cast address it has been programmed to accept –Accepts all frames in “promiscuous (eavesdropping) mode”

Transmitter Algorithm If adapter has a frame to send: Line is idle-sends its frame-up to 1500 bytes Line is busy-wait til line goes idle and send the frame Collisions These are detected as overvoltage AT AN ADAPTER Sends 32 bit jamming sequence and shuts down-purpose of jamming frame is to cause a collision at the sender So, the frame will be truncated-could be as small as 96 bits (64 bit preamble+32 bit jamming sequence) Called a “runt” frame

Worst-case collision scenario: a)A transmits at time t=0 b)B sends frame just before t=d(delay) c)B detects collision and sends runt frame d)A detects collision at time 2d (if its still sending!) e)Frame must be 2d=51.2 seconds long for collision to occur at A f)Frame must be at least 512 bits long at 10 Mbps

Exponential backoff collision recovery Wait k*52.1  s where k=0,1,…,2 n -1 –n=number of unsuccessful attempts to transmit –delay selected at random in each attempt

The ethernet advantage It’s simple—no routing tables to maintain It’s flexible—just plug in your PC It’s cheap –Cable—$ 0.12/ft –Adaptors—$ 20 –Hubs--$ 30/port –Switch--$ 50/port

Token Ring Shared medium similar to ethernet –Distributed algorithm controls transmission –All nodes see all frames Token=small circulating frame To send: Remove token and send frame Sender: removes message when it comes around and inserts token

Token ring specification IBM Token Ring IEEE Fiber Distributed Data Interface (FDDI)

Relays (normally closed) in the network adapter protect the ring from host failures

Multi-station access unit (MSAU) create a star-like topology