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 802.3

• 10 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-X1X2:X3X4:X5X6:X7X8:X9X10:X11X12

– 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,…,2n-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 802.5

• 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