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