Token Ring, FDDI

• Ethernet, continued
• Token Ring
• FDDI intro

Switched Ethernet

• use switches instead of hubs to connect hosts (or hubs) to each other
• switches forward packets only on link to destination (once route has been learned)
• ==> reduces collisions, allows more traffic

Fast Ethernet

• 100 Mb/s Ethernet
• Ethernet protocol (packet format, backoff)
• Maximum ethernet diameter?
• 100Base-T:
  • three pairs of wires
  • ternary encoding (3 voltage values)
  • 3^3 = 27 symbols, of which 16 used for transmission
  • 25 MHz transmission
  • additional pair of wires transmits collision signal

Fast Ethernet

• 100Base-TX:
  • two pairs of wires
  • full duplex (I think): one pair of wires in each direction (no collision)
  • 4B/5B at 125 MHz
• 100Base-FX:
  • two fibers
  • full duplex: one fiber in each direction (no collision)
  • up to 2000m. long

Gigabit Ethernet

• 1 Gb/s throughput
  • two fibers
  • full duplex: one pair of wires in each direction (no collision)

Ethernet Questions

• Can ethernet packets be received out of order?
• Can ethernet packets be received with incorrect bits?
• Can ethernet packets be received truncated?
• Can ethernet packets be received extended?
• Can ethernet packets be lost?

Token Ring

• Ring Topology
• Each interface forwards the packet and buffers it if it's for the attached computer
• When an interface has a packet to send and receives an "empty packet" (the token), it sends out the packet
• When the packet returns to the sender, sender assumes it's been received, removes it, sends out the token.
• What happens if no token is ever received?
• What happens if there are multiple tokens?

Care and Feeding of Tokens

1. Never kill a token
2. Monitor the time since the last token, send out a token if the time is "too long".
3. If you see a token from someone other than yourself and with a lower-numbered address, drop your own token when it gets back to you.

What happens if a cable breaks?

• With an ethernet, the network is partitioned. Halves can still communicate (as long as the wires will carry the signal)
• With a token ring, all communication is interrupted
• So why not a dual token ring?
• ==> FDDI

FDDI

• Fiber Distributed Data Interface
• Dual Ring Topology, one ring in each direction
• In case of a break, the two rings are automatically merged to provide partial connectivity.
• 100 Mb/s
• Longer total distances than Ethernet

Homework

• 7-3
• 4-5
• Scenario from 4-9: write down the algorithm (pseudo-code) for (a) the base station, and (b) the outlying stations