FDDI, Wireless Networks

• FDDI:
  • Protocol
  • Token Rotation Time
  • Synchronous Traffic
  • Station Management
• Wireless LANs:
  • Cell Networks
  • Ad-Hoc Networks

FDDI

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

FDDI Protocol

• 4B/5B over fiber
• Frame bracketed by a starting delimiter and an ending delimiter
• Frame format: destination address (2/6 bytes), source address (2/6 bytes), data (0 to 4500 bytes), CRC (4 bytes)
• Token passing:
  • When I receive a token,
  • I can replace it with my data
  • Followed by the token
  • I must remove my data when it comes back to me

Real-Time Features

• Data is Synchronous (real-time) or Asynchronous
• Hosts negotiate a Target Token Rotation Time (TTRT = Sigma[i in nodes] Si, where Si is the time required by node i for synchronous transmission)
• Goal is to be able to transmit synchronous data within at most 2 TTRT
• Host algorithm uses timers TRT (token rotation time) and THT (token holding time)

FDDI Host Algorithm

1. THT = TTRT - TRT (THT starts decrementing)
2. TRT = 0 (TRT starts incrementing)
3. host sends all its synchronous data
4. host sends asynchronous data while THT > 0
5. release the token

Notes:

FDDI Host Algorithm

• TTRT >= Sigma[i in nodes] Si
• if nodes only transmit synchronous traffic, TRT <= TTRT
• a node can only transmit asynchronous (low priority) traffic for time TTRT - TRT[-1] - Si
• So when TRT >= TTRT, nodes cannot send asynchronous traffic
• Maximum asynchronous traffic is when TRT[-1] = 0, when nodes can send asynchronous data for a total time of TTRT

Station Management

• Normally, traffic can use either ring (total bandwidth 200Mb/s)
• Cheaper interfaces only interface to one ring
• Ring interruption is detected by TRT >> 2 * TTRT
• Ring interruption can be identified by absence of token generations
• Dual-homed station nearest break can automatically reconfigure ring
• If ring comes back up, dual ring can automatically be re-established

FDDI Questions

• Who uses FDDI, and for what?
• Why do people prefer 100Mb/s ethernet?
• Why do people prefer ATM?
• Do people use the synchronous feature of FDDI?
• Do people use the long distance that FDDI provides?
• Is the ring topology helpful?
• If there is no synchronous traffic, what TTRT do you negotiate?
• Since the network is self-healing, how do you:
  • Become aware of a broken link?
  • Identify the broken link?

Wireless LANs

• Base stations (hubs) connected by land lines
• Hosts (nodes) connected only by radio
• Cellular LANs:
  • Hosts only talk to base stations
  • Host cell selection protocol
  • Example: satellite networks
  • Example: cellular telephones
• Ad-hoc packet radio LANs:
  • Hosts talk to other hosts or base stations
  • Routing protocol
  • Difficult management
  • Example: laptops in a lecture room
• Power consumption

Wireless LANs: Performance

• Throughput often only a few Kb/s
• High error rates due to varying power levels, noise, collisions
• Spread-spectrum techniques can help with noise and collisions
• Data-link level acknowledgement and retransmission for reliability
• Retransmission can lead to high latency
• Reservations can reduce collisions

Cellular LANs

• Hosts choose the base station with the strongest signal, or base station that receives the host most strongly tells host which cell it is in
• Problems with asymmetric connectivity
• When a host moves from one cell to another, host and base stations must negotiate a handoff
• Overhead related to selecting cell
• Within a cell, a host usually has a reserved frequency, which could lead to low utilization

Ad-Hoc Wireless LANs

• Security issues (can I send my homework to the teacher through my neighbor's laptop?)
• Routing: hosts (and obstacles to radio waves) can move, so information must be updated frequently, but we only have little bandwidth available to distribute the information
• Management: what do I do if I lose connectivity? Who is responsible?
• Fairness/cost: nodes nearer the base stations have to use more of their power to send data to other nodes.