Lecture 10: Token Ring and FDDI Networks

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Lecture 10 Token Ring and FDDI Networks

• Homework for Chapter 2 All problems,
  particularly time lines for sliding window
  protocol
• Exam 2 October 16, 2000

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Token Ring Review

• Shared medium similar to ethernet
• Distributed algorithm controls transmission
• All nodes see all frames

Tokensmall (24 bit) circulating frame To send
Remove token and send frame Sender removes
message when it comes around and reinserts token
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Key Concept

• Token Holding Time (THT)gttransmission time of a
  packet
• Token Rotation Time (TRT)lt ActiveNodes x
  THTRingLatency Note TRT is load dependent

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Fiber Distributed Data Interface (FDDI)
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Single Attachment Stations (SAS) attached to a
concentrator (DAS)Star-like topology
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FDDI physical properties

• 100 Mbpsbit time is only 10 ns
• Network adaptor buffers-9-80 bits-retransmit when
  half full
• Queuing delay50 ns for 10 bit buffers
• Up to 500 stations
• 2 km between stations (propagation delay10 ms)
• 100 km ring circumference--200 km total fiber
  length
• 4B/5B encoding
• Packet size18,200 bytes

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FDDI physical properties (cont.)

• Complete Ring
• Ring Latency24 (10 ns)50050 ns0.5 ms0.53 ms
• Packet transmission time1.45 ms
• Every node should see a valid transmission (token
  or data frame) every 1.98 ms (or less). This is
  called the idle time
• Broken ring
• Ring Latency 24 (10ns)100050ns1.0ms1.05 ms
• Idle time2.5 ms

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Key Parameters

• Target token rotation time (TTRT)-upper bound to
  TRT as seen from any node
• So, TTRT is a measure of the maximum possible
  delay as seen by each station
• TTRT is negotiatedthe station with the most
  sensitive application sets TTRT (more later)
• Measured TRT-every station measures current value
  of TRT

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Media Access Algorithm(Timed Token Algorithm)

• If Measured TRT gt TTRT, the token is
  delayedstation is not permitted to send
• Measured TRT lt TTRT, station can send
  TTRT-Measured TRTTHT
• Ideally, this algorithm provides feedback needed
  to keep the ring from being overloaded

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Access Problems

• Station are allowed to start packets that cause
  THTgt TTRT-Measured TRT
• If you are downstream from such a node, you may
  never be able to send your packets
• Two traffic categories
• Synchronouse.g. time sensitive-Total synchronous
  traffic limited TTRT per ring rotation
• Asynchronous-governed by Timed Token Algorithm

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Worst Case Performance

• Token Rotation Time2TTRT
• Does not happen on successive rotations

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FDDI Token Maintenance

• Recall idle time of a broken ring is 2.5 ms
• Stations send claim if idle longer than 2.5 ms
• Claim contains a bid for TTRT that is the maximum
  that is acceptable to by sending station
• Bid is passed on by nodes with higher TTRT-they
  reset their TTRT to the bid value
• Bid is replaced by nodes with lower TTRT
  requirements
• Eventually, TTRT is set to the largest value that
  satisfies all stations.

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FDDI frame format
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Wireless LAN - 802.11

• LAN - Local Area Network. A network based on any
  physical network technology that is designed to
  span distances of up to a few thousand meters.
• IEEE 802 LAN-Shared mediumpeer-to-peer
  communications network that broadcasts
  information to all stations to communicate

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Wireless LAN - 802.11

• OSI Layer 1 - Physical
• Encoding/Decoding
• Preamble generation/removal
• Bit transmission/reception
• OSI Layer 2 - Data Link
• Medium Access Control/Arbitration
• Addressing
• Protocol Data Unit/Frame

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Wireless LAN - 802.11

• Design issues
• Distance
• Performance
• Reliability
• Security
• Administration
• Scalable

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Wireless LAN - 802.11

• Technology
• LAN Emulation
• Frequency Hopping
• Direct Sequencing
• Collision
• Distribution
• Frame Format
• Other

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Wireless LAN - 802.11

• Frequency Hop Spread Spectrum
• 2.4Ghz, 1 and 2 Mbps transmission
• Hop over 79 channels (N.America)
• Pseudo-random hopping
• Interference Immunity

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Wireless LAN - 802.11

• Direct Sequence Spread Spectrum
• 2.4Ghz, 1 and 2 Mbps transmission
• 11 Chip Barker Sequnce
• Interference Immunity

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Wireless LAN - 802.11

• Baseband Diffuse Infrared
• 1 and 2 Mbps transmission
• Interference Immunity
• Light
• Reflection

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Wireless LAN - 802.11

• Baseband Diffuse Infrared
• 1 and 2 Mbps transmission
• Interference Immunity
• Light
• Reflection

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Wireless LAN - 802.11

• Direct Sequencing
• Chipping Code
• Spread signal across frequency
• Increase in bandwidth determined by bit size of
  chipping code
• 802.11 chipping code 11-bits

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Four-bit chipping sequence
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Wireless LAN - 802.11

• Collision Avoidance
• Unlike Ethernet, not all nodes can see each
  others signal
• Use of RTS/CTS/ACK handshake between sender and
  receiver

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Example of a wireless network
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Wireless LAN - 802.11

• Components
• Access Point
• Central wireless base Unit
• Client
• Remote Node
• Distribution System
• Interconnecting LAN (Like Ethernet)

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Access points connected to a distribution network
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Node Mobility
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IEEE 802.11 frame format
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Wireless LAN - 802.11

• Other related Topics
• IEEE 802.11b - 11Mbps Wireless LAN
• Currently only DSS support from vendors
• IEEE 802.11a - Future (51Mbps?) Wireless LAN

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Wireless LAN - 802.11
Other References (outside of class
text) http//grouper.ieee.org/groups/ 802/11/
Tutorial/index.html Stallings, W Data and
Computer Communications - 5th ed.,
Prentice-Hall, 1997