CSC 336 Data Communications and Networking

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CSC 336Data Communications and Networking

• Lecture 7c Local Area Networking
• (Token Bus 802.4)
• Dr. Cheer-Sun Yang
• Spring 2001

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Topologies

• Bus A single communication line, typically a
  twisted pair, coaxial cable, or optical fiber,
  represents the primary medium.
• Ring packets can only be passed from one node to
  its neighbor.
• Star A hub or a computer is used to connect to
  all other computers.
• Tree no loop exists (logical connection).

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Token Bus

• Token Ring (802.5) P. 183, Section 6.3
• Token Bus (802.4) P. 186, Section 6.4

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Token Bus vs. Token Ring

• The token bus operates on the sam eprincipal as
  the token ring. The stations are organized into a
  ring and a token passes among them. A station
  wanting to send something must wait for the token
  to arrive.
• The stations communicate via a common bus in an
  Ethernet.

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Token Bus vs. Token Ring

• Generally, a station receives a token from its
  predecessor and sends a token to its successor.
• Token bus stations must know their predecessor
  and successor.

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Token Bus vs. Ethernet Bus

• The token bus specifies signals to be sent over a
  baseband cable or a broadband cable in one of the
  three possible modulation schemes continuous
  FSK(baseband), phase coherent FSK(baseband), and
  multilevel duobinary AM/PSK(broadband).
• Ethernet bus does not use the same modulation
  schemes. Ethernet MAC layer protocol is a
  contention protocol and there is not token
  involved.

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Token Bus Operations

• Removing stations
• Adding stations

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Lost Tokens

• Claim token frame is sent out is only one station
  detects that a token is not received in a
  reasonable amount of time.
• If more than one station detect this, a
  contention-resolving algorithm is invoked.

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Ring Initialization

• Each station must go through the process of
  claiming a token and times out until one station
  gets it.
• The token passing algorithm is then in effect.
• Each station begins to join the ring.
• Consequently, the station in the ring eventually
  sends a solicit successor frame and another
  station enters the ring.
• Over a period of time solicit successor frames
  are sent, thus, allowing all of the waiting
  stations to enter the ring.

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Prioritizing Frames

• The token bus protocol 802.4 does not prioritize
  stations, but it does define priorities for data
  frames.
• Each station maintains four priorities class 0,
  class 2, class 4, and class 6.
• Class 0 frame has the lowest priority class 6
  frame has the highest priority.
• When a station gets a token, it sends class 6
  frames first. If there are no class 6 data to be
  sent, it tries class 4, etc. (Figure 6.23 and
  6.24)

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Prioritizing Frames

• Token Holding Time(THT) the maximum time a
  station may spend sending class 6 frames.
• Token Rotation Time(TRT) the maximum time for a
  token to rotate around the ring. It also
  determines the maximum time allowed for a station
  to send frames in the lower classes.

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Two Conditions

• Two conditions allow a station to send lower
• priority frames
• If one or more stations do not send the
• maximum number of class 6 frames.
• If TRT gt n THT.

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Priority Performance

• If each station spends the full time allowed by
  the THT sending class 6 frames, the token will
  take at least n THT time to circulate the ring
  once.
• Consequently, class 6 frames get 100 of the
  buss bandwidth.

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Priority Performance

• If all stations have the maximum number of class
  6 frames to send, we can guarantee a percentage
  of bandwidth equal to 100 n THT / TRT for
  them.
• For example, if we need to guarantee 75 of the
  bandwidth for sending class 6 frames, we need to
  maintain the relationship of TRT 4n THT/3.
• This type of control over the timer adds
  flexibility to the token bus in order to adopt to
  real-time situations.

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Reading

• Chapter 6 section 6.4