1. CSC 336 Data Communications and Networking
Lecture 7c: Local Area Networking
(Token Bus – 802.4)
Dr. Cheer-Sun Yang
Spring 2001

3. 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 it’s neighbor.
Star: A hub or a computer is used to connect to all other computers.
Tree: no loop exists (logical connection).

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

6. 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.

7. 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.

8. 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.

10. Token Bus Operations
Removing stations
Adding stations

12. 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.

13. 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.

14. Prioritizing Frames
The token bus protocol 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)

17. 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.

18. 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 > n * THT.

21. 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 bus’s bandwidth.

22. 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.

23. Reading
Chapter 6: section 6.4