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CSC 336Data CommunicationsandNetworking Lecture 7c: Local Area Networking (Token Bus – 802.4) Dr. Cheer-Sun Yang Spring 2001
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).
Token Bus • Token Ring (802.5) : P. 183, Section 6.3 • Token Bus (802.4) : P. 186, Section 6.4
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.
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.
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.
Token Bus Operations • Removing stations • Adding stations
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.
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.
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)
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.
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.
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.
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.
Reading • Chapter 6: section 6.4