CS 635 Lecture 5, Part 1

Token Bus General Description

CSMA/CD does not feet well with factory automation. It is probabilistic and the worst case is unbounded. The 802.3 frames doe not have priority and are unsuitable for real time systems
In a token ring system with n stations and T sec to send a frame, no frame will wait more than nTsec to be sent (this is the upper bound that is missing in 802.3). However, the physical implementation of a ring does not fit the linear topology of most assembly lines.
Token bus has the robustness of 802.3 broadcast cable, but the known worst-case behavior of a ring
Physically, the token bus is a linear or tree-shaped cable. Logically, the stations are organized into a ring with each station knowing the address of the station to its "left" and "right".
The highest numbered station may send the first frame. After it's done, it passes permission to its immediate neighbor by sending it a token.
Physical order in which stations are attached to the cable is not important
MAC protocol has provisions for adding stations to, and deleting station from the ring. Physical layer uses 75 ohm broadband coaxial cable used for cable TV. Speeds of 1,5, and 10 Mbps is possible
Protocol is more complex than 802.3. The protocol maintains 10 different timers, 2 dozen internal variables, and consists of more than 200 pages
IEEE 802.4 determines the logical ring of the physical bus by the numeric value of addresses. An MAC or LLC data unit provides the utility for the lowest address to hand the token to the highest address. Then, the token is passed from a predecessor station to its successor station.
The token is passed from station to station in descending numerical order of station address.
When a station hears a token frame addressed to itself, it may transmit data frames
When a station has completed transmitting data frames, it passes the token to the next station in the logical ring
When a station has the token, it may temporarily delegate its right to transmit to another station by sending a request-with-response data frame
After each station has completed transmitting any data frames it may have, the station passes the token to its successor by sending a token control frame.
After sending the token control frame, the station listens for evidence that its successor has heard the token frame and is active.
If the sender hears a valid frame following the token, it assumes that its successor has the token and is transmitting.
If the token sender does not hear a vlid frame following its token pass, it attempts to asses the status of the network and may implement measures to pass around the problem station by establishing a new successor.
If the successor does not transmit, the sending station normally assumes the successor is not operable. The sender then transmits a "who follows" frame with it successor's address in the frame.
The failed station is bypassed by all stations comparing this address with the address of their predecessor.
The station whose predecessor's address matches the "who follows" address then sends a "set successor" frame with its address.
In this manner, the failed station is bridged out of the network.

Stations are added to an 802.4 bus by an approach called response windows:

While holding the token, a node issues a solicit-successor frame. The address in the frame is between it and the next successor station
Token holder waits one window time (slot time, equal to twice the end-to-end propagation delay)
If no response, the token is transferred to the successor node
If response, a requesting node sends a set-successor frame and token holder changes its successor node address. Requested receives token, sets its addresses, and proceeds.

A node can drop out of the transmission sequence. Upon receiving a token, it sends a set-successor frame to the predecessor, which orders the next node to give the token hereafter to its successor

MAC Sublayer

Token Bus MAC sublayer

The MAC sublayer consists of four major functions:

The interface machine (IFM)
The access control machine (ACM)
The receive machine (RxM)
The transmit machine (TxM)\

ACM

the heart of the token-bus system. It determines when to place a frame on the bus and cooperates with the other stations. ACMs control access to the shared bus.
Responsible for initialization and maintenance of the logical ring, including error detection and fault recovery
In controls the admission of new stations and attempts recovery from faults and failures

TxM

transmit the frame to the physical layer
accepts frames from the ACM and builds MAC PDUs

RxM

accepts data from physical layer
identifies a full frame by detecting the SD and ED
checks the FCS field to validate transmission
if the received frame is an LLC type (data) it is passed to the IFM. IFM indicates its arrival and then delivers it to the LLC

Token Bus MAC Frame

Priority Scheme

When the ring is initialized, stations are inserted into it in order of station address, from highest to lowest
The token bus defines four priority classes, 0,2,4,6 for traffic, with 0 the lowest and 6 the highest
Conceptually, each station internally being divided into four substations, one at each priority level
As input comes into the MAC sublayer from above, data are checked for priority and routed to one of the four substations. Thus each substation maintains its own queue
When the token comes into the station over the cable, it is passed internally to the priority 6 substation, which may begin transmitting frames, if it has any
When it is done (or when its timer expires), the token is passed internally to the priority 4 substation...
Setting the timer properly we can ensure that a guaranteed fraction of the total token holding time can be allocated to priority 6 traffic
The lower priorities will have to live with what is left over
Priority 6 traffic is guaranteed a known fraction of the network bandwidth and can be used to implement real-time traffic
Example: 50 station network, running at 10 Mbps adjust 1/3 of bandwidth to priority 6 traffic. Each station has a guaranteed 67 Kbps for priority 6 traffic (1 digital voice channel)

THT = token holding time; the maximum time that a station can hold the token to transmit class 6 data
TRTi = token rotation time for class i, (i=4,2,0); the maximum time that a token can take to circulate and still permit class i transmission

In N station ring, the max amount of time available for class 6 transmission is N * THT.
After transmitting class 6 data, or if there were no class 6 data transmit, the station may transmit class 4 data only if the amount of time for the last circulation of the token (including any class 6 data just sent) is less than TRT4
Class 2 and 0 are handled in the same way
Preference is given to class 6

Principal disadvantage of token bus is complexity. Second disadvantage is the overhead - under lightly loaded condition a station may have to wait a while to transmit.

Can overcome the disadvantage by using multiple priority levels; different stations maybe allowed to hold the token different amounts of time

Advantages
There is no minimum packet length. No need to listen and talk. Superior performance under heavy load.

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