CS 635 Lecture 5, Part 1
Token Bus General Description
Stations are added to an 802.4 bus by an approach called response windows:
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
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
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.
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
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
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.
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)\
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
transmit the frame to the physical layer
accepts frames from the ACM and builds MAC PDUs
Token Bus MAC Frame
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
THT = token holding time; the maximum time that a station can hold the
token to transmit class 6 data
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
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)
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
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.
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
Can overcome the disadvantage by using multiple priority levels; different
stations maybe allowed to hold the token different amounts of time
There is no minimum packet length. No need to listen and talk. Superior
performance under heavy load.