Token Bus (IEEE 802.4) : Definition
Token bus is a physical bus
that operates as a logical ring
(From: Local Area Networks - Fourzan)
Introduction to Token Bus
Token Bus (IEEE 802.4) combines features of Ethernet
(a bus topology) and Token ring.
Stations are logically organized into a ring. A token is
passed among the stations. If a station wants to send data,
it must wait and capture the token.
Token Bus is limited to factory automation and process
control and has no commercial applications in data
Token Bus: Introduction
The logical ring is formed based on the MAC address
of the station in descending order.
Each station considers the immediate lower address
as next station and the station with immediate
higher as the previous station.
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Token Ring (IEEE 802.5)
Token ring requires that stations take turns sending data.
Each station may transmit only during its turn and may
send only one frame during each turn.
The mechanism that coordinates this rotation is called
Token Bus Layers
The Token Bus protocol operates in the :-
a) Data Link Layer:
1. LLC Sublayer
2. MAC Sublayer
b) Physical Layer
The logical link control (LLC) is the upper part
sublayer of the data link layer (logical addresses,
control information, and data)
Media Access Control (MAC) sublayer resolves
the contention for the shared media.
Token Bus Layers
It uses the token passing access method over a physical bus topology.
Frame Format – general format of fields in the frame at
the MAC layer
1. Preamble: Consists of one or more bytes of a
pre-defined pattern to synchronize the sender and
2. Start delimiter: Its 1 byte long and is used to
alert the receiving station when a frame is arriving.
3. Frame control: Its 1 byte long and defines the
type of the frame: if the 1st 2 bits are 00 - MAC control frame
if 01 – data frame from LLC layer.
if 10 – Data from other protocols.
Mac Sublayer (Frame Control Field)
A MAC control frame can be:
000000 – Claim token
000001 – Solicit successor- 1
000010 – Solicit successor- 2
000011 – Who- follows FC for
000100 – Resolve contention frames
001000 – Token
001100 – Set-successor
000 – request – with -no- response
001 – request – with response Data
010 - response
4. Destination address: Contains the physical
address of the frame’s next destination. The DA may
or may not be valid based on the type of frame.
FRAME Destination Address
Claim token IGNORED
Solicit successor- 1 ADDRESS OF SUCCESSOR
Solicit successor- 2 ADDRESS OF SUCCESSOR
Who- follows IGNORED
Resolve contention IGNORED
Token ADDRESS OF SUCCESSOR
Set-successor ADDRESS OF STATION THAT SENT SOLICIT-SUCCESSOR
Data ADDRESS OF RECEPIENT OF DATA
5. Source Address: Its 2 to 6 bytes long and
contains the physical address of the sending station.
Its PRESENT in all types of frames
6. Data: It contains data coming from the LLC layer
for a data frame or extra information required for
some control frames
7. FCS: Its 4 bytes long and contains a CRC-32 error
8. End delimiter: Its 1 byte long and is used to alert
the receiving station to the termination of a frame. It
serves as a flag. The field may be 1 or N (non-data) bit.
DATA FIELD CONTENTS:
Claim token Length of frame
Solicit successor- 1 Missing
Solicit successor- 2 Missing
Who- follows Missing
Resolve contention Missing
Set-successor New address of successor
Data Data coming from LLC level
ED FIELD FORMAT:
N N 1 N 1 1 1 1