The document discusses Token Bus, which combines features of Ethernet and Token Ring. It operates as a physical bus but with stations logically organized into a ring, passing a token among them. Only the station holding the token can transmit data. Token Bus was limited to industrial applications and saw no commercial use for data communication. It uses the token passing mechanism over a physical bus topology at the data link layer.

1. Token Bus (IEEE 802.4) : Definition
 Token bus is a physical bus
that operates as a logical ring
using tokens.
(From: Local Area Networks - Fourzan)

2. 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
communication.

3. 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.
 Eg:
20 44 65 90

4. 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 passing.

5. Token Passing

6. Token Passing

7. Token Passing

8. Token Passing

9. Service Classes

10. Service Classes

11. Ring Management

12. Ring Management: Removing Stations
1. Voluntary Leaving:

13. Ring Management: Removing Stations
2. Unexpected Leaving:

14. Ring Management: Adding Stations
 CASE 1

15. Ring Management: Adding Stations
 CASE 1 (Contd….)

16. Ring Management: Adding Stations
 CASE 2

17. Ring Management
 Token Recovery

18. Ring Management
 Removing Duplicate Tokens

19. Ring Management
 Ring Initialization

20. 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.

21. Token Bus Layers
LLC Data
Link
MAC Layer
Physical Layer
Transmission Medium
MAC Sublayer
It uses the token passing access method over a physical bus topology.

22. Mac Sublayer
 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
receiver.
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.

23. 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
control
000100 – Resolve contention frames
001000 – Token
001100 – Set-successor
000 – request – with -no- response
FC for
001 – request – with response Data
frames
010 - response

24. Mac Sublayer
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
FRAME
Data ADDRESS OF RECEPIENT OF DATA

25. 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
detection sequence
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.

26. DATA FIELD CONTENTS:
FRAME DATA
Claim token Length of frame
Solicit successor- 1 Missing
Solicit successor- 2 Missing
Who- follows Missing
Resolve contention Missing
Token Missing
Set-successor New address of successor
Data Data coming from LLC level
ED FIELD FORMAT:
N N 1 N 1 1 1 1

27. Physical Layer