IEEE 802.4 defines the Token Bus network standard. A Token Bus network uses a physical bus topology with stations attached to a main cable, but logically organizes the stations in a ring. One station at a time is allowed to transmit after capturing a circulating token; this prevents collisions. When initialized, stations are inserted into the logical ring based on their MAC addresses from highest to lowest. The token is passed between stations in sequence, allowing the current holder to transmit for a set time before passing it to the next station.

1. IEEE 802.4 – Token Bus
By – SAGAR ADHIKARI
ROLL NO. - 129
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2. IEEE 802.4 – Token Bus
Definition:
• Physically, the token bus is a linear or tree-shaped cable to which the
stations are attached .
• Logically the stations are organized into a Ring .
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3. Introduction
• Token Bus is a physical bus that operates as a logical ring in LANs using
Tokens .
• Token Bus (IEEE 802.4) combines the features of Ethernet and Token
Ring .
• IEEE 802.4 token bus networks are constructed with 75-ohm coaxial
cable using a bus topology.
• A token is passed among a stations. If a station wants to send a data, it
must wait and capture the token.
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4. • The token is passed from one user to other in a Sequence (clockwise
or anticlockwise).
• A station can only transmit the data when it has the token.
• The logical ring is formed based on the MAC address of the station in
a Decreasing order.
• n stations takes T sec to send the frame . Station take nT sec to wait .
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5. Token Bus
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6. Token Bus Logical Ring Organization
• When the ring is initialized stations are inserted
into it in order of station address from
highest to lowest .
• Whenever a station acquires the token
it can transmit frame for a specific amount
of time .
• If a station has no data it passes the token immediately
upon receiving it .
• Only one station can hold the token at a time .
• Do not occur any collision.
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7. • Each station is internally divided into four substations, one at each
priority level 0, 2, 4, and 6 for traffic.
• When a token comes into the station, it passed internally to the priority
6 substation, which can begin transmitting its frames.
• When its time expires, the token is passed to priority 4 substation. This
process continues until either the priority 0 substation has sent all its
frames or its time expires.
• After this the token is passed to the next station in the ring.
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8. Frame Format of Token Bus
1.Preamble – It is used for bit synchronization.
2.Start Delimiter – These bits marks the beginning of frame.
3.Frame Control – This field specifies the type of frame – data frame and control
frames.
4.Destination Address – This field contains the destination address.
5.Source Address – This field contains the source address.
6.Data – If 2 byte addresses are used than the field may be upto 8182 bytes and
8174 bytes in case of 6 byte addresses.
7.Checksum – This field contains the checksum bits which is used to detect errors
in the transmitted data.
8.End Delimiter – This field marks the end of frame.
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9. Thank you
References:-
• https://www.geeksforgeeks.org/token-bus-ieee-802-4/
• https://www.youtube.com/watch?v=OpdwfKSjhn8&list=LLy17dJ0u9ir
5pTs9K9rsZgQ&index=2&t=190s
• https://ecomputernotes.com/computernetworkingnotes/computer-
network/
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