1. TOKEN BUS & TOKEN RING

2. IEEE 802 Subgroups and their Responsibilities
802.1
Internetworking
802.2
Logical Link Control (LLC)
802.3
CSMA/CD
802.4
Token Bus LAN

3. IEEE 802 Subgroups and their Responsibilities (Cont.)
802.5
Token Ring LAN
802.6
Metropolitan Area Network
802.7
Broadband Technical Advisory Group
802.8
Fiber-Optic Technical Advisory Group

4. IEEE 802 Subgroups and their Responsibilities (Cont.)
802.9
Integrated Voice/Data Networks
802.10
Network Security
802.11
Wireless Networks
802.12
Demand Priority Access LANs
Ex: 100BaseVG-AnyLAN

5. Local Area Network Technology
There are two types of token-passing architectures:
Token Bus is similar to Ethernet because all clients are on a common bus and can pick up transmissions from all other stations
Token Ring is different from Token Bus in that the clients are set up in a true physical ring structure

6. Token Passing Standards
IEEE 802.5
For the token-ring LANs
IEEE 802.4
For the token-bus LANs
A FDDI protocol is used on large fiber-optic ring backbones

7. INTRODUCTION
Token Ring defines a method for sending and receiving data between two network-connected devices
To communicate in a token-passing environment, any client must wait until it receives an electronic token
The token is a special frame that is transmitted from one device to the next

8. TOKEN RING
Token ring LAN are logically organized in a ring topology with data being transmitted sequentially from one ring station to the next with a control token circulating around the ring controlling access.
This token passing mechanism is shared by ARCNET, token bus, and FDDI, and has theoretical advantages over the stochastic CSMA/CD of Ethernet.

9. TOKEN RING
Token ring local area network (LAN) technology is a protocol which resides at the data link layer (DLL) of the OSI model.
It uses a special three-byte frame called a token that travels around the ring.
Token
Data packet that could carry data
Circulates around the ring
Offers an opportunity for each workstation and server to transmit data.

10. IEEE and Token Ring
Proposed in 1969 and initially referred to as a Newhall ring.
Token ring :: a number of stations connected by transmission links in a ring topology. Information flows in one direction along the ring from source to destination and back to source.
Medium access control is provided by a small frame, the token, that circulates around the ring when all stations are idle. Only the station possessing the token is allowed to transmit at any given time.

11. Token Ring Operation
When a station wishes to transmit, it must wait for token to pass by and seize the token.
One approach: change one bit in token which transforms it into a “start-of-frame sequence” and appends frame for transmission.
Second approach: station claims token by removing it from the ring.
Frame circles the ring and is removed by the transmitting station.
Each station interrogates passing frame, if destined for station, it copies the frame into local buffer.

12. IEEE Token Ring
4 and 16 Mbps using twisted-pair cabling with differential Manchester line encoding.
Maximum number of stations is 250.
Waits for last byte of frame to arrive before reinserting token on ring {new token after received}.
8 priority levels provided via two 3-bit fields (priority and reservation) in data and token frames.
Permits 16-bit and 48-bit addresses (same as 802.3).

13. Token Ring
Under light load – delay is added due to waiting for the token.
Under heavy load – ring is “round-robin”
The ring must be long enough to hold the complete token.
Advantages – fair access
Disadvantages – ring is single point of failure, added issues due to token maintenance.

14. Token Maintenance Issues
What can go wrong?
Loss of token (no token circulating)
Duplication of token (forgeries or mistakes)
The need to designate one station as the active ring monitor.
Persistently circulating frame
Deal with active monitor going down.

15. A Token Bus Network

16. Token Passing in a Token Bus Network

17. Token Passing in a Token Bus Network

18. Token Bus Server Token Client Client Client
A token is distributed to each client in turn.

19. TOKEN FRAME
When no station is transmitting a data frame, a special token frame circles the loop.
This special token frame is repeated from station to station until arriving at a station that needs to transmit data.

20. TOKEN FRAME
When a station needs to transmit data, it converts the token frame into a data frame for transmission. Once the sending station receives its own data frame, it converts the frame back into a token.

21. TOKEN FRAME PRIORITY
Token ring specifies an optional medium access scheme allowing a station with a high-priority transmission to request priority access to the token.
8 priority levels, 0–7, are used.
When the station wishing to transmit receives a token or data frame with a priority less than or equal to the station's requested priority, it sets the priority bits to its desired priority. The station does not immediately transmit; the token circulates around the medium until it returns to the station.

22. FRAME FORMAT
A data token ring frame is an expanded version of the token frame that is used by stations to transmit media access control (MAC) management frames or data frames from upper layer protocols and applications.

23. IEEE 802.5 Token and data frame structure
Token Frame Format SD AC ED
Data Frame Format 1 1 1
2 or 6
2 or 6 4 1 1
Destination
Address
Source
Address SD AC FC
Information
FCS ED FS
Starting
delimiter
J K J K 0
J, K non-data symbols (line code)
Access
control
PPP Priority; T Token bit
M Monitor bit; RRR Reservation
P P P T M
R R R
Frame
control
FF frame type
ZZZZZZ control bit
F F
Z Z Z Z Z Z
Ending
delimiter
I intermediate-frame bit
E error-detection bit
J K J K 1 I E
A address-recognized bit
xx undefined
C frame-copied bit
Frame
status A C
x x A C
x x

24. Token Bus Data Pickup A token is sent from one node to the other
The client wanting to transmit grabs an empty token
Data is attached
Token leaves for the next node and its travel on the bus until it reaches the address to which the data is destined

25. Token Bus Data Delivery
Token delivers the data to the addressee
Acknowledgement is returned to the sender
Token is passed to the next node
The process continues
If there is an error in delivering the information, a request for retransmission attached to the token and it is sent to the sender

26. Token Bus Standard and Applications
IEEE 802.4
It can be used in both broadband and baseband transmission

27. Token Passing Protocol in Operation
Chapter 23
8/7/2018
Token Passing Protocol in Operation
Circulating
Token
Workstation
Server
No collisions
Workstation
(c) Copyright N Ganesan

28. Comparison with CSMA/CD
Absence of collision
Offers a systematic method of transmitting information
In theory, it is superior to CSMA/CD
More sophisticated to implement
Protocols used in the newer and most popular networks are, however, based on CSMA/CD

29. The Transmitting Workstation
Waits for a free token in order to be able to attach the data to be transmitted to the token
On finding a free token, attach the following:
Sender’s address
Receiver’s address
Data block to be transmitted
Error checking details
etc.

30. At the Receiving End Data is received and checked for errors
Outcomes at the receiving end
Data received without errors
Date received with errors

31. Error-free Delivery of Data
An acknowledgment is attached to the token
Acknowledgment is passed to the sender
Token is set free for other nodes to transmit information
At this time, the next workstation on the ring will receive an opportunity

32. Correcting Errors in Delivery
A request for retransmission is attached to the token
Token carries the message for retransmission to the sender
The data is thus retransmitted

33. Token Regeneration
The token is regenerated at regular intervals to sustain the timing of circulation of the token

34. Usage of Token Passing Used extensively in ring LANs
Especially in the IBM token-ring LAN
A version of this protocol is also used on certain types of bus LANs
Token-bus networks
Used in large fiber-optics backbones
Used for the construction of very large networks

35. Usage in Practice Used in backbones Uses in a number of IBM shops
Overall, the usage of Ethernet surpasses the usage of Token-Ring networks that are based on the Token-Passing protocol