The document discusses IEEE 802 subgroups and local area network (LAN) technologies such as token bus and token ring. It provides details on: - IEEE 802 subgroups and their responsibilities for various networking standards - How token passing works on token bus and token ring networks, with stations passing a token frame that allows the holder to transmit data - Standards such as IEEE 802.4 for token bus and IEEE 802.5 for token ring - Key aspects of token ring and token bus networks, including frame formats, priority schemes, and how data is transmitted and errors are handled.

1. TOKEN BUS
TOKEN RING
Mrs.R.Shanthi Prabha M.Sc., M.Phil.,
Assistant Professor,
DEPARTMENT OF Computer Science

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

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
Circulating
Token
Server Workstation
Workstation
•No collisions

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