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.
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
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.
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