WHAT IS PROJECT 802 ?
OVERVIEW OF TOKEN BUS
TOKEN RING OPERATIONS
TOKEN RING FRAME FORMAT
COMPARISON OF TOKEN RING AND TOKEN BUS
APPLICATIONS AND REASONS FOR WHY IT IS NOT IN
USE THESE DAYS.
Project 802 defined network standards for the
physical components of a network. It got started in the
month February of the year 1980 , that is why the
project name 802.
TOKEN PASSING PROTOCOLS
Before workstation can transmit it must possess the one and
TOKEN BUS : IEEE STANDARD 802.4
This standard 802.4(Dirvin and Miller and IEEE, 1986)
described a LAN called a token bus.
A simple system in which stations takes turns sending a frame
was introduced know as a TOKEN BUS SYSTEM.
The token bus is linear tree shaped cable onto which stations
Logically stations are organized into a ring with each station
knowing the address of the station to its left and right.
During logical ring initialization the highest number station
may send the first frame.
It passes permission to its immediate neighbor by sending the
neighbor a special control frame called a TOKEN.
Only the token holder is permitted to transmit frames.
Only one station at a time holds the token therefore no collision
It uses a broadcast medium for each station to receive frames .
IBM created TOKEN RINGS to compete with DIX
(DEC/Intel/Xerox) standard of Ethernet to improve upon its
previous token bus technology.
IEEE 802.5 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. Can both be implemented using star as well as ring
topologies but basically it uses ring topology logically and
star topology physically.
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.
IEEE 802.5 TOKEN RING
There is a point to point link
between stations that form a ring.
Physical Layer Topology: Ring
Stations connected in a loop
Signals go in only one direction,
In a token ring a special bit
format called a token circulated
around all the stations.
TOKEN RING OPERATION
When a station wishes to transmit, it must wait for
the token to pass by and seize the token.
One approach: change one bit in token which transforms it into a
normal data frame and appends frame for transmission.
Second approach: station claims token by removing it from the
The data frame circles the ring and is removed by the
Each station interrogates passing frame. If destined
for station, it copies the frame into local buffer.
Ring interfaces have three operating modes.
Repeater: the input bits are simply copied to output with a delay
of 1 bit time.
Insert: the interface breaks the connection b/w the input &output
entering its own data onto the ring.
Copy: the interface or the station simply copies data into its local
buffer as it is the destined station.
As bits have propagated around the ring & they come back, they
are removed from the ring by the sender.
TOKEN RING NETWORK
WITH STAR TOPOLOGY
Wiring center Or the multistation attachment unit
Multistation Attachment Unit
Earlier there was problem of segmentation using just
by - pass relays , but that problem got easily solved
using multistation attachment unit because there were
many stations attached to this single unit . Also the
problem of going down of stations got corrected.
Token Frame Format
Start Frame Delimiter
End Frame Delimiter
000 – lowest
111 - highest
0 – Token frame
1 – nontoken
0 = “clean” frame
1 = “orphaned" frame
3-bit field used for
reserving a token
at a prescribed
IEEE 802.5 Data Frame Structure
Data Frame Format
2 or 6
Destination Source Information FCS
2 or 6
PPP Priority; T Token bit
M Monitor bit; RRR Reservation
J K1 I
A addressrecognized bit
C frame-copied bit
00000000 = duplicate address
test; used if two
lobes have the same
00000010 = beacon; used to
locate breaks in the
00000011 = claim token; used to
recover from an
00000100 = purge; used to
reinitialize the ring.
00000101 = active monitor
present;used by the
active monitor to
inform all lobes that
it is alive.
00000110 = standby monitor
present; used to
notify all lobes that
monitor is present.
00xx00xx = address not
10xx10xx = address
01xx01xx = address not
11xx11xx = address
Starting delimiter and ending delimiter mark the
beginning & ending of the frame.
Access control consist of token bit, monitor bit,
Destination address & source address fields gives the
Checksum field is used to detect transmission errors.
Frame status field
When a frame arrives at the interface A bit is turned on.
If the interface copies the frame to the station the C bit is
A=0 C=0 : Destination not present or not
A=1 C=0 : destination present but frame not
A=1 C=1 : Destination present and frame copied.
This increases reliability and acts as automatic
STAR TOPOLOGY OR WIRE CENTER
• Physically each station is connected to the Multistation
Attachment Unit by a cable. While logically it is still a ring.
• A bypass relay that are energized by the current from the
station are inside the Multistation Attachment Unit.
• When station goes down the station is bypassed from the rest
of the ring.
• Multistation Attachment Unit improves the reliability and
Here we use a MONITER STATION that oversees the
Active Monitor Station Selection takes place using MAC
Active Monitor Station Observes the ring for its correct
Tasks Performed by Active Monitor
Upstream Neighbour Determination
Sending purge-frame in the ring
Loss of token (no token circulating.
Garbled frames appears.
Length of the ring.
Monitor cannot handle breaks in the ring.
GARBLED AND ORPHAN FRAMES
Both actually refers to the corrupted frames. For e.g.
sometimes what happens that in case a station after
sending data got corrupted. Now as it is the duty of the
sending station to release the token so as there is no
sending station so this frame becomes orphaned
frame. And that needs to be removed by the monitor
station . Similar is the case with garbled frame .
Monitor Station changes monitor bit to 1 and when it
again receives it , it removes it from the ring.
802.4 Vs 802.5
802.4 does not use any monitor like 802.5.
Which is more reliable ? (depends upon the situation)
Token ring & token bus both have priorities but not 802.3.
Adding new station to token bus was difficult as compared to
the addition in token ring.
In token bus there is no relation between the physical location
of a station on the bus to its address whereas it was not so in
Both uses broadcasting addresing technique.
Basically token ring is primarily used for control of industrial
and factory automation processes.
Main problem : No active Monitor Station.
Used in large Local Area Networks.
Provides security and save time as there was no packet
Why It Got Down:
Single point of Failure.
Very High Cost.
The era of Switching technique gave ETHERNET an
edge over it.