This document summarizes Project 802 and token passing network protocols such as token ring and token bus. Project 802 established standards for local area network physical components. Token passing protocols like token ring and token bus ensure only one station can transmit at a time by passing a token. Token ring networks arrange stations in a logical ring topology, though the physical topology can be a star. The token frame format and operations of token ring networks are described, including the role of the active monitor station in maintenance. Comparisons are made between token ring and token bus networks, and their applications are discussed.
2. CONTENTS
WHAT IS PROJECT 802 ?
TOKEN PROTOCOL
OVERVIEW OF TOKEN BUS
TOKEN RING
TOKEN RING OPERATIONS
TOKEN RING FRAME FORMAT
MAINTENANCE
COMPARISON OF TOKEN RING AND TOKEN BUS
APPLICATIONS AND REASONS FOR WHY IT IS NOT IN
USE THESE DAYS.
2
3. PROJECT 802
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.
3
4. TOKEN PASSING PROTOCOLS
Before workstation can transmit it must possess the one and
only token.
Two types:
Token ring
Token bus
4
5. 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
are attached.
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.
5
6. 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
occurs.
It uses a broadcast medium for each station to receive frames .
6
7. TOKEN RING
IBM created TOKEN RINGS to compete with DIX
(DEC/Intel/Xerox) standard of Ethernet to improve upon its
previous token bus technology.
7
8. 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.
8
9. 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,
station-to-station
In a token ring a special bit
format called a token circulated
around all the stations.
9
10. 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
ring.
The data 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.
10
11. Ring interfaces have three operating modes.
Repeater
Insert
Copy
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.
11
12. TOKEN RING NETWORK
WITH STAR TOPOLOGY
A
Wiring center Or the multistation attachment unit
E
B
C
D
12
13. 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.
13
14. Token Frame Format
S
AC
(1 Byte)
E
(1 Byte)
(1 Byte)
Start Frame Delimiter
End Frame Delimiter
P
Priority Bits
000 – lowest
111 - highest
Access Control
P
P
Token Bit
0 – Token frame
1 – nontoken
frame
T
M
R
R
R
Monitor Bit
0 = “clean” frame
1 = “orphaned" frame
Reservation Bits
3-bit field used for
reserving a token
at a prescribed
priority level
14
15. IEEE 802.5 Data Frame Structure
Data Frame Format
1
SD
1
1
AC
FC
Starting
delimiter
2 or 6
Ending
delimiter
Frame
status
4
Destination Source Information FCS
Address
Address
J
Access
control
Frame
control
2 or 6
K
0
J
M
PPP T
FF
J
A
K
C
Z
1
K 0
Z
Z
A
0
Z
C
J, K
1
ED
FS
non-data symbols
PPP Priority; T Token bit
M Monitor bit; RRR Reservation
RRR
J K1 I
xx
0
1
Z
E
x x
Z
FF
ZZZZZZ
I
E
frame type
control bit
intermediate-frame bit
error-detection bit
A addressrecognized bit
xx undefined
C frame-copied bit
15
16. Frame
control
FC
00000000 = duplicate address
test; used if two
lobes have the same
address.
00000010 = beacon; used to
locate breaks in the
ring
00000011 = claim token; used to
recover from an
inactive monitor
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
a standby
monitor is present.
Frame
Status
FS
00xx00xx = address not
recognized;
nothing copied.
10xx10xx = address
recognized;
nothing copied
01xx01xx = address not
recognized; data
copied(major
error)
11xx11xx = address
recognized; data
copied (implies
transmission was
successful).
16
17. Starting delimiter and ending delimiter mark the
beginning & ending of the frame.
Access control consist of token bit, monitor bit,
priority bit.
Destination address & source address fields gives the
address.
Checksum field is used to detect transmission errors.
17
18. 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
turned on.
3 combinations:
A=0 C=0 : Destination not present or not
powered.
A=1 C=0 : destination present but frame not
accepted.
A=1 C=1 : Destination present and frame copied.
This increases reliability and acts as automatic
acknowledgement.
18
19. 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
maintainability.
19
20. MAINTANAINCE
Here we use a MONITER STATION that oversees the
ring.
Active Monitor Station Selection takes place using MAC
addresses.
Active Monitor Station Observes the ring for its correct
functioning.
20
21. Tasks Performed by Active Monitor
Station
•
•
•
•
•
•
Upstream Neighbour Determination
Sending purge-frame in the ring
Loss of token (no token circulating.
Garbled frames appears.
Orphan frames.
Length of the ring.
Monitor cannot handle breaks in the ring.
21
22. 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.
22
23. 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
token ring.
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.
23
24. Applications :
Used in large Local Area Networks.
Provides security and save time as there was no packet
collision.
Why It Got Down:
Single point of Failure.
Very High Cost.
The era of Switching technique gave ETHERNET an
edge over it.
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