- Another way to share a broadcast medium:
- regulated transmissions...
- The use (= transmission)
of the LAN is
regulated
by well-defined rules.
Because nodes do not content (compete) for access, this type protocol are called "contention-free".
- regulated transmissions...
- Right to use (= transmit):
- The right to use
is conveyed in a
special messages
from one node
to another
- The right is
passed
in a round robin manner
(for fairness reasons).
- The special message is called a token
- The right to use
is conveyed in a
special messages
from one node
to another
- There are 2 types of
contention-free MAC protocols:
- Token ring (IEEE 802.5)
- Token bus (IEEE 802.4)
- Architecture:
- Computers are organized into a
physical ring:
- Messages
are circulated through the
ring network:
- Computers are organized into a
physical ring:
- Preliminaries:
- The "token"
is nothing more
than a bit
in the
message header
When this special bit is set (= 1), the message is a token.
- The node that has
received a
token message is
called the
token holder
- The token holder
is permitted to transmit
only one message
- A transmitted messages
will circulated through the entire ring
and returned back to
the sender
- The "token"
is nothing more
than a bit
in the
message header
- Reliability protocol in IEEE 802.5 token ring:
- The header of the message
also has an
ACK bit
- Initially, the
ACK bit is
reset (0)
When the intended destination received the message, it sets (= 1) the ACK bit
- When the message returns to the sender, it can detect whether the message was received
- The header of the message
also has an
ACK bit
- The Token ring protocol:
- Graphically:
- When a node has a messages to send, it waits for the token.
- The node removes the token from the ring - this way,
other nodes in the network will not start a transmission.
(Easy: the node simply do not forward the token further)
- When the node receives the token, it transmits the messages.
- The nodes waits for the message to return
back !!
- After the messages returns, the node "issues" a new token on the ring.
- When a node has a messages to send, it waits for the token.
- Graphically:
- Weaknesses:
- Poor performance
in lightly loaded situation:
- When most nodes are idle, the token will circulate by many nodes "in vain"
- Addition and
deletion of
nodes (computers)
from the token ring
is very labor intensive
- Cable breakage
will cause the
entire network to fail...
(Because the ring is severed....)
NOTE:
- Locating the section of the cable that had the disruption is not easy...
- Poor performance
in lightly loaded situation:
- Strengths:
- Good performance
in highly loaded situation
- Built-in
ACK scheme
(Since the packet has to come back to the sender before the sender can release the token, the message frame might as well take on the role of an ACK frame...)
- Good performance
in highly loaded situation
- The star-shaped ring
topology was proposed to
overcome the
cable failure problem:
The actual shape of the hub or concentrator is a box:
The connection forms a physical ring !!!
- Suppose a section of the
Token Ring
was disrupted:
Locating the cable disruption is now relatively easy:
- Just find the 2 end points that
does not
for a closed loop:
Fixing the token ring is also relatively easy:
- Omit
the disconnected segment:
- Just find the 2 end points that
does not
for a closed loop:
- The IEEE 802.4 Token bus
architecture addresses the
node addition/deletion
problem in the
IEEE 802.5 token ring
- Architecture of the IEEE 802.4 Token bus:
Notes:
- Stations (computers)
are connected using a bus
(like Ethernet)
- Stations (computers)
form a logical ring
- The order of the nodes
in the ring is always
descending
according to the
ID of the nodes
that are in the ring
- Each station maintains:
- the ID of
its logical predessor
(The ID of the predecessor is needed for station (node) removal)
- the ID of
its logical successor
(The ID of the successor is needed for general operation and station insertion)
- the ID of
its logical predessor
- Stations (computers)
are connected using a bus
(like Ethernet)
- Protocol
- When a node receives the
token message,
it can
transmit
upto 1 messages
- Unlike in the Token ring system,
the token holder does
not wait
for the message to return
- The token holder sends the token to its successor
- When a node receives the
token message,
it can
transmit
upto 1 messages
- Node insertion procedure:
- From time to time, a node
that is
not the
last member of the ring,
sends out a
Solicit_successor1(myID, mySucc)
message
Example:
- A node with
ID between
1 and
4
that wants to join
will then reply.
Example:
- The node is inserted by the
inviting node
by sending an
Set_successor() message.
Example:
- The inserted station must
make itself known to
it's downstream neightbor
Example:
- From time to time, a node
that is
not the
last member of the ring,
sends out a
Solicit_successor1(myID, mySucc)
message
- Node insertion procedure - last node:
- When the last node
(it can tell by:
my ID < succ ID),
it sends a
different Solicitation message
- called
Solicit_successor2()
Example:
The Solicit_successor2(0,4) message will invite nodes:
- nodes with an ID < 0, or
- nodes with an ID > 4
- Beside this difference, the insert procedure is identical to the one described abovbe...
- When the last node
(it can tell by:
my ID < succ ID),
it sends a
different Solicitation message
- called
Solicit_successor2()
- When multiple nodes are invited, there will be collision
Example:
Responses from nodes 2 and 3 will collide:
- Collision Resolution Protocol:
Executed by nodes that are involved in a collision:
- Select the first 2 bits
of your own ID
Let this value be x
- Transmit for
(x+1) × &tau
time
- If the medium
is still busy
after
(x+1) × &tau
time,
remove yourself
from the contention
(I.e., you have lost the competition)
- If just before (x+1) × &ta,
you
only sensed your own transmission,
declare yourself
to be the winner
- Otherwise
(you sensed another transmission
just before you ended):
- Select the next 2 bits
from your own ID
- Repeat the above precedure...
- Select the next 2 bits
from your own ID
- Select the first 2 bits
of your own ID
- Effect of the Collision Resolution Protocol:
- The node that has the largest value for ID will be the winner of the Collision Resolution "tournament"....
- Example:
- Nodes 2 and
3
enters the "tournament":
- Round 1:
Result: undecided
Select the next 2 bits in the address....
- Round 2:
- Nodes 2 and
3
enters the "tournament":
- When a node wants to
leave the
token bus,
it sends a
Set_successor(my Successor)
message to its
predecessor.
Example:
- The notified node will then
make itself known
to its
new successor:
Example:
- When the token ring
is first started,
there are no stations
in the ring...
- When the first node
on the token bus
comes up, it sends a
Claim_token
packet to initialize the ring.
- If more than one station
send this packet at the same time, there is a
collision.
- The Collision Resolution Protocol
(tournament) is run...
And the station with the largest ID will be the winner.
- This station
will form a ring
consisting of
one node...
- Then the station invites the other stations into the ring
- When a node fails,
it will not pass the token on
and the
whole token ring
is dead...
- Responsibility of the token holder:
- When the token holder
passes the token to
its
successor,
it sets a timeout
- When the timeout expires
without any network activity,
the token holder will
retransmit the token
again.
- When the token holder
times out for the
second time,
it will assume that its
successor has failed
.... and initializes a recovering protocol
- When the token holder
passes the token to
its
successor,
it sets a timeout
- Recovery Protocol:
- The Token holder send a
Who_follows(x)
message, with
x equals to the ID of the
down node
- The successor
of the down node x
(say node C),
will now respond with:
- Set_successor(C)
- The token holder will set its successor node to C, and the logical ring is restored.
- The Token holder send a
Who_follows(x)
message, with
x equals to the ID of the
down node