- Token holder:
- Token holder =
the node that has
received the
token frame
- There is only
one token holder in the
entire
token ring
- There is only
one token holder in the
entire
token ring
- Only the token holder is allowed to transmit a frame !!!
- Token holder =
the node that has
received the
token frame
- Note:
- The token frame is
also a
frame
- Only the token holder is allowed to transmit the token frame (to another node)
- The token frame is
also a
frame
- Comment:
- When nodes are
idle
(= have no data frames to
transmit),
the nodes will
transmit
token frames to
each other:
- When nodes are
idle
(= have no data frames to
transmit),
the nodes will
transmit
token frames to
each other:
- Operation:
- The token holder
is allowed to
transmit
(only)
one message
- A transmitted messages
will circulated through the entire ring
and returned back to
the sender
- A non-destination node will:
- Forward the data frame to the next node
- When the transmitted message arrives
at the destination node:
- The destination node
will:
- Copy the
data frame
- Set the
frame copied
indicator bit
at the end of the
data frame:
- Forward the data frame to the next node
- Copy the
data frame
- The destination node
will:
- When the data frame returns to the
sender:
- The sender transmits a token frame to the next node
- The token holder
is allowed to
transmit
(only)
one message
- The token ring has a
"built-in"
ACK method
because:
- A
frame with the
frame copy
indicator bit
that has been set by
the receiver,
will travel
back to the
sender !!!
- This works like an acknowledge frame !!!
- A
frame with the
frame copy
indicator bit
that has been set by
the receiver,
will travel
back to the
sender !!!
- The frame copied bit explained
graphically:
Explanation:
- Suppose
node A is the
token holder
- Node A can
transmit a
frame
(intended for node D)
- When node D received
the message, it
sets the
ACK bit in the
tail of
the message
- Node D can do this while the message passes by the node !
- The message will
return back to
the sender
node A
- Then the node A transmits
a token frame (message)
- The next node (B)
in the ring
will become the
token holder
- If node B has
no message to
transmit, it will transmit
a token frame
Otherwise, it sends a data frame and follow the procedure above.
- The next node (B)
in the ring
will become the
token holder
- Suppose
node A is the
token holder
- Strengths:
- Good performance
in highly loaded situation
- Has a built-in ACK scheme
- Good performance
in highly loaded situation
- Weaknesses:
- Poor performance
in lightly loaded situation:
- When most nodes are idle, the token will circulate by many nodes "in vain"
- Difficult to
add/delete
nodes (computers)
from the
token ring
- You need to wired the node into the ring !!!
- Cable breakage
will cause the
entire network to fail...
(Because the ring is disconnected !!!!)
- Poor performance
in lightly loaded situation: