- Architecture of the
IEEE 802.4 Token bus:
- The nodes in
IEEE 802.4 are:
- Physically
connected using
a bus network
(e.g., using Ethernet cables)
- Logically organized into a ring struture
Example:
- Physically
connected using
a bus network
(e.g., using Ethernet cables)
- The nodes in
IEEE 802.4 are:
- Schematically:
- Information maintain
by each node:
- nodeID =
the ID of
the node
- pred =
the ID of
its logical predecessor
- Predecessor =
the node with
the next smaller ID that
is a member of
the token bus
(Except for the node with the smallest ID
The predecesser of the node with the smallest ID is the node with the largest ID that is a member of the token bus)
- Predecessor =
the node with
the next smaller ID that
is a member of
the token bus
- succ =
the ID of
its logical successor
- Successor =
the node with
the next larger ID that
is a member of
the token bus
(Except for the node with the largest ID
The predecesser of the node with the largest ID is the node with the smallest ID that is a member of the token bus)
- Successor =
the node with
the next larger ID that
is a member of
the token bus
Note:
- Some nodes may
not participate
in the token bus !!!
- A node must be
inserted
into the Token bus
to become a member node of
the Token bus
- Node insertion uses a insertion protocol
- nodeID =
the ID of
the node
- The order of the nodes
in the logical ring
is always:
- by descending node ID
Example:
The ring consist of the following nodes (in this specific order):
- Node #4
- Node #1
- Node #0
Node #0 is the last node in the token bus
- Important Property:
- The successor ID of
a node is
always smaller
that the node's nodeID
(Because the ordering is descending in nodeID)
Exception:
- The last node in the token ring !!!!
- The successor ID of
a node is
always smaller
that the node's nodeID
- "Normal" nodes
(= non-last nodes) will
have:
- node ID > successor ID
- Only
the last node will have:
- node ID < successor ID
- Example:
- Frame format:
(Unit = # bytes)
Meaning of the fields:
- Preamble =
used to synchronise
the receiver's clock.
- Start Delimeter
and End Delimeter =
used to mark the
start and
end of the
frame
- Frame control = indicates the type of the frame (see below)
- Preamble =
used to synchronise
the receiver's clock.
- Sample
token Bus Control Frames:
Frame control field Name Meaning 00000000 Claim_token Claim the token during ring initialization 00000001 Solicit_successor_1 Invite stations to enter the ring 00000010 Solicit_successor_2 Invite stations to enter the ring (used by the last node) 00000011 Who_follows Recover a lost token (due to node failure) 00000100 Resolve_contention Used when multiple stations want to enter the ring 00001000 Token Pass the token
Also used as a Set_predecessor control frame00001100 Set_successor Allow stations to leave the ring (used in various protocols) 01MMMPPP Data_frame Data frame (PPP are priority level bits) Note:
- The use of the control frames will be explaned later...