Page 81 - Intelligent Communication Systems

64 INTELLIGENT COMMUNICATION SYSTEMS
In Case 1, cycle time is about 10 microseconds. In case 2, cycle time is about 1,000
microseconds. Transmission time of a frame is 200 microseconds in both Case 1
and Case 2, so in Case 1 the transmission time of a frame is 20 times the cycle time
and the transmission performance is not affected in the single-token method. In Case
2, the transmission time of a frame is one-fifth the cycle time. Twenty percent of
the transmission capacity is used and 80% is unused.

7.6.6 Token-Bus Method
A source node issues a message both ways through a bus network. Because a bus
network is not a ring network, it is difficult to circulate a message in the network.
Therefore a logical ring network is constructed virtually on the bus network. Each
node is assigned a sequential number. A token is transmitted from one node to the
adjacent node. A token is a control frame composed of a source address and a des-
tination address. A node that has a request to send a message waits for the arrival
of the token that is designated to the node. Once the node receives the token, it is
possible to send a message during the allotted time. When it finishes sending a mes-
sage or the allotted time has passed, it issues a token that is designated to its adja-
cent node. And it waits for the arrival of a new token. In the token-bus method,
priority control of transmission is performed by the time token control method.
When traffic is light, token cycle time is short; when traffic is heavy, token cycle
time is long. When a token circulates at high speed, the network has light traffic,
so the possibility is great that a low-priority node can issue a message. When a
token circulates at low speed, the network has heavy traffic. Then a low-priority
node has little chance to issue a message.
To summarize, here are the main characteristics of the token-bus method and
the token-ring method.
(1) In the token-ring method, a token has an identifier whose size is 8 bits.
But because a prefix of 8 bits and a postfix of 8 bits are attached to the token, the
size of a token is 24 bits. In the token-bus method, a token consists of a token iden-
tifier, a source address, and a destination address.
(2) In the token-bus method, the source node that issued the message moni-
tors the completion of transmission of the message to the next node.
(3) In the token-bus method, the network configuration changes with the addi-
tion of a new node, the removal of a node, or the malfunction of a node.
(4) In the token-ring method, the network is closed, and it is necessary to
remove the information that is circulated through the network.
(5) In the token-bus method, the network is passive. Therefore it is reliable in
comparison to the ring network. The token-bus method has the characteristics not
only of a bus network but also of a logical ring network. The token-bus method has
been standardized as IEEE 802.4.