Page 77 - Intelligent Communication Systems

60 INTELLIGENT COMMUNICATION SYSTEMS
division multiplication method is used to make communication efficient and effec-
tive. This type of topology is used for the metropolitan two-way cable television
system.
To construct a bus network with one-way transmission lines such as optical
cables, the star bus topology is used. In the center of the network, a star coupler is
installed. Each node in the network has a transmitter and a receiver. The star cou-
pler has input terminals and output terminals. The transmitter of each node is linked
to an input terminal, and the receiver is linked to an output terminal. The signal
issued by a node is sent to all of the nodes via the star coupler. This behaves just
as in the bus network.
There is another ring network: Two wires are linked to the center node, which
is called a concentrator. This network topology composes a ring network via the
concentrator. When a node is in trouble, it is bypassed; communication can be per-
formed except for the troubled node.
Recently, wireless local area networks have evolved. Using frequencies up to
10 Mbps, communication is conducted between a mobile terminal and a base sta-
tion positioned at the ceiling. It is used for in-house communication. The network
topology is a star network that provides 1 to « communication channels.

7.6.3 Media Access Control Method
The transmission media in a bus or ring network is used as a multicasting trans-
mission line accessed by every node. A source node deblocks a message into sev-
eral blocks, adds the source address and its destination address to each block of
the message to compose an information block, and sends the information block to
the transmission line with time-division mode. The information block is called a
frame. Each node in the network receives a frame and accepts the frame that is des-
ignated to the node while deleting the other frames. Through this action, each node
exchanges information.
For each node to share the same transmission media in a consistent way, an
access control method is needed. This is called the media access control. The fun-
damental control principle is equivalent to the media access control method of packet
switching networks. An access control method is one of the main features of the
local area network. The typical methods are described next.

7.6.4 CSMA/CD
CSMA/CD is a typical protocol for the Ethernet and is a simple and effective com-
munication method in a local area network. In this method, any source node issues
one frame at a time. When only one node issues a frame at a particular time, the frame
is sent to its destination successfully. However, when two or more source nodes
issue their frames at the same time, a collision occurs and the frames are destroyed.
The nodes that issued their frames at the same time back off and reissue their frames
at random times until transmission succeeds.