Page 78 - Intelligent Communication Systems

CHAPTER 7 / NEWLY DEVELOPED TELECOMMUNICATION SERVICES 6 1
CSMA/CD is carried out in the following sequence.
(1) Sending a frame: Using a carrier detection technique, each node monitors
the transmission medium. If a carrier is detected and another node is sending a
frame, the first node must wait until the transmission medium is idle. When the
transmission medium becomes idle, any node can issue its frame into the medium
after waiting for the time slot in which a minimum frame is transmitted. If no col-
lision occurs during the transmission, the node finishes the sending operation and
waits for the next request to send a frame.
(2) Collision detection and recovery: When a node issues a frame, it tries to
detect a collision. When a collision is detected, it immediately stops sending, issues
a jam signal to the transmission medium, and informs it of the occurrence of the
collision. After stopping its operation, it backs off and waits a while before trying
to reissue the frame. The waiting time is determined by the Mh power of 2, where
N is the number of the collision. This is called the binary exponential back-off
method.
(3) Recovering a frame: The receiving operation starts with the detection of
a start frame delimiter by getting the preamble of the frame. The operation ends
after checking the frame address, frame length, and frame check sequence (PCS),

CSMA/CD is a well-known method that gives efficient and effective trans-
mission when traffic is scarce. That is, when the number of source nodes is small,
high-speed transmission is possible because full use is made of the transmission
network.
When traffic is heavy, that is, when it occupies more than 70-80% of trans-
mission capacity, performance decreases substantially because of collision of the
frames. This makes it is difficult to predict the completion time for the frame's
transmission. This defect of CSMA/CD makes it inappropriate for applications such
as production management and real-time control of a factory. However, it is good
for the intraoffice environment where requirements are not severe.

7.6.5 Token Ring Protocol

In the token ring method, a token is circulated through the network. A source node
gets the right to transmit a frame once it acquires the token. A node that has a frame to
transmit waits to acquire the token, at which time it begins to transmit the frame. It
releases the token back into the network after finishing the transmission. As the
right to transmit a frame is moved from one node to another in the token ring, no
conflicts among nodes can occur. Therefore, performance is very high when traffic
is heavy in the network. The token ring method is divided into two submethods
depending on the network topology.
(1) Token ring method: This is applied to the ring network.
(2) Token bus method: This is applied to the bus network. Since a bus
network is not a ring topology, a logical ring is constructed and a token is
circulated in the logical ring network.