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The Unidirectional Dual Bus architecture is a popular
configuration for LAN's and MAN's, and several access schemes
have been suggested for this topology, e.g., Fasnet,
Expressnet, Simple, DQDB and CRMA. The IEEE 802.6 - Distributed
Queue Dual Bus (DQDB) - for Metropolitan Area Networks (MAN)
has been proposed. It is based on a unidirectional dual bus
architecture. Fasnet is proposed in 1980's and it channel
utilization is decides by the distribution of stations. In some
case, the line utilization is low. D-Net and Expressnet are
proposed to improve the utilization. Fasnet uses the cycle
concept to transmit segments. However, there are wasted slots
between two consecutive cycles. These wasted slots reduce the
utilization of Fasnet. We introduce a modified protocol to
reduce the number of such wasted slots. CRMA control also
access to the channel by means of cycles, and a full
utilization can be achieved. The performance of the CRMA can be
improved by applying the concept of slot reuse. For slot reuse,
Huang and Chiou proposed a reservation scheme based on the
region concept to improve the utilization and reduce access
delay. Released slots can be resued in a cycle. However, slots
can not be reused in the same region. We propose a refined
scheme which allows slots to be reused in the same region, thus
cycles generated are shorter and the channel utilization is
increased. It has been shown that the optimization of slot
reuse of slot reuse of the CRMA is NP-complete. Simulation
results shown that the refined FR reservation scheme we
proposed dose result in a much shorter average cycle length and
obtains better utulization and a lower access delay. However,
the refined FR scheme is not optimal.
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