TAILIEUCHUNG - Optical Networks: A Practical Perspective - Part 59

Optical Networks: A Practical Perspective - Part 59. This book describes a revolution within a revolution, the opening up of the capacity of the now-familiar optical fiber to carry more messages, handle a wider variety of transmission types, and provide improved reliabilities and ease of use. In many places where fiber has been installed simply as a better form of copper, even the gigabit capacities that result have not proved adequate to keep up with the demand. The inborn human voracity for more and more bandwidth, plus the growing realization that there are other flexibilities to be had by imaginative use of the fiber, have led people. | 550 Network Survivability Note that this protection scheme easily handles failures of links transmitters receivers or nodes. It is simple to implement and requires no signaling protocol or communication between the nodes. The capacity required for protection purposes is equal to the working capacity. This will turn out to be the case for the other ring architectures as well. The main drawback with the UPSR is that it does not spatially reuse the fiber capacity. This is because each bidirectional connection uses up capacity on every link in the ring and has dedicated protection bandwidth associated with it. Thus there is no sharing of the protection bandwidth between connections. For example suppose each connection requires 51 Mb s STS-1 of bandwidth and the ring operates at 622 Mb s OC-12 . Then the ring could support a total of twelve 51 Mb s connections. The BLSR architectures that we will study next do incorporate spatial reuse and can support aggregate traffic capacities higher than the transmission rate. UPSRs are popular topologies in lower-speed local exchange and access networks particularly where the traffic is primarily hubbed from the access nodes into a hub node in the carrier s central office. In this case we will see that the traffic carrying capacity that a UPSR can support is the same as what the more complicated ring architectures incorporating spatial reuse can support. This makes the UPSR an attractive option for such applications due to its simplicity and thus lower cost. Typical ring speeds today are OC-3 STM-1 and OC-12 STM-4 . There is no specified limit on the number of nodes in a UPSR or on the ring length. In practice the ring length will be limited by the fact that the clockwise and counterclockwise path taken by a signal will have different delays associated with them which in turn will affect the restoration time in the event of a failure. A UPSR is essentially 1 1 protection implemented at the path layer in a ring. Bidirectional

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