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Connection Oriented Networks - Perros H.G

Perros H.G Connection Oriented Networks - John Wiley & Sons, 2005. - 359 p.
ISBN 0-470-02163-2
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Let us consider a lightpath from user A to user B (see the solid line in Figure 9.6). This lightpath is routed through nodes 1, 2, and 3. Assume that the lightpath fails on the link between nodes 2 and 3. In this case, the protection mechanism will switch the lightpath over to the protection fiber from nodes 2 to 3 (see the dotted line labeled “span switching” in Figure 9.6). If the working fiber from nodes 2 to 3 fails as well, then all of the lightpaths will be switched onto its protection fiber from nodes 2 to 3, as in the
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WAVELENGTH ROUTING OPTICAL NETWORKS
Figure 9.6 A four-fiber optical bidirectional link sharing ring (4F-OBLSR).
case of the lightpath above. This is known as span switching. When all four fibers are cut between nodes 2 and 3, then the traffic will be diverted to the working fibers in the opposite direction. This is known as ring switching. In this case, the lightpath from A to B will be diverted; that is, it will be routed back to node 1, and then to nodes 4 and 3. (See the dotted line labeled “ring switching” in Figure 9.6.)
9.2.3 Mesh Optical Networks
A mesh network can employ both path and link protection. Link protection can be implemented using the point-to-point 1 + 1, 1:1, and 1:N schemes (see Section 9.2.1). Path protection uses dedicated or shared back-up paths. Alternatively, an arbitrary mesh topology can be organized into a set of WDM optical rings, which permits ring-based protection schemes.
The 1 + 1 path protection scheme is the simplest form of protection. It is also the most expensive and bandwidth-inefficient. The user signal is split into two copies, and each copy is transmitted simultaneously over two separate lightpaths. The lightpaths might be diversely routed (i.e. they follow different geographical paths) or they might go through the same OXCs but use different fibers. The receiver monitors the quality of the two signals and selects the best of the two. If one lightpath fails, then the receiver continues to receive data on the other lightpath.
In the case of the 1:1 path protection, the user signal is carried over a working lightpath. The back-up protection lightpath has also been established, but it is not used. If the working lightpath fails, the source and destination switches to the protection lightpath. Since the bandwidth allocated to the protection lightpath is not utilized during normal operation, it can be shared by multiple working lightpaths. This is the 1:N path protection scheme.
An important concept in these protection schemes is the concept of the shared risk link group (SRLG). An SRLG is a group of links that share the same physical resources, such as a cable, a conduit, and an OXC. Failure of these physical resources will cause failure of all of the links. Each common physical resource is associated with an identifier called the SRLG. When setting up a working and a protection lightpath, care is taken so that the two lightpaths are not routed through the same SRLG. For example, let us consider the optical network shown in Figure 9.7. The working lightpath from OXC 1 to OXC 2
THE ITU-T G.709 STANDARD - THE DIGITAL WRAPPER
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that uses links {1, 6,11} and its protection lightpath that uses links {3, 8,13} do not use the same SRLG. That is, they are SRLG-disjoint.
The concept of SRLG can also be used in the 1:N shared protection scheme. For instance, in Figure 9.7, the two working lightpaths {1, 6,11} and {2, 7,12} from OXC 1 to OXC 2 are SRLG-disjoint. Therefore, it makes sense that they both use the same SRLG-disjoint protection lightpath {3, 8, 13}. This is because a single failure of a physical resource along the path of either working lightpaths (excluding the originating and terminating OXCs) will not cause both working lightpaths to fail at the same time. That is, in this case, the protection lightpath will only be used by one of the two working lightpaths.
In protection schemes, the backup protections routes are pre-planned and the necessary resources (e.g. wavelengths, fibers, and bandwidth within an OXC) are allocated in advance. During the normal operation of the network, these resources are either kept idle, or they are used to transmit low priority traffic which can be preempted any time a failure occurs. This guarantees a fast recovery from a failure at the expense of inefficient resource utilization. An alternative strategy, known as dynamic restoration, is to calculate a protection path and allocate resources for recovery at the moment when a network failure occurs. This approach has a more efficient resource utilization but the recovery time is longer than in the case of a protection scheme. Dynamic restoration is a promising new approach that is being further studied.
9.3 THE ITU-T G.709 STANDARD - THE DIGITAL WRAPPER
Information on a lightpath is typically transmitted using SONET/SDH framing. Also, Ethernet frames can be transmitted over an optical network. In the future, it is expected that information will be transmitted over the optical network using the new ITU-T G.709 standard, otherwise known as the digital wrapper. This standard defines the network node interfaces between two optical network operators, or between subnetworks of vendors within the same network of an operator. The following are some of the features of the G.709 standard:
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