10.3. Routing with Physical Diversity

Route diversity is an important constraint often imposed on multiple connections between the same end points in an optical network. In particular, the requirement on link diversity is common, that is, two connections may not be routed over a common link. Occasionally, a more stringent node diversity requirement might arise, that is, two connections may not be routed via any common node.

Physical diversity between connections is driven by the need to prevent connections from being affected by the same failure. Two connections are said to be diverse if no single failure would affect both of them. In traditional transport networks, the dominant failure mode is a failure in the interoffice plant, such as a fiber cut. Data network operators have relied on their private line providers to ensure diversity so that IP routing protocols do not have to deal directly with the problem.

The concept of Shared Risk Link Group (SRLG) [Hjálmtysson+00] is used to capture diversity. An SRLG identifier is a number assigned to a set of links that are realized over the same physical resources and hence subject to the same failure. A given link may be assigned more than one SRLG identifier depending on the underlying physical resources. The SRLGs associated with a link can thus be viewed as an abstraction of different physical resources whose failure may affect the service on that link. The following are different forms of physical diversity captured by SRLGs:

• Cable (conduit) diversity. A cable (conduit) is a physical enclosure for a collection of fibers. Two fibers are said to be cable (conduit) diverse if they are carried over different cables (conduits). Two connections are cable (conduit) diverse if they are routed over fibers that are cable (conduit) diverse. This type of diversity helps to protect against the vulnerability of connections to “ordinary” cable cuts (technically known as backhoe fades). A set of cable (conduit) diverse paths may be computed by associating a unique SRLG identifier to each cable (conduit), assigning that SRLG value to each link realized over that cable (conduit) and choosing links with different SRLG values for each path.

• Right of way (ROW) diversity. ROW denotes the physical path (terrestrial or underwater) of one or more conduits. ROW diversity helps to protect against larger scale disasters such as ship anchor drags, train derailments, and so on. A set of ROW diverse paths may be computed by associating a unique SRLG identifier to each ROW, assigning that SRLG value to each link traversing that ROW, and choosing links with different SRLG values for each path.

• Geographic route diversity. This type of diversity helps to protect against various larger scale disasters such as earthquakes, floods, tornadoes, hurricanes, and so on. For this purpose, geography can be approximately described by a piecewise set of latitude/longitude or Universal Transverse Mercator (UTM) coordinate pairs. A set of geographic route diverse paths may be computed by associating a unique SRLG identifier to each geographic area, assigning that SRLG value to each link traversing that area, and choosing links with different SRLG values for each path.

The SRLG concept can be adopted to cover node diversity. In this case, a “node” can be one network element, a set of network elements, or even an entire network. From the point of view of node diverse routing, specific entities of interest include nodes, physical points of presence (e.g., a switching center), geographic areas, service provider networks, and so on. A set of node diverse paths may be computed by assigning a unique SRLG identifier to each node and choosing nodes with different SRLG values for each path.

Dealing with diversity is an unavoidable requirement for routing in optical networks. It requires the ability to deal with constraints in the routing process but, more importantly, additional state information. Specifically, the SRLG relationships between links and the precise paths of existing circuits must be available so that a diverse path for a new circuit can be computed.

At present, SRLG information cannot be self-discovered. Instead, the information must be manually configured for each link. Furthermore, in a large network it is very difficult to maintain accurate SRLG information. The problem becomes particularly daunting when multiple administrative domains are involved, for instance, after the acquisition of one network operator by another.