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Unformatted text preview: 718 Deployment Considerations with time division multiplexing. These boxes perform statistical aggregation of the incoming data signals before mapping them into SONET time slots on their line sides. Finally, there are MSPs that do not have any time division capabilities at all, carrying all incoming traffic over a packet-switched network such as IP. Like SONET rings, most MSPs are deployed in ring configurations and include built-in restoration capabilities, which are based on SONET mechanisms for the most part. Ring configurations work well for metro networks, as the fiber is mostly laid in rings. Laying fiber in ring configurations is economical, compared to using other configurations, such as a star (also called a hub and spoke) configuration. A star configuration requires two disjoint fiber routes to be laid between each access node and the central office. In contrast, multiple access nodes can be combined on a single fiber ring, and additional nodes can be added to the ring as needed, without having to lay new fiber routes each time a new node needs to be added. Some MSPs also include built-in WDM interfaces with optical add/drop (OADM) capabilities. Passive optical networks (PONs) are also emerging as potential candidates to deliver services to small and medium users of bandwidth. We studied PONs in Chapter 11. WDM is being used in metro networks, though at a slower rate because its economics are not as compelling as in long-haul networks. More on this is presented in Section 13.2.8. 13.2 Designing the Transmission Layer We will next look at the choices that service providers have to make in choosing the right transmission layer. The historical trend has been to increase capacity in the network and at the same time drive down the cost per bit of bandwidth. Service providers generally look for at least a fourfold increase in capacity when planning their networks. As a rule of thumb, they expect to get this fourfold increase in capacity at about 22.5 times the cost of current equipment. There are fundamentally three ways of increasing transmission capacity. 1. The first approach is to light up additional fibers or to deploy additional fibers as needed. We can think of this as the space division multiplexing (SDM) approach: keep the bit rate the same but use more fibers. 2. The other traditional approach is to increase the transmission bit rate on the fiber. This is the TDM approach. 3. The third approach is to add additional wavelengths over the same fiber. This is the WDM approach. 13.2 Designing the Transmission Layer 719 Note that the three techniques are complementary to each other and are all needed in the network for a variety of reasons. For instance, using SDM, particularly when existing fibers are close to being exhausted, can be viewed as a long-term way of building up infrastructure; WDM and TDM can be viewed as providing the ability to turn up services rapidly over existing fiber infrastructure. Electronic TDM is required for grooming traffic at lower speeds in the network, where optics is not cost-effective.for grooming traffic at lower speeds in the network, where optics is not cost-effective....
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- Spring '09