Optical Networks - _6_1 SONET_SDH_74

Note that for a sonet sts 1 m v connection

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connections, and VC-3-5v is composed of five VC-3 connections. Note that for a SONET STS-1- M v connection, intermediate SONET nodes can be unaware that a particular STS-1 is a member of the VCG. This is an important property of virtual concatenation since it permits seamless deployment over the vast SONET installed base with only the endpoints being aware of VCAT, and has been one of the primary factors behind its adoption. Of course, this also applies to SDH.
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378 Client Layers of the Optical Layer A VCG could consist of lower-order tributaries (e.g., VT-1.5/VC-11 or VC- 12) or higher order tributaries (e.g., VC-3/STS-1 or VC-4/STS-3c). Accordingly, we have low-order or LO-VCAT and high-order or HO-VCAT. Typically, 10 Mb/s and 100 Mb/s Ethernet traffic use LO-VCAT and Gigabit Ethernet traffic uses HO-VCAT. For example, SONET VT1.5-64v can carry 100 Mb/s Ethernet and STS-1-21v can carry 1 Gb/s Ethernet, which for both cases is only a 2% overprovisioning. VCAT also allows the members of a VCG to be routed on different physical paths. Thus, a VCAT connection can be made up of constituent connections from different parts of the network, which can further reduce the amount of stranded unused bandwidth. Combining VCG members that are routed on different physical paths poses a practical problem since the latencies of the paths could be widely different. The difference in latencies is known as the differential delay . In order to combine these byte streams properly, buffers are required at the end of the connection to compensate for the delay. The standards allow a differential delay between any two members of a VCG to be up to 128 ms. This corresponds to about the maximum latency we can expect when routing between any two points on the globe! Practical implementations tolerate differential delays of 32 to 64 ms. The Link Capacity Adjustment Scheme (LCAS) is a companion to VCAT that allows for hitless resizing of bandwidth in a VCAT connection when adding or removing members of a VCG. This can be useful for managing the capacity of a VCAT connection for applications such as using the connection as an IP link. Then the link capacity can be adjusted to changes in IP traffic loads. LCAS can also be used to implement hitless regrooming, which can occur when traffic must be rerouted to optimize bandwidth use or avoid a network component that will undergo maintenance. 6.1.3 SONET/SDH Layers The SONET layer consists of four sublayers—the path, line, section, and physical layers. Figure 6.4 shows the top three layers. Each layer, except for the physical layer, has a set of associated overhead bytes that are used for several purposes. These overhead bytes are added whenever the layer is introduced and removed whenever the layer is terminated in a network element. The functions of these layers will become clearer when we discuss the frame structure and overheads associated with each layer in the next section.
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  • Spring '09
  • Boussert
  • Bit rate, Synchronous optical networking, SLM SLM SLM

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