Optical Networks - _6_1 SONET_SDH_74

Spe path overhead vt group sts 1 spe sts 1 sts 3c spe

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(SPE + path overhead) VT group STS-1 SPE STS-1 STS-3c SPE STS-3c DS1 1.544 Mb/s E1 2.048 Mb/s DS1C 3.152 Mb/s DS2 6.312 Mb/s DS3 44.736 Mb/s ATM 48.384 Mb/s E4 139.264 Mb/s ATM 149.760 Mb/s STS- N + 4 + 1 + 7 + N + N /3 + 2 + 3 Figure 6.2 The mapping of lower-speed asynchronous streams into virtual tributaries in SONET. STS payload pointer VT pointer STS-1 SPE VT SPE Transport overhead Payload Figure 6.3 The use of pointers in a SONET STS-1 signal carrying virtual tributaries (VTs). The STS payload pointer in the transport overhead points to the STS-1 synchronous payload envelope (SPE) and the VT pointer inside the STS-1 SPE points to the VT SPE.
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6.1 SONET/SDH 377 6.1.2 VCAT and LCAS As we discussed earlier, SONET has the option of locking or concatenating multi- ple STS-1 payloads to carry client signals. Commonly supported concatenations are STS-3c, STS-12c, STS-48c, and STS-192c, which correspond to the line rates shown in Table 6.2. A drawback of concatenation is that the constituent payloads must be contiguous. Thus, if there are two STS-1s that are adjacent but a third STS-1 that is not, the three could not be concatenated together to form an STS-3c. This can leave stranded unused bandwidth. Another drawback is that since there are a limited number of concatenated connection rates, STS-3c, STS-12c, . . . , there can be a mismatch between the client signal rate and the available SONET/SDH connection rates. For example, the smallest SONET concatenated connection that can carry a Gigabit Ethernet link is a 2.5 Gb/s STS-48c connection, which is an overprovision- ing by 150%. Yet another drawback is that older SONET DCS equipment switch at STS-1 rates and cannot switch larger payloads. In this case, implementing con- tiguous concatenated services will require upgrading intermediate SONET switching equipment, which can be expensive. Virtual Concatenation (VCAT) addresses these problems by allowing noncon- tiguous payloads to be combined as a single connection. Such a grouping is referred to as a virtual concatenation group (VCG). VCAT is an inverse multiplexing tech- nique that combines multiple connections into a single connection at the aggregate bandwidth. For example, STS-1-12v is a SONET VCAT connection with the same data rate as an STS-12c and is composed of 12 STS-1 payloads, which are possibly noncontiguous. Here, the "v" in STS-1-12v means virtual concatenation . Another SONET VCAT connection with the same data rate is an STS-3c-4v, which is com- posed of four STS-3c connections. The VCAT notation for SONET is STS- N - M v, where N is the size of a member and M is the number of members in a VCG. The values of N are the standard concatenated payload sizes, and commonly STS-1 and STS-3c. The M values have fewer restrictions than contiguous concatenation, and as a result the right-sized bandwidth can be provisioned for a data application. Going back to our Gigabit Ethernet application, VCAT can provide a 1.05 Gb/s STS-3-7v connection, which is an overprovisioning of only 5%. SDH also has virtual concatenation. The VCAT notation for SDH is VC- N - M v, for example, VC-4-7v is composed of seven VC-4 connections, and VC-3-5v is composed of five VC-3 connections.
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