1 When small variations between
the clock rates of the DS3 signal and the SONET network build up over time, the
SONET network simply shifts the location of the SPE (and the DS3 it carries) inside the
STS-1 payload and adjusts the pointer. Figure 11.1 illustrates the STS-1 frame and its
relationship to the SPE.
The STS-1 frame structure represents the basic building block for SONET signals.
Fixed multiples of STS-1 signals may be byte-interleaved to form higher-rate signals such
as STS-3, STS-12, and STS-48, etc. (see Table 11.1). This increases the number of STS-1
payloads that a SONET interface can support. As a way to increase the payload size (not
just the number of STS-1 payloads), the payloads of N STS-1 signals (N = 3, 12, 48, 192,
and 768) may be concatenated into a single STS-Nc SPE. Most routers use some form of
payload concatenation (e.g., STS-48c) on their Packet over SONET (PoS) interfaces.
1 SONET systems carry DS1s within the STS-1 SPE by first mapping them within synchronous virtual
tributaries (VTs); an STS-1 SPE carries up to 28 VT1.5 signals (and therefore up to 28 DS1s). As with STS-1
signals, each VT has a corresponding SPE and uses a pointer to locate the SPE within the VT payload capacity.
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