PDF of this tutorial- mapping—used when tributaries are adapted into VTs by adding justification bits and POH information
- aligning—takes place when a pointer is included in the STS path or VT POH, to allow the first byte of the VT to be located
- multiplexing—used when multiple lower order path-layer signals are adapted into a higher-order path signal, or when the higher-order path signals are adapted into the line overhead
- stuffing—SONET has the ability to handle various input tributary rates from asynchronous signals; as the tributary signals are multiplexed and aligned, some spare capacity has been designed into the SONET frame to provide enough space for all these various tributary rates; therefore, at certain points in the multiplexing hierarchy, this space capacity is filled with fixed stuffing bits that carry no information but are required to fill up the particular frame
One of the benefits of SONET is that it can carry large payloads (above 50 Mbps). However, the existing digital hierarchy signals can be accommodated as well, thus protecting investments in current equipment.
To achieve this capability, the STS SPE can be sub-divided into smaller components or structures, known as VTs, for the purpose of transporting and switching payloads smaller than the STS–1 rate. All services below DS–3 rate are transported in the VT structure.
Figure 19 illustrates the basic multiplexing structure of SONET. Any type of service, ranging from voice to high-speed data and video, can be accepted by various types of service adapters. A service adapter maps the signal into the payload envelope of the STS–1 or VT. New services and signals can be transported by adding new service adapters at the edge of the SONET network.
Figure 19. SONET Multiplexing Hierarchy
Except for concatenated signals, all inputs are eventually converted to a base format of a synchronous STS–1 signal (51.84 Mbps or higher). Lower-speed inputs such as DS–1s are first bit- or byte-multiplexed into VTs. Several synchronous STS–1s are then multiplexed together in either a single- or two-stage process to form an electrical STS–N signal (N >= 1).
STS multiplexing is performed at the byte interleave synchronous multiplexer. Basically, the bytes are interleaved together in a format such that the low-speed signals are visible. No additional signal processing occurs except a direct conversion from electrical to optical to form an OC–N signal.
