In brief, SONET defines optical carrier (OC) levels and electrically equivalent synchronous transport signals (STSs) for the fiber-optic based transmission hierarchy. The standard SONET line rates and STS-equivalent formats are shown
below.
The next figure 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 virtual tributary (VT). New services and signals can be transported by adding new service adapters at the edge of the SONET network. 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 virtual tributaries. Several synchronous STS-1s are then multiplexed together in either a single- or two-stage process to form an electrical STS-n signal (n = one or more).
- 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
STS-1 is a specific sequence of 810 bytes (6,480 bits), which includes various overhead bytes and an envelope capacity for transporting payloads. It can be depicted as a 90-column by 9-row structure. With a total frame length of 125 µs (8,000 frames per second), STS-1 has a bit rate of 51.840 Mbps -- viz.(9) x (90 bytes/frame) x (8 bits/byte) x (8,000 frames/s) = 51,840,000 bps = 51.840 Mbps
The order of transmission of bytes is row-by-row from top to bottom and from left to right (most significant bit first). As shown in the figure below, the first three columns of the STS-1 frame are for the transport overhead. The three columns contain 9 bytes. Of these, 9 bytes are overhead for the section layer (for example, each section overhead), and 18 bytes are overhead for the line layer (for example, line overhead). The remaining 87 columns constitute the STS-1 envelope capacity (payload and POH).
In addition to the STS-1 base format, SONET also defines synchronous formats at sub-STS-1 levels. The STS-1 payload may be subdivided into virtual tributaries, which are synchronous signals used to transport lower-speed transmissions. There are three sizes of VTs:
SONET uses a concept called “pointers” to compensate for frequency and phase variations. Pointers allow the transparent transport of synchronous payload envelopes (either STS or VT) across plesiochronous boundaries, i.e., between nodes with separate network clocks having almost the same timing. The use of pointers avoids the delays and loss of data associated with the use of large (125-microsecond frame) slip buffers for synchronization.
For future services, the STS-1 may not have enough capacity to carry some services. SONET offers the flexibility of concatenating STS-1s to provide the necessary bandwidth. The next illustrates SONET flexibility by concatenating three STS-1s to 155.52 Mbps to provide the capacity to transport an H4 digital-video channel. STS-1s can be concatenated up to STS-3c. Beyond STS-3, concatenation is done in multiples of STS-3c. For ATM services, concatenation is done in multiples of STS-12c.
A Simple Asynchronous Signal -- terminal serial communication.
A Basic Synchronous (or Plesiochronous) Signal -- High-Level Data Link Control (HDLC) frame structure.
- Asynchronous -- simple, cheap and inefficient (slow)
- Synchronous -- complex, expensive and efficient (fast)
