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Line-Switched Rings

These involve either two or four fibers. In the single ring implementation, traffic moves in one direction, with the other fiber acting as backup. In the event of a network failure, the backup ring is activated to allow transmission in the reverse direction. A 4-fiber implementation supports transmission in one direction over 1 fiber in each of the 2 rings. By way of example, Fibers #1 in Rings #1 and #2 might transmit in a clockwise direction, while Fibers #2 in Rings #1 and #2 would transmit counterclockwise. The second fiber in each ring acts to support transmission in the reverse direction in the event of a failure in the primary ring. Line-switched rings smaller that 1200 kilometers offer standard restoral intervals of 50 milliseconds or less [10-19].

Paths, Tributaries and Channels

The SONET pipe can carry information at Gbps speeds, with excellent performance characteristics, including error performance and network management. The pipe can also carry any variety of asynchronous and synchronous information (voice, data, video, and image) and presented in a number of frame sizes—concatenation is a clear advantage. The pipe consists of Virtual Paths, Tributaries, and Channels, as reflected in Figure 10.4.


Figure 10.4  Relationship of Path, Virtual Path and Virtual Channel.

Virtual Paths, or Virtual Containers
are simply end-to-end communications paths, or routes or circuits, that carry traffic from one end point to another. The path is not fixed or dedicated, neither is it dedicated to a particular conversation or user. A virtual path consists of many virtual tributaries.
Virtual Tributaries
carry one form of signal, such as a DS-1, DS-2 or DS-3 signal within a byte-interleaved frame. Virtual tributaries can be mapped into a single virtual path. A virtual tributary may be channelized (e.g., a 48-channel T-1 for voice) or unchannelized (e.g, a clear channel DS-1 for full motion video). Virtual tributaries are sized to accommodate the originating signal and in consideration of the legacy digital hierarchy. For instance, VT1.5 operates at 1.544 Mbps (T1), VT2 at 2.048 Mbps (E1), VT3 at 3.152 Mps (T1c), VT6 at 6.312 Mbps (T2). Individual VTs are distinguished by the use of a pointer, which identifies the position of the VT within the STS frame; the pointer also provides synchronization in a SONET environment.
Virtual Channels, or Tributary Units
exist within virtual tributaries. For example, a virtual tributary might carry a T1 frame. Within that tributary, there might exist 24 channels with each channel carrying a single voice or data communication in multiple time slots.

Sonet Frame Format

The STS-1 frame (Figure 10.5) is the basic building block for SONET, much as is the DS1 frame in a T/E-carrier environment. The STS-1 frame can be considered logically as a matrix of 9 rows of 90 octets, yielding 810 octets in total. The data are transmitted from top to bottom, one row at a time and from left to right. SONET accommodates payloads (data content) in increments of 765 octets, logically organized in matrixes of 9 rows × 85 columns. The payload is contained within a Synchronous Payload Envelope in increments of 774 bytes (9 rows × 86 columns), with the additional column attributable to Path Overhead (POH). Where superrate services require more than a single STS-1, they are mapped into a higher level, concatenated STS-N, with the constituent STS-1s being kept together [10-20]. For example, a 135 Mbps B-ISDN H4 frame requires huge amount of contiguous, unbroken bandwidth. SONET accommodates this requirement by linking 3 STS-1s into an STS-3c, with the c indicating concatenation [10-21].


Figure 10.5  SONET frame structure.

The SPE, which contains the payload data, actually floats within the SONET frame. While SONET is a synchronized network, with all devices relying on a common clocking signal, variations in clocking can occur. These clocking variations can be the result of different master clocks in different national networks, thermal expansion and contraction in individual fiber optic cables, and other phenomena. Rather than buffering individual frames to effect synchronization, floating mode operation allows the network to adjust to frame float with the SPE being identified by the pointer. The floating mode reduces cost and delay which would be caused by the use of buffers to exactly synchronize each frame and SPE [10-21].

The SONET overhead structure mirrors that of the existing digital carrier network for purposes of non-intrusive, end-to-end network management. Overhead layers include Path Overhead, which is further divided into Section Overhead and Line Overhead, and Path Overhead.

Section Overhead (SOH)

SOH of nine (9) bytes provides for management of optical network segments between Section Terminating Equipment (STE) in the form of repeaters. The repeaters can be standalone or can be built into switches, such as Digital Cross Connect Systems. At the Section Layer, every repeater in the network performs the section overhead functions which include framing, span performance, and error monitoring, and STS ID numbering. These functions are similar to those performed by traditional point-to-point protocols such as SDLC and LAP-D. The 9 bytes of Section Overhead include 1 byte STS-1 signal ID, 2 bytes Bit Interleaved Parity (BIP) for error monitoring, 1 byte Orderwire (connection request), and 3 bytes DCC (Data Communication Channel).

Line Overhead (LOH)

LOH of eighteen (18) bytes controls the reliable transport of payload data between network elements. A DXC (Digital Cross-Connect System) would perform Line Layer functions, including error control, switching and multiplexing, order-wire, express orderwire (priority connection request), automatic protection switching to back-up circuits, insertion of payload pointers, and synchronization control. The 18 bytes of Line Overhead comprise 3 bytes STS-1 pointer, 1 byte BIP (error monitoring), 2 bytes automatic protection switching, 9 bytes DCC, 1 byte orderwire, and 2 bytes reserved for future use.

Path Overhead (POH)

POH, not to be confused with Pooh (the bear), of nine (9) bytes comprises all aspects of end-to-end performance monitoring and statistical reporting. Path management is an essential responsibility of the Add/Drop Multiplexers (ADMs). Functions performed at the Path Layer include end-to-end performance monitoring, statistical reporting, STS Mapping, and DS-to-OC mapping. The 9 bytes of Path Overhead comprise 1 byte trace, 1 byte BIP, 1 byte Payload ID, 1 byte maintenance status, 1 byte user ID, 1 byte frame alignment, and 3 bytes reserved for future use.


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