Biblio5

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Figure 17.6 STS-3c concatenated SPE. (From Ref. 2, courtesy of Hewlett-Packard.)

VT configurations are shown in Figure 17.9. In the 87-column by 9-row structure of the STS-1 SPE, the VTs occupy 3, 4, 6, and 12 columns, respectively.

There are two VT operating modes: floating mode and locked mode. The floating mode was designed to minimize network delay and provide efficient cross-connects of transport signals at the VT level within the synchronous network. This is achieved

Figure 17.7 Transport overhead assignment showing OC-3 carrying an STS-3c SPE. (From Ref. 1, Figure 3–8, copyright 1994 Bellcore. Reprinted with permission.)

17.2 SONET 517

Figure 17.8 The virtual tributary (VT) concept. (From Ref. 2, courtesy of Hewlett-Packard.)

by allowing each VT SPE to float with respect to the STS-1 SPE in order to avoid the use of unwanted slip buffers at each VT cross-connect point.5 Each VT SPE has its own payload pointer, which accommodates timing synchronization issues associated with the individual VTs. As a result, by allowing a selected VT1.5, for example, to be cross-connected between different transport systems without unwanted network delay, this mode allows a DS1 to be transported effectively across a SONET network.

The locked mode minimizes interface complexity and supports bulk transport of DS1 signals for digital switching applications. This is achieved by locking individual VT

Figure 17.9 The four sizes of virtual tributary frames. (From Ref. 2, courtesy of Hewlett-Packard.)

5Slips are discussed in Chapter 6.

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SPEs in fixed positions with respect to the STS-1 SPE. In this case, each VT1.5 SPE is not provided with its own payload pointer. With the locked mode it is not possible to route a selected VT1.5 through the SONET network without unwanted network delay caused by having to provide slip buffers to accommodate the timing/ synchronization issues.

17.2.2.5 Payload Pointer. The STS payload pointer provides a method for allowing flexible and dynamic alignment of the STS SPE within the STS envelope capacity, independent of the actual contents of the SPE. SONET, by definition, is intended to be synchronous. It derives its timing from the master network clock. (See Section 6.12.2.)

Modern digital networks must make provision for more than one master clock. Examples in the United States are the several interexchange carriers which interface with local exchange carriers, each with their own master clock. Each master clock (stratum 1) operates independently. And each of these master clocks has excellent stability (i.e., better than 1 × 10 11 per month), yet there may be some small variance in time among the clocks. Assuredly they will not be phase-aligned. Likewise, SONET must take into account the loss of master clock or a segment of its timing delivery system. In this case, switches fall back on lower-stability internal clocks. This situation must also be handled by SONET. Therefore synchronous transport must be able to operate effectively under these conditions, where network nodes are operating at slightly different rates.

To accommodate these clock offsets, the SPE can be moved (justified) in the positive or negative direction one octet at a time with respect to the transport frame. This is achieved by recalculating or updating the payload pointer at each SONET network node. In addition to clock offsets, updating the payload pointer also accommodates any other timing phase adjustments required between the input SONET signals and the timing reference at the SONET node. This is what is meant by dynamic alignment, where the STS SPE is allowed to float within the STS envelope capacity.

The payload pointer is contained in the H1 and H2 octets in the line overhead (LOH) and designates the location of the octet where the STS SPE begins. These two octets are illustrated in Figure 17.10. Bits 1 through 4 of the pointer word carry the new data flag, and bits 7 through 16 carry the pointer value. Bits 5 and 6 are undefined.

Let us discuss bits 7 through 16, the actual pointer value. It is a binary number with a range of 0 to 782. It indicates the offset of the pointer word and the first octet of the STS SPE (i.e., the J1 octet). The transport overhead octets are not counted in the offset. For example, a pointer value of 0 indicates that the STS SPE starts in the octet location that immediately follows the H3 octet, whereas an offset of 87 indicates that it starts immediately after the K2 octet location.

Payload pointer processing introduces a signal impairment known as payload adjustment jitter. This impairment appears on a received tributary signal after recovery from a SPE that has been subjected to payload pointer changes. The operation of the network equipment processing the tributary signal immediately downstream is influenced by this excessive jitter. By careful design of the timing distribution for the synchronous network, payload pointer adjustments can be minimized, thus reducing the level of tributary jitter that can be accumulated through synchronous transport.

17.2.2.6 Three Overhead Levels of SONET. The three embedded overhead levels of SONET are:

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Figure 17.11 SONET section, line, and path definitions.

1. Path (POH);

2. Line (LOH); and

3. Section (SOH).

These overhead levels, represented as spans, are illustrated in Figure 17.11. One important function is to support network operation and maintenance (OAM).

The POH consists of 9 octets and occupies the first column of the SPE, as pointed out previously. It is created and included in the SPE as part of the SPE assembly process. The POH provides the facilities to support and maintain the transport of the SPE between path terminations, where the SPE is assembled and disassembled. Among the POH specific functions are:

•An 8-bit-wide (octet B3) BIP (bit-interleaved parity) check calculated over all bits of the previous SPE. The computed value is placed in the POH of the following frame;

•Alarm and performance information (octet G1);

•A path signal label (octet C2); gives details of SPE structure. It is 8 bits wide, which can identify up to 256 structures (28);

•One octet (J1) repeated through 64 frames can develop an alphanumeric message associated with the path. This allows verification of continuity of connection to the source of the path signal at any receiving terminal along the path by monitoring the message string; and

•An orderwire for network operator communications between path equipment (octet F2).

Facilities to support and maintain the transport of the SPE between adjacent nodes are provided by the line and section overhead. These two overhead groups share the first three columns of the STS-1 frame (see Figure 17.1). The SOH occupies the top three rows (total of 9 octets) and the LOH occupies the bottom 6 rows (18 octets).

The line overhead functions include:

•Payload pointer (octets H1, H2, and H3) (each STS-1 in an STS-N frame has its own payload pointer);

•Automatic protection switching control (octets K1 and K2);

•BIP parity check (octet B2);

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Figure 17.13 The SPE disassembly process. (From Ref. 2, courtesy of Hewlett-Packard.)

17.2.3 Line Rates for Standard SONET Interface Signals

Table 17.1 shows the standard line transmission rates for OC-N and STS-N.

17.2.4 Add–Drop Multiplex (ADM)

The SONET ADM multiplexes one or more DS-n signals into the SONET OC-N channel. An ADM can be configured for either the add–drop or terminal mode. In the ADM mode, it can operate when the low-speed DS1 signals terminating at the SONET derive timing from the same or equivalent source as (SONET) (i.e., synchronous) but do not derive timing from asynchronous sources.

Figure 17.14 is an example of an ADM configured in the add–drop mode with DS1 and OC-N interfaces. A SONET ADM interfaces with two full-duplex OC-N signals and one or more full-duplex DS1 signals. It may optionally provide low speed DS1C, DS2, DS3, or OC-M (M ≤ N) interfaces. There are nonpath-terminating information payloads from each incoming OC-N signal, which are passed the SONET ADM and transmitted by the OC-N interface at the other side.

Timing for transmitted OC-N is derived from either an external synchronization source, an incoming OC-N signal, from each incoming OC-N signals in each direction (called through-timing), or from its local clock, depending on the network application. Each DS1 interface reads data from an incoming OC-N and inserts data into an outgoing OC-N bit stream as required. Figure 17.14 also shows a synchronization interface for local switch application with external timing and an operations interface module (OIM) that provides local technician orderwire, local alarm, and an interface to remote

Table 17.1 Line Rates for Standard SONET Interface Signals