Special Report, April 1998 (View With Acrobat Reader - Shows Illustrations)

Clocking SONET Equipment

by Ronald G. Todd

The popularity of Synchronous Optical Network (SONET) technology is growing rapidly as more products demand the high bandwidth, connectivity, and scalability that SONET offers. This enabling technology is necessary in many cases for products to reach "next-generation" performance levels. SONET is thus not only well-established within the traditional telecommunications industry, but equipment vendors outside this community are now - or soon will be - offering SONET interfaces on their products.

SONET equipment requires precise clocking to function properly and to be compliant with SONET standards. However, clocking is an area that is often misunderstood, even by people specifying and deploying SONET equipment. This article will cover the major issues related to understanding and designing a clocking system for SONET equipment.

The need for synchronization

SONET networks were originally designed to transport voice traffic in an efficiently scalable way. The legacy time-division multiplex (TDM) hierarchy, consisting of digital signals DS-0, DS-1, DS-3, and so on, incurs an increasing loss of efficiency as the hierarchy rate increases. This is because the TDM system is asynchronous and thus requires additional overhead each time one multiplexes up to a higher rate in the hierarchy, in order to rate match each lower-rate asynchronous source into the new higher rate.

SONET overcomes this scalability limitation by using a synchronous hierarchy, resulting in an overhead percentage that does not vary as the rate increases. Each time one multiplexes to a higher rate, the lower-rate signals are synchronously byte interleaved to produce the higher rate. Since the lower-rate signals are synchronous to one another, no additional overhead is needed (with stuffing bytes and associated signaling) to support rate matching.

SONET has a built-in overhead of 4.4%, which can decrease slightly in the special case where one transports concatenated payloads (such as with an OC-3c payload rate of 149.76 Mbits/sec versus an OC-3 payload rate of 148.61 Mbits/sec). The initial mapping of a payload into the SONET synchronous payload envelope can result in additional losses. For instance, a frame of a DS-3 (44.736 Mbits/sec) is usually mapped into a full SONET 52-Mbit/sec STS-1 (Synchronous Transport Signal 1 - the fundamental SONET rate and format) frame, yielding an effective loss to overhead of 13.7%. But when this STS-1 carrying a DS-3 is multiplexed to higher rates - for instance, to an STS-192 (corresponding to OC-192) at 10 Gbits/sec - no further losses to overhead occur.

SONET standards emanate from two different organizations: the American National Standards Institute (ANSI) and Bell Communications Research (Bellcore). In general, the Bellcore requirements are derived from the ANSI standards. Clocking requirements are distributed throughout numerous standards. However, the key requirements common to all systems are contained within a manageable subset that includes:

• ANSI T1-101: Synchronization Interface Standard 
• Bellcore GR-253-core: Synchronous Optical Network (SONET) Transport Systems: Common Generic Criteria 
• Bellcore GR-1244-core: Clocks for the Synchronized Network: Common Generic Criteria 
• Bellcore GR-436-core: Digital Network Synchronization Plan 
• Bellcore GR-378-core: Generic Requirements for Timing Signal Generators
• Bellcore GR-499-core: Transport Systems Generic Requirements (TSGR): Common Requirements 

Other clocking standards usually pertain to specific equipment types, so you must search for those standards when dealing with a specific equipment type.

Stratum levels

Network clocks are divided into stratum levels based on their accuracy, stability, and other parameters, according to Bellcore GR-1244-core. Stratum levels are expressed as a number, sometimes along with a letter. The better the clock, the lower the stratum level. The primary references used in the network meet the Stratum 1 requirements. As a clock is distributed across a network and among equipment, impairments are introduced that reduce the stability and result in the clock being classified at a higher stratum level. SONET equipment must either be synchronized with a Stratum 3 or better clock or, if the equipment is not a digital crossconnect, clocked from an oscillator with a minimum accuracy of ±20 parts per million.

The defined clock stratum - along with the basic accuracy requirement for that stratum - for an oscillator when it is not locked to a higher stratum level clock (free-run accuracy) is indicated in the table. The table also shows the accuracy with which the last frequency produced must be maintained if connection to the higher stratum level clock is lost (holdover stability).

SONET network timing architecture

The SONET network in the United States is timed from a limited number of Stratum 1 primary reference sources (PRSs) distributed across the country, forming timing domains. Historically, synchronization was distributed as an analog 2.048-MHz signal. With the advent of the digital network, this distribution occurred via DS-1 (1.544-Mbit/sec) signals passed from the PRSs to network elements (NEs), which then distributed timing to other NEs in a hierarchical fashion. This architecture is evolving such that the SONET network will distribute the network timing instead of using the tdm network. In the new architecture, one point of the SONET network within a timing domain will be synchronized with the prs; the SONET network will then distribute timing to other nodes of the SONET network and other non-SONET NEs from DS-1s timed from the SONET network.

The hierarchy allows a small number of expensive, high-accuracy clocks to be used to time an expansive network, providing reference to lower-accuracy clocks, which then provide reference to yet lower-accuracy clocks. An advantage of this system is that a detected failure of a clock high in the hierarchy will cause the clocks it feeds to enter holdover mode. This will still maintain a clock for that level and lower levels that is more accurate than the free-running clock of the level just below the failure.

Equipment timing sources

There are five different ways that SONET equipment can be timed. A subset of these is applicable in a given application:

External timing - Building integrated timing supply (BITs) is the name given to the single master clock within a telephone company central office. The BITs clock is derived from a timing reference that feeds the central office, typically a DS-1. DS-1s carried on SONET cannot be used for network synchronization distribution because they fail to meet ANSI T1.101 synchronization interface specifications due to jitter introduced by SONET network pointer adjustments. When SONET is used to distribute a timing reference, timing is derived from the SONET line rate, not from within the payload. As distributed within an office, the BITs clock can assume one of two formats: a composite clock (CC) signal or a DS-1.

The CC conveys both bit and byte synchronization (64 and 8 kHz, respectively) used in DS-0 (64-kbit/sec) signals all on a single waveform. The CC signal consists of a 64-kHz, 5/8 duty cycle, return-to-zero, bipolar signal with a bipolar violation every eighth bit. Data is clocked out on the leading edge (departure from 0V) of this signal and sampled on the trailing edge (return to 0V).

DS-1 signals used for bits typically consist of a framed "all-ones" sequence, with a bipolar return-to-zero line format, and can be either in the super-frame or extended-super-frame format.

In either case, the bits clock is multiplied up using a phase-locked loop (PLL) to produce the bit- and byte-rate frequencies for SONET transmission.

Line timing - This method is used, for instance, with an add/drop multiplexer. This kind of NE has a bidirectional SONET interface on both sides. Timing is extracted from one incoming side and used as a reference for both outgoing sides.

Loop timing - This is a special case of line timing and is applicable to line terminating equipment. Here the NE has only one bidirectional connection to the SONET network. The transmit timing is derived from recovered receive timing.

Through timing - This mode is mostly used for regenerators. Timing recovered from the incoming signal in one direction is used to clock the transmitted signal continuing in the same direction. The same holds for the opposite direction.

Free running - If an oscillator not refer- enced back to a network timing source is used, both the oscillator and the mode are referred to as free running.

Of the various ways that SONET NEs can be synchronized, the preferred order for clock selection is as follows: bits (external timing), received timing (loop, line, or through), and local timing (free running).

Clock switching

If multiple clock sources, such as external timing, loop timing, and an internal free-running clock, are available to an NE, then a mechanism must be implemented to select the appropriate synchronization source. The selection can be statically provisioned or can be under hardware or software control, based on the real-time health of the various clock sources.

One must be conscious of preventing timing loops in a network (such as when NE #1 uses NE #2 as its timing reference, but NE #2 is already using NE #1 for its reference). This can be an issue if an NE can switch between external and line timing, such as upon an external reference failure. This is not a problem with network terminating equipment, since downstream equipment within the network would not look to the terminating equipment for a reference. Thus, terminating equipment can be designed to switch between external and line-timing sources. Switching can be either revertive or nonrevertive. Revertive switching implies that there is a preferred clock source, and that if it has failed (whereupon one switches away from it) and then recovers, one should switch back to the preferred source. With nonrevertive switching, once you switch to a new clock source you stay with that source until it fails, even if the clock you switched away from recovers. Nonrevertive switching is recommended by Bellcore in most circumstances.

SONET implements a messaging protocol, via synchronization status messages, to identify to the receiving NE the stratum level traceability of the clock that was used to create a given SONET signal. With this information, an NE can select the best synchronization reference from a set of available references. This can aid in automatic reconfiguration of line-timed rings and in troubleshooting synchronization problems.

Ronald G. Todd is vice president of engineering and technology at Kalman Saffran Associates, Inc. in Newton, MA. 
 
 

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