Data Transmission Timing and Synchronization
Stratum Levels 1, 2, 3, 3e, 4, and 4e
Data transmission must be clocked so that the sending and receiving stations can reliably communicate. Standards dictate the speeds, but what can control and manage the speeds? For SONET systems, this is done by the most accurate of timing sources - the atomic clock, or "cesium clock".
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:
Other clocking standards usually pertain to specific equipment types, so you must search for those standards when dealing with a specific equipment type.
- 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
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).
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.
Statum Levels Defined
An American National Standards Institute (ANSI) standard entitled "Synchronization Interface Standards for Digital Networks" (ANSI/T1.101-1987) was released in 1987. It is being revised in Committee T1X1 for release as ANSI/T1.101-1998 [Reference 4]. This document defines the stratum levels and minimum performance requirements. The requirements for the stratum levels are shown in Table A, which provides a comparison and summary of the drift and slip rates for the strata clock systems.
Stratum 1 is defined as a completely autonomous source of timing which has no other input, other than perhaps a yearly calibration. The usual source of Stratum 1 timing is an atomic standard or reference oscillator. The minimum adjustable range and maximum drift is defined as a fractional frequency offset f/f of 1 x 10-11 or less. At this minimum accuracy, a properly calibrated source will provide bit-stream timing that will not slip relative to an absolute or perfect standard more than once every 4 to 5 months. Atomic standards, such as cesium clocks, have far better performance.
Using a trickle-down method, the atomic clock runs a Stratum 1 time server, which sends it's signals into Stratum 2 servers, which in turn feed Stratum 3 servers, etc - as shown below. Each successice Stratum time server is a bit less accurate the the one before it - however, since they are all clocked from a Stratum 1 source (the most accurate), their timing is precise, and the only possible exception is found in a very tiny amount of jitter that can occur on any instantaneous timing pulse. However, the jitter is then corrected, since the clock will synchronize on every pulse, with the "father clock source":
Stratum 1 clock system and GPS - the Stratum 1 clock is an example of a Primary Reference Source (PRS) as defined in ANSI/T1.101 [Reference 4]. Alternatively, a PRS source can be a clock system employing direct control from Coordinated Universal Time (UTC) frequency and time services, such as Global Positioning System (GPS) navigational systems. The GPS System may be used to provide high accuracy, low cost timing of Stratum 1 quality. GPS receivers are available for this application.
Stratum 2 clock system - tracks an input under normal operating conditions, and holds to the last best estimate of the input reference frequency during impaired operating conditions. A Stratum 2 clock system requires a minimum adjustment (tracking) range of 1.6 x 10-8. The drift of a Stratum 2 with no input reference is less than 1.6 x 10-8 in one year. The short term drift of the system is less than 1 x 10-10 in 24 hours. If one interprets this specification as a drift of 1 x 10-10 each 24 hours, this amounts to a frame slip rate of approximately 1 slip in 7 days when the Stratum 2 clock system is in the hold mode. Larus has a Stratum 2 clock with a drift of less than 2.5 x 10-11 per day, resulting in a time to the first frame slip of more than 2 months.
Stratum 3 clock system - tracks an input as in Stratum 2, but over a wider range. A Stratum 3 clock system requires a minimum adjustment (tracking) range of 4.6 x 10-6. The short term drift of the system is less than 3.7 x 10-7 in 24 hours. This amounts to approximately 255 frame slips in 24 hours while the system is holding. Some Stratum 3 clock equipment is not adequate to time SONET network elements.
Stratum 3E - defined in Bellcore documents [References 3, 7 and 8], is a new standard created as a result of SONET equipment requirements. Stratum 3E tracks input signals within 7.1 Hz of 1.544 MHz from a Stratum 3 or better source. The drift with no input reference is less than 1 x 10-8 in 24 hours. Larus provides a Stratum 3E Enhanced clock that provides a drift of less than 5 x 10-9 in 24 hours, which is better than the 3E designation. This is less than four frame slips in 24 hours, compared to 255 slips for Stratum 3. Typical performance of the Larus 3E Enhanced clock is less than one slip in 36 hours or 9 x 10-10 /day.
Stratum 4 - defined as a clock system which tracks an input as in Stratum 2 or 3, except that the adjustment and drift range is 3.2 x 10-5. Also, a Stratum 4 clock has no holdover capability and, in the absence of a reference, free runs within the adjustment range limits. The time between frame slips can be as little as 4 seconds.
Stratum 4E - a proposed new customer premises clock standard which allows a
holdover characteristic that is not free running. This new level, intended for
use by customer provided equipment in extending their networks, is not yet
Table A. Clock Strata Requirements
|Stratum||Accuracy, Adjustment Range||Pull-In-Range||Stability||Time To First Frame Slip *|
|1||1 x 10-11||N/A||N/A||72 Days|
|2||1.6 x 10-8||Must be capable of synchronizing to clock with accuracy of +/-1.6 x 10-8||1 x 10-10/day 1||7 Days 2|
|3E 3||4 x 10-7||4.6 x 10-6||5 x 10-9/day||7 Hours 4|
|3E||1.0 x 10-6||Must be capable of synchronizing to clock with accuracy of +/-4.6 x 10-6||1 x 10-8/day||3.5 Hours|
|3||4.6 x 10-6||Must be capable of synchronizing to clock with accuracy of +/-4.6 x 10-6||3.7 x 10-7/day||6 Minutes (255 in 24 Hrs)|
|4E||32 x 10-6||Must be capable of synchronizing to clock with accuracy of +/-32 x 10-6||Same as Accuracy||Not Yet Specified|
|4||32 x 10-6||Must be capable of synchronizing to clock with accuracy of +/-32 x 10-6||Same as Accuracy||N/A|
* To calculate slip rate from drift, one assumes a frequency offset equal to the above drift in 24 hours, which accumulates bit slips until 193 bits have been accumulated. Drift rates for various atomic and crystal oscillators are well known, and are not usually linear or not necessarily continually increasing.
A Network Slip, What Happens?
If a frame slip occurs due to a clock system in the holdover condition, what is the penalty? Does the connected equipment stop working? Not usually. Voice equipment tends to re-acquire frame synchronization quickly, resulting in a pop or click, which is not usually a problem. Data circuits lose some number of bits depending on the data rate being transmitted, and on whether or not forward error correction is being used.
Some multiplex equipment that provides add and drop services interrupt all output trunks while a new source of synchronization is acquired. Such interruptions, if due to circuit noise, may render a network temporarily useless, as the slip causes further slips downstream (error or slip multiplication).
A clock system provides a stable frequency source during circuit impairments. The connected equipment will not be affected until the clock holdover drift results in a slip. A stable clock will change a network that experiences problems two or three times a day to one that maintains timing through a major trunk outage. The network will continue to operate without impairment until the outage is repaired, as long as the repair time is comparable to the time of the first frame slip (see Table A)
Since occasional slips will always occur, the best one can do is to minimize their rate of occurrence. Through careful network engineering of the clock systems, near perfect timing may be achieved at a reasonable cost with excellent reliability and maintainability.
Timing Q and AQ. If voice traffic is still intelligible to the listener in a relatively poor communication channel, why isn't it easy to pass it across a network optimized for data?
Q. How does synchronization differ from timing?
A. Data communication requires very low Bit-error Ratio (BER) for high throughput but does not require constrained propagation, processing, or storage delay. Voice calls, on the other hand, are insensitive to relatively high BER, but very sensitive to delay over a threshold of a few tens of milliseconds. This insensitivity to BER is a function of the human brain's ability to interpolate the message content, while sensitivity to delay stems from the interactive nature (full-duplex) of voice calls. Data networks are optimized for bit integrity, but end-to-end delay and delay variation are not directly controlled. Delay variation can vary widely for a given connection, since the dynamic path routing schemes typical of some data networks may involve varying numbers of nodes (for example, routers). In addition, the echo-cancellers deployed to handle known excess delay on a long voice path are automatically disabled when the path is used for data. These factors tend to disqualify data networks for voice transport if traditional public switched telephone network (PSTN) quality is desired.
Q. If I adopt sync status messages in my sync distribution plan, do I have to worry about timing loops?
A. These terms are commonly used interchangeably to refer to the process of providing suitable accurate clocking frequencies to the components of the synchronous network. The terms are sometimes used differently. In cellular wireless systems, for example, "timing" is often applied to ensure close alignment (in real time) of control pulses from different transmitters; "synchronization" refers to the control of clocking frequencies.
Q. If ATM is asynchronous by definition, why is synchronization even mentioned in the same sentence?
A. Yes. Source Specific Multicasts (SSMs) are certainly a very useful tool for minimizing the occurrence of timing loops, but in some complex connectivities they are not able to absolutely preclude timing loop conditions. In a site with multiple Synchronous Optical Network (SONET) rings, for example, there are not enough capabilities for communicating all the necessary SSM information between the SONET network elements and the Timing Signal Generator (TSG) to cover the potential timing paths under all fault conditions. Thus, a comprehensive fault analysis is still required when SSMs are deployed to ensure that a timing loop does not develop.
Q. Most network elements have internal stratum 3 clocks with 4.6ppm accuracy, so why does the network master clock need to be as accurate as one part in 10^11?
A. The term Asynchronous Transfer Mode applies to layer 2 of the OSI 7-layer model (the data link layer), whereas the term synchronous network applies to layer 1 (the physical layer). Layers 2, 3, and so on, always require a physical layer which, for ATM, is typically SONET or Synchronous Digital Hierarchy (SDH); thus the "asynchronous" ATM system is often associated with a "synchronous" layer 1. In addition, if the ATM network offers circuit emulation service (CES), also referred to as constant bit-rate (CBR), then synchronous operation (that is, traceability to a primary reference source) is required to support the preferred timing transport mechanism, Synchronous Residual Time Stamp (SRTS).
Q. What are the acceptable limits for slip and/or pointer adjustment rates when designing a sync network?
A. Although the requirements for a stratum 3 clock specify a free-run accuracy (also pull-in range) of 4.6ppm, a network element (NE) operating in a synchronous environment is never in free-run mode. Under normal conditions, the NE internal clock tracks (and is described as being a traceable to) a Primary Reference Source that meets stratum 1 long-term accuracy of one part in 10^11.
This accuracy was originally chosen because it was available as a national primary reference source from a cesium-beam oscillator, and it ensured adequately low slip-rate at international gateways.
Note: If primary reference source (PRS) traceability is lost by the NE, it enters holdover mode. In this mode, the NE clock's tracking phase lock loop (PLL) does not revert to its free-run state, it freezes its control point at the last valid tracking value. The clock accuracy then drifts elegantly away from the desired traceable value, until the fault is repaired and traceability is restored.
Q. Why is it necessary to spend time and effort on synchronization in telecom networks when the basic requirement is simple, and when computer LANs have never bothered with it?
A. When designing a network's synchronization distribution sub-system, the targets for sync performance are zero slips and zero pointer adjustments during normal conditions. In a real-world network, there are enough uncontrolled variables that these targets will not be met over any reasonable time, but it is not acceptable practice to design for a given level of degradation (with the exception of multiple timing island operation, when a worst-case slip-rate of no more than one slip in 72 days between islands is considered negligible). The zero-tolerance design for normal conditions is supported by choosing distribution architectures and clocking components that limit slip-rates and pointer adjustment rates to acceptable levels of degradation during failure (usually double-failure) conditions.
Q. How many stratum 2 and/or stratum 3E TSGs can be chained either in parallel or series from a PRS?
A. The requirement for PRS traceability of all signals in a synchronous network at all times is certainly simple, but it is deceptively simple. The details of how to provide traceability in a geographically distributed matrix of different types of equipment at different signal levels, under normal and multiple-failure conditions, in a dynamically evolving network, are the concerns of every sync coordinator. Given the number of permutations and combinations of all these factors, the behavior of timing signals in a real-world environment must be described and analyzed statistically. Thus, sync distribution network design is based on minimizing the probability of losing traceability while accepting the reality that this probability can never be zero.
Q. Is synchronization required for non-traditional services such as voice-over-IP?
A. There are no defined figures in industry standards. The sync network designer must choose sync distribution architecture and the number of PRSs and then the number and quality of TSGs based on cost-performance trade-offs for the particular network and its services.
Q. Why is a timing loop so bad, and why is it so difficult to fix?
A. The answer to this topical question depends on the performance required (or promised) for the service. Usually, Voice-over-IP is accepted to have a low quality reflecting its low cost (both relative to traditional PSTN voice service). If a high slip-rate and interruptions can be accepted, then the voice terminal clocks could well be free-running. If, however, a high voice quality is the objective (especially if voice-band modems including Fax are to be accommodated) then you must control slip occurrence to a low probability by synchronization to industry standards. You must analyze any new service or delivery method for acceptable performance relative to the expectations of the end-user before you can determine the need for synchronization.
Q. What is the difference between SONET and SDH?
A. Timing loops are inherently unacceptable because they preclude having the affected NEs synchronized to the PRS. The clock frequencies are traceable to an unpredictable unknown quantity; that is, the hold-in frequency limit of one of the affected NE clocks. By design, this is bound to be well outside the expected accuracy of the clock after several days in holdover, so performance is guaranteed to become severely degraded.
The difficulty in isolating the instigator of a timing loop condition is a function of two factors: first, the cause is unintentional (a lack of diligence in analyzing all fault conditions, or an error in provisioning, for example) so no obvious evidence exists in the network's documentation. Secondly, there are no sync-specific alarms, since each affected NE accepts the situation as normal. Consequently, you must carry out trouble isolation without the usual maintenance tools, relying on a knowledge of the sync distribution topology and on an analysis of data on slip counts and pointer counts that is not usually automatically correlated.
Q. What is hair pinning, and why would I want to use it?
A. The lowest SDH speed is STM-1, which is equivalent to SONET's STS-3 at 155.52 Mbps. So there is no SDH speed = SONET STS-1 (STS1=OC1). The first level in the SDH hierarchy is STM-1 (Synchronous Transport Mode 1) has a line rate of 155.52 Mb/s. This is equivalent to SONET's STS-3c. Then comes STM-4 at 622.08 Mb/s and STM-16 at 2488.32 Mb/s. The other difference is in the overhead bytes which are defined slightly differently for SDH. A common misconception is that STM-Ns are formed by multiplexing STM-1s. STM-1s, STM-4s and STM-16s that terminate on a network node are broken down to recover the virtual circuits (VCs) they contain. The outbound STM-Ns are then reconstructed with new overheads.
Q. Doesn't a two fiber bi-directional line switched ring (BDLSR) waste half of the line rate bandwidth?
A. Hair pinning is bringing traffic in on a tributary and instead of putting it on the high speed OC-N line you direct it out another low speed tributary port. You might want to do this if you have interfaces to two interexchange carriers (IXCs) on different nodes. If one of your IXCs goes down, you can hair pin the other to pick the traffic, assuming the spare capacity exists on the tributary. Hairpin cross-connections allow local drop of signals, ring extensions supported by a ring host node, and allow passing traffic between two ring interfaces on a single host node. In this case, no high speed channel is involved and the cross-connections are entirely within the interfaces.
Q. What is the difference between TSA and TSI?
A. No. It can be shown that in all cases the aggregate bandwidth on a two fiber BDLSR is no less than the aggregate bandwidth on a path switched ring. In some cases that exemplify an inter-office transport ring, it can actually be shown that the aggregate bandwidth of a two fiber BDLSR can be larger than that of a path switched ring.
Q. What are some timing rules of thumb?
A. Time Slot Assignment (TSA) allows for flexible assignment for add-dropped signals but not for through path signals. Once a signal is multiplexed onto a time slot it stays in that time slot until it is dropped. Time Slot Interchange (TSI) is more flexible in that it allows a signal passing through a node to be placed in another time slot if desired. Equipment that provides neither TSA or TSI is said to be hard wired. This pass-through grooming, which is not supported by systems limited to TSA, allows in-transit bandwidth rearrangements for maximum facility utilization. This grooming is most useful for networks with intersite routing (for example, interoffice or private networks) and networks with significant churn (service removal as well as new service installation).
Q. What are some advantages of timing from an OC-N line?
A. Here are some basic points:
- A node can only receive the synchronization reference signal from another node that contains a clock of equivalent or superior quality (stratum level).
- The facilities with the greatest availability (absence of outages) should be selected for synchronization facilities.
- Where possible, all primary and secondary synchronization facilities should be diverse, and synchronization facilities within the same cable should be minimized.
- The total number of nodes in series from the stratum 1 source should be minimized. For example, the primary synchronization network would ideally look like a star configuration with the stratum 1 source at the center. The nodes connected to the star would branch out in decreasing stratum level from the center.
- No timing loops may be formed in any combination of primary.
Q. What is the advantage of using the DS1 timing output instead of a multiplexed DS1 as the timing reference?
A. OC-N timing distribution has several potential advantages. It preserves transport bandwidth for customer services and guarantees a high-quality timing signal. Also, as the network architecture evolves to replace Digital Signal Cross Connect (DSX) interconnects with SONET interconnects and direct OC-N interfaces, OC-N distribution becomes more efficient than multiplexing DS1 references into an access facility. A previous drawback to using OC-N timing distribution was that network timing failures could not be communicated to downstream clocks via DS1 Alarm Indication Signal (AIS), since the DS1 signal does not pass over the OC-N interface. A standard SONET synchronization messaging scheme to convey synchronization failures is in place. With this option, clock stratum levels can be passed from NE to NE, allowing downstream clocks to switch timing references without creating timing loops, if a network synchronization failure occurs. If a quality timing reference is no longer available, the NE sends AIS over the DS1 interface. If the local OC-N lines fail, the NE outputs AIS on the DS1 output or an upstream NE enters holdover. Although an ideal source of timing, OC-N timing distribution, through a DS1 timing output, cannot be used to provide timing in all applications. In cases where the local equipment is not provided with an external timing reference input, or in some private networks where the timing is to be distributed from another private network location, timing may be distributed via traffic-carrying DS1s. In these applications, a stable DS1 timing source can be achieved by ensuring that all elements in the SONET network are directly traceable to a single master clock via line timing.
Note: Synchronous operation via line timing eliminates the generation of virtual terminal (VT) pointer adjustments, thus maintaining the phase stability needed for a high-quality DS1 timing reference. Cross-connecting at the STS-1 level also eliminates the VT pointer adjustments. It is recommended that, where possible, the DS1 sources (switch, private branch exchange [PBX], or other equipment) be traceable to the same timing source used to time the SONET NE. Multiplexed DS1 reference transport is also consistent with current planning and administration methods (but you better know exactly what is happening to that multiplexed DS1).
Q. Can a DS1 carried over SONET ever be used as a timing reference?
A. The DS1 timing output is derived from the optical line rate and is superior because the DS1 is virtually jitter-free. Synchronization messages guarantee the traceability of the timing. Administration of traffic DS1s for timing is eliminated
Q. Are there any specific concerns when using a DS1 carried over SONET to time equipment such as a switch remote or DLC?
A. Yes. In many applications there is no other choice. Most switch remotes, for instance, obtain their timing from a specific DS1 signal generated by their host switch; so these remotes must line or loop time from the DS1 signal. In addition, digital loop carrier (DLC) equipment, channel banks, and PBXs are not likely to have external references and may be allowed to line or loop time from a DS1 carried over SONET. Five years ago all the literature however answered no to this question. See the next question for more information.
Q. How many SONET NEs can I chain together in an add or drop configuration before the timing becomes degraded?
A. Yes. The major concern is to make sure all the equipment is synchronous to each other to prevent pointer adjustments. For example should you have an OC-N that goes through multiple carries, a LAN Emulation Client (LEC) and interexchange carrier (IXC) for example, and one of clock is a stratum 1 while the other is being timed from some stratum 3 holdover source, you will have pointer adjustments that will translate into DS1 timing jitter.
Q. Why are there more issues related to timing with SONET equipment than there is with asynchronous equipment?
A. The stratum level traceability of the nth node in an add or drop chain is the same as that in the first node. Also, while timing jitter theoretically increases as the number of nodes is increased, the high quality timing recovery and filtering should allows add or drop chains to be extended to any practical network limit without detectable increases in jitter levels. In practice, the only effects on timing at the nth node will occur whenever high-speed protection switches occur in any of the previous n-1 nodes.
A. SONET equipment was designed to work ideally in a synchronous network. When the network is not synchronous, mechanisms such as pointer processing and bit-stuffing must be used and jitter or wander increases.