Timing the Triple Play

As service providers introduce more technically sophisticated real-time services, precise timing becomes a service quality differentiator. Accurate timing facilitates the seamless transport of information, as well as the smooth transmission of applications delivered over the network. This ensures end customers consistently get the best quality for services such as the triple play of voice, video and data.

Although there are nuances, time alignment and frequency alignment often are grouped together and generically called synchronization. Time alignment refers to time transfer or time of day, and is needed to support QoS and traffic engineering. Frequency alignment refers to the steady pace of information through the network to minimize loss of data. This data loss, or slips, results in either the retransmission of packets (excess bandwidth usage) for non-real-time applications, or service quality degradation in real-time applications.

Voice
Information to and from a human consumer (audio and video) is analog in nature. If the conversion clocks for the A/D and D/A converters have a frequency offset, impairments result. This often is referred to as the pitch modification effect, and is illustrated in Figure 1. The human auditory system is remarkably tolerant of such impairments; voice-band modems are not. The proliferation of such devices as fax machines necessitates robust synchronization between all endpoints.

Figure 1
Pitch Modification Effect 

 

The input analog signal, x(t), is converted to digital format and the resulting digital samples delivered across the network for conversion back to analog. Even if the network faithfully delivers all the samples intact, and in order, the resulting analog signal, y(t), will differ from the original analog signal x(t) if there is a frequency offset between the conversion clocks.

For example, consider the two primary types of VoIP a provider can make available. VoIP over the public Internet is a best-effort proposition, and users may experience a variety of quality issues. Alternatively, VoIP packets can pass through a gateway and over the PSTN. To maintain a quality of service normally associated with the legacy PSTN, the gateway must be synchronized to a primary reference source, as depicted in Figure 2.

Figure 2
Synchronizing the Media Gateway
 

For carrier-class voice-band services using VoIP, the A/D and D/A converters must be provided with an accurate clock (better than 50ppb) or must incorporate sophisticated relay functionality. It is possible that these converters will be in the customer premises integrated access device, and the service provider is responsible for ensuring the availability of clock reference.

Video
The converged access medium, with limited available bandwidth, can act as a triple-play bottleneck. Because the triple play includes video (e.g. IPTV), good synchronization is necessary to maintain the quality of experience to which end users have become accustomed. With broadcast video, if the timing/latency of a video stream is off by a few seconds, viewers probably will not notice. But the interactive nature of IPTV (e.g. channel changes) requires tight constraint on time latency.

IPTV is based on MPEG. MPEG technologies are employed to encode and decode audio-visual content and require the decoder synchronize to the encoder, a requirement similar to that depicted in Figure 1 for voice. This is achieved by utilizing time stamps within the MPEG stream. However, packet delay variation (PDV) in the network introduces jitter in these time stamps, making it difficult to achieve proper synchronization. The options available are to engineer the network to keep the PDV to a guaranteed minimum, or distribute the requisite timing information by other means.

Data
Transfer of non-real-time data does not require precise synchronization. Applications such as file transfer and e-mail, for example, represent the type of transactions for which IP networks were designed. However, in the context of the triple play, the data component can introduce impairments in the other (real-time) services being provided over the converged medium because, even if data packets are assigned a low priority for transmission, once they are in the pipe they force any high-priority packet to wait.

Synchronization Alternatives
Every network element has three principal modes of operation from the viewpoint of (frequency) synchronization. Each element can “free run”; it can derive its frequency reference from an incoming (traffic bearing) signal, or it can accept an external reference. Likewise, for time of day it can “free run” (requiring a routine, maybe infrequent, operator intervention to “set” the time); it can derive its time from a server (e.g. NTP), or it can have a dedicated feed that provides time (and frequency). Figure 3 demonstrates the main modes and Table 1 shows some of the technologies for delivering a timing reference.

Figure 3
Three Main Synchronization Modes

Table 1
Some Technologies Associated With Synchronization and Timing Distribution

Timing Accuracy
Timing plays a critical role in the successful rollout of triple-play networks. Without an accurate time base, transfer of data between networks can suffer latency and losses. Ultimately, the quality of timing you put into your network directly impacts the quality of services that come out of your network. Needless to say, the requirements must be established to satisfy the most demanding service. As new services and applications emerge, accurate sync provides increased functionality as well as becoming a service quality differentiator.

Dr. Kishan Shenoi is chief technologist of R&D at Symmetricom Inc.

Jim Olsen is director of market development at Symmetricom. They can be reached at kshenoi@symmetricom.com or jolsen@symmetricom.com.

Symmetricom Inc. www.symmetricom.com

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