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1998-04-20 23:21:12


GPS Rollback and Network Degradation or Collapse



This lengthy 1996 sales document from Hewlett Packard shows how crucial GPS time signals are to maintaining telecommunications networks, and how widely they have been adopted

On August 22, 1999, the GPS system loses 1,024 weeks. All software dependent on the GPS must be rewritten and installed before this date to compensate for the rollback.

* * * * * * * *

To keep pace with dramatic subscriber growth and consumer demand for next-generation voice, data and video services, telecommunication carriers are offering an increasing array of features and applications. Cost, quality, reliability and breadth of service are expected to become the key factors by which customers decide which providers to use.

Of these, though, the most critical factors -- the make-or-break issues -- are quality and reliability. Even the most desirable features or lowest prices won't draw customers if a given communication network doesn't provide dependable, high-quality service. The fact is, as service providers scramble to be the first to offer new services based on emerging digital technologies, it's imperative that they address the serious timing and synchronization problems that may degrade service quality.

When Timing is Off, Problems are Many

Why are timing and synchronization so important? For wireline telephone carriers -- who are beginning to deploy on a large scale the ISDN, video and other broadband data services based on next-generation SONET/SDH and ATM technologies -- the potential problems are many. When timing or synchronization is off, quality issues range from distorted, unreadable faxes and corrupted or lost data to frozen images on video-conference screens and unintelligible encryption messages requiring retransmission.

Similarly, wireless carriers -- who are increasing capacity and moving from older, analog technologies to next-generation CDMA and TDMA digital technologies -- need to address the timing problems that cause static and blocked or dropped calls. Because of the unprecedented growth of wireless communications, the challenge of providing reliable service promises to become even more demanding. . . .

To help telecommunication carriers successfully enter the digital age, HP offers a total network-synchronization solution built around HP's worldwide and world-class service, support and consulting capabilities. . . . .

Today's Challenges and Solutions

Communication networks -- whether wireline or wireless -- include timing and synchronization subsystems that provide the network with consistent pulses and time intervals to keep digitized information moving from one node to the next. A time-interval standard or clock determines epoch (time or time of day) and provides a stable, precise time interval (time between events, independent of a starting point) called frequency or timing.

Both wireless and wireline networks need an accurate frequency source, but there are significant differences in the technology that underlies these two types of networks. As a result, wireline and wireless have very different timing and synchronization needs -- which demand technology solutions optimized for each.

The Hierarchy of Wireline Timing/Synchronization

On a typical wireline telecommunications network, one or two primary reference sources -- located at the top of the network hierarchy (level 1) -- provide a precise frequency that is sent to lower offices or nodes in the network. Standard specifications require at least one of these sources to be cesium, traditionally the most accurate -- and most expensive -- frequency source. The second specified source might also be cesium or GPS (global positioning system), which is less expensive and, until now, less reliable.

Until recently, network providers have successfully relied upon less accurate -- and less expensive -- clocks for timing at lower levels of the network (levels 2 and 3). However, in the transition to digital technology, these lower-level clocks do not provide sufficient reliability and accuracy to support next-generation technologies. . . .

The Critical Interplay of Timing and Synchronization

An accurate and reliable frequency source is not sufficient in and of itself to guarantee network reliability. In fact, another critical issue comes into play: synchronization. Synchronization is the means of keeping all digital equipment in a communications network operating at the same rate.

For digital transmission, information is coded into discrete data frames, or packets. A clock located at the network node whence the packet originates controls the rate at which the packet is transmitted from the node. A second clock located at the receiving node controls the rate at which the data frame is read. The objective of network synchronization is to keep the source and receive clocks in step so that the receiving node can properly interpret the digital frame.

Differences in timing at nodes within a network will cause a "slip" in which the receiving node either drops or must reread information sent to it. Poor synchronization is caused either by differences in timing accuracy among the network's level 1, 2 and 3 clocks or by phase movements called "jitter" and "wander" that distort the network's timing references as they are distributed from one node to another. In both instances, the installation of HP PRS technology will minimize the problem.

As wireline networks are expanded, the resulting longer synchronization distribution chains cause increased degradation of the timing reference. . . .

Using its HP SmartClock technology and global positioning system (GPS) precise time, HP designs systems to solve the serious timing and synchronization problems of wireline networks. . . .

Wireless networks operate very differently from wireline networks in that the timing signal, as well as any calls or data transmitted, is transferred by radio frequency rather than by cable between the customer and the local office or base station. Unlike wireline callers, who are stationary, the wireless caller can be mobile. The wireless call may need to be handed off, or transferred, between base stations as the caller changes location. HP's SmartClock technology and GPS provide low-cost, high-quality timing to support the needs of wireless networks for both synchronization and successful call handoff. . . .

GPS -- GPS is the U.S. Department of Defense (DoD) satellite system that provides continuous access to precise time and frequency anywhere on earth. It is available for commercial use at no cost. . . .

Global Positioning System Precise Time -- The global positioning system (GPS) provides continuous access to precise time and frequency, at low cost-anywhere on earth. The U.S. Department of Defense's 24-satellite global positioning system transmits data needed to determine precise time and position (latitude, longitude and altitude). Each of the satellites carries two cesium and two rubidium clocks for synchronized timing output and completes two earth orbits per day. The signal -- a composite of a time signal transmitted by the satellites' clocks and by atomic clocks located in international time-standards bureaus on earth -- is accurate to within 300 nanoseconds of Universal Coordinated Time (UTC).


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