Growing Vulnerability of the Public Switched Networks Implications for National Security Emergency Preparedness (1989) / Chapter Skim
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Appendix B: Issues in Digital Network Time and Frequency Synchronization
Appendix B: Issues in Digital Network Time and Frequency Synchronization
Pages 91-116
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From page 91... ...
It is based on briefings presented to the National Research Council's Committee on Review of Switching, Synchronization and Network Control by several organizations, including the American Telephone and Telegraph Company (AT&T) , Bell Communications Research, MCI Communications Corporation, US Sprint, CONTEL/ASC, and the U.S.
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From page 92... ...
The final three sections contain the committee assessment of the impact of synchronization impairments on the National-Leve! Program/National Security Emergency Preparedness (NEP/NSEP)
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From page 93... ...
The UT-0 day of 24 hours is defined as the mean sidereal day converted to mean solar day by ephemeris tables. The UT-1 day is determined from the UT-0 day by including regular corrections on the order of 3G ms due to seasonal changes in winds and tides.
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From page 94... ...
Primary Frequency Standards In order that both atomic and civil time can be coordinated throughout the world, it is expected that national administrations will operate publicly available primary time and frequency standards and maintain UTC cooperatively by observing various radio transmissions and through occasional use of portable atomic clocks. A primary frequency standard is an oscillator that can maintain extremely precise frequency relative to a physical phenomenon, such as a transition in the states of an orbital electron.
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From page 95... ...
The current time-scale formats used by NIST radio broadcast services do not include provisions for advance notice of leap seconds, so this information must be determined from other sources. Various specification and standards documents stipulate that the primary timing sources used by digital networks must be verifiable with respect to UTC; however, for digital network synchronization, only the frequency information is used—the time information is not used.
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From page 96... ...
words transmitted over an interval of 30 s. The time code information includes the UTC time of year, satellite position, and UT-1 correction.
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From page 97... ...
4. SECONDARY TIMI: AND FREQUENCY DISTRIBUTION Present time and frequency dissemination services operated by NIST are not sufficiently ubiquitous and reliable as the basis of digital network synchronization in the United States.
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From page 98... ...
Station timekeeping within a chain is usually better than 50 ns relative to the master cesium clock; however, monitored deviations of 100 ns or more are indicated in the transmitted signals by ~blinking" certain pulses. The master cesium clocks may drift 60 us on a dayto-day basis, but are maintained within 2.5 ,us of NIST standard time using corrections determined manually and published weekly.
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From page 99... ...
Signal monitors necessary for LORAN-C guided nonprecision approaches will be installed and become operational in 1989. By 1990, additional transmitting stations will be installed to complete signal coverage over the 48 conterminous states.
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From page 100... ...
Satellite-based systems such as GPS can provide differential time measurements to an extraordinary precision; however, with current DoD policy the accuracy achievable with GPS for civil users is in the same range as LORAN-C. Portable Clocks and Transfer Standards6 Portable cesium clocks have been constructed for the purpose of calibrating local time and frequency standards when other means are not available and as a backup for these means when available.
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From page 101... ...
Disciplined Frequency Standards Quartz crystal oscillators have been used as frequency references for many years, since they are compact, relatively inexpensive, and stable. A suitably designed and temperature-stabilized crystal oscillator should be stable within a few parts in 10~° per day and be adjustable to a precise reference, such as a cesium clock.
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From page 102... ...
By industry agreement through the American National Standards Institute (ANST T-1 Committee) , all digital synchronization networks must be controlled by a primary reference standard (PRS)
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From page 103... ...
Stratum-2 and stratum-3 clocks must have diverse primary and secondary timing sources, show little effect of source switching, and have accurate holdover in case of complete loss. Stratum-3 and stratum-4 clocks must switch sources if the current timing signal source is defective (for example, out-of-frame synchronization)
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From page 104... ...
The accuracy achieved by the slave clock also depends on the synchronization path to the master. This path may be disturbed by any one or more of several mechanisms, including facility error bursts and short outages, frame-jitter, phase hits (sudden phase changes)
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From page 105... ...
carriers today. American Telephone and Telegraph Company The existing AT&T synchronization network is based on the AT&T Basic Synchronization Reference Frequency PRS of 2.048 MHz, which consists of three cesium atomic clocks that maintain stratum-1 accuracy of 10-~.
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From page 106... ...
Each BOO I`EC has one or more synchronization coordinators whose function it is to keep the maps current and provide technical help as required. Bell Communications Research, Incorporated Prior to divestiture the RBOC facilities were an integral part of the AT&T synchronization network.
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From page 107... ...
Some of the BOCs make use of AT&T facilities, while others maintain a PRS using cesium clocks or a DFS slaved to LORAN-C. In 1986 Bell Communications Research, Incorporated (BelIcore)
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From page 108... ...
The design of the clock distribution equipment (CDE) includes provisions to smooth and "deglitch" the received timing signal, usually in the form of one or more DS-1 signals, and distribute it within the facility over a loop-free synchronization tree with backup.
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From page 109... ...
depend strongly on the type and severity of the underlying cause and on the particular user application. In the following subsections, the effects of synchronization impairments will be assessed on transrn~ssion, network elements, and user applications.
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From page 110... ...
of a DS-1 bit stream, occurs if the timing source of the transmitting network equipment at the sending end of a digital link is not synchronized with the timing source of the network equipment at the receiving end of that link. The interval between controlled slips is inversely proportional to the frequency offset between the two timing sources.
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From page 111... ...
Synchronization impairments due to controlled slips result in a loss or replication of a 125-us frame. The impact on DTMF signaling could be a missed digit if a minimum 50-ms DTMF signal was being transmitted.
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From page 112... ...
Tables B-8 and B-9 summarize these effects for three levels of transmission impairment for typical user applications. The three transmission impairments considered in the table are isolated controlled slips, isolated burst errors (for example, a 100 ms period with a BER Of 10-2 every 4 s)
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From page 113... ...
8. SENSITIVITY OF NATIONAL SECURITY EMERGENCY PREPAREDNESS TO SYNCHRONIZATION lMPAIRMENTS,2 The preceding discussion has emphasized the mechanism and ejects of synchronization impairments within the telephone network itself
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From page 114... ...
. The main motivation in this paper is to determine whether there may be problems due to synchronization impairments in the public switched networks that could lead to unacceptable performance or loss of useful NSEP capabilities of any of the various services required by the National Communications System (NCS)
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From page 115... ...
9. CONCLUSION AND RECOMMENDATION From the foregoing analysis the committee reaches the following conclusion and recommendation: No significant synchronization timing issues for national security emergency preparedness appear to exist, because timing is set by the connected surviving access tandem.
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From page 116... ...
Technical Advisory TA-NP~000436. Livingston, N.J.: Bell Communications Research, Incorporated.
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