ÿWPCL ûÿ2BJ|xÐ ` ÐÐÌÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿH øÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÌÐÐ °°°è ÐÑ Âx„|ü@Ž ÑÐ Å°6Ø'°6Ø'Å Ð3.ÁHÁÓÓÃÃRecommendation G.811ÄÄ Ð À ÐÁàÐ'ÁTIMING REQUIREMENTS AT THE OUTPUTS OF PRIMARY REFERENCE CLOCKS SUITABLE FOR ÁàÐ2ÁƒPLESIOCHRONOUS OPERATION OF INTERNATIONAL DIGITAL LINKSƒ 1.ÁHÁÃÃGeneralÄÄ 1.1ÁHÁÃÃInternational connections and network synchronization considerationsÄÄ ÁHÁNational digital networks, which may have a variety of internal synchronization arrangements, will usually be connected by international links which operate plesiochronously. International switching centres (ISCs) will be interconnected directly or indirectly via one or more intermediate ISCs, as indicated in the hypothetical reference connection (HRX) shown in Figure 1/G.801. ÁHÁInternational connections terminate on synchronous network nodes that may or may not be co©located with a primary reference clock. Such network nodes may include slave clocks. Therefore, synchronous network node clock specifications are essential to ensure satisfactory operation of plesiochronous international digital links. ÁHÁFigure 1/G.811 illustrates the two alternative international connections described above. 1.2ÁHÁÃÃPurpose of this RecommendationÄÄ ÁHÁThe purpose of this Recommendation is to specify requirements for primary reference clocks, promote understanding of associated timing requirements for plesiochronous operation of international digital links, and to clarify the relationship of the requirements for synchronous network nodes, constituent clocks and the use of satellite systems. Ð X ÐÁHÁAdministrations may apply this Recommendation, at their own discretion, to primary reference clocks other than those used in connection with international traffic. 1.3 ÃÃInteraction between plesiochronous and synchronous international operationÄÄ ÁHÁIt is important that the Recommendations for plesiochronous operation should not preclude the possibility of the later introduction of international synchronization. ÁHÁWhen plesiochronous and synchronous operations coexist within the international network, the nodes will be required to provide for both types of operation. It is therefore important that the synchronization controls do not cause short©term frequency departures of the clocks which are unacceptable for plesiochronous operation. The magnitudes of the short©term frequency departures should satisfy the specifications in ÀÀ 2.2. a)ÁHÁÃÃCase 1: Synchronous network node including primary reference clockÄÄ b)ÁHÁÃÃCase 2: Synchronous network node including slave clockÄÄ ÃÃNote 1ÄÄ © PRC Primary reference clock ÁHÁSC Slave clock ÁHÁDE Digital equipment such as digital exchange or digital muldex ÁHÁIDL International digital link ÃÃNote 2ÄÄ © Other cases are for further study. ÁàÀIÁFIGURE 1/G.811ƒ ÁàÀPÁƒ ÁàÀ/ÁÃÃInternational connections terminating on synchronous network nodesÄă 1.4ÁHÁÃÃMaximum time interval error and relationship with frequency departureÄÄ ÁHÁMaximum time interval error (MTIE) is the maximum peak©to©peak variation in the time delay of a given timing signal with respect to an ideal timing signal within a particular time period (Figure 2/G.811), i.e. MTIE(ÃÃSÄÄ) = max ÃÃxÄÄ(ÃÃtÄÄ)©min ÃÃxÄÄ(ÃÃtÄÄ) for all ÃÃtÄÄ within ÃÃSÄÄ. ÁàÀPÁƒ ÁàÀPÁƒ ÁàÀPÁƒ ÁàÀPÁƒ ÁàÀPÁƒ ÁàÀPÁƒ ÁàÀPÁƒ ÁàÀPÁƒ ÁàÀPÁƒ ÁàÀPÁƒ ÁàÀPÁƒ ÁàÀPÁƒ ÁàÀPÁƒ ÁàÀPÁƒ ÁàÀPÁƒ ÁàÀPÁƒÔ ñ,ÔŒÁàÀIÁFIGURE 2/G.811ƒ ÁàÀPÁƒ ÁàÀ<ÁÃÃDefinition of maximum time interval errorÄă ÁHÁLong©term frequency departure (ÀˆÀÃÃfÄÄ/ÃÃfÄÄ) is determined by the MTIE divided by the observation interval ÃÃSÄÄ, as ÃÃSÄÄ increases. ÃÃNote 1ÄÄ © The rigorous definition and measurement of long©term frequency departure for clocks is a subject for further study. 2.ÂXHÂÃÃLong©term frequency departure and phase stability of primary referenceÄÄ ÃÃclocksÄÄÆÆ ÁHÁA primary reference clock controls the synchronization performance of the overall network. It is necessary to specify the long©term frequency departure and phase stability of a primary reference clock, and to provide guidance concerning issues associated with degradation and unavailability performance. The definition of a primary reference clock is given in Recommendation G.81y. ÃÃLong©term frequency departureÄÄ ÁHÁA primary reference clock should be designed for a long©term frequency departure of not greater than 1 x 10Ãé11ÄÄ. The long©term frequency departure of 1 x 10Ãé11ÄÄ is about two orders of magnitude larger than the uncertainty of Coordinated Universal Time (UTC). Therefore UTC should be the reference for long©term frequency departure (see CCIR Report 898). ÁHÁThe theoretical long©term mean rate of occurrence of controlled frame or octet slips (i.e. the design rate of slips based on ideally undisturbed conditions) in any 64 kbit/s channel is consequently not greater than one in 70 days per digital international link (see Recommendation G.822). ÃÃNote 1ÄÄ © Some administrations support a primary reference clock long©term frequency departure of not greater than 7 x 10Ãé12ÄÄ based upon current primary reference clock technology. ÃÃNote 2ÄÄ © Caesium©beam technology is suitable for primary reference clocks complying with the above specification. 2.2ÁHÁÃÃPhase stabilityÄÄ ÁHÁThe phase stability of a clock can be described by its phase variations, which in turn can be separated into a number of components: ÁHÁ©Âà  Âphase discontinuities due to transient disturbances;ÆÆ ÁHÁ©Âà  Âlong©term phase variations (wander and integrated frequency departure);ÆÆ ÁHÁ©Âà  Âshort©term phase variations (jitter).ÆÆ ÁHÁA phase stability model for primary reference clocks is described in the annex to this Recommendation. 2.2.1ÁHÁÃÃPhase discontinuitiesÄÄ ÁHÁPrimary reference clocks need a very high reliability and are likely to include replication of the equipment in order to ensure the continuity of output. However, any phase discontinuity, due to internal operations within the clock or network node, should only result in a lengthening or shortening of the timing signal interval and must not cause a phase discontinuity in excess of 1/8 of a unit interval at the clock output. (This refers to output signals at 1544 kbit/s or 2 048 kHz, of ÀÀ 4. Specification of other interfaces is under study.) 2.2.2ÁHÁÃÃLong©term phase variationsÄÄ ÁHÁThe maximum permissible long©term phase variation of a primary reference clock (whether sinusoidal or pulse) is expressed in MTIE. ÁHÁThe MTIE over a period of S seconds shall not exceed the following limits: ÁHÁa)Á   Á100S ns for the interval 0.05 < S ÃÃ<ÄÄ 5 ÁHÁb)Á   Á(5S+500) ns for the interval 5 < S ÃÃ<ÄÄ 500 ÁHÁc)Á   Á(.01S+X) ns for values of S > 500. ÁHÁThe asymptote designated 10Ãé11ÄÄ refers to the long©term frequency departure specified in Section 2.1. ÁHÁThe value of X is under study. It is provisionally recommended that X = 3000 ns. Certain administrations support a value of 1000 ns. ÃÃNote 1ÄÄ © For measurement of long©term phase variations, the use of 10 Hz low© pass filter is suggested. ÃÃNote 2ÄÄ © The MTIE Recommendation requires further study. ÃÃNote 3ÄÄ © The overall specification is illustrated in Figure 3/G.811. 2.2.3ÁHÁÃÃShort©term phase variationsÄÄ ÁHÁClock implementations exist today which may have some high©frequency phase instability components. The specification of maximum permissible short© term phase variation of a primary reference clock due to jitter is under study. 3.ÁHÁÃÃDegradation of the performance of a primary reference clockÄÄ ÁHÁTo achieve the required high reliability a primary reference clock includes redundancy, i.e. by incorporating several (caesium beam) oscillators, the output of only one of these being used at any given time. If a clock frequency departs significantly from its nominal value, this should be detected and switching to an undegraded oscillator should then be effected. This switching should be accomplished before the MTIE specification is exceeded. ÁHÁWith current technology, the performance of a primary reference clock is statistically well below the MTIE specification of Figure 3/G.811. 4.ÁHÁÃÃInterfacesÄÄ ÁHÁThe preferred interface for the timing output is in accordance with Recommendation G.703, 10, i.e. an interface at 2048 kHz. By agreement between operators or manufacturers of equipment, the timing signal may also be delivered at various other physical interfaces (e.g., 1544 kbit/s primary rate signal, 1 MHz, 5 MHz, or 10 MHz). ÁàÀPÁƒ ÁàÀPÁƒÔ ñ,ÔŒÁàÀPÁƒ ÁàÀPÁƒ ÁàÀPÁƒ ÁàÀPÁƒ ÁàÀPÁƒ ÁàÀPÁƒ ÁàÀPÁƒ ÁàÀPÁƒ ÁàÀPÁƒ ÁàÀPÁƒ ÁàÀPÁƒ ÁàÀPÁƒ ÁàÀPÁƒ ÁàÀPÁƒ ÁàÀPÁƒ ÁàÀPÁƒ ÁàÀPÁƒ ÁàÀPÁƒ ÁàÀPÁƒ ÁàÀPÁƒ ÁàÀPÁƒ ÁàÀPÁƒ ÁàÀPÁƒ ÁàÀPÁƒ ÁàÀPÁƒ ÁàÀPÁƒ ÁàÀPÁƒ ÁàÀPÁƒ ÁàÀPÁƒ ÁàÀPÁƒ ÁàÀPÁƒ ÁàÀPÁƒ ÁàÀPÁƒ ÁàÀPÁƒ ÁàÀIÁFIGURE 3/G.811ƒ ÁàÀPÁƒ ÁàÀ9ÁÃÃPermissible maximum time interval error (MTIE)Äă ÁàÀ8ÁÃÃdue to long©term phase variation as a function ofÄă ÁàÀ7ÁÃÃobservation period S for a primary reference clockÄă 5.ÂXHÂÃÃUse of satellite systems in an international plesiochronous digitalÄÄ ÃÃnetworkÄÄÆÆ ÁHÁIt is recommended that the link be operated in a plesiochronous mode using high accuracy (1 x 10Ãé11ÄÄ) source for the satellite TDMA timing. The international satellite links will be terminated on network nodes whose timing is in accordance with Recommendations G.823 and G.824. 6.ÁHÁÃÃGuidelines concerning the measurement of jitter and wanderÄÄ ÁHÁVerification of compliance with jitter and wander specifications requires standardized measurement methodologies to eliminate ambiguities in the measurements and in interpretation and comparison of measurement results. Guidelines concerning the measurement of jitter and wander are contained in Supplement No. 3.8 (O©Series) and Supplement No. 35. ÁàÀMÁANNEX Aƒ ÁàÀPÁƒ ÁàÀDÁ(to Recommendation G.811)ƒ ÁàÀPÁƒ ÁàÀ3ÁÃÃCharacterization of primary reference clock phase stabilityÄă ÁHÁThe following phase stability model may be employed to characterize primary reference clocks. Let ÃÃxÄÄ(ÃÃtÄÄ) represent the time interval error of a clock synchronized at ÃÃtÄÄ = 0, and free©running against UTC thereafter. ÃÃxÄÄ(ÃÃtÄÄ) may be defined as: ÁàÀCÁÃÃxÄÄ(ÃÃtÄÄ) = yÃÃ0ÄÄt + (D/2)tÃÃ2ÄÄ + e(t)ƒ ÁàÀPÁƒ where: ÁHÁDÁ  Áis the normalized linear frequency drift per unit time (ageing), ÁHÁyÃÃ0ÄÄÁ   Áis the initial frequency departure with respect to UTC, and ÁHÁÃÃeÄÄ(ÃÃtÄÄ)Á øÁis the random error component. ÁHÁThe estimate of the standard deviation of ÃÃxÄÄ(ÃÃtÄÄ) may be obtained, and used for characterization of phase instability. Ãà ÄÄ À%ÀÃÃxÄÄ(t) = (D/2)ÃÃtÄÄÃÃ2ÄÄ + t ÀÀÀ%ÀÃÃyÄÄ ÃÃ2ÄÄ + À%ÀÃÃyÄÄÃÃ2ÄÄ (À)À = ÃÃtÄÄ)                                            ÃÃOÄÄ where: Ð ð ÐÂHHÂÂX  ÂÁ€ HÁÁ€ÁÁHÁÀ%ÀÃÃyÄÄÃÃ2ÄÄ Á øÁis the two©sample variance of the initial frequency departure, andÆÆ ÁHÁÀ%ÀÃÃyÄÄÃÃ2(À)À)ÄÄ is the two©sample Allan variance describing the random frequency instability of the clock.