Digital communication networks are now appearing in increasingly greater numbers. The digital networks are as complex as a switched network providing national telecommunications or as simple as a private data network having several private branch exchange (PBX) equipments interconnected by a T1.5 line. In the digital network, a master or reference clock provides continuing timing signals to all other clocks in a digital network. This permits all digital equipment on the network to operate from a common time base. Synchronization is made possible by designating one or more master clocks as the common time base and distributing their timing information to all equipment in the network. As a result, all interconnected network equipment are expected to operate at a rate related to the rate of the designated master clock.
Proper synchronization of digital equipment on the network would be nearly impossible without such clock signals and network performance would be seriously degraded at best. A poorly sychronized network would experience severe sychronization slips and error bursts, both of which adversely affect throughput, quality, and delay performance for data and other traffic and services carried by the digital network.
When two digital communication equipments are transmitting information back and forth to each other at different rates, a slower receiving equipment will not be able to keep pace with the faster transmitting equipment. As a result, the receiving equipment must drop or discard some of the information in order to keep up with the current information being received. Depending on equipment characteristics, the speed of one of the equipments will be adjusted or synchronized to the speed of the other so that both equipments appear to be operating from the same clock signal. It should be noted that a speed difference causing a timing loss as small as one second every 300 years is intolerable according to some telecommunication network standards.
Typical problems resulting from timing or synchronization errors can vary from exchange of incorrect or incomplete information to complete blockage and communication failure of the network. Synchronization errors can also propagate downstream through cascaded equipments as a result of a transient timing phase error. For encrypted data systems, loss or slippage of synchronization can adversely affect security of the network by requiring key retransmission. For facsimile transmission on the digital network, synchronization errors can lead to blurred or even illegible images causing the user to generate a need for facsimile retransmission. In digital video services, synchronization errors can cause picture segments to be distored and blanked for periods extending up to several seconds.
When occuring in industry-specific digital networks, the effect of synchronization errors is further magnified. Banking networks may lose monetary transaction information; aviation control networks may be impaired in dealing with the volumes of instantaneous flight information; military and national security can be compromised; hotel, automobile, and airline reservation information may be lost; securities exchange information may include erroneous data; and internal business communications may become flawed, if occurring at all.
To address some of these problem areas, various organizations have attempted to set standards concerning synchronization and clock precision for networks interconnecting digital equipment. See, for example, American National Standard for Telecommunications--Synchronization Interface Standards for Digital Networks, ANSI T1.101-1987. While these standards state the error performance of different clock signals, there is no attempt to teach or suggest the manner in which one should develop and optimize a sychronization plan for an entire network to ensure end-to-end network reliability within the acceptance criteria of the standards.