Digital trunks between two connecting nodes require clock synchronization in order to ensure proper framing of the voice channels. The sending switch clocks the bits in each frame onto the transmission facility. They are clocked into the receiving switch at the other end of the facility. Digital facility interfaces use buffering techniques to store the incoming frame and accommodate slight variation in the timing of the data sent between the two ends. A problem arises if the other digital switch that is connected to the facility has a clock signal that is out of phase with the first switch. The variation in clock signals eventually causes errors in identifying the beginning of a frame. This condition is known as slip, and it results in buffer overrun or buffer underrun. Buffer overrun occurs if the frequency of the sending clock is greater than the frequency of the receiving clock, discarding an entire frame of data. Buffer underrun occurs if the frequency of the sending clock is less than the frequency of the receiving clock, repeating a frame of data. Occasional slips do not present a real problem for voice calls, although excessive slips result in degraded speech quality. However, they are more detrimental to the data transfer, in which each bit is important. Therefore, synchronization of time sources between the digital switches is important. Because digital transmission facilities connect switches throughout the network, this requirement escalates to a network level, where the synchronization of many switches is required.
There are various methods of synchronizing nodes. One method involves a single master clock source, from which other nodes derive timing in a master/slave arrangement. Another method uses a plesiochronous arrangement, where each node contains an independent clock whose accuracy is so great that it remains independently synchronized with other nodes. You can also use a combination of the two methods by using highly accurate clocks as a Primary Reference Source (PRS) in a number of nodes, providing timing to subtending nodes in the network.
The clocks’ accuracy is rated in terms of stratum levels. Stratums 1 through 4 denote timing sources in order of descending accuracy. A stratum 1 clock provides the most accurate clock source with a free-running accuracy of ±1 x 10 -11, meaning only one error can occur in 1011 parts. A stratum 4 clock provides an accuracy of ±32 x 10-6.
Since the deployment of Global Positioning System (GPS) satellites, each with a number of atomic clocks on-board, GPS clocks have become the preferred method of establishing a clock reference signal. Having a GPS clock receiver at each node that receives a stratum 1-quality timing signal from the GPS satellite flattens the distributed timing hierarchy. If the GPS receiver loses the satellite signal, the receiver typically runs free at stratum 2 or less. By using a flattened hierarchy based on GPS receivers, you remove the need to distribute the clock signal and provide a highly accurate reference source for each node. Figure 5-9 shows an example that uses a stratum 1 clock at a digital switching office to distribute timing to subtending nodes, and also shows an example that uses a GPS satellite clock receiver at each office.
