tailieunhanh - Signaling System No.7 Protocol Architecture And Sevices part 13

Network Timing 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. | Network Timing 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 .