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SYNCHRONIZATION OF DIGITAL FACILITIES
5-3
NOTE: The deletion or repetition of a single frame is termed a slip or a controlled slip. For an
individual digital bit stream, slips are serious impairments since digital switching systems with
improperly synchronized clocks will eventually suffer slips on every received digital bit stream.
Notice in figure 5-1-B, Options for Synchronization, that the overrun or underrun problem can be
prevented by forcing the channel bank transmitter to operate at the same clock rate as the receive
portion of the channel bank. This arrangement is called channel-bank loop timing. This becomes
more complex when two digital switches terminate a digital transmission facility. Figure 5-1-C,
Options for Synchronization, shows this configuration.
In figure 5-1-C, Options for Synchronization, each switching system transmits at a rate determined by
a reference clock. Unless the received digital bit stream arrives at the same clock rate as its internal
clock rate, slips will occur. To prevent or minimize slips, it is necessary to force both switching
systems to synchronize on a common reference clock rate (F0). Both will then be part of a
synchronized network, and will read and write their buffers as they should.
The primary objective of network synchronization is to minimize the slip rate. This is done by
synchronizing the clocks associated with the switching nodes so that all transmissions from these
nodes have the same average line rate. For short-term intervals, the switching-system receive buffers
absorb the difference between the line rate and the average rate. These short term variations are
called jitter. A long-term difference will result in a slip.
The impact of slips on a customer varies with the services used. For example, voice services are
insensitive to slips. Slip rates as high as 20 per second are barely perceptible; therefore, speech is not
considered a limiting factor in setting slip-rate standards. However, data services are much more
sensitive to slips since each bit of data is a discrete piece of information. For data applications, a slip
(at any slip rate) requires retransmission and will adversely affect the throughput and degrade
performance of data transmission facilities.
Slips can occur for two basic reasons:
1.
2.
Lack of or loss of frequency synchronization among the network nodes (for example, when slips
occur at a constant, regular rate)
Phase modulation of the transmitted digital bit streams owing to environmental variations of
the transmission facilities (such as temperature variations that affect the electrical length of a
transmission line)
Therefore, even if all network nodes are synchronized, slips can still occur owing to transmission
impairments.
SYNCHRONIZATION HIERARCHY
Within North America, all private digital telecommunications facilities that connect to the public
digital network must obtain synchronization by the hierarchical method. This method is based on:
1.
Controlling the slip rate to provide satisfactory service to the customer
2.
Maximum reliability