 much more desirable than to have an uncontrolled slip that can occur anywhere. Slips occur for two basic reasons: 1. Lack of frequency synchronization among clocks at various network nodes 2. Phase wander and jitter on the digital bit streams Thus, even if all the network nodes are operating in the synchronous mode and synchro- nized to the network master clock, slips can still occur due to transmission impairments. An example of environmental effects that can produce phase wander of bit streams is the 6.12 DIGITAL NETWORK daily ambient temperature variation affecting the electrical length of a digital transmis- sion line. Consider this example: A 1000-km coaxial cable carrying 300 Mbps (3 X 108 bps) will have about 1 million bits in transit at any given time, each bit occupying about one meter of the cable. A 0.01% increase in propagation velocity, as would be produced by a 1°F decrease in temperature, will result in 100 fewer bits in the cable; these bits must be absorbed to the switch's incoming elastic store buffer. This may end up causing an underflow problem forcing a controlled slip. Because it is underflow, the slip will be manifested by a frame repeat; usually the last frame just before the slip occurs is repeated. In speech telephony, a slip only causes a click in the received speech. For the data user, the problem is far more serious. At least one data frame or packet will be corrupted. Slips due to wander and jitter can be prevented by adequate buffering. Therefore adequate buffer size at the digital line interfaces and synchronization of the network node clocks are the basic means by which to achieve the network slip rate objective (Ref. 9). 6.12.2.2 Methods of Network Synchronization. There are a number of methods that can be employed to synchronize a digital network. Six such methods are shown graphically in Figure 6.24. Figure 6.24a illustrates plesiochronous operation. In this case each switch clock is free-running (i.e., it is not synchronized to the network master clock.) Each network nodal sw