bit rate is extremely close to the nominal bit rate. To achieve this, network synchronization is necessary. Network synchronization can be accomplished by synchronizing all switch (node) master clocks SO that transmis- sions from these nodes have the same average line bit rate. Buffer storage devices are judiciously placed at various transmission interfaces to absorb differences between the actual line bit rate and the average rate. Without this network-side synchronization, slips will occur. Slips are a major impairment in digital networks. Slip performance require- ments are discussed in Section 6.12.3.5. A properly synchronized network will not have slips (assuming negligible phase wander and jitter). In the next paragraph we explain the fundamental cause of slips. As mentioned, timing of an outgoing bit stream is governed by the switch clock. Suppose a switch is receiving a bit stream from a distant source and expects this bit stream to have a transmission rate of F(0) in Mbps. Of course, this switch has a buffer of finite storage capacity into which it is streaming these incoming bits. Let's further suppose that this incoming bit stream is arriving at a rate slightly greater than F(0), yet the switch is draining the buffer at exactly F(0). Obviously, at some time, sooner or later, that buffer must overflow. That overflow is a slip. Now consider the contrary condition: The incoming bit stream has a bit rate slightly less than F(0). Now we will have an underflow condition. The buffer has been emptied and for a moment in time there are no further bits to be streamed out. This must be compensated for by the insertion of idle bits, false bits, or frame. However, it is more common just to repeat the previous frame. This is also a slip. We may remember the discussion of stuffing in Section 6.8.1 in the description higher-order multiplexers. Stuffing allows some variance of incoming bit rates without causing slips. When a slip occurs at a switch port buffer, it can be controlled to occur at frame boundaries. This is