ustification and intermediate amount of jitter in frames containing commands of negative justification. 11 The maximum amount of justification rate per tributary is shown in Table 6.3. 6.9 LONG-DISTANCE PCM TRANSMISSION 6.9.1 Transmission Limitations Digital waveforms lend themselves to transmission by wire pair, coaxial cable, fiber-optic cable, and wideband radio media. The PCM multiplex format using an AMI signal was first applied to wire-pair cable (see Section 5.5). Its use on coaxial cable is now deprecated in favor of fiber-optic cable. Each transmission medium has limitations brought about by impairments. In one way or another each limitation is a function of the length of a link employing the medium and the transmission rate (i.e., bit rate). We have discussed loss, for example. As loss increases (i.e., between regenerative repeaters), signal-to-noise ratio suffers, directly impacting bit error performance. The following transmission impairments to PCM transmission are covered: jitter, distortion, noise, and crosstalk. The design of long-distance digital links is covered in Chapter 9. 11 Jitter, see Section 6.9.2. 6.9 LONG-DISTANCE PCM TRANSMISSION 6.9.2 Jitter and Wander In the context of digital transmission, jitter is defined as short-term variation of the sampling instant from its intended position in time or phase. Longer-term variation of the sampling instant is called wander. Jitter can cause transmission impairments such as: Displacement of the ideal sampling instant. This leads to a degradation in system error performance. Slips in timing recovery circuits, manifesting in degraded error performance. Distortion of the resulting analog signal after decoding at the receive end of the circuit. Think of jitter as minute random motion of a timing gate. Timing gates are shown in the lower-right-hand corner of Figure 6.11. An analogy is a person with shaky hands trying to adjust a screw with a screwdriver. The random phase modulation, or phase jitter, introduced at each regenerative repeater accumulates