2 048 mbit s pbx 12 the second loopback mode is an offline all channel loopback from the t1 1 544 mbit s input to the 2 048 mbit s pbx 12 rate back to the 1 544 mbit s output the third loopback mode is an online channel by channel loopback from the 2 048 mbit s pbx 12 input to the 2 048 mbit s side of the rate conversion circuitry 14 back to the 2 048 mbit s pbx 12 output the first two loopback modes mentioned above are used for offline testing and the last loopback mentioned above is used for online performance verification data passing through the pcm converter 14 is digitially attenuated or amplified on a channel by channel basis to provide level matching parallel data from a microprocessor can be substituted in channel 24 for signaling purposes the basic clock rate of the t1 link can be different from the basic clock rate of the pbx 12 without destroying the quality of sound of the communications system 10 basically the pcm conversion circuitry 14 implements a four frame buffer in both directions for clean frame slips and de jitter performance this eliminates the need for external jitter attenuators optimally the t1 data input output and the pbx output input are separated by two frames if the separation becomes as small as one frame the pcm conversion circuitry 14 will wait until the beginning of the frame and repeat the last frame bringing the difference to two frames if the separation is as large as three frames the pcm conversion circuitry 14 will wait.