1,544,000 stream transports 1,536,000 payload bits, or 192,000 payload bytes and 8000 framing bits, or 1000 bytes. Each individual 64-Kbs channel is referred to as DSO, meaning digital signal level 0. The composite serial bitstream is referred to as digital signal 1 or DS1. DS1 is considered a multiplex level and while the bitrate and framing is the same, DS1 is often con- fused with the T1 carrier or channel and its interface. See Table 4-1 for examples of the different multiplexing level and transmission chan- nel interface rates. 125 Microseconds 193 Bits 192 Bits (24 Bytes) (1 Sample From Each Channel @ 8Khz Rate) FIGURE 4-3 T-Carrier Digital Channel Timing and Framing PRACTICAL IP AND TELECOM FOR BROADCAST ENGINEERING AND OPERATIONS TABLE 4-1 Payload Clear Aggregate Channel Signal Multiplex Channel channel multiplex interface level input count Bitrate rate* Region of use T1/J1 24xDSO 1.536 1.536 1.544 NA & Japan 2xDS1 3.072 3.152 E Countries 4XDS1 6.144 6.312 E Countries 30xDS0 1.920 1.920 2.048 E Countries 7.680 8.448 E Countries 16XE1 30.720 30.720 34.368 E Countries T3/J3 28XDS1 41.216 43.008 44.736 NA & Japan 64XE1 115.200 139.264 E Countries 6xDS3 258.048 274.176 NA & Japan *The minimum amount of bandwidth available. It is also precisely equal to the number of 64 Kbs DSO channels removed from the next higher order channel. Actual payload in practice is dependent on design requirements. For example, a DS3 requires a minimum of 526,306 bits for framing, leaving 44,209,694 out of 44,736,000 bits available for payload in unchannelized service. BASIC FRAMING Framing bits are also applied when a basic 1.544 Mbs stream is multiplexed with another stream or additional streams into higher order aggregate signal. Additional bits are added in specific timeslots and designated as framing bits to enable the receiving equipment to recover the original clock and separate the payload, first into the next lower order bitstream and then ultimately down through the multi- plex hierarchy to the original 1.544 Mbs payload a