Data Communications Networking Devices - 4th Ed.pdf

4.10 DIGITAL SERVICE UNITS ______________________________________________________ 509Figure 4.84 DDSframing formats: a) 56 kbps; b) 2.4, 4.8, 9.6 kbps; c) 19.2 kbpsF frame bit, D data bits, C control bit, P&EC parity and error correction bits)bit byte 8000 times per second, this framing format results in 8 kbps of controlbits being added to the 56 kbps customer data rate.The construction of DS0 signals from the 2.4, 4.8 and 9.6 kbps DDS subrates isillustrated in Figure 4.84b). As indicated, customer data is inserted into 8-bit byteswith six bits of user data framed by a frame bit F) and a control bit C). Once 2.4,4.8 or 9.6 kbps DDS data streams are framed, one of two methods is used to placethe framed data onto a DS0 channel. When `byte stuf®ng' is used, the frame bit isset to `1' and the customer data are repeated the required number of times to createa 64 kbps DS0 signal. Thus, the 8-bit byte containing six bits of user data isrepeated at 5, 10, and 20 times to enable 9.6, 4.8 and 2.4 kbps DDS data to beplaced on a 64 kbps channel. When the F bit is set to `1' the frame format illustratedin Figure 4.84b), is referred to as a DS0-A format. Thus, DS0-A data can bypassthe ®rst level TDM illustrated in Figure 4.83 and be fed directly into the secondlevel TDM.The second method of placing 2.4, 4.8 or 9.6 kbps DDS data onto a DS0 channelinvolves the use of the ®rst level TDM illustrated in Figure 4.83. When this occurs,®ve 9.6, ten 4.8, or twenty 2.4 kbps formatted signals are multiplexed onto a singleDS0 channel. To distinguish between the repeating of the same data resulting frombyte stuf®ng and the multiplexing of different DDS signals, the framing bit isaltered from all ones in byte stuf®ng to a subrate framing pattern to indicate multiplexingof different DDS data sources. When this framing pattern occurs, theresulting framing format is referred to as a DS0-B format. Obviously, DS0-B