Data Communications Networking Devices - 4th Ed.pdf

464 _________________________________ WIDE AREA NETWORK TRANSMISSION EQUIPMENTenables users to carry on a normal conversation over the PSTN while transmittingdata at rates up to 33.6 kbps. The second method results in the transmission ofvoice in its native, analog format, while reducing the data rate to provide separatechannels for voice and control information. This method is commonly referred toas analog simultaneous voice and data ASVD). Although ASVD can be moreaesthetically pleasing to a listener's ear in certain situations, most implementationsof a SVD capability work digitally to obtain a higher data transfer operating ratecapability. Two competitive and incompatible techniques for obtaining asimultaneous voice and data transmission capability were being offered byMulti-Tech Systems and Rockwell Semiconductor Systems when this book waspublished.Multi-Tech Systems markets a series of DSVD modems using codecs codersdecoders)to digitize the full spectrum of audio bandwidth from 0 to 3300 Hz.Using what is referred to as the Multi-Tech Supervisory Protocol MSP) forcommunications between modems, voice, data, and supervisory information areseparately packetized for transmission. All three types of packets are multiplexedonto a single stream of data which enables a common circuit to support voice anddata simultaneously; however, to eliminate potential voice distortion voice packetsreceive priority over data packets. Figure 4.58 illustrates the MSP packet structure.Note that each packet uses one 8-bit SYNC byte header for synchronization.The ID/LI Information byte includes an ID bit number 7) and a Packet LengthIndicator LI) ®eld which ranges in value from 01h to 07Fh for data and quali®edpackets. Voice packets are preset to 24 or 39 bytes in length based upon the dataoperating rate. Thus, the ID/LI Information byte is set to a value of 00h in voicepackets.Figure 4.58Multi-Tech Supervisory Prtotocol packet structure