Data Communications Networking Devices - 4th Ed.pdf

514 _________________________________ WIDE AREA NETWORK TRANSMISSION EQUIPMENTand are generated by the network and used by the DSU to illuminate anappropriate indicator on the device.DDSIIDuring 1988, AT&T introduced a new version of its DDS facility commonlyreferred to as DDS II. One of the key advantages of DDS II is its capability toprovide a diagnostic channel along with the primary subrate channel. Thisdiagnostic channel is obtained through a modi®cation to the framing used by DDSand requires the use of special DSU/CSU that support the new channel.Through the use of DDS II, end-users can perform non-disruptive testing or usethe channel for network management purposes. The key to obtaining the ability toderive a secondary channel on DDS is the use of the network control or C bit.In conventional DDS the C bit, which is bit eight in each DDS 8-bit byte, istransmitted as a binary one whenever a DTE requests access to a channel byturning its request to send RTS) signal on. With the C bit continuously set to a onethe DTE can transmit an unrestricted stream of data to include continuous zerossince every eighth bit will be automatically set to a one. By robbing this bit onceevery third byte, AT&T established a virtual path for diagnostic use.The diagnostic channel data rate for 56 kbps DDS II is obtained by multiplyingthe full DS0 rate of 64 kbps by 1/8 which represents the C bit's portion of the DS0rate to obtain 8 kbps. Next, since the bit robbing occurs every third byte, theresulting data rate becomes 8000 2 3 , or 2666 2 3bps. Similarly, dividing 2666 2/3bps by the number of 19.2, 9.6, 4.8 or 2.4 kbps channels multiplexed onto a DS0channel results in the diagnostic data rate for DDS II at those data rates.AT&T added diagnostic channel capability throughout the DDS network.Although older DSU/CSU devices can support transmission on DDS II, thosedevices cannot support the use of the secondary channel capability provided by thismodi®cation to DDS. To do so requires the use of newer DSU/CSU devices thatsupport the multiplexing of diagnostic data onto every third C bit position.Analog extensions to DDSAT&T provides an 831A data auxiliary set which allows analog access to DDS forcustomers located outside the DDS servicing areas. The 831A connects the EIARS-232 interfaces between a data service unit 500A-type) and a voice-band dataset or modem. The 831A contains an 8-bit elastic store, control, timing, and testcircuits which allow loop-back tests toward the digital network. The elastic store isa data buffer that is required by the DSU to receive data from the modem in timewith the modem's receive clock. The data is then held in the elastic store until theDSU's transmit clock requests it. Thus, the buffer serves as a mechanism toovercome the timing differences between the clocks of the two devices. In thereverse direction, no buffer is required when the DSU's receive clock is used as themodem's external transmit clock. When the modem cannot be externally clocked orwhen one DSU is connected to a second DSU or a DTE that cannot accept an