Data Communications Networking Devices - 4th Ed.pdf

466 _________________________________ WIDE AREA NETWORK TRANSMISSION EQUIPMENTcombines dial control and data over a common RS-232 interface. Concerning thelatter, there are currently several methods competing to become de facto industrystandards.Hayes Microcomputer Products' proposed standard is the Hayes SynchronousInterface HSI). This interface consists of a hardware independent connectionbetween a user's application program and a synchronous communications driverwhich directly controls the hardware. Under the HSI method, dialing commandsto a synchronous modem are issued asynchronously, with the modem directed tochange into a synchronous mode of operation once a conncetion with a distantdevice is made. Obviously, this method is only applicable for use with modems thatcan work both synchronously and asynchronously; however, it does permit theubiquitous Hayes AT command set to be used.A second method being used to perform automatic dialing operations withsynchronous modems is based upon the use of the Racal-Vadic Synchronous Auto-Dial Language SADL). Under SADL special modem controlling messages inboth bisynchronous and SDLC protocols are prede®ned. This enables softwaredevelopers to modify application programs to support the use of SADL compatiblemodems for dialing on the PSTN.The third method being used for synchronous dialing is similar to the previouslydescribed SADL technique. This method is called `SyncUp', which is sponsoredby Universal Data Systems and is restricted to sending dialing commands underthe bisynchronous protocol.4.4 MULTIPORT MODEMSOperationThe integration of modems and limited-function multiplexers into a device knownas a multiport modem offers signi®cant bene®ts to data communications users whorequire the multiplexing of only a few channels of data. Users who desired tomultiplex a few high-speed data channels prior to the introduction of multiportmodems were required to obtain both multiplexers and modems as individual unitswhich were then connected to each other to provide the multiplexer and datatransmission requirements of the user. Since multiplexers are normally designed tosupport both asynchronous and synchronous data channels, the cost of the extracircuitry and the additional equipment capacity was an excess burden for manyuser applications.The recognition by users and vendors that a more cost-effective, less wastefulmethod of multiplexing and transmitting a small number of synchronous channelsfor particular applications led to the development of multiport modems. By thecombination of the functions of a time division multiplexer TDM) with thefunctions of a synchronous modem, substantial economies over the past datatransmission methods can be achieved for certain applications.A multiport modem is basically a high-speed synchronous modem with a built-inTDM that uses the modem's clock for data synchronization, rather than requiring