Data Communications Networking Devices - 4th Ed.pdf

4.3 INTELLIGENT MODEMS ________________________________________________________ 463upon establishment of a carrier signal it will initiate handshaking by transmitting anegotiation frame. If the distant modem either lacks a negotiation capability or hasthat feature disabled, the negotiation frame passes through the modem as it isassumed to represent demodulated data. Unfortunately, the datacryptor alsoassumes it is demodulated data and initiates its decryption operation. Not only doesthis result in false decrypted data being passed to the mainframe but, in addition, itresults in a loss of synchronization between the pair of datacryptors. Thus, whenthe notebook user attempts to transmit a request to the mainframe, data is encodedbased upon one position in a pseudo-random key sequence generated by thedatacryptor built into the notebook, and decoded at the mainframe by thedatacryptor using a different position in a pseudo-random key sequence since thenegotiation frame preceded the data. Needless to say, the mainframe receives whatappears to be `gobbledegook'.Another common example of modem negotiation havoc can occur in a data PBXenvironment. When one modem transmits negotiation frames and the other eitherhas its negotiation capability disabled or lacks a negotiation capability, the framesare passed through the modem as data. In this network con®guration the datacryptorperforms its intended operation, resulting in false decrypted data being sentto the mainframe which results in a loss of synchronization between datacryptors.If a data PBX is behind a modem, the negotiation frame could be misinterpreted asa routing code, and could cause a misrouting before a user could enter a correctrouting code. To correct such situations, you can enable error detection andcorrection and data compression features on modems at the other end of the communicationslink, if those features were previously disabled, or replace modemsthat don't support those features with modems that do. A third option is to disablethose features on the modem transmitting the negotiation frame. However, doingso eliminates the possibility of using those features.Simultaneous voice and data operationsThe growth in `telecommuting' has resulted in many people working from theirhomes, accessing LAN servers and corporate mainframes to perform work thatmight otherwise require a long commute into the of®ce. Accompanying the growthin telecommuting is an increase in the requirement to communicate verbally withother telecommuters or with staff at the of®ce while maintaining a data transmissionsession. Although the Integrated Services Digital Network ISDN)provides the ability to maintain two separate conversations on one pair of telephonewires, enabling a voice and a data version to coexist simultaneously, the technologyis relatively expensive in comparison to the cost of using the analog PSTN and is farfrom ubiquitous. Recognizing those limitations, as well as the requirements oftelecommuters to converse simultaneously with a distant party while transmittingdata, resulted in the development of simultaneous voice and data SVD) capabilityinto certain modem products.Currently two methods are used to provide a simultaneous voice and datacapability. One method, referred to as digital simultaneous voice and data DSVD),uses speech compression technology to digitize voice into a low bit rate which