Data Communications Networking Devices - 4th Ed.pdf

4.2 MODEMS _____________________________________________________________________ 379Figure 4.23 Multiport modem. Containing a limited function time division multiplexer, amultiport modem combines the input of a few synchronous input data streams fortransmissionmore than one synchronous data stream over a single transmission line, as shown inFigure 4.23.In contrast with typical multiplexers, the limited function multiplexer used in amultiport modem combines only a few high-speed synchronous data streams,whereas multiplexers can normally concentrate a mixture of asynchronous andsynchronous, high-and low-speed data streams. A further description, as well asapplication examples, will be found in Section 4.4.Security capabilityTo provide an additional level of network protection for calls originated over thePSTN several vendors market `security' modems. Some of these modems contain abuffer area into which a network administrator enters authorized passwords andassociated telephone numbers. When a potential network user dials the telephonenumber assigned to the security modem that device prompts the person to enter hispassword. If a valid match between the entered and previously stored passwordoccurs the security modem disconnects the line and then dials the telephonenumber associated with the password. A modem with a security capability thusprovides a mechanism to verify the originator of calls over the PSTN by histelephone number. A second type of security modem veri®es the location of the calloriginator. To do so this type of modem is ®rst con®gured by associating callbacktelephone numbers to passwords. Once a caller establishes a connection with thistype of security modem and enters his or her password, the distant modem breaksthe connection and initiates a callback using the telephone number associated withthe password. Based upon this type of operation, this feature is referred to ascallback security. A further description of security modems is contained in Section4.6.Multiple speed selection capabilityFor data communication systems which require the full-time service of dedicatedlines but need to access the switched network if the dedicated line should fail ordegrade to the point where it cannot be used, dial backup capability for the modemsused is necessary. Since transmission over dedicated lines usually occurs at a higher