TERMINATING THE MODEM ANALOG SIGNAL
As a general rule, voice and data service providers attempt to terminate the modem analog signal and convert it to a digital signal as soon as practicable. Figure 5-4 shows three scenarios in how the analog modem termination can be handled.[2] The goal is provide this termination as close to the subscriber as possible, which allows more efficient techniques to be used on the remainder of the link to the Internet or corporate node. The shadowed boxes in the figure that are numbered 1, 2, or 3 symbolize the demarcation point at which the modem signals (modem connection) are terminated. Thereafter, digital signaling is employed at the physical layer, with Frame Relay or ATM operating over the physical layer, and IP then operating over Frame Relay or ATM.
[2] Additional information on these operations is available from Telesis, Issue 102, published by Nortel, [email protected]
Figure 5-4. Possible modem terminations
In scenario 1, the modem termination occurs at the trunk side of the terminating central office (CO) that is servicing the ISP. A data access switch is installed as a trunk-side peripheral at this CO. This configuration bypasses the line side of the remote CO, but this configuration is not a conventional method to terminate the modem interfaces because it entails sending the analog signal through the telco backbone, which is usually digital-based. Nonetheless, if parts of the telephone network are still analog-oriented, this configuration is used.
In scenario 2, the modem termination occurs at the trunk side of the originating CO. The benefit of this approach is that it reduces or eliminates the need to upgrade the network's trunks to handle data traffic. The traffic is diverted to data networks before it hits the voice network. The data networks are designed to handle asynchronous data traffic. However, with this approach, the CO voice switch is still involved in the processing of the data call, which results in tying up a port and resources on the local circuit switch.
Scenario 3 is the best approach. It terminates the analog connection before it enters the local voice switch. This example uses a front-end processor to intercept the user signal (this processor is not shown here but is explained later in this chapter). If the called party is an ISP customer, the call is diverted away from the CO facilities and sent directly to the ISP.