Comparing Modem Passthrough and Cisco Modem Relay

Of all the transport options available for fax and modem communications, modem passthrough enjoys the most widespread support among the Cisco voice gateways. All the Cisco IOS voice gateways support modem passthrough and all the non-IOS voice gateways. From a Cisco voice gateway interoperability standpoint, modem passthrough is always safe to use. However, when it comes to third-party voice gateway integration, the proprietary NSE-based switchover of modem passthrough is not supported by other vendors' products.

Cisco modem relay, on the other hand, is more restrictive. Because of the proprietary nature of the Cisco modem relay protocol and its NSE-based switchover, it is not supported by third-party gateways either. In addition, certain Cisco voice gateways, such as the 6608/ 6624, VG248, ATA, and any NextPort-based DSP gateway (5350, 5400, and 5850), also do not support Cisco modem relay. On Cisco voice gateways that do support modem relay, DSPs must use high complexity or flex mode. Compared to a modem passthrough call, a modem relay call consumes more DSP resources, as discussed previously in the section "Resource Utilization."

From a protocol interoperability perspective, both modem passthrough and Cisco modem relay work no matter what voice signaling protocol is used. As long as the voice gateway supports either feature, the voice signaling protocol does not matter. This is the main benefit of an NSE-based switchover, and it allows for modem passthrough and Cisco modem relay to be an effective transport method whether the voice signaling protocol is H.323, SIP, MGCP, or SCCP.

When deciding on whether to use modem passthrough or Cisco modem relay in a design situation, a number of factors concerning each of these transport methods should be studied. Table 7-15 summarizes some of the key differences between modem passthrough and Cisco modem relay.

If the design criteria for transporting modem traffic over IP has already been determined, Table 7-15 should assist in ascertaining the best choice. Both modem passthrough and Cisco modem relay have their advantages and disadvantages, but every network design is somewhat different.

In cases where both modem passthrough and Cisco modem relay are valid options, Cisco modem relay is the recommended transport option. Cisco modem relay is specifically engineered to transport modem communications over IP, whereas modem passthrough adapts a voice codec in an effort to accurately sample modulated data. Therefore, Cisco modem relay is more efficient and robust in maintaining successful modem over IP transmissions.