After years of delay, carriers are installing an all-IP network infrastructure, but the move to IP is forcing them to make a huge decision regarding IMS, according to a new report from Heavy Reading -- "Network Transformation: Moving Class 5 Services to IP Networks."

    "After more than 100 years of limited disruption, the subscriber side of the PSTN is rapidly transforming, and by the end of this decade will bear little resemblance to the existing infrastructure," says Joe McGarvey, senior analyst with Heavy Reading and author of the report.

    "The toughest choice now facing carriers is whether to make a one- or two-step transition to IMS," McGarvey writes.

    By moving directly to an IP Multimedia Subsystem (IMS)-based session control architecture, operators can skip the intermediary step of installing a softswitch. However, operators may choose to make a two-step migration from TDM to a next-generation network softswitch environment, then finally to IMS.

    The two-step approach ensures that carriers will be able to offer the same features on an IP network that they did on the old TDM network, all the while giving the IMS standard more time to harden before moving it into their networks.

    Carriers must also decide what strategy they will pursue in transforming their network infrastructures, whether it be a replacement strategy, an overlay strategy, or a combination of the two.

    With a replacement strategy, carriers remove Class 5 TDM switches and replace them with IP-based call/session control. Meanwhile, using an overlay strategy, carriers could shift subscribers to an IP-based, multimedia-enriched environment with less capex than that demanded by a replacement strategy. However, an overlay strategy means that operators must continue to support legacy infrastructure.

    Either way, the transformation will be a boon for equipment providers, McGarvey writes: "The replacement of the Class 5, or end-office, portion of the old public switched telephone network involves thousands of switches and billions of dollars in capex."

    While the movement away from TDM-based softswitches means there is an opening for smaller, IP-based vendors to grab share in the market, McGarvey sees the transformation benefiting large equipment providers the most. "With the carrier community all over the board in terms of a migration path toward IP-based Class 5 services, equipment vendors that support a wide array of upgrade options will have the advantage," he writes.