Algorithm cuts VoIP bandwidth requirement

Looking to reduce edge-access bottlenecks, Effnet Inc. has developed a licensable version of the Compressed Real-Time Protocol (CRTP) algorithm to support the real-time delivery of VoIP. The CRTP algorithm can compress a typical IPv4 header from 40 bytes to as little as 2 bytes, allowing a T1 line to increase its call capacity over 250 percent, the company said.

Header compression complements the voice payload compression typically handled by the G.7xx vocoder algorithms. "However, header compression becomes increasingly important as the size of the payload shrinks," said Rich Stamm, marketing director at Effnet. "CRTP compression will lower the bandwidth requirement by about 60 percent."

CRTP was developed in 1991 by Cisco Systems Inc. employees Steve Casner and Van Jacobsen for real-time applications. Defined by Internet Engineering Task Force RFC 2508, CRTP is used on a link-by-link basis to compress Internet Protocol, User Datagram Protocol (UDP) and Real-Time Protocol headers. "While it can be used to reduce the bandwidth, it can also be used to allow bigger voice samples for a given bandwidth, thereby improving voice quality," said Stamm.

Cisco and to a lesser extent Motorola have been the only companies supporting CRTP to date, Stamm said. "So if edge-access equipment wants to support compressed voice for VoIP, they have to purchase a Cisco router and can then cut the VoIP bandwidth in half," he said. "With our licensable CRTP implementation, anyone who's building access equipment can quickly build in CRTP and immediately be able to talk to the Cisco routers."

According to Effnet's calculations, "CRTP can increase the number of voice channels for a given T1 [1.5Mb] line from 90 to 234," said Stamm.

Available this quarter, the CRTP algorithm is priced at $25,000 for an evaluation/development license, and on a per-unit basis from there, up to $500,000.

Patrick Mannion

EE Times