Question and Answer

Bob McCollum, President, Comtech EF Data

November 10, 2003
How can Internet-enabled satellite modems reduce a company’s required VoIP bandwidth?

With the advent of Voice over IP (VoIP) and IP videoconferencing, there has been a major push toward convergence in terrestrial networks. With overhead challenges associated with IP encapsulation of voice and video, plus the inability to manage Quality of Service (QoS), the satellite industry’s move toward convergence has been slow.

A breakthrough in convergence for satellite users materialized with the introduction of IP-enabled satellite modems. With bandwidth optimization capabilities and QoS metrics, satellite users can now migrate to native IP networks, capable of carrying all of their voice, video and data traffic. Advanced codecs, low overhead link layer framing plus UDP/IP/RTP and TCP/IP header compression also achieves efficient bandwidth and spectral utilization.

Typically, G.729 coded voice when carried in IP, requires:

* 20 byte IP header * 8 byte UDP header * 12 byte RTP header * 20 byte payload

Of the 60 bytes, 40 are just overhead. With innovative IP-enabled satellite modem approaches to UDP/IP/RTP header compression, this 40-byte header overhead is reduced to 1 to 2 bytes. For comparison, cRTP overhead is between 4 to 5 bytes. When combined with low overhead link layer framing of CDM-IP, a G.729 coded voice stream can be carried in 10.8 kbs. Even Voice over Frame Relay requires 11 kbs. Header compression technologies are particularly beneficial when applied to small packets where the ratio of payload to header bytes is very high. With header compression capabilities, the bandwidth for VoIP is reduced by up to 60 percent. Additionally, Web/HTTP is reduced by an additional 10 percent via TCP/IP header compression.

Using QoS in IP-enabled satellite modems minimizes jitter and latency for real-time applications, such as VoIP. QoS enables bandwidth grooming and optimization. Once traffic traversing the satellite link is identified, it is categorized and applied with QoS algorithms to ensure the highest priority traffic gets through and the lowest priority traffic is dropped if satellite capacity is unavailable. VoIP traffic should be assigned the highest priority, so the associated packets are always forwarded first. Finally, deploying IP-enabled satellite modems in networks with VoIP traffic increases efficiency. For more information go to www.comtechefdata.com