Advanced Voip Gateway Technology

By editor@aims.co.il
Increased demand for video-based applications in the mobile network drive the deployment of 3G networks. Inherent complexities in traditional video gateways cause performance bottlenecks and inflate costs. If the industry expects to absorb the impending explosion in demand these proposed cost-saving and complexity-reducing changes are needed.

Unrelenting market demand for video-based applications over mobile networks is driving the increased deployment of 3G networks.
The 3G-324M protocol is used by 3GPP and 3GPP2 standards organizations to combine voice video data and control into a single 64kbps stream of circuit-switched data. This is a departure from the standard Real-Time Transport Protocol/Real-Time Transport Control Protocol (RTP/RTCP) used to transport real-time video and voice over IP networks.
The industry’s use of traditional video gateways to bridge video in the mobile network (3G-324M) and in the IP network (video/voice over RTP) has multiple deficiencies impeding migration to 3G networks including:
• Excessive system complexity
• Excessive costs per port
• Separate voice and video gateways systems adds to costs and complexity
• Degraded customer experience
State of Traditional 3G-324M–to–IP Video Gateways
Current Mobile-to-IP video gateways convert video and audio signals from their state in the mobile network into the state required for transmission over the IP network. Once in the IP network video content is carried by three separate UDP/IP streams:
1.A compressed video stream encapsulated in an RTP stream and inserted into a UDP/IP packet.
2.A compressed audio stream encapsulated in an RTP stream and inserted into a UDP/IP packet.
3.Signaling and control information transported using SIP or H.323.
In the mobile network 3G-324M–based video content is transported using a single H.223 64kbps stream that multiplexes audio video data and control information. The video portion of the H.223 protocol is usually MPEG-4 and the audio portion uses NB-AMR compression. The channel parameter exchange and session control portion uses the H.245 protocol.
The complexities involved in delivering these services result in significant performance derogation and increased operational costs.
For the industry to absorb the impending explosion in demand for video-based applications over mobile networks radical cost-saving and complexity-reducing changes are needed.
Those changes require a gateway capable of handling the circuit-switch–to–packet-switch layer while offloading other tasks to a separate media server or to the IP videophone itself.

The Solution
The key to improving VoIP gateway performance and lowering costs is a high-performance video gateway with the following properties:

1.It is not used for IP video protocol termination (3G-324M).
2.It is not used for IP video protocol termination (video/audio over RTP with SIP/H.323 for control).
3.It hands voice and video transcoding over to a media server.
Items 1 and 2 will result in a reduction in operational complexity while item 3 will significantly reduce per-port costs and increase video quality and service flexibility.
The following methods (3G-324M–over–IP) provide the ability to implement a streamlined and cost-saving solution.
Method 1 - IP conversion done by IP videophone
1.Mobile and IP end points implement video sessions using the 3G-324M protocol which is terminated end-to-end. On the mobile side video is transmitted as a standard 64kbps stream. On the IP side the 64kbps stream is encapsulated in UDP/IP packets. The end point on the IP side is implemented in the IP videophone.
2.Encapsulation of 3G-324M into IP packets is similar to encapsulation of voiceband data into IP packets. It could involve breaking up the 3G-324M stream into frames of a given size and then adding RTP heads with optional redundancy and forward error correction mechanisms to increase transport reliability.
3.RTP encryption should be used.
4.In messaging and video surveillance applications where end-to-end delay is not a concern transportation via reliable UDP packets such as the SPRT protocol used in ITU-V.150 may be substituted.

Method 2 - IP conversion is done by the media server
1.The mobile end point implements video sessions using the 3G-324M protocol and the IP video end point implements video sessions using video/voice over RTP. SIP/H.323 is used for signaling and control. The media server in the IP network converts IP video into a H.223 64kbps stream that is encapsulated over RTP/UDP.
Steps 2 3 and 4 are the same as method 1.

Implementation
This powerful solution is simply a regular VoIP gateway operating in VBD (Voice Band Data) mode with these requirements and features:
a.Media type set to transparent pass-through of 64kbps data from the IP/UDP/RTP side to the E1/T1 side and vice versa.
The VoIP gateways enter this mode of operation under any of these scenarios:
• The gateway was configured to automatically operate in this mode.
• The gateway receives PSTN signaling information in the user-to-user information field of the ISDN-PRI signaling.
• The gateway receives IP signaling such as MGCP within the media gateway controller.
• The gateway receives modem or fax tones. The 3G-324M stack of the mobile phone must transmit a modem tone in order to force the gateway into pass-through mode as if it were handling a modem call.
b.The gateway’s jigger buffer must adapt slowly or be configured to a fixed depth of 80 ms in order to avoid unwanted frame loss.

c.The RTP payload type carrying the 3G-324M stream can be the same as regular G.711 voice.

d.The same redundancy and FEC mechanisms can be used for modem transmissions over G.711 (VBD mode).
e.The IP payload frame size should be set to the smallest possible value permitted by all components on the IP side of the system. 10ms is recommended.
f.The E1/T1 interface must be a link without attenuation pads and robbed-bit signaling to ensure that a transparent 64kbs stream flows from the mobile phone to the IP network.

Summary Benefits for This Approach
1.Low gateway costs.
2.Easy upgrade path for voice gateway manufacturers supporting video when bridging between IP and mobile networks.
3.Media server manufacturers can support the 3G-324M protocol without physically supporting T1/E1 circuit-switched interfaces.
4.Decoupling the IP-to-TDM conversion process from the video processing makes IP-to-TDM conversion as simple as implementing a G.711 VOIP gateway and offloading the heavy video processing to TDM-based networks.
5.Reduced end-to-end delay provides better conversational video experience for IP videophones supporting 3G-324M-over-IP.
6.Shorter call setup time.

Avi Fisher is CTO of Surf Communications. He is an expert on Triple Play DSPs and was the first contributor to the ITU V.150 standard. A longer version of this article on VoIP Gateways with illustrations as well additional resources is available at www.surf-com.com

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