AltiGen comm ACM 5.1 manual Understanding VoIP Bandwidth Requirements

Chapter 25: Enterprise VoIP Network Management

Understanding VoIP Bandwidth Requirements

Before starting VoIP related configurations, it is helpful to have some understanding of VoIP bandwidth requirements, so that you can plan your VoIP deployment properly. Also see Chapter 24, “Network Configuration Guidelines for VoIP” on page 315.

The data network bandwidth required to carry VoIP depends on the following factors:

•Codec and Compression—This is the encoding of analog voice to digital form, decoding of digital form to analog wave form, and compression of digital form to a smaller size. AltiWare supports three type of codec: G.711 (64Kbps), G.729AB (8Kbps), G.723.1 (6.4Kbps)

•Packet Length (Frame Size)—The size of the voice frame data (payload) transmitted in a packet. For G.711 and G.729, you have choice of 10, 20, and 30ms lengths. For G.723.1, the packet length is a fixed 30ms. A larger packet length decreases the transmission overhead. However, it will increase the latency and have a negative effect on the voice quality if a packet is lost during transmission. For G.711 and G.729, 20ms is efficient and recommended.

•IP Header—The IP/UDP/RTP header adds 40 octets per packet. With a packet length of 20ms, the IP headers will require 16kbps of bandwidth in addition to whatever codec is being used.

•Transmission Medium—In order to travel through the IP network, the IP packet is wrapped in another layer by the physical transmission medium. The transmission medium, such as Ethernet, will add its own header, checksums, and spacers to the packet. With a packet length of 20ms, the transmission medium requires additional 15.2kbps of bandwidth to carry the packets to their destination.

•Silence Suppression—You can suppress the transmission of data during periods of silence. This can reduce the demand for bandwidth by as much as 50 percent. However, it may have a negative impact on the voice quality. Some users may feel the conversation is not "natural" when artificial comfort noise is generated during periods of silence.

The following table lists bandwidth requirements for various transmission media with different codecs and frame sizes. It assumes silence suppression is not turned on. (The same table appears on page 315.)

VoIP Bandwidth requirement for WAN connection varies depending on the type of WAN. Bandwidth requirement typically is less than Ethernet requirement.

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