computebandwidth.txt

语音编码的带宽计算,对于VoIP中的各种语音编码所占用的带宽做了详细的说明

VOIP Bandwidth consumption naturally depends on the codec used.  
VOIP消耗的带宽一般取决于所使用的语音编码.

When calculating bandwidth, one can't assume that every channel is used all the time. Normal conversation includes a lot of silence, which often means no packets are sent at all.So even if one voice call sets up two 64 Kbit RTP streams over UDP over IP over Ethernet (which adds overhead), the full bandwidth is not used at all times.  
计算带宽时,不能假设每一个通道都处于使用状态.正常的通话过程包括一系列的静音,也就意味着并不是一直都有包在传送.所以一个语音呼叫建立两个经过UDP,IP和以太网的64Kbit的RTP流(总开销),全部带宽并末一直被使用.

A codec that sends a 64kb stream results in a much larger IP network stream. The main cause of the extra bandwidth usage is IP and UDP headers. VoIP sends small packets and so, many times, the headers are actually much larger than the data part of the packet.  
一个传送64kb流的语音编码很大程度上都是IP网络流的结果.额外的带宽使用主要是IP或UDP头的增加.VOIP只传送少量的包,很多时候,实际上是包头远远大于包数据.
Codec      BR      NEB
G.711    64 Kbps 87.2 Kbps
G.729     8 Kbps 31.2 Kbps
G.723.1 6.4 Kbps 21.9 Kbps
G.723.1 5.3 Kbps 20.8 Kbps
G.726    32 Kbps 55.2 Kbps
G.726    24 Kbps 47.2 Kbps
G.728    16 Kbps 31.5 Kbps
iLBC     15 Kbps 27.7 Kbps  
BR = Bit rate
NEB = Nominal Ethernet Bandwidth (one direction) 
根据我的使用经验，8K的G.729加上IP封装后达到32K,为了防封杀，还有的用户使用IP Sec设备将语音做成VPN，这样G.729加上IP封装，再加上VPN会达到60多K。