VOIP Bandwidth consumption naturally depends on the codec used.

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.

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.

IAX2 trunking helps with the IP overhead, but only when you are sending more than 2 or so calls between the same Asterisk servers. John Todd has done some useful practical testing, named IAX2 trunking: codec bandwidth comparison notes and results.

The bandwidth used depends also on the datalink (layer2) protocols. Several things influence the bandwidth used, payload size, ATM cell headers, VPN headers, use of header compression and IAX2 Trunked. You can see the influence of some of this factors using the Asteriskguide bandwidth calculator.

Teracall has the table which shows how the codec's theoretical bandwidth usage expands with UDP/IP headers:

 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)

• Asterisk guide bandwidth calculator, it calculates 32xCodecs, 13xL2-Protocols, IAX Trunked, ATM, cRTP, VPN
• Asteriskguru's bandwidth calculator
• Bidirectional Throughput VoIP Test - Utility to test if your network is ready for VoIP Service - Throughput, Capacity, Latency, and SIP & MGCP Port Tests
• Cisco.com whitepaper: Per call bandwidth consumption This Cisco whitepaper explains how you can calculate bandwidth with VOIP.
• Erlang.com online bandwidth calculators: http://www.erlang.com/calculator/
• IAX2 trunking: codec bandwidth comparison notes and results
• IPhone App: VoIP Bandwidth Calc and app store link
• Android App: VoIP Bandwidth Calc
• Newport Networks Online VoIP Bandwidth Calculator and White Paper
• Packetizer.com: Bandwidth calculators
• Speex.org: Codec feature comparisons
• VoIP traffic bandwidth calculator This bandwidth tool support many codec (g711, g729, g723_6.3Kbps, g723_5.3Kbps, g726_32Kbps, g726_24Kbps, g728_16Kbps, g722, ilbc 15.2Kbps, ilbc 13.33Kbps), many signaling protocol (sip, mgcp, h323, iax), transport protocol (RTP, cRTP), layer 2 protocol (Ethernet or non ethernet)

Mobile App

• VoIP bandwidth calculator designed by network engineer Supports many signaling protocol (sip, mgcp, h323, iax), L2, L3, tunnelings, codecs. Universal app so works on iphone/ipod/ipad.

See also

• Bandwidth Meter
• Codecs
• RTP
• TestYourVoIP.com