Friday, 27 November 2009

How much bandwidth does a SIP call use?

A one-hour call contains 70 to 80 MB of data, with data transmissions typically approaching 100 Kb per second and per direction (upstream and downstream) and therefore 200 Kb per second total. It is important to consider the upstream and downstream usage seperately as an Internet connection will usually have very different capabilities for one direction of traffic.
This means a standard 'ADSL Max home' connection is at best capable of 4 simultaneous SIP calls under perfect conditions. With an ADSL connection the limitation will always be the upload speed (400Kbps in this case). However, 'perfect conditions' cannot be relied upon, and in most situations we would only recommend a maximum of 2 simultaneous calls over ADSL Max home. ADSL Max business (700Kbps upload) could theoretically support upto 7 simultaneous calls under perfect conditions, but in reality is only likely to be able to cope with 3-4 calls. Additionally the contention ratio of your connection will dictate how much of this bandwidth is shared with other users. This means, especially at peak times, it is not possible to guarantee an ADSL connections bandwidth.

ConnectionUpload speedDownload speedMaximum callsNotes
ADSL Max Home448 kbps max8 Mbps4. Realistically 2Contention ratio 50:1
ADSL Max Business832 kbps max8 Mbps8. Realistically 3-4Contention ratio 20:1
ADSL2+1.3 Mbps24 Mbps13. Realistically 5-6Contention ratio varies
SDSL2 Mbps2 Mbps10. Realistically 8Contention ratio 10:1