ViBE - VoIP BandWidth Enhancement
On the face of it, broadband should be a medium which is ideally suited for linking multiple sites for the purposes of voice and data, as well as VoIP links to external telephony providers. However, there are some very real problems associated with using voice and data over an ADSL network. • The asynchronous nature of ADSL means that there is much lower bandwidth upstream when compared to down. Since voice calls are by their very nature symmetrical in terms of bandwidth requirements, the number of simultaneous conversations that can be maintained is limited by the much slower upload speed ( typically 256Kbit/s. ) • Voice information can be compressed ( using similar techniques to MP3 music compression ) to use only 8kbit/s per call. However, due to the way in which it is packaged to be sent over a broadband network, the reality is that each call will use at least 42kbit/s – significantly higher. • In order to allow voice and data to share the same network, QoS has to be employed. Standard QoS techniques do not work very well for voice carried over links with less than about 1000Kbit/s, and are not ideal even well above that. The effect of this is that voice information gets delayed because it has to wait for a free slot on the link in order to be sent – since peoples voices do not generally incorporate such delays this causes issues with voice quality. In addition, standard QoS techniques can not allocate more than 70% of the available link bandwidth reliably. This means that on a standard broadband link, three calls is the absolute maximum that can sensibly be achieved, and the quality of those calls will still suffer where data is present. • On a standard BT Wholesale broadband link, no QoS can be applied in the download direction. • Broadband is usually part of a public or at least shared network, making security a concern.