The problems of ADSL and voice
The asynchronous nature of ADSL means that there is much lower bandwidth upstream when compared to downstream which the number of simultaneous calls that can be maintained.
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Voice information can be compressed to use only 8kbit/s per call, however, due to the way in which it is packaged the reality is that each call will use at least 42kbit/s – significantly higher.
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Standard QoS techniques do not work very well for voice carried over links with less than about 1000Kbit/s, and standard QoS techniques can not allocate more than 70% of the available link bandwidth reliably; on a standard broadband link three calls is the maximum that can sensibly be achieved - even the quality of those calls will suffer where data is present.
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On a standard BT Wholesale broadband link, no QoS can be applied in the download direction.
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Broadband is usually part of a public or at least shared network, so security is a concern.
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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...
How ViBE Technology helps:
ViBE (Voice over IP Broadband Enhancement) solves the problems associated with running VoIP over ADSL links in several ways:
  • Quality of Service is implemented in a completely different way to traditional methods, being much more suited to slower links.
  • Voice is no longer treated as simply another type of data which happens to have a high priority. Instead, it is treated as a data stream with very specific requirements not only in terms of priority, but also in terms of spacing between packets.
  • The bandwidth budget required is reduced to that of the compression format used, so that a G.729 call really does only use 8Kbit/s over the ADSL link rather than the 42Kbit/s which is more usual. Rather than a maximum of three calls, twenty-eight calls are now practical on a single broadband line.
  • Voice and data can readily coexist on a single link. Data transfer rates are not compromised by the fact that voice is present. Classes of data can be given their own share of available bandwidth in a much more granular way than traditional QoS.
  • Sites that have Vibe links can be joined together to form private networks, which means that security is easier to control.
  • Backup links can be implemented which can be activated without losing calls in progress.
  • Much more of the capacity of the link can be used.
  • Future development of the technology will allow multiple links to be used as a single large bandwidth connection, offering a viable alternative to costly private circuits.
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IP PBX vendors can provide guaranteed quality calls over existing ADSL lines
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ISP's can increase VoIP throughput by over 500% on the same bandwidth
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