Voice over Cable

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Voice over Cable ( VoC ) or cable telephony refers to a special variant of IP telephony (VoIP) via cable television networks or broadband cable networks .

Implementation of voice services in the VoC architecture

Fixed components of the DOCSIS architecture are a cable modem on the customer side and a cable modem termination system (CMTS) in the provider's cable headend. The modem digitizes and encodes the voice information and forwards it to the CMTS. For historical reasons, the H.323 protocol is usually used, which requires a gatekeeper (the cable modem). Telephony can now also be implemented using the SIP protocol. In both cases, the access data in the modem is configured by the provider via remote maintenance. Telephony is technically limited to the provider's network via geotargeting or to the approved hardware via checking the MAC address , so "nomadic use" is excluded.

Since DOCSIS only specifies the lowest two layers in the OSI model , the actual addressing and transmission of the voice data on layer 3 ultimately takes place with regard to the route as in IP telephony, i.e. H. the voice information is not transmitted via a fixed connection (as in the conventional circuit-switched telephone network ), but in a packet-oriented manner via the cable network .

Telephone services

If data compression using data reduction is used, ISDN protocols that rely on transparent 1-to-1 transmission - such as fax and data services - cannot work. Runtime delays ( IP telephony # runtime (latency) and jitter ) can also cause disruptions in Internet-based telephony services (including Skype). A VoIP connection with "ISDN features" is therefore not a real ISDN connection according to the DSS1 protocol and is now mostly only advertised as a "convenience connection" or the like with the cable network operators .

The fully provider -controlled end-to-end quality of service should enable the reliability of the fixed network (often referred to as "first-line quality"), but with the exclusion of an emergency power supply in the event of a power failure , as is still guaranteed on the network side with analog telephony or ISDN. The emergency call so it works due to the system only in normal operation. Fax transmissions require the use of the T.38 protocol ( Fax over IP ) or, within the framework of guaranteed quality of service, the use of uncompressed audio codecs such as G.711 .

Web links

Individual evidence

  1. ↑ Technical article " Voice over Cable: A safe thing " ( Memento from May 18, 2013 in the Internet Archive ) (PDF; 46 kB) Kabel Deutschland, accessed on September 2, 2011
  2. ↑ Article " H.323 and SIP in comparison " Electronics Compendium, accessed on September 1, 2015
  3. ↑ IP Telephony Tips for Cable Internet Customers.