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Teracom Tutorial: VoIP over Cable or DSL:
Telephone Service over Cable Modem

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Teracom Tutorial: VoIP over Cable or DSL:
Telephone Service over Cable Modem

Note: This is an archived article that appeared in the Teracom newsletter January 2004, and this article has not been updated to reflect technology developments since then. But reading it, we think you will agree that in hindsight, this analysis written in 2004 is highly accurate... and gives you a good idea of the quality and durability of Teracom's training courses.

Please be assured that our training courses have been updated since the time of this article!

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1.07  Voice over DSL/Cable
Local and long-distance voice
User: VoIP over Cable or DSL
ISP: Access network in metro area
IXC: Internet or managed IP network
Gateway to standard telephony at far end
Requires adapter at customer premise
Packetizes voice
Negotiates with call management servers
Bypass the LEC at origination
Bypass the regulators: “Internet” telephony
More efficient use of LD trunks
Voice over IP over cable modem or DSL broadband internet access has started to become mainstream. This is essentially an initiative by the cable companies to move into the telephony business – which forces the incumbent telephone companies to follow suit lest they lose a large chunk of their business.
This service supports Plain Ordinary Telephone Service (POTS) at both ends. At the originating end, the user must have an adapter into which they plug their regular phone and their computer. This adapter in turn connects to the cable or DSL modem, which links the user to the Internet Service Provider's access network in the metropolitan area. The ISP's access network is connected to the Local Exchange Carrier (LEC) to terminate local phone calls, and to an IP network for long distance.
In theory, the “long-distance IP network” is the Internet. If you are getting phone service from your cable company or from your phone company, this is unlikely, due to the total lack of Quality of Service (QoS) guarantees on the Internet and resulting total lack of guaranteed voice quality. It is much more likely that the calls will be carried over a managed IP network, which uses the same protocols as the Internet, but where the traffic, loading and quality are carefully controlled. The main tool for guaranteeing service levels is called MPLS.
If you are getting phone service from a third party, such as Vonage or Eric's Fly-By-Night Internet Phone Service, then the voice packets may well be delivered along with Internet traffic, where the service level is not guaranteed. It will probably be good. Maybe not.
It is even possible that initially, the VoIP calls will be converted at the earliest opportunity to regular telephony and carried long-distance like any other telephone call. This would be the simplest and cheapest way to guarantee voice quality.
The IP network will have to provide a number of services, including Authorization, Accounting and Administration (AAA), call management features and gateways to convert the IP telephony to standard telephony for the far end.
This type of residential telephony is cheaper than POTS for a number of reasons:
First, the switched access charges added by the LEC to any long-distance phone call are avoided.
Second, this service is being portrayed by the carriers as “Internet” telephony, avoiding regulation and fees imposed on POTS. Third, if variable-rate codecs are used, i.e. no packets are transmitted if no noise is coming out of the speaker's face, the long-distance trunk circuits are used more efficiently, thus costing less to the carrier.
This last point is the least important, as the wholesale cost of long-distance capacity is very close to zero due to recent technology-based exponential increases in capacity.
There are several hidden drawbacks to this scenario… discussed in Chapter 2 of Course 120.
Source: Teracom Course 130, Voice over IP, SIP, Security, 5G and the Internet of Everything , slide 1.07
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