[Sussex] VoDSL - Waiting in the wings

Iain Stevenson iain at iainstevenson.com
Wed Sep 17 09:14:01 UTC 2003


--On Tuesday, September 16, 2003 11:33 pm +0100 Jon Fautley 
<jon at geekpeople.net> wrote:

> I wonder if VoDSL is just a branding name, or if it is actually different
> to VoIP. I'm guessing not. Will we start seeing terms such as VoLL (Voice
> Over Leased Line) or Vo56k, VoS (Voice Over Sattelite), VoWiFi etc...?
>

VoDSL is different to VoIP.  It was most popular with the US Competitive 
Local Exchange Carriers (CLECs) ... before most of them went out of 
business.  VoDSL partitions a voice channel at the ATM layer or even at the 
physical layer - hence the capacity available for data is reduced but 
capacity is guaranteed for voice.  VoDSL was primarily targeted at SOHO 
applications.  Now that most of the proponents of VoDSL have gone out of 
business and VoIP solutions are stronger almost all carriers interested in 
integrated voice and data solutions are looking at VoIP over DSL.


>> Three sets of equipment are required to implement VoDSL:
>>
>> * An integrated access device (IAD) at the customer premises
>> * A DSLAM (DSL access multiplexer) at the carrier's local exchange
>> * A voice gateway, which transfers the voice traffic from the DSL
>> network to the public switched telephone network (PSTN).
>
> OK. The IAD is a router with a phone stuck in the back. Not a difficult
> piece of technology.
> DSLAM - You have one of these if you have xDSL.
> Voice Gateway... ah ha! That's interesting. A voice gateway is a box that
> effectively has voice in one one side and a connection to the PSTN on the
> other side. A bit like that one sitting in the corner of my room running
> Linux. (http://www.asterisk.org/)

The voice technology (ie media gateways and softswitches) needed for 
carrier class VoIP are well established.  The real issue is getting IP QoS 
in the local loop.  Today's ADSL services weren't designed to do this and 
need some re-engineering.

>> This enables computers, telephones and fax machines all to be connected
>> to the single line. 'The IAD takes the voice and data through the DSL
>> line to the DSLAM,' says Mahendra Soneji, director of Product Marketing
>> at Polycom, a US IAD manufacturer.
>> 'The DSLAM can have thousands of lines coming in from various customers,'
>> says Soneji. 'The device bifurcates (splits) the information and sends
>> the voice traffic - which can be compressed - to a voice gateway
>> connected to a class 5 switch and over to the PSTN, and sends the data
>> to the Internet via an ISP.'
>
> Right. That sounds like a clever way of saying 'It routes IP traffic'..
> considering if it didn't, it'd be pretty useless.
>

Not necessarily.  If separate ATM virtual connections are used the 
"bifurcation" is done at Layer 2 ie below IP.  Similarly for solutions 
using physical layer partitioning.


>
> hmm, "unlikely that large corporates would ever see the benefit"... does
> this mean that the company that wrote this article doesn't have the
> infrastructure to cope with lots of calls at once? Interesting...
>

... also seems to ignore the fact that more and more large companies are 
looking at DSL for branch networks right now.


  Iain







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