IP telephony

Iljitsch van Beijnum iljitsch at muada.com
Wed Sep 17 10:28:37 UTC 2003


On dinsdag, sep 16, 2003, at 23:38 Europe/Amsterdam, Christopher Bird 
wrote:

> There has been much buzz of late about using IP telephony solutions in
> place of the more common analog based solutions.

> Traditional telephony has very high reliability (many years without 
> loss
> of dial tone for some companies). From what I have seen in this group
> about networking, router behaviors, etc. it seems to me that the IP
> networks that exist aren't yet ready for the prime time of IP 
> telephony.
> As we move buildings, my company is looking at installing an IP
> telephony based solution (packet switched) instead of a traditional
> analog based solution (circuit switched). I am worried that the
> reliability will likely be lower than I am used to.

First of all, I don't use IP telephony myself (yet) so the usual 
disclaimers and more apply.

The telephony world may claim very high reliability, but they don't 
deliver when you need it the most. During 9/11 the phone network had 
huge problems. Not so much because lines and equipment were destroyed, 
but simply because too many people picked up the phone at the same 
time. The same thing happened at a smaller scale during last months 
blackouts.

The individual parts in an IP network are probably less reliable than 
the individual parts in a POTS network, so you're likely to see more 
outages. On the other hand, IP networks use a distributed model, while 
POTS networks use a centralized model. So when something fails in an IP 
network the consequences are usually not too serious (given good 
network design of course) while with POTS you're usually in trouble.

Note that you need to make the distinction between IP-only calls and 
calls that originate or terminate in the public POTS network. For the 
former, you should be able to reach reliability levels that are no 
worse than that of good PABX equipment, as long as you don't use the 
public internet as part of your IP path. For the latter, you're not 
going to improve upon a PABX. You may be able to achieve good 
reliability for outgoing calls by building in huge amounts of 
(geographic) redundancy in the POTS gateways, but I don't think this is 
possible for incoming calls.

If you want to make calls over the public internet you're not going to 
be very reliable. However, you can avoid 90% of the trouble by making 
sure your calls go over the network of a single service provider (by 
connecting all your offices to that ISP) as most of the trouble that 
isn't simple outages (get more lines to protect against taht) has to do 
with the interconnects between networks in one way or another. Of 
course this ISP has to be a reliable one.




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