VoIP and WAN Bandwidth

ISDN is pretty reliable, it's also fairly expensive though. It's important to bear in mind, ISDN is not an IP connection itself. It's digital as opposed to the analog PSTN but it's not IP. A call over ISDN uses a channel, the codec is part of the ISDN standard, you don't get to choose one (and I'm trying to remember the particulars but can't). One call per channel, ISDN comes in 2 channel and 30 channel at the low end (though BT will sell you a 30 channel line with only some channels activated - similar to a 2Mbit connection provisioned over 10Mbit fibre, expansion potential is there).

The routing is done by dial plans (at least in callmanager it is). You configure the outbound lines to ring what you want, could be a single internal extension, or all internal extensions, whatever really...
 
When you make a call using an ISDN line it establishes a direct circuit to the landline then? Rather than a typical IP connection with your ISP?

So talking about the bottom line... exactly where do these 'cost savings' come from when people talk about VoIP. I guess if the majoriy of your external calls are long range, then renting ISDN lines could effectively be cheaper than paying your usual telecomms bill? Or am I barking up the wrong tree?

I understand it's easier to manage (although probably more difficult to troubleshoot) than a PBX network.
 
VoIP is not ISDN but you can use them togeather
VoIP is any calls over IP based networks ie your LAN to a PBX that is connected to ISDN/PSTN or over your LAN over the internet to your SIP Provider (ISP etc.)
these guys are talkin about having a ISDN line as a backup for their internet service (afaik)
we've just got the internet bit and the savings come from cheaper calls
 
I was under the impression that unless you switch it to PSTN or ISDN yourself locally then you'll have to pay to have your service provider do so?

I'm thinking specifically about calls that are made to external landline numbers, not VoIP phones.

I really need to get a better understanding on this ha-ha :D
 
well I'm only semi literate on this but from what I can see thats true, the saving is made from only having x number of lines rather than one per phone?
 
I'm not followijng you.. with the exactly comment lol.

I'm trying to workout if it is supposed to be more cost effective having an ISDN line rather than a typical phone line, which doesn't really seem likely. Although you'll be saving money on any calls to branch offices as they will be VoIP to VoIP calls and will take place entirely over a WAN VPN link.
 
So I think you mean....

VoIP to VoIP calls are effectively free anyways, but if your WAN links goes down then it will switch to ISDN.

All external calls will still be required to use the ISDN links though? Or can you just somehow switch it to a normal landline?

Does that make it a little clearer what i'm talking about? How do the make completely external phone calls, from a VoIP phone. For example, ringing your house from work..
 
in my office we've got landline and VoIP lines, the VoIP lines work for incoming and outgoing but its also connected to the normal line incase of the internet going down, all the outgoing is VoIP via out SIP provider and hence cheaper (for all calls)
 
Wardie said:
So I think you mean....

VoIP to VoIP calls are effectively free anyways, but if your WAN links goes down then it will switch to ISDN.

All external calls will still be required to use the ISDN links though? Or can you just somehow switch it to a normal landline?

Does that make it a little clearer what i'm talking about? How do the make completely external phone calls, from a VoIP phone. For example, ringing your house from work..
Click to expand...


Ok [deep breath] I'll explain the call routing (as simply as possible) and this should help you understand how a VoIP system works.

Connectivity wise you will need:

LAN - For connecting IP phones or Softphones on PC's to an IP network. This will include switches/routers and QoS etc depending on complex you need it.

IP PBX - this IS your phone system - i.e Asterisk or Cisco Callmanager or a.n.other. The IP PBX is used to provide ALL telephony features like Voicemail, unified messaging (voicemail to email inbox), a dial plan (what is allowed to be called, i.e 0845, 0870 etc) and loads more!

A transport mechanism - IP to IP calls within the LAN are easily dealt with, but you need a way to get an IP based call, outside of your environment to a customer/supplier. You can use an ISP that provides an IP to PSTN translation service (if the call needs to go to the PSTN network) or you can use ISDN to do the translation for you. Most businesses will use both for resiliency and use least cost routing to determine which route (IP or PSTN) to take at the time the call is placed. Since most calls are cheaper via IP and your ISP this is the default route, however if you lose your broadband connection ISDN is used to route the call to the destination over the PSTN network. You need as many ISDN "channels" as you want simulataneous calls to be made to an external party. We chose 8.

Now then. There are benefits to using ISDN over IP, namely reliability but its more costly than an ISP would charge in terms of rental (like line rental for a phone line essentially) and call cost. For a business that relies upon telephony you would be a fool if you didn't consider redundancy both in terms of your IP PBX and your transport mechanism.

So this is how a call to an external party works for us:

Call is initiated by the users softphone or deskphone that has an IP address. It contacts the IP PBX that sets up a SIP channel to facilitate the call. If the call is routed via ISDN the PBX also opens an ISDN/PSTN channel to facilitate connection to the external party. The PBX at the other end responds to the request and the call is answered, at which point the data (voice) flows. When a user hangs up the PBX's at both ends release the channels (SIP and either IP or PSTN) that were in use.

Does that help?
 
You must log in or register to reply here.
Back
Top Bottom