VoIP Gateway

Basher

Basher

Basher

Soldato
Joined
18 Oct 2002
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9,158
Hi,

I'm very new to the VoIP infrastructure scene and am trying to get to grips with the fundaments, so ... Gateways - I seem to be getting a few conflicting descriptions so could someone (nice and simple!) explain:

  • The purpose of a Gateway device
  • Would one typically be needed per site or just in one central location to bridge to PSTN?

Many Thanks,
James
 
Well an official definition:

A network device that converts voice and fax calls, in real time, between the public switched telephone network (PSTN) and an IP network. The primary functions of a VoIP gateway include voice and fax compression/decompression, packetization, call routing, and control signaling. Additional features may include interfaces to external controllers, such as Gatekeepers or Softswitches, billing systems, and network management systems.
Click to expand...
 
Ok, so in real terms what would that mean - one per site or one in a central location to route from/to PSTN? I've also seen gateways described as the bridge from the current (non IP) telephone network internally to an IP network - so that wouldn't fit with that explanation.
 
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The term can be quite generalised depending on who you talk too, yes IP enabled PBX systems can act as VoIP gateways or from a Cisco point of view such devices are described as Unified Border Elements which act as VoIP gateways and generally terminate analogue or ISDN connections before handing them off to a Call Manager platform. They also sell smaller platforms that can interface directly with the PSTN network and provide call processing functionlity.

Take for example a company with 6 sites joined together using an MPLS network, the call processing system is located at the hub sites and all calls from spoke sites are routed via the Call Manager at the hub site. The Call Manager sets up and tears down the call but the breakout to the PSTN network is managed by the VoIP gateway or "Border Element"

SIP Service providers also use similar devices for breaking out onto the PSTN network, the customer mearly operates user agents capable of supporting the SIP protocol for call setup and teardown, all interaction between the provider and end user is purley IP based.


Umm i hope i make some kind of sense its been a long day!

:)
 
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Ok so if an employee at site A wanted to make a call to someone outside of the company (so via PSTN) would that go:

- Out of site A straight onto the PSTN
- Via the MPLS network to the main site and then out to the PSTN?

Or would it just be a case on individual configurations?

Cheers
J
 
You could have a single site or multiple sites, it doesnt have to be MPLS but it aids in gurateeing delivery and voice quality other technologies such as IPSec VPN's can work if provisionined correctly.

- Out of site A straight onto the PSTN - Yes using an IP Enabled PBX
- Via the MPLS network to the main site and then out to the PSTN? - Yes using a centralised model with a IP Enabled PBX at the main site.
 
Ok - so what would be the benefit of site A going directly onto the PSTN rather than via the central site. Surely this introduces the need for more hardware = more cost?
 
If you had a local gateway to the PSTN in site A, you'd get the oppertunity to use a local number, though with newer calling plans, different toll rates aren't so important anymore, but a local gateway at a site, would allow breakout for local calls. SRST (Cisco) is one good reason to have a local gateway - even if you only have a FXO card in it, if your wan link goes down, at least you'll still be able to make emergency calls - change that to an E1 type circuit, that's 30 voice calls to your local PSTN.

/edit

The cost might be a factor, but if you're running a CME at a site, then adding the VWIC card into the router wouldn't be that expensive. If you're running a centrallised deployment, then it's good practice to put in a router for SRST, if you don't have a local IP PBX such as Call Manager on site, that way if your wan link goes down, you can still make calls within the site at the least, then add a VWIC for PSTN connectivity...
 
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No problem :)

Breakout for local calls, was more important a few years ago - before phone companies were falling over themselves to give you cheaper rates (it might still be important, but we get uber cheap rates as we're part of a telco - which can distort things a bit)

Essentially, you'd have two or more sites and a nice IP network in the middle, and a PSTN gateway at each end, this would allow you to make free (IP) calls to your other site, and by using local breakout at each end, make cost savings. So if a person at site A wanted to call a phone in the same area code as site B, you could route that call over your IP network (no long distance) and pop out on the nice cheap PSTN connection at Site B and make it a local call.
The other advantage for local breakout is having a local number, same as at your home really - though there are pros and cons to this.
Plus, if your PSTN connectivity is generally done via a SIP provider and your internet connection goes down, how do you make calls to the outside world - particularly emergency calls? Thus having a local breakout even if it's an FXO card with two ports on it, would be an advantage.

SRST is survivable remote telephony - I'm talking about a cisco world as this is what I know (ish!), but there will be something similar for other vendors no doubt...

If you have a call processing model where all of your call control and siganlling is done at another location, so Site A contains the IP PBX (Cisco Call Manager in this case) and site B contains a few deskphones that are registered over the WAN to the call manager at site A, if the WAN link fails, it renders the phones useless as they don't have anything to control the calls as all of that is done at site A.
SRST is essentialy a router with very basic call processing features on it, so in the event of a WAN link failure, the phones know (though configuration) to fail to SRST this will allow the basic functions of making calls within the site. If you have a PSTN connection on this router then the people at the site can make calls to the PSTN, or at the very least make emergency calls.
SRST can be implemented on an existing cisco router platform if it's powerfull enough, so you might not need to purchase any additional hardware (other than a vwic or memory)
 
no problem at all :)

I'm just starting to kick off my study for the CCNA Voice (would have started CCVP but Cisco changed the track slightly so need to get CCNA voice before I can make a start on my CCVP) but I've been playing with Cisco Call Manager for a few years, and hopefuly we'll get to upgrade this year :)

I'd totally recommend getting the Cisco Press CCNA Voice book, so you can get an idea of all the bits - it's aimed towards CCME (call manager express which is a router based platform), but gives a good grounding in VoIP stuff. Also try and get hold of some of the CBT nuggets on it - even if you're not going down the certifcation track yet, it's good knowledge :)
I'd be able to get past chapter 3 of the book if I wasn't so busy at work :(
 
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