How to make the move from ISDN to SIP

ISDN is fast becoming a technology of the past. Today's telcos have networks that bypass traditional telephony signalling technologies for IP networks: the hardest thing they do is present a “legacy” connection such as an analogue line or an ISDN connection to a customer, as layering a non-IP service on an IP network is non-trivial at a technical level.

On the customer end, supporting traditional telco technology is equally tedious: running up an IP PBX with SIP access trunks is an incredibly simple thing to do – not to mention cheap with Open Source technologies such as Asterisk. You only really add complexity and expense using an ISDN card and adding the necessary functionality to interface the system to the public phone network (PSTN).

The move from ISDN to SIP is one that we're seeing more and more these days. So let’s look at what you need to consider and the actions you need to take in order actually to achieve it.

Selecting a provider

Ask Google for “SIP providers” and the list is enormous, including many that provide “free” and “unlimited” services.

If something's free the chances are that it doesn't come with a great deal of service guarantee. That's probably fine if you're just using it for personal or perhaps home office use – after all, you probably have a mobile phone that you can use relatively inexpensively for the average call in the event that your SIP service has died. For corporate links, though, you'll want more.

Billing
This sounds like a trivial point, but if you're looking for a business service you'll want it to be billed like a business service i.e. with real time billing and usage and access to reports? I once worked with a business whose small (15 or so people) Texas office used one of the popular VoIP services for all its landline communications. It was a nightmare because not only did we receive 15 separate bills, but we had to pay by credit. For business, you'll want integrated billing that works on a post-paid regime. Just like a normal phone bill, in other words.

Support
Are you happy to have semi-formal tech support 9-5 on weekdays? Or do you need something better, more responsive, and / or with a formal service level agreement wrapped around it? The more expensive the service, the better the tech support will be: test it out before you make your choice, because support is one of those things where you're happy to save money on the less-cover-for-less-money option until the moment something goes pear-shaped!

Charging clarity
There are relatively few IT-related services whose charges can't be predicted in advance. Telephony is one of the few items whose charging is based on usage, and so:
(a) You need to be clear on the cost of any given type of call
(b) Aim to pick a provider where you can check your usage and your predicted charges through the month. Visibility gives you control: look for it in a service provider.

Service quality guarantees
Do you want perfect service? Generally, you don't: you just want something that satisfies your expectations. As mentioned, service quality tends to be directly proportional to price. In the case of SIP services, though, you have more to consider than with ISDN-based telephony.

Yes, I can go out and get (say) a 20Mbit/s MPLS connection from London to Glasgow and throw 20Mbit/s reliably down it all day, every day. But that costs real money; for a fraction of the cost I can get a 20Mbit/s internet connection and rely on best-efforts forwarding to get 20Mbit/s most of the time.

Is the latter acceptable? In most cases, yes. It's all about expectations. The chances are that the average provider will be able to give you certain guarantees: these are usually for the components in their network and perhaps those closely connected or which form part of networks with which they have formal partnership agreements. Some more remote locations may fall outside the guarantee.

PSTN integration
The fact remains, of course, that in a SIP world you're still going to have to talk to people who don't have the ability to talk SIP. ISDN is still plentiful around the world, after all. You therefore need a provider that brings a balance of SIP and ISDN – with less expensive SIP used where both end parties support it, and PSTN integration to handle SIP-to-ISDN and vice versa.

Again, consider your calling profile against that of your service provider and take advantage of their abilities. For instance if you make lots of Europe-to-USA calls an operator with points of presence in London, New York and Paris, could use their trans-Atlantic SIP connectivity to break out calls to the PSTN close to the destination.

How's the service delivered?

Your ISDN service is delivered through a piece of electric string that terminates in an RJ45 in your comms room. But how does your service provider deliver SIP?

Direct connect - supplier-provided
If the supplier you've chosen is local to you, it may well be able to deliver the service natively in the same way as the ISDN service – as a point-to-point link into your office. Note that it is unlikely to use precisely the same copper, but will most likely run something more robust capable of handling higher speeds.

Direct connect – third party-provided
For semi-local service providers there's also an option of a direct point-to-point circuit between you and a SIP provider, installed either by them or by an independent telco (think Amazon's AWS Direct Connect service – it's the same idea). The approach is the same – a direct, guaranteed bandwidth link from you to the provider – but the interconnect is provided by someone else.

Internet connectivity
The next option is to use an internet connection to provide indirect connectivity from your premises to the service provider. The quality is less guaranteed here, because your voice traffic is sharing the same connection as your normal Internet traffic, so it's important to monitor bandwidth usage and traffic types closely and, if necessary, expand the link to a fatter pipe and/or employ bandwidth shaping tools to prioritise the voice streams.

Hybrid connectivity
If your SIP provider is able to provide a hybrid service carrying both SIP and Internet traffic, the chances are that they can implement specific bandwidth allocations for the two traffic types, so that the less critical Internet traffic can’t overwhelm the SIP connection and destroy the call quality. It’s a good option if it’s available to you as it gives a useful compromise between the expensive direct-connect and the uncertainty of using an Internet connection.

Link resilience

I used to run resilient ISDN lines in the head office of a previous company. We had two PBXs in separate locations, with an ISDN30 into each and a metro Ethernet between the two. It worked absolutely great, but it's sadly not very common for a telco to be able to provide such resilience.

SIP is just an application on an IP network, and IP networks can be built with inherent resilience and failover. Providers can make judicious use of global load balancing (GLB) on top of networks where service providers peer with each other using BGP and the like to provide multiple paths through the global network of networks. It's a whole lot easier to fail over the landing point of a SIP service to a far-distant location than it traditionally was with ISDN.

As usual there will be a cost to adding resilience, but the same applies to ISDN. The difference is that you have more options with SIP. What we’ve mentioned above is the resilience you’d expect the SIP provider to have in its network, but as long as your provider has more than one location where you can hook into the network, you can look to have dual links between your network and theirs too. Because of the way IP networking works it’s generally easier to run a pair of links in active/passive mode (where link A carries all the traffic until something fails and when a problem is detected it’s flipped to link B). By default this will generally protect you against a failure in the provider’s point of presence, but by working with the provider you can often implement protection against a failure at your end too.

Phone numbers

Telephone numbers are a quaint concept in these days of URLs and IP addresses, but you'll still generally find one on every business card that anyone hands you. Hence you'll need some concept of a phone number for your SIP service.

Unless your SIP service is coming from the telco that provided your ISDN service, the chances are that you can't port your existing phone numbers. If you're lucky then you may be able at least to get numbers within your own dialling code, but much of the time you won't Non-geographic numbers tend to be the way forward in such cases.

Dealing with the move
Helpfully Ofcom, the UK telecoms regulator, is living in the 21st century and even though you can’t necessarily take your existing phone numbers from one traditional telco service to another, Ofcom has decreed that you can port your ISDN phone numbers to your SIP service . All you need to check is that the new SIP provider supports the porting process, as some of the smaller ones don’t.