ENUM : e164 Number Translation

What is ENUM?

ENUM stands for e164 NUMber translation. e164 is the industry standard format for a PSTN telephone number. ENUM translates PSTN telephone numbers to "something else". That "something else" can be pretty much any service that you like. There are some industry standard definitions of what "something else" could be - SIP, H323 and email services are three of them for example. I'm just going to consider SIP for the moment although all of these address records basically work in the same way.

How does that help?

To understand exactly what ENUM does and how it impacts voice over IP we can see how it might affect an example call flow.

Bob is with VoIPUser and he wants to call Bill who is with Sipgate. Bill has an e164 (PSTN) number which is +44 12 3456 7890. The call flow without ENUM is going to look something like this:-

• Bob dials +44 12 3456 7890
  
• VoIP User SIP Server says "that's an e164 number" and routes it to the PSTN gateway
  
• The call enters the PSTN network cloud. The clock starts ticking at this moment and VoIPUser are incurring call charges, on a per minute basis.
  
• The PSTN network systems say "that belongs to SIPgate" and route the call to the SIPgate gateway
  
• The SIPgate gateway says "that's for Bill" and routes the call to the SIPgate SIP server for the attention of Bill.
  

The key with this kind of call flow is the way that the route is forced into the PSTN incurring call charges for the calling party.

ENUM, with its lookup table of PSTN numbers to SIP records can help here. Here's how the call flow would change with the addition of ENUM:-

• Bob dials +44 12 3456 7890
  
• VoIP User SIP Server says "that's an e164 number" and performs an ENUM lookup
  
• The ENUM server says "that's Bills number, you can reach him direct at SIP:bill@sipgate.co.uk".
  
• VoIP User routes the call straight to the SIPgate server for the attention of Bill.
  

Here we've completely skipped the PSTN and route the call direct, peer to peer (or through SIPgates proxy in the event of NAT or a firewall being in the way).

With the call completely being off-PSTN the only transit is over the internet, where we don't have per minute charges.

In practice, particularly with inter-carrier peering, not all calls will be routed peer to peer, but some may be routed to a network Border Controller (SBC). This means that the provider network still carries media traffic, in turn meaning expense and infrastructure requirement. It does, however, still remove the per minute model of the PSTN, replacing it with a bandwidth charge model (for the VoSP network operator).

There are several problems with ENUM which are going to need to be addressed in order to make it viable. The single biggest factor is trust; who's going to be in control of all that data?

That's going to be a topic for another thread, but in the meantime, if you're intereted in the latest state of play, at least as far as the UK is concerned, start here:-

http://www.voipuser.org/forum_topic_8891.html

If you're looking for a little more technical "meat", in particular with reference to Number Portability and VoIP Peering, I recommend reading Lennart Maris' Masters Thesis on Infrastructure Enum (.pdf):-

http://isoc.nl/files/ScriptieLennartMaris.pdf