Normalization and Translation with Gateways and SfB

Skype for Business has a number of areas where you can normalize or translate a number. When we’re dealing with users, normalization is the process of taking the number that they’ve entered or “dialed”, and converting it to a standard format for further processing. This normalization allows the user to maintain local dialing habits, and convert them to a standard format. That standard format is E164, which also ensures the number is globally unique. When we’re dealing with calls received from the telco or PBX, normalization refers to the process of converting the called number into E164 standard format.

Normalization is a form of translation, but not all translation is normalization. When we’re sending a call out of SfB, we might need to translate to a format that’s expected by the device we are sending the call to. That format might not be standard or globally unique. This translation can take place on both the called and calling numbers. This translations is configured in Trunk Translation Rules.

Trunk Translation rules can only match and translate same field. For example, SfB cannot match on Calling Number and translate the Called Number. However you can match and modify a called number and match and modify the calling number, on the same call.

Outside of SfB, number translation can happen

  • At a PSTN gateway or SBC
  • At an analog gateway
  • At a PBX
  • At the telco (few and far between, but some do!)

The options you have within these systems will vary. Generally speaking, the gateways and SBCs are very flexible, PBXs somewhat flexible, and telcos are something that you shouldn’t count on.

Depending on what are we’re talking about, there are a number of overlapping options available, so which ones should you use?

In general, you’ll want to avoid a smattering of different translations. Where ever possible, keep all of your translations in one system. Which system? Usually SfB, because most organizations have more people that understand SfB more thoroughly than gateways. For example, when you’re sending a call to +14258675309, the telco probably wants you to send only 4258675309 to them. You could strip the +1 within SfB before you had the call to the gateway, or you could strip it at the gateway before you hand it to the telco. SfB is my preferred option here.

Sometimes, your hand will be forces. Let’s say you want all calling numbers (your caller ID) to be changed when you call to a certain area code. You need to match on the called number, but change the calling number. SfB doesn’t handle complex translations like this, so you’ll have to use a gateway.

When you’re considering analog devices dialing through SfB, you can perform normalization in the gateway or in SfB.  If you use a dialplan within SfB, you can take advantage of the “select” feature to select normalization rules that already exist in SfB. Generally, you would want to use the same rules that are in the Dialplan that apply to the local users.

There’s some confusion over how to assign a Skype for Business Dial Plan to an analog device. Technet states “you can then manage the analog device by assigning policies and dial plans to the contact.” It turns out that you can’t apply a dial plan to a CsAnalogDevice. Well, you can apply it, it just won’t work.

When you look at how user level dial plans work, this makes sense. User dial plans are applied at the device level: your SfB client on your PC, your mobile client, or your AudioCodes 440HD phone – any SfB client. An analog phone isn’t an SfB client, nor is the gateway that it plugs into, so there is no place for the normalization rules to be downloaded to and applied. Instead, you need to use a pool level, site level, or global dial plan. These dial plans will apply at the server level for inbound calls, but will also be used for users if a user level dial plan doesn’t exist.

For gateways, I’ll use the Global Dial Plan in smaller environments, and Pool level Dial Plans in more complex or geographically diverse environments. The Global Dial Plan can quickly get messy if you try to stuff in enough rules to handle larger, global deployments.

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