PSTN call routing

UPDATE: The bit I was missing was NPAC. All answered now.

So tonight, we ported over Katherine’s phone number from one provider to another. This got me thinking about number porting in general, and specifically the call routing implementation employed within the NANPA (North American Numbering Plan Administration) – the people responsible for assigning and managing USian/Canadian/other (now) non-Mexico NA numbers similar to the ARIN for the Internet – between carriers. I need to know.

In the case of the Internet, ARIN assigns top level netblocks, ASes, and other routable entities, and then various top-level network routers and ISP equipment broadcast routes for their 16-and-32-bit ASes. The global routing table is large but can still (mostly) fit in memory on big beefy routers because not everybody has their own AS (network). Instead, most people have a number assigned within a providers non-portable netblocks. When it comes to tele carriers, this isn’t true under modern NANPA. You can freely move your local phone number between carriers as you wish. The fact that you have a “617″ (Boston/Cambridge/etc.) or “415″ (best coast regional) number really means very little in practice as you might have moved a million times, and changed carrier too. Therefore, your number is neither really regional, nor a non-portable carrier assigned number.

Reasoning tells me that carriers can’t just announce routes for particular “blocks” of phone numbers any more because these are rapidly fragmented and hop between carriers. Nor does it seem to be practical to advertise announcements for each number individually. And yet, that seems to be the only way to truly do this right. One possibility is that such a level of route announcement is done, but at the local exchange level (the 759 in 617-759-XXXX) and even if I move regionally, there will still be an entry sitting in that exchange like in the good old days. But is it still like that? How does the routing between large telecommunications companies work in reality? I need to know. Preferably, I “need” an extremely large book that details this and the protocols involved in a ridiculous level of detail. Thanks!


NOTE: This falls under Obsessive Compulsive Need To Know. The kind of reasoning that has me signed up to real-time alerts from my regional ISO whenever power generation within MA falls below certain levels. No normal person would care about this level of detail in their life, and I know this ;)

One Response to “PSTN call routing”

  1. Dominic Hargreaves says: should tell you some of what you want, I think.

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