Introduction of Traditional_Telephony
Internationally we have the international telecom or Telecommunications Union telecom sector more affectionately known as the ICU T. And they are responsible for a lot of standards that govern the public switched telephone network. They’re global formed by the United Nations and they’re really big on pushing standards for use in third world countries and things like that for us in the VoIP world.
We’re particularly interested in the codex which is defined in the G and H series but we’ll see that there are a lot more standards that they’re worried about including the one that we’re going to talk about today the FCC or the Federal Communications Commission is big here in the United States obviously formed by the Communications Act of 1934. They are very big in setting prices deciding who can sell what course that makes it the regulation and all the standards that we use here. Now the only reason I mentioned these two standards bodies this time is kind of we’re going to be talking about a particular standard today to start us off we’re going to work with this general topology.
This entire diagram is a sort of representative of the PSTN or the public switched telephone network. It is comprised of local exchange carriers and interexchange carriers and then the phones themselves are connected to what we call the local loop and I’ll talk about the local loop later on in future videos but I just wanted to give you the architecture that we’re going to be using here.
So as we make a phone call or we’re worried about is actually the structure of that phone number as I said the Hi to you T is responsible for almost all of our standards governing communications on the public switched telephone network and the baby that we’re going to talk about today is e dot 164 which describes the numbering plan for your phone number in the United States.
North American Numbering Plan
which isn’t exactly e than 164 but it is compliant with it at 164. And so here in the states that are our structure right there.
We have a one followed by an n value and then the Xs and then and xx. And then finally x x xx. Now the N can be of value anywhere from two to nine and the X can be a value anywhere from zero tonight. Now, of course, we’re not adding the country codes in this case but we are going to talk about what each one of these sections means. The last thing that I want to mention before we leave this slide is that telephone numbers are highly geographic. That’s because we’re talking about circuits and where they run.
And so when you pick up a phone number or when you dial a phone number you’re getting pretty close to that person’s actual physical location within limits of course so let’s break this out a little bit wider. Our first number there actually the n x acts of the first number is the area code. And this also represents our geographic assignment and we’ll see what that means here. Second, you’re all familiar with the idea of area codes. And this really points to the section of the country that you’re calling to or from the middle part of our phone number here is called the office code. Also considered to be what we call the switch code.
We’re used to talking about at least in the networking side we’re used to talking about routers and switches and access points and things like that but on the telco side, there’s also something called a switch. Probably the most popular telcos which are something called a class 5 switch. And one particular model is a five E S S switch very popular. And these are the switches that connect phone calls. So that second set of numbers there identifies the office code.
Now, this is also another way of referring to your central office or your local exchange carrier. So the reason that you don’t have to dial an area code when you’re calling somebody that’s close by is that you are in the same office code and the last four numbers there identify the phone. Now we think of those as identifying the phone but that’s not really what they do. They really identify the port or circuit that that phone is connected to and whether or applying signals or sending ringing voltage down that line you’re actually lighting up a particular circuit that’s attached to a switch at the office. Now another related term is a laptop or local access transport area.
There are about 200 of these in the United States and they loosely very loosely correspond to the area go. And I’ll tell you what I mean here in a second but they’re definitely not the same thing. So the United States is split up into these geographic areas and to these geographic areas we assign or we have operating a local exchange carrier which again is part of our PSTN and is the partner to the interexchange carrier
Now picture worth a thousand words. So here’s an example from Chapter Two. This is figure two point five. This is Kansas and here on the left, you can see the two ladders that are assigned. But in that same geographic area, we have our area codes and because area codes are attached to our phone numbers as population changes densities change things of that sort. It’s really really tough to root to keep area codes static here in western New York. We split area codes a couple of years ago because we have too many people in this one particular area code to come up with a unique number for everybody. So basically what we have today is that we talked about our phone number. And if you take the phone number and bring it back to the PSTN structure we can sort of understand how we have a geographic assignment that goes down to a switch that actually winds up at a particular circuit or a station number. And next time we’ll talk about what that local loop and what that local exchange connection is actually like.