POTS; was: Anyone here from Oregon USA to help with collection pickup?

Alexey Toptygin alexeyt at freeshell.org
Mon Oct 2 13:36:47 CDT 2006

On Mon, 2 Oct 2006, Sean Conner wrote:

>  But what about cell phones and roaming?  Last year I went to Las Vegas
> with a friend of mine.  My cell phone is in the 561 area code (Palm Beach
> County), my friend's phone is in the 205 area code (Tuscaloosa, Alabama).
> Las Vegas is 702.  My Dad was able to call me (760) while I was in Las
> Vegas, and I was able to call my friend (205 area code, although phyically
> five feet away) from my cell phone.
>  How do they handle *that*?

This is getting dangerously OT, but what the hell, I used to test these 
things :-)

All calls from the PSTN to a particular cell number go to that number's 
'home' switch. This switch owns the enclosing block of numbers in the PSTN 
sense. It also keeps track of which switch is the 'serving' switch for a 
particular subscriber at any time. When your phone moves into a new 
location area, it preforms a location updating procedure; this procedure 
places the phone into the serving switch's list of visitors, the serving 
switch then lets the home switch know the current serving switch's 
address, and the home switch tells the old serving switch to purge the 
number from it's list of visitors. When a call comes in for a cell 
subscriber, the PSTN routes it to the home switch based on the number 
alone; the home switch looks up the address of the serving switch for this 
subscriber in it's database and routes the call to the serving switch, 
which then delivers it to the phone. Calls from the cell phone are usually 
handled directly by the serving switch (i.e. they're sent into the PSTN 
and if the recipient is also a cell number the call delivery procedure 
above is used by the recipient's home switch).

As a result, your mobile-to mobile call with both of you roaming can have 
either 2 to 4 legs (maybe more if forwarding is in use). It's 2 legs (you 
-> serving switch -> your buddy) if you're on the same serving switch and 
it can figure out that the call recipient is on the same switch as the 
sender. It's 4 legs (you -> your serving switch -PSTN-> your buddy's home 
switch -PSTN-> your buddy's serving switch -> your buddy) if you're not on 
the same serving switch or your serving switch doesn't have enough 
information to figure out that it's also your buddy's serving switch.

More or less the same thing is done for number portability, except there 
can be 3 switches: the switch that owns the number in the PSTN sense, the 
switch that is the new 'home' and tracks the location of the phone, and 
the serving switch to which the phone is actually talking. The owns number 
switch to new home switch mapping is static (updated by people, not 
software) and the protocols used for number portability are not the same 
as those used for roaming, but the idea is basically the same.

Now take that explanation, stir in tons of telco jargon, make it about 5 
times more complicated due to various features (call forwarding, anyone?), 
route optimizations (saving money on trunk use is a big deal for carriers) 
and security procedures, and you'll get what's actually going on :-P


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