[VoiceOps] Question about SS7 routing

Mary Lou Carey marylou at backuptelecom.com
Thu Sep 3 13:55:52 EDT 2020

I'll try to make this as short and sweet as possible even though it's 
pretty complicated. Point Codes are the 10 digit addresses for a 
particular switch and LRNs are the 10 digit addresses for a particular 
connection point that switch is associated with. In the PSTN world, all 
connections are dedicated and implemented by LATA / Tandem area for 
Local / IntraLATA traffic. When you get your first NPA-NXX for a LATA / 
tandem area, you enter it in the LERG (National Routing Database) and 
populate the tandems (Local, IntraLATA and FGD) that you are connecting 
to. Then you assign a 10 digit phone number from your NXX block to be 
your LRN. You add that to both the LERG and NPAC (National Porting 

Once you've published all your switch information in the LERG and NPAC, 
then you establish your ISUP trunks with each ILEC you're 
interconnecting with. You can set up additional trunks with other 
carriers if you want a cheaper option for routing traffic, but the 
minimum required is the ILEC. Each carrier's switch will have a distinct 
point code associated with it so you'll order ISUP trunks to each switch 
(point code route) you need to be connected to. You'll also associate 
the distinct LRN for that LATA / carrier tandem area with that trunk 
group. Usually there's multiple trunk groups per LATA / tandem area so 
you'll program your routing tables with the NPA-NXXs each trunk group 
serves. That way when a customer originates a call, your switch can do 
the LNP dip to find the LRN and send it over the route that the NPA-NXX 
of the LRN is associated with.  Routing tables can get complicated 
depending on how many carriers you're connected to. Companies that 
operate in more than one ILEC area or LATA usually purchase Least Cost 
Routing software so they can send their originating traffic out over the 
cheapest route.

IXC traffic is routed a little differently because it is routed by CIC 
(4 digit code that identifies the IXC) rather than by NPA-NXX. They 
connect to all the ILEC carriers just like the CLECs do, but they 
populate their routing information in the SMS database instead of the 
NPAC database. Once the call is dipped, the traffic is delivered in 
pretty much the same way.....by dedicated trunk group / tandem area.

BackUP Telecom Consulting
Office: 615-791-9969
Cell: 615-796-1111

On 2020-09-02 04:46 PM, Ross Tajvar wrote:
> Hi all,
> I'm trying to understand how routing works in SS7-land. I am familiar
> with portability, and I know (at least in the US) the first step in
> routing a call is doing an LNP dip to get the LRN.
> However, it looks like addresses in MTP3 are "point codes" (PCs) which
> are assigned to switches. Calls are set up with ISDN-UP, which is
> transported via MTP3. So in order for a call to be set up, the
> destination switch's PC must be known. How is the destination PC
> determined from the destination LRN?
> Thanks,
> Ross
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