PDF of this tutorialDonor Switch and Recipient Switch
There are two switches, a donor switch (to which the call goes initially) and a recipient switch (the switch handling the second number). When the first switch receives a call, it tells the call, in effect, that the recipient is not here, it has been forwarded, and here is the new number. The donor switch presents the new number, and the network tears down the call and reroutes it, sending it to the recipient switch, which then processes the call, sending it to the second number.
Because up to 20 percent of the population change providers each year, this means that 20 percent of all calls must be switched twice. This is a significant increase in the amount of call processing required, forcing the switch load way up.
Location Number Routing
Fortunately, a long-term solution is emerging. As we shall see, this solution is tied to the issue of testing unbundled loops. The new method is LNR. In March 1998, it was implemented in one site per regional Bell operating company (RBOC), and in December 1998, in each of the 100 largest metropolitan markets.
A National Portability Administration Center (NPAC) will soon be organized and all ILECs and CLECs will need to install and maintain an LNR database. Connected to the NPAC via signal transfer points (STPs), these databases will update NPAC with changes on a daily basis. The NPAC will administer LNR, accepting number changes and synchronizing the databases. Considering the number of customers who change providers each year, the number of changes NPAC must synchronize will be large, indeed.
When someone dials a number, the STP looks in the routing database to find out where the call goes—to New York City, Chicago, Omaha, etc. With this method there is now a single process, but the call still gets where it needs to go. As we will see, to do adequate testing of the local loop, the LNR has to be tied to testing equipment in some way.
LNR has important implications for other areas, besides remote call forwarding. At this time, calls made through LNR are copper to copper. In the future, however, LNR will drive the use of new media, including wireless local loops and hybrid fiber coax.
Testing in the Local Loop
We have mentioned some of the established and new phone services that make testing the local loop a necessity. Telcos have experimented with a number of methods of responding to this need for testing; most of these methods have significant drawbacks.
