On 02/08/2011 11:01 AM, Neil Harris wrote:
They did indeed, but they did it by centrally precomputing and then
downloading centrally-built routing tables to each exchange, with
added statically-configured routing between telco provider domains,
and then doing step-by-step call setup, with added load balancing and
crankback on the most-favoured links in the static routing table at
each stage.
All this works fine in a fairly static environment where there are
only a few, well-known, and fairly trustworthy officially-endorsed
entities involved within each country, and topology changes could be
centrally planned.
BGP is a hack, but it's a hack that works. I'm not sure how PSTN-style
routing could have coped with the explosive growth of the Internet,
with its very large number or routing participants with no central
planning or central authority to establish trust, and an
endlessly-churning routing topology.
Still, every good old idea is eventually reinvented, so it may have
its time again one day.
The way LNP works is a good example of PSTN style routing scaling. Each
carrier has to have at least one NPA/NXX pair per switch, of which they
pick one number they will never port out, and never assign to an end
user, and declare that number as their LRN. There's nothing super
special about this LRN, except that it's part of that NPA/NXX that's
directly allocated to the carrier's switch as the original assignee.
When a phone call is made, a TCAP query is launched by the originating
switch to a set of STPs that then route it to an LNP database, that has
a full list of every ported number, and its LRN, and a few other tidbits
of info. The switch then sees the LRN in the response, sets the called
party number to the LRN, and then sets the "Generic Address Parameter"
parameter in the ISUP message to the originally dialed number.
This tunnels it through a network that is unaware of LNP, and when the
terminating switch sees its own LRN as the destination, it moves the
Generic Address Parameter back to the Called Party Number and continues
processing.
-Paul
