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Re: IPv6 news

  • From: Andre Oppermann
  • Date: Tue Oct 18 08:37:51 2005

Paul Jakma wrote:
On Tue, 18 Oct 2005, Andre Oppermann wrote:
On a local level however it's not more than a DNS name mapping to some real on-net identifier.
Within a telco?
No.  Within a region.  Normally area codes are a region.  Sometimes
entire country codes are a region in this sense.  Depends on the size
of the region/country though.  In some cases there is even more than
one area code for the same region.

For every area code there is a 'default' carrier.  Usually this is
the incumbant.  They've got the obligation to forward inbound calls
to the true carrier of that particular number holder.  However this
only works if the target carrier has some kind of interconnect with
said default carrier.  On top of this forwarding doesn't come for
free.  Depending on my call volume with that area I have to look into
direct termination of ported numbers.  Usually the regulator or some
party designated by the regulator runs a number portability registry
with the true carrier information for every number.  If I want to
optimize my call routing I have to periodically synchronize the call
routing tables on my switches with that registry.  In a very competitive
area this lead to 30-50% of all numbers being ported and thus showing
up in my routing table.  As we know from the Internet DFZ the routing
table becomes very large.  Fortunatly for the TDM network I have to
do the routing table lookup only once when setting up the call, not
for every voice sample.  Thus I pay the lookup cost only once for
x minutes of voice traffic.  Very DNS like.  And because of the area
code hierarchy even more so.

That's why number portability is normally only offered within the same
area code or region.  So you can't take your NY fixed line phone number
to LA.  Unless of course you have someone picking up the call in NY and
transporting it to you in LA.

For non-US mobile networks the number portability works the same.  The
mobile network(s) have got their own area codes.

There's a myriad of ways to do it afaict. The case I know best though involved calls inbound to an operator specific prefix (there are a set of 4 or so major telco peering exchanges in Ireland, where the domestic and transit telcos /must/ be avilable for peering). The operator used custom software to map specific numbers to X.25 "addresses" (I forget the exact X.25 jargon) on their own network to deliver the calls to various locations on our network.
You can forget that X.25 stuff.  It's only used for SS7 message routing
and doesn't have anything to do with call routing as such.

The telco peering points is just a technicality.  It's there just for
optimization.  Most regulators have set up an "easy interconnection"
policy to prevent your favorite incumbant from offering 'peering' only
on lands end.

Outgoing are not affected because the TDM network always sets up parallel in/out path's. The return channel for your outgoing call doesn't come back through your former mobile operator.
I didn't know that, but sounds exactly what you want.
Sure.  However this is the main difference between the TDM network
and the Internet.  Due to this fact many things work on the phone
network like carrier pre-selection, phone number portability, etc.,
that do not work on an IP network.

Now compare this to the Internet and IP routing. See some little differences and diffculties here and there? Yea, I thought so.
There are huge differences in the details, obviously. The basic concepts though are at least interesting to consider, if not directly applicable to IP (technically at least - operationally/politically is another question):

1. Providers servicing these prefixes must peer and exchange the
prefix information
On the phone network the prefix information is not dynamically exchanged.
There are number portability registries whose data you can download
every night or so and then dump it into your own switch or IN platform.

2. Providers must be prepared to carry other providers traffic into
   the area
Only one of them.  The 'default' carrier.  There are many phone networks
and carriers carrier who do not have 100% coverage.

2a. The providers within the area have to figure out how to bill for
    the difference of this traffic.
No.  Usually the tariff is set by the regulator based on some fixed
interconnection charge and network element usage.

Conclusion: Applying the phone number portability to the Internet is broken by design.
Right, cause phone number portability is up and running for several sets of prefixes in various regions across the world[1], so there's definitely nothing we can learn from them. ;)
Well, we can learn from them that circuit switched networks are different
than packet switched networks.  Beyond that not much.

1. Does the US have number portability anywhere? If so, that would be a /huge/ region, and very interesting to examine to see how they manage it.
See above for an explanation.

To summarize the differences between PSTN and Internet routing:

 o  PSTN ports numbers only within regions/area codes
 o  PSTN routes the return path along the forward path (symetric)
 o  PSTN calls have pre-determined characteristics and performance (64kbit)
 o  PSTN has static routing with periodic sync from porting database
 o  PSTN pays the routing table lookup only once when doing call setup
 o  PSTN call forwarding and peering is not free or zero settlement