Subject: RE: TRIP

From: James Yu <[email-masked]>
Date: Sun, 5 Dec 1999 23:02:48 -0600

Jonathan,

I'll explain using an example.  Switch A of local exchange carrier (LEC,
e.g., the local telehpone company instead of the long distance or
international telephone companies) X serves 212-455-0000 to 212-455-9999
(I'll ignore the country code for the time being because number portability
is supported within the U.S. or Canada; therefore, the LRN discussed in all
the number portability documents is a North American Numbering Plan Number)
before number portability is supported.  Switch B of local exchange carrier
Y serves 212-466-0000 to 212-466-9999.  Now due to the need to support
number portability, Switch A is given a LRN of 212-455-0000 (only the first
6 digits of the LRN are used for routing within the SCN in the U.S. or
Canada).  Switch B is given a LRN of 212-466-0000.  We'll call these two
LRNs LRNA and LRNB respectively.  Assume that one customer whose number is
212-455-1234 switches from LEC X to LEC Y.  There is a process among LEC X,
LEC Y and NeuStar (as the Number Portability Administration Center or NPAC)
to confirm the transfer and uppdate all the Number Poratbility Databases
(NPDBs) about the change.  Basically, the LRN associated with 212-455-1234
in the NPDBs is changed from LRNA to LRNB.  Please note that this customer's
home/business phone would be pysically connected with Switch B instead of
Switch A after the phone number is "ported."

North American Numbering Plan numbers have the format of NPA+NXX+XXXX where
NPA (Numbering Plan Area) is the area code and has the format of NXX (The
area codes used to have the format of N0/1X where the second digit can be
either 0 or 1. That restriction has been removed to allow more area codes to
be assigned.). N is any digit from 2 to 9 and X is any digit from 0 to 9.
The NXX after NPA is called "office code."  Normally, a switch is assigned
the whole block of NPA+NXX (e.g., 212-455-0000 to 212-455-9999).  Now
"number pooling" will allow several LECs to share one NPA+NXX.  That is, a
LEC will be assigned, for example, 212-477-1 (block from 212-477-1000 to
212-477-1999) and another LEC assigned 212-477-2 (block from 212-477-2000 to
212-477-2999).  We can talk about "number pooling" later if you or others
have questions.  The focus of this discussion is on LRN and number
portability.

When any number in a NPA+NXX block is ported out to another switch, this
particular NPA+NXX is marked as a portable NPA+NXX.  Calls to numbers in the
portable NPA+NXX blocks will trigger NPDB queries.  This is to avoid NPDB
queries on all calls.

Assume someone in a different state (e.g., California) in the U.S. dials
1-212-455-1234.  The originating switch in California will deliver the call
to the caller's presubscribed interexchange carrier (IEC, e.g., long
distance telephone company).  The IEC performs a NPDB query with the called
party number because 212-455 is a portable NPA+NXX.  The response will
contain the LRN assocaited with 212-455-1234 which is LRNB.  Certainly, for
numbers that have not been ported out of 212-455, the same "called party
number" is returned in the response because the NPDB presently does not
store information about the "non-ported" numbers.  After receiving the
response, the IEC switch sets an indicator in the ISUP Initial Address
Message (IAM) to indicate that NPDB has been dipped.  This will prevent
subsequent NPDB dips from happening at any subsequent switches.  For a
ported number, the dialed number, 212-455-1234, will be carried in a ISUP
Generic Address Parameter (GAP) since the called party number parameter now
carries the LRNB.  All the subsequent switches route the call by analyzing
the first six digits of the called party number.  The call will be routed to
Switch B where it retrieves the dialed number, 212-455-1234, from the GAP in
the received IAM.  

For a ported number, the called party number in the IAM contains the LRN
associated with the serving switch of the ported number.  For a non-ported
number, the called party number is the susbcriber number.  In either case,
the first six digits of the subscriber number or LRN are the same.  

If the TRIP information contains the reachability of subscriber number
ranges, A GW that can reach Switch A but not Switch B will have to indicate
that it can reach +1-212-455-0000 to +1-212-455-1233 and
+1-212-455-1235-+1-212-455-9999 (if only one number has been ported out of
Switch A).  The problem of this is that many ported-out numbers need to be
singled out from the reachable phone number ranges ranges.  This is a minor
problem when comparing to the bigger problem of updating the TRIP
information every time a number changes its assoicated LRN after been ported
to a new LEC.

For calls routed to the North America, it would be much simpler for a GW to
indicate the switches (e.g., LRNs (plus the country code 1)) it can reach
becuase the information is static.

James

-----Original Message-----
From: Jonathan Rosenberg [mailto:[email-masked]]
Sent: Saturday, December 04, 1999 12:21 AM
To: James Yu
Cc: [email-masked]'
Subject: Re: TRIP

James Yu wrote:
> 
> The telephone number ranges used for routing decisions need to consider
the
> impacts of number portability when routing calls to the SCNs where number
> portability is supported.  For example, the subscriber number (e.g.,
dialed
> number) and the network routing number (e.g., the Location Routing Number
or
> LRN) are in two different domains in the North America.  In Europe, the
> routing prefix plays a similar role as the LRN (one difference is that the
> LRN identifies a specific switch while the routing prefix may identify a
> switch or just a network).
> 
> If TRIP is used to pass routing information, it should be decided whether
> the "telephone number" is the subscriber number or the LRN/routing prefix.
> My opinion is that it should be the LRN/routing prefix.  This will keep
the
> routing information static rather than dynamic (e.g., changes every time a
> number is ported).  Any other thoughts?

I have a question. In your other message, you mention that the LRNs
identify switches, and that they do overlap with the subscriber numbers.
Thats actually OK, so long as the overlap is such that the LRN is for
the switch serving those subscribers whose numbers the switch overlaps.
In other words, if a switch has an LRN that is +1 212 455, that OK so
long as this switch serves the subscribers with numbers +1 212 455-0000
to +1 212 455-9999. The reason that this is OK is that a longest prefix
match on the LRN or a subscriber would still get you the right gateway.
So, my question is: is this the case? Or, is it the case that the LRN
has no relationship with the subscriber numbers it overlaps.

-Jonathan R.
-- 
Jonathan D. Rosenberg                       200 Executive Drive
Chief Scientist                             Suite 120 
dynamicsoft                                 West Orange, NJ 07052
[email-masked]                     FAX:   (732) 741-4778
http://www.cs.columbia.edu/~jdrosen         PHONE: (732) 741-7244
http://www.dynamicsoft.com

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From: Jonathan Rosenberg <[email-masked]>
Date: Mon, 06 Dec 1999 19:32:19 -0500

James,

Thanks for the excellent description of the process. Comments below:

> have questions.  The focus of this discussion is on LRN and number

Hmm, sounds familiar.... CIDR?

> information every time a number changes its assoicated LRN after been ported

I think TRIP should carry subscriber numbers, but that the LNP dip
should happen before the TRIP database is accessed. So, returning to
your example, a gateway reaching only switch A would advertise
+1-212-455* and a gateway reaching only switch B +1-212-466*. A PC users
calls +1-212-455-1234. The INVITE is sent to its local proxy, which
first does an LNP dip. It translates this to an LRN of +1-212-456, which
is looked up in TRIP. The longest prefix match (which is an exact match
for the LRN) is found. The INVITE is then sent to the next hop towards
that LRN as learned through TRIP. The original dialed number will be
carried in the To field. 

Note that we could also allow switch B to advertise +1-212-455-1234 in
TRIP. If the LNP dip is not done, the proxy will pick this because of
longest prefix matching, and the call still completes. However, I would
recommend doing this, since it will result in a completely flat TRIP
routing table.

-Jonathan R.

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From: "Hussein F. Salama" <[email-masked]>
Date: Mon, 06 Dec 1999 16:55:33 -0800

Jonathan Rosenberg wrote:
> first does an LNP dip. 

The N-1 agreement in classic telephony makes the (N-1)th network (i.e.
the next to last network), on the path towards the destination,
responsible for making the LNP database query. I wonder what's the
reason for this? Wouldn't executing the LNP database query at the
originating network have resulted in more efficient routing?    

> It translates this to an LRN of +1-212-456, which

Looks like a SIP extension is required to indicate to other signaling
points on the call path that an LNP query has already been done for this
destination, in order to avoid multiple LNP queries for the same call.
Is such a flag already there?

Hussein

-- 
Hussein F. Salama
Cisco Systems
Mail Stop SJC6/3, 170 W. Tasman Drive, San Jose, CA 95134
Voice: +1 (408) 527-7147, Fax: +1 (408) 527-1714

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