NLT - Number Length Table

The NLT should be used when an exchange has a remote LIM in a different area (with an other area code).

For the US market the NLT can also be used to distinguish between conflicting numbers using a combination of timer and number length analysis.

The table is not mandatory and if the dialed number is not found in the data base the analysis proceeds to the next table, the DNT1.

Area Code Prefixing

If an exchange consists of LIMs (remote LIMs) situated in different areas (with different area codes), a dialed local public number intends to address different public subscribers depending on in which LIM the number is dialed.

An example: A call to the local public number 20802 from a LIM that is situated in the area code 0455, has to be separated from a call to the local public number 20802 from a LIM that is situated in the area code 0451. When the caller dials only the local public number 20802 (without the area code) this information is not enough to route that call correctly, the exchange that the LIMs belong to cannot see that the calls originated in different area codes. To solve this the dialed number must be modified t o also include the area code of the originating LIM before routing the call.

It is strongly recommended not to use customer numbers (parameter CUST in command RODDI) to indicate what LIM the call originated in, and then use this information to control the routing of the call. Instead, the NLT should be used to insert the LIMs own area code between the LAC and the subscriber number of the dialed local public number, that is, all dialed local public numbers should be translated into the national format. Then there is no need to treat calls differently depending on which LIM the call originated in when breaking out to the public network. The number will always be in the national format and thus uniquely define the called subscriber, no matter what area code the call originated in.

If the call is to be routed within a private network, the routing should be set up to always forward the dialed number including LCR access code and trunk code (area code) to the next exchange in the private network.

The area code flag, ACF, should be set to Y (as in Yes) to indicate that a dialed number should be prefixed with the own area code:

LCDDI:TAB=NLT,ENTRY=002,.............,ACF=Y; All public numbers beginning with digit 2 (in this example public numbers beginning with digit 2 can only be to local public destinations) will be prefixed with the own area code.

LCLDI:LIM=x,........,AC=0455; The own area code for LIM x is initiated with the LCLDI command, in this example the AC is 0455. This initiation must be done for all LIMs in the exchange, with their respective area codes.

IPGDI:DOMAIN="z",LIM=x,........,AC=0477; The own area code for domain "z", which belongs to LIM x is initiated with the IPGDI command, in this example the AC is 0477.

The result is that the dialed local public number 002... is modified into 0004552... if it originates in LIM x.

And it is modified into 0004772... if it originates in domain "z".

This initiation must of course be done for all LIMs and domains in the exchange, with their respective area codes.

In a branch office scenario, the initiation of domain area codes for the branch offices of course requires the initiation of a LIM area code for all LIMs in the main site exchange in order to keep consistency between IP and non IP extensions.

Conflicting Numbers

For the US application the NLT also offers extended analysis of conflicting numbers.

In the US numbering plans a complete public number consists of 10 digits:

• The first three digits represents the area code (AC).
• The following three digits represents the office code (OC).
• The last four digits represents the subscriber number.

The length of the public number is 3+3+4=10 digits. Add the LAC (9) and the total length will be 11 digits.

The shape or pattern of the ACs and the OCs are specific, but when interchangeable ACs or OCs are introduced conflicts may occur.

Below is an example where the number 525-2591 either can be a complete number in the own area:

• 525 is the OC
• 2591 is the subscriber number

  The total length of the dialed number (including the LAC) will be 8 digits.

or a long distance number:

• 525 is the AC
• 259 is the OC
• 1 is the first digit of four in the subscriber number.

  The total length of the dialed number (including the LAC) will be 11 digits.

To be able to distinguish between these two cases a timer is used to decide when the whole number is dialed, and then the length of the number is analyzed. The concluded number length is used by the system in the further handling of the call.

When initiating the number the parameter MIN should state the total length of the number in the own area, that is, 8 digits. The parameter MAX should state the total length of the long distance number, that is, 11 digits.

LCDDI:TAB=NLT,ENTRY=95252591,CONF=Y,MIN=8,MAX=11; The number 95252591 is initiated as a conflict number, with two possible lengths, 8 or 11 digits.

After the number length has been determined to either of the two, the call will proceed to a DNT where the number will be transformed, and so on.

The number length analysis can also be combined with the insertion of the own area code. In this case the insertion of the own are code takes place if the length of the dialed number is the same as the length initiated in the parameter MIN. If more digits are dialed the area code is not inserted.

LCDDI:TAB=NLT,ENTRY=95252591,CONF=Y,MIN=8,ACF=Y; The number 95252591 is initiated as a conflict number, when this number is dialed the own area code is inserted. If more digits are dialed the own area code is not inserted and the call proceeds to a DNT for further analysis.