The Mysterious World of the LERG
by Tom Morrow 3.0
The LERG is the Local Exchange Routing Guide.
Basically, this is the document that helps assist telcos route calls. In order to better understand what the LERG is, some definitions are in order.
Please note that since deregulation these terms are less defined, however it helps to think of the different responsibilities of the different types of companies.
CLLI: Common Language Location Identification. An 11-character alphanumeric code used to identify physical locations of equipment such as buildings, COs, antennas, telephone poles, etc.
CO: Central Office. These house Class 5 switches. These are nondescript buildings that house both the wiring frame and the telephone switch(es). They serve small geographical areas and connect to other central offices through tandem offices or to other central offices via direct interoffice trunks.
IXC: Interexchange Carrier. Long distance carriers like AT&T, Sprint, MCI, and the like. They provide inter-LATA calls.
LATA: Local Access and Transport Area. Basically, this is the area where a LEC can carry calls. If a call is to move from one LATA to another, the call must be handed off to an IXC.
LEC: Local Exchange Carrier. I am not going to differentiate between Competitive LECs (CLEC), Incumbent LECs (ILEC), Bell Operating Companies, etc. This is the local phone company who provides dial tone. They provide local and intra-LATA calls.
LNP: Local Number Portability. This is the ability to terminate a phone number away from its "homed" CO. This is done using SS7 in North America to check a database to see if the call should be routed to its home CO or to another using the LRN.
LRN: Local Routing Number. This is a 10-digit number indicating the network address of the terminating CO. It is generally of the form of "home" NPA-NXX-9999 or -0000 (or other variation). It basically says, "to route this LNP'd number, route it like this LRN number." LNP is complex and will be the subject of another article by me.
Tandem: Also can be known as a Class-4 switch. These connect central offices to each other when no direct interoffice trunks exist. In the purest sense, they do not serve end users, only COs.
It is important to know the LERG is only a database and not an application.
It is up to the user of the LERG to take the data and process it to meet their own needs. It is primarily used for:
- Routing inter-LATA calls by IXCs.
- Routing intra-LATA calls based on "what is local."
One example of this is can be seen at the beginning of most phone books where there is a chart that shows where calls can be made that are still local and not charged long distance rates. An NPA-NXX on the west side of town can usually call further west than a centrally (or otherwise) located NPA-NXX can.
There are 14 sections in the LERG:
LERG 1: Contains the Operating Company Number (OCN) of all the carriers used throughout the LERG. This can be used to link the company name with other records in the LERG. It also contains contact information for each company in the record. (Example)
LERG 2: Contains country codes. It does not contain city codes. It is used only to route international calls out of North America properly. (Example)
LERG 3: Has information on Numbering Plan Areas (NPA). This is just a fancy way of saying area codes. This used to be in the form of N[0/1]X, but now is in the form of NXX. This section is important because of all the area code overlays and splits going on in these last few years. It includes the effective date of the NPA, permissive dialing periods, time zones, and in some cases test telephone numbers. (Example)
LERG 4: Has SS7 point code assignments. A point code is a unique number identifying a network node. It is of the form of XXX-YYY-ZZZ with XXX being the network, YYY the cluster, and ZZZ the member. This assignment is cataloged to a certain company, rather than the specific node. The information is often in ranges. You will not be able to determine the point code of your local CO from this database. (Example)
LERG 5: Has LATA information. This information includes LEC region, LATA name, and the associated NPAs within the LATA. An NPA may be split between two LATAs. (Example)
LERG 6: Is one of the more interesting sections in the LERG. Given the NPA and NXX, the "home" switch can be identified. This includes the switch CLLI code, rate center location state, OCN, and LATA. Also shown here is the Central Office Code (COC), which determines the type of CO. The three most common are End Office Code (EOC), Public Mobile Carrier (PMC), and Access Tandem Code (ATC). LATA ATCs can be found in the LERG 6 ATC. (Example)
LERG 7: Expands on the information provided in LERG 6. Given the CLLI, one can find the LATA, LATA name, equipment type (5ESS, DMS-100, others), OCN, and physical location (including street address, city, ZIP Code, and Vertical & Horizontal Coordinates). (Example)
LERG 8: Contains rate center information. This lists rate center identifiers, dates of any changes, LATA, NPAs, localities served, geographical limits to LNP, if a split is set, and if there is an embedded overlay of an NPA. This section is useful for setting up and maintaining switch rate tables. (Example)
LERG 9: Lists to whom switches are "homed" to. Given a LATA and tandem switch, one can see all the "homed" offices connected to it. Listed are long distance Feature Group B/C/D, Operator Services Tandems, end offices, etc. This can be used to map connections between offices. (Example)
LERG 10: Has operator access codes. Operator services include Directory Assistance (DA), inward, toll terminal, toll station, T&C callback (Time & Charges), and many others. (Example)
LERG 11: Presents the data in LERG 10 in different ways, specifically given a location name (from LERG 6) the operator access codes are given. (Example)
LERG 12: Lists LRNs by LATA. Remember, when a number is LNP'd to another switch (not its home switch), the LNP database will return with a LRN for routing. If done correctly, no fast busies or intercept recordings should occur. (Example)
LERG 13: Is the database relating to number pooling. Number pooling allows COs to share a whole NPA-NXX. This helps better distribution of scarce numbers amongst COs without needing to create more NPAs. An NPA-NXX is 10,000 numbers. NPA-NXX are usually shared at 1000 number blocks.
LERG 14: Is the final section and lists Feature Group D Tandems for information: NPA-555-1212
The LERG is the best way for a LEC to determine how to route calls. Here is an example:
You arrive at Orlando International Airport and you need to call your friend staying at the Contemporary Resort Hotel at Walt Disney World. After picking up your bags near the car rental counters, you pick up the payphone nearby. From 407-514-8500 you call to 407-824-1000, the resort's main number.
Using LERG 6 the NPA-NXX of 407-514 the switch CLLI is ORLDFLERDS0 in LATA 45808 (most LATAs are 3-digits, but Florida is an exception). This end office has an OCN of 7391. Using LERG 1 we find that OCN belonging to Sprint Metropolitan Networks, Inc. Using LERG 7, ORLDFLERDS0 is a Lucent 5ESS switch located at 200 E. Robinson St., Orlando, FL 32801.
Again using LERG 6, the NPA-NXX of 407-824, the switch CLLI is LKBNFLXBDS0 in LATA 45807. Checking the front of the phone book at the payphone, you know this is a local call. The OCN of LKBNFLXBDS0 is 0330. From LERG 1, the OCN of 0330 is Smart City Telecom, LLC. Again using LERG 7, LKBNFLXBDS0 is a DMS-100/200 located at 3100 Bonnet Creek Rd., Lake Buena Vista, FL 32830.
Now if ORLDFLERDS0 does not have inter-machine trunks directly to LKBNFLXBDS0, the call must go to the LKBNFLXBDS0 tandem. Using the above methods, we find that this tandem is a DMS-100 colocated at the same facility as LKBNFLXBDS0 with the OCN of 0330.
With the LERG, one can learn a lot about the current state and future changes of the telecom network in the USA. Every end office and tandem is listed, along with CLLIs, addresses, and owners of the offices. The relationship between end offices (Class 5) and tandems (Class 4) are addressed. Changes in the network are occurring at a fast pace. If you have ever called a valid new number in your local area but gotten an intercept recording, now you can understand why and appreciate the difficulty some companies have in dealing with the changes.
Sources: Telcordia LERG Routing Guide, Telcordia Notes on the Networks SR-2275
