Location Area Codes (LAC)

Location Area Codes (LAC)

The desire to create databases containing MCC/MNC/LAC/CellID may seem a good idea but might produce inaccurate or erroenous results depending upon the way in which the information is used. It is known that LACs (and CellIDs for that matter) change and therefore a database becomes out of data fairly quickly.

LACs change for various reasons. Commonly LAC Dimensioning can be implemented due to heavy paging load or many LACs under one BSC causing frequent Location Updates are just two examples. The outcome of Dimensioning can require reprogramming BSC (GSM) / RNC(WCDMA) for each cell. Moreover, reprogramming at the MSC. On some occasions re-parenting at the MSC maybe required that requires re-programming LACs. Cell shapes, size and traffic capacity can change from the material time.

Unlike MCCs and MNCs, LACs are not published. LAC identifiers can by assigned by the network operator. It is essential as with all broadcast data to understand what the data mean and the data relevance to mobile telephone evidence.

Location Area Code (LAC) which is a fixed length code (of 2 octets) identifying a location area within a GSM PLMN. This part of the location area identification can be coded using a full hexadecimal representation except for the following reserved hexadecimal values:

0000, and FFFE

These reserved values are used in some special cases when no valid LAI exists in the MS.

The Location Area Identification is a type 3 information element with 6 octets length.
....8......7......6......5......4......3......2......1....
+-----------------------------------------------+
│ ...Location Area Identification IEI..│ octet 1
+-----------------------------------------------│
│ ....MCC digit 2 │ MCC digit 1..... │ octet 2
+-----------------------+-----------------------│
│ .....MNC digit 3 │ MCC digit 3.....│ octet 3
+-----------------------+-----------------------│
│ ...MNC digit 2 │ MNC digit 1...│ octet 4
+-----------------------------------------------│
│ .......................LAC......................│ octet 5
+-----------------------------------------------│
│ ............LAC (continued)............│ octet 6
+-----------------------------------------------+
Figure 10.5.3: Location Area Identification information element


Table 10.5.3: Location Area Identification information element
+----------------------------------------------------------------------+
MCC, Mobile country code (octet 2 and 3)
The MCC field is coded as in CCITT Rec. E212, Annex A.

If the LAI is deleted the MCC and MNC shall take the
value from the deleted LAI.

In abnormal cases, the MCC stored in the mobile
station can contain elements not in the set
{0, 1 ... 9}. In such cases the mobile station should
transmit the stored values using full hexadecimal
encoding. When receiving such an MCC, the network
shall treat the LAI as deleted.

MNC, Mobile network code (octet 3 bits 5 to 8, octet 4)
The coding of this field is the responsibility of each
administration but BCD coding shall be used. The MNC
shall consist of 2 or 3 digits. For PCS 1900 for NA, Federal
regulation mandates that a 3-digit MNC shall be used.
However a network operator may decide to use only two
digits in the MNC in the LAI over the radio interface. In this
case, bits 5 to 8 of octet 3 shall be coded as "1111".
Mobile equipment shall accept LAI coded in such a way.

Note 1: In earlier versions of this protocol, the possibility to use a one digit MNC in LAI was provided on the radio interface. However as this was not used this possibility has been deleted.

Note 2: In earlier versions of this protocol, bits 5 to 8 of octet 3 were coded as "1111". Mobile equipment compliant with these earlier versions of the protocol may be unable to understand the 3-digit MNC format of the LAI, and therefore unable to register on a network broadcasting the LAI in this format.

In abnormal cases, the MNC stored in the mobile
station can have
- digit 1 or 2 not in the set {0, 1 ... 9}, or
- digit 3 not in the set {0, 1 ...9, F} hex.
In such cases the mobile station
shall transmit the stored values using full hexadecimal
encoding. When receiving such an MNC, the
network shall treat the LAI as deleted.

The same handling shall apply for the network, if a
3-digit MNC is sent by the mobile station to a network using
only a 2-digit MNC.

LAC, Location area code (octet 5 and 6)
In the LAC field bit 8 of octet 5 is the most
significant bit and bit 1 of octet 6 the least
significant bit.

The coding of the location area code is the
responsibility of each administration except that
two values are used to mark the LAC, and hence the
LAI, as deleted. Coding using full hexadecimal
representation may be used. The location area code
consists of 2 octets.

If a LAI has to be deleted then all bits of the
location area code shall be set to one with the
exception of the least significant bit which shall be
set to zero. If a SIM is inserted in a Mobile Equipment
with the location area code containing all zeros, then
the Mobile Equipment shall recognise this LAC as part
of a deleted LAI
+-----------------------------------------------------+

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