TAC / LAC Decoder & TAI Builder
Decode a Tracking Area Code (TAC) or Location Area Code (LAC) and build the full TAI or LAI by entering a PLMN (MCC + MNC). Supports 2G, 3G, 4G LTE, and 5G NR.
PLMN — optional, needed to build full TAI
TAC / LAC Area Code Reference
Each generation of mobile network uses a different code to identify the paging area a UE belongs to. All use the same 3-byte BCD-encoded PLMN prefix — see the CGI Decoder for full cell-level identifiers including the LAC, or the IMSI Decoder to look up MCC and MNC for a specific operator.
| Generation | Area type | Code name | Bit width | Hex width | Identity name | Core node |
|---|---|---|---|---|---|---|
| 2G GSM | Location Area | LAC | 16-bit | 4 hex | LAI | BSC / MSC |
| 3G UMTS | Routing Area | LAC + RAC | 16 + 8 bit | 4 + 2 hex | RAI | RNC / SGSN |
| 4G LTE | Tracking Area | TAC | 16-bit | 4 hex | TAI | MME |
| 5G NR | Registration Area | TAC | 24-bit | 6 hex | TAI | AMF |
4G LTE — TAI hex structure
5G NR — TAI hex structure
PLMN BCD encoding (3 bytes · 6 hex) — shared by all generations
Byte 0 : MCC digit 2 (high nibble) | MCC digit 1 (low nibble)
Byte 1 : MNC digit 3 / 0xF (high nibble) | MCC digit 3 (low nibble)
Byte 2 : MNC digit 2 (high nibble) | MNC digit 1 (low nibble)
MNC digit 3 = 0xF when MNC is 2-digit (3GPP TS 24.008 §10.5.1.13).
Frequently Asked Questions
What is a Tracking Area Code (TAC)?▾
TAC is a 16-bit (LTE) or 24-bit (5G NR) identifier for a Tracking Area (TA), a group of cells that share paging resources. The MME (4G) or AMF (5G) pages all cells in a TA when it needs to reach a device in RRC-Idle. TAC 0x0000 and 0xFFFE are reserved in LTE. In 5G NR the TAC is extended to 24 bits to support more granular TA configurations.
What is a TAI (Tracking Area Identity)?▾
TAI = PLMN + TAC. The PLMN (3 bytes, BCD-encoded MCC+MNC) combined with the TAC uniquely identifies a Tracking Area globally. It appears in NAS Registration Accept, TAU Accept, and Handover messages.
What is the difference between TAC in 4G and 5G?▾
In LTE, TAC is 16 bits (range 0x0001–0xFFFE). In 5G NR, TAC is extended to 24 bits (range 0x000001–0xFFFFFE) to support larger registration areas and more precise TA lists per UE. Both versions are encoded in big-endian byte order.
What is a Location Area Code (LAC)?▾
LAC is the 2G/3G equivalent of TAC. It identifies a Location Area (set of cells) in GSM and UMTS. The LAI (Location Area Identity) = PLMN + LAC. In 3G, a Routing Area (RAI = PLMN + LAC + RAC) adds finer granularity for packet-switched services.