ASN.1 (Abstract Syntax Notation One) is an ITU-T standardized data description language (X.680). In mobile telecom, it defines the structure of RRC and NAS signalling messages, encoded in PER (Packed Encoding Rules) to minimize frame size on the radio interface.
What is ASN.1?
The universal language that structures signalling messages in 2G through 5G mobile networks.
Detailed explanation
ASN.1 separates the abstract data definition from its concrete encoding. An ASN.1 schema describes a message structure (fields, types, constraints) independently of how it will be transmitted. Encoding rules (PER, BER, DER, XER) then define how to convert this abstract structure into a byte sequence.
In mobile telecom, ASN.1 is ubiquitous. The RRC protocol uses ASN.1 for every message exchanged between device and base station: RRC Setup, RRC Reconfiguration, Measurement Report. NAS also uses ASN.1 for attachment, authentication and session management messages.
PER (Packed Encoding Rules) is preferred on the radio interface because it produces the most compact messages. Unlike BER which transmits full structure (type, length, value), PER exploits schema constraints to eliminate redundant information. A PER-encoded RRC message can be 5 to 10 times smaller than its BER equivalent.
The 3GPP specifications publish ASN.1 schemas for each protocol: TS 36.331 for LTE RRC, TS 38.331 for NR RRC, TS 24.301 for EPS NAS, TS 24.501 for 5GS NAS. Decoding a message requires the matching schema version, making manual analysis complex without dedicated tools.
ASN.1 encoding rules
| Encoding | Full name | Primary usage |
|---|---|---|
| PER | Packed Encoding Rules | Radio interface (RRC, NAS) |
| BER | Basic Encoding Rules | SNMP, LDAP, network protocols |
| DER | Distinguished Encoding Rules | X.509 certificates, security |
| XER | XML Encoding Rules | Readability, debugging, logs |
How HiCellTek decodes ASN.1
The L3 Decoder module in HiCellTek integrates 3GPP ASN.1 schemas for RRC (LTE and NR) and NAS (EPS and 5GS) protocols. Decoding is automatic: every signalling message captured via the Qualcomm DIAG interface is decoded in real time and displayed in a structured view. Decoded messages are exportable in standard formats.
Frequently asked questions
Why is ASN.1 used in telecom?
What is the difference between ASN.1 BER, DER and PER?
How do you decode an ASN.1 message from a mobile network?
HiCellTek automatically decodes RRC and NAS messages encoded in ASN.1 PER on Android smartphones.