RRC vs NAS: Understanding 3GPP Signaling Protocols
Complete guide to differentiating the two pillars of mobile signaling: RRC (radio access layer) and NAS (non-access layer). Layers, messages, encoding and use cases.
Quick Comparison
| Criterion | RRC | NAS |
|---|---|---|
| Full Name | Radio Resource Control | Non-Access Stratum |
| Layer | Layer 3. Access Stratum (AS) | Layer 3. Non-Access Stratum |
| Endpoints | UE ↔ eNB / gNB | UE ↔ MME / AMF |
| Encoding | ASN.1 UPER (LTE/NR), PER (UMTS) | Binary TLV (Tag-Length-Value) |
| Specifications | TS 36.331, TS 38.331, TS 25.331 | TS 24.301, TS 24.501, TS 24.008 |
| Primary Function | Radio resource management | Attachment, authentication, sessions |
| Transport | Directly on SRB (PDCP → RLC → MAC) | Encapsulated in RRC container |
RRC Protocol in Detail
RRC (Radio Resource Control) operates at Layer 3 of the radio access stack (Access Stratum). It handles all signaling between the mobile device (UE) and the base station (eNodeB in LTE, gNodeB in 5G NR, NodeB/RNC in UMTS).
Key RRC Messages
Connection Setup
RRC Connection Setup / Setup Complete, establishes the radio link between UE and network.
Reconfiguration
RRC Connection Reconfiguration, handover, carrier addition/removal, measurements.
System Information
MIB, SIB1–SIB24, cell parameters broadcast to all devices in the cell.
Measurements
Measurement Report, device reports RSRP, RSRQ, SINR of neighboring cells.
RRC Channels by Technology
| Technology | Logical Channels | Encoding |
|---|---|---|
| 5G NR | BCCH-BCH, BCCH-DL-SCH, DL-CCCH, DL-DCCH, UL-CCCH, UL-DCCH | ASN.1 UPER |
| 4G LTE | BCCH-BCH, BCCH-DL-SCH, PCCH, DL-CCCH, DL-DCCH, UL-CCCH, UL-DCCH, MCCH, SC-MCCH | ASN.1 UPER |
| 3G UMTS | BCCH-BCH, BCCH-FACH, PCCH, CCCH, DL-DCCH, UL-CCCH, UL-DCCH, MCCH, MSCH | ASN.1 PER |
| 2G GSM | CCCH, SACCH | CSN.1 |
NAS Protocol in Detail
NAS (Non-Access Stratum) is the end-to-end signaling protocol between the device and core network. Unlike RRC which terminates at the base station, NAS passes through the radio access network transparently to reach the MME (LTE) or AMF (5G NR).
NAS Sub-protocols
EMM / 5GMM
EPS / 5GS Mobility Management, attachment, authentication, TAU, Registration Update.
ESM / 5GSM
EPS / 5GS Session Management. PDN/PDU session setup, QoS, guaranteed bitrate.
MM / GMM (3G/2G)
Mobility Management. IMSI attach, location update, UMTS/GSM authentication.
SM / CC / SS
Session Management, Call Control, Supplementary Services, data sessions, CS calls, USSD.
How RRC and NAS Interact
NAS messages are encapsulated inside RRC containers. When a device needs to send a NAS message (e.g., Attach Request), the protocol stack works as follows:
This is why a complete decoder must support both: RRC to understand the radio context, and NAS to analyze the network logic.
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Frequently Asked Questions. RRC vs NAS
What is the main difference between RRC and NAS?
RRC manages radio signaling between the device and base station (access stratum). NAS manages end-to-end signaling between the device and core network (non-access stratum): attachment, authentication, session management.
Do RRC and NAS use the same encoding?
No. RRC messages use ASN.1 UPER (LTE/5G NR) or PER (UMTS). NAS messages use binary TLV (Tag-Length-Value) encoding defined by 3GPP in TS 24.x specifications.
Can you decode RRC and NAS with the same tool?
HiCellTek is the only online decoder combining RRC + NAS multi-RAT (2G, 3G, 4G, 5G). Most tools only cover one protocol.
Is NAS encapsulated inside RRC?
Yes. NAS messages are carried in RRC containers (DL/UL Information Transfer). The device first decodes the RRC, then extracts and processes the encapsulated NAS message.
What are the most common RRC messages?
RRC Connection Setup, RRC Connection Reconfiguration (handover, measurements), SIBs (System Information Blocks), and RRC Connection Release.
What are the most common NAS messages?
Attach Request/Accept, Authentication Request/Response, Tracking Area Update, PDN Connectivity Request, and Detach Request.