Skip to content

LTE EPC Cheat Sheet: Control & User Plane Separation

1. Core Components & Roles

Component Plane Key Responsibilities
MME Control Mobility management, authentication, session establishment, NAS signaling, handover.
SGW-C Control Session management, bearer setup/modification, GTP-C signaling with MME/PGW-C.
SGW-U User User data routing, GTP-U tunnels, QoS enforcement.
PGW-C Control IP allocation, QoS/policy enforcement, PCRF interaction, session anchoring.
PGW-U User Data routing to external networks (Internet/IMS), NAT, filtering, QoS.

2. Key Interactions

MME ↔ SGW-C

  • Purpose: Session/bearer management, mobility (handover, TAU).
  • Protocol: GTP-C (Create/Modify/Delete Session Requests).
  • Example Flows:
  • UE attach: MME → SGW-C (Create Session Request).
  • Handover: MME → SGW-C (Modify Bearer Request).

MME ↔ PGW-C

  • Purpose: IP allocation, QoS/policy enforcement, PDN connectivity.
  • Protocol: GTP-C (for session management) + Diameter (PCRF interaction).
  • Example Flows:
  • IP assignment: MME → PGW-C (Create Session Request → DHCP/IPv6).
  • QoS update: PGW-C → PCRF (Diameter) → MME.

SGW-C ↔ PGW-C

  • Purpose: End-to-end session management, QoS coordination.
  • Protocol: GTP-C.

SGW-U ↔ PGW-U

  • Purpose: User data forwarding.
  • Protocol: GTP-U (tunneling).

3. Protocols Summary

Protocol Used Between Purpose
GTP-C MME ↔ SGW-C ↔ PGW-C Control plane signaling (session/bearer).
GTP-U SGW-U ↔ PGW-U User plane data tunneling.
Diameter PGW-C ↔ PCRF, MME ↔ HSS Policy/Qos (PCRF), authentication (HSS).
NAS UE ↔ MME Non-Access Stratum signaling (attach, TAU).

4. Common Procedures

UE Attach Procedure

  1. UE → MME: Attach Request (NAS).
  2. MME → SGW-C: Create Session Request (GTP-C).
  3. SGW-C → PGW-C: Create Session Request (GTP-C).
  4. PGW-C → PCRF: Policy Request (Diameter).
  5. PGW-C → SGW-C → MME: Create Session Response (IP assigned).
  6. MME → UE: Attach Accept (NAS).

Handover Procedure

  1. Source eNodeB → MME: Handover Required.
  2. MME → Target SGW-C: Modify Bearer Request (GTP-C).
  3. Target SGW-C → PGW-C: Modify Bearer Request (update paths).
  4. PGW-C → Target SGW-C → MME: Modify Bearer Response.
  5. MME → Target eNodeB: Handover Command.

5. 5G Evolution

  • MME → AMF (Access and Mobility Management Function).
  • SGW-C + PGW-C → SMF (Session Management Function).
  • SGW-U + PGW-U → UPF (User Plane Function).

6. Quick Troubleshooting Tips

  • UE cannot attach?
    Check MME ↔ HSS (Diameter) for authentication issues.
    Verify SGW-C/PGW-C GTP-C connectivity.
  • No data connectivity?
    Inspect SGW-U ↔ PGW-U GTP-U tunnels.
    Confirm PGW-U NAT/firewall rules.
  • QoS not applied?
    Validate PGW-C ↔ PCRF (Diameter) communication.