PDCP (Packet Data Convergence Protocol) is used for user data transmission in LTE networks.

RLC (Radio Link Control) is responsible for radio link control in LTE networks.

GTP (GPRS Tunneling Protocol) is used for signaling and control in LTE networks.

MAC (Medium Access Control) is responsible for medium access control in LTE networks.

MME (Mobility Management Entity) is used for mobility management in LTE networks.

EPC (Evolved Packet Core) is the core network architecture for LTE networks.

EAP (Extensible Authentication Protocol) is used for authentication and authorization in LTE networks.

HSS (Home Subscriber Server) is responsible for storing and managing subscriber data in LTE networks.

SIP (Session Initiation Protocol) is used for session management in LTE networks.

PCRF (Policy and Charging Rules Function) is responsible for policy and charging in LTE networks.

X2 is used for inter-eNB communication in LTE networks.

SGW (Serving Gateway) is responsible for routing and forwarding user data in LTE networks.

S1 is used for communication between the MME and the SGW in LTE networks.

PGW (Packet Gateway) is responsible for routing and forwarding user data between the LTE network and the external network.

S5 is used for communication between the SGW and the PGW in LTE networks.