In LTE architecture, core network includes: Mobility Management Entity (MME), Serving Gateway (SGW), Packet Data Network Gateway (PDN GW) whereas E-UTRAN has E-UTRAN NodeB (eNB). With increased data rates, improved spectrum efficiency and packet-optimized system, LTE technology is set to drive machine to machine technology and data intensive applications.

The figures shown below provide mapping of protocols to corresponding specifications.

Protocol structure of control plane in between UE & MME is shown below.

Non-Access-Stratum (NAS) Protocol

The non-access stratum (NAS) is highest stratum of the control plane between UE and MME at the radio interface. Main functions of the protocols that are part of the NAS are the support of mobility of the user equipment (UE) and the support of session management procedures to establish and maintain IP connectivity between the UE and a packet data network gateway (PDN GW).

• NAS control protocol performs followings:
• EPS bearer management;
• Authentication;
• ECM-IDLE mobility handling;
• Paging origination in ECM-IDLE;
• Security control.

Radio Resource Control (RRC)

RRC protocol layer exists in UE & eNodeb, It is part of LTE air interface control plane. The main services and functions of the RRC sublayer include:

• Broadcast of System Information related to the non-access stratum (NAS);
• Broadcast of System Information related to the access stratum (AS);
• Paging;
• Establishment, maintenance and release of an RRC connection between the UE and E-UTRAN
• Security functions including key management;
• Establishment, configuration, maintenance and release of point to point Radio Bearers;
• Mobility functions
• QoS management functions;
• UE measurement reporting and control of the reporting;
• NAS direct message transfer to/from NAS from/to UE.

Packet Data Convergence Protocol (PDCP)

PDCP protocol layer exists in UE & eNodeb, it is part of LTE air interface control and user planes.

The main services and functions of the PDCP sublayer for the user plane include:

Header compression and decompression:

• ROHC only;
• Transfer of user data;
• In-sequence delivery of upper layer PDUs at PDCP re-establishment procedure for RLC AM;
• Duplicate detection of lower layer SDUs at PDCP re-establishment procedure for RLC AM;
• Retransmission of PDCP SDUs at handover for RLC AM;
• Ciphering and deciphering;
• Timer-based SDU discard in uplink.
• The main services and functions of the PDCP for the control plane include:
• Ciphering and Integrity Protection;
• Transfer of control plane data.

Radio Link Control (RLC)

RLC protocol layer exists in UE & eNodeb, it is part of LTE air interface control and user planes.

The main services and functions of the RLC sublayer include:

• Transfer of upper layer PDUs;
• Error Correction through ARQ (only for AM data transfer);
• Concatenation, segmentation and reassembly of RLC SDUs (only for UM and AM data transfer);
• Re-segmentation of RLC data PDUs (only for AM data transfer);
• In sequence delivery of upper layer PDUs (only for UM and AM data transfer);
• Duplicate detection (only for UM and AM data transfer);
• Protocol error detection and recovery;
• RLC SDU discard (only for UM and AM data transfer);
• RLC re-establishment.

Medium Access Control (MAC)

The main services and functions of the MAC sublayer include:

• Mapping between logical channels and transport channels;
• Multiplexing/demultiplexing of MAC SDUs belonging to one or different logical channels into/from transport blocks (TB) delivered to/from the physical layer on transport channels;
• Scheduling information reporting;
• Error correction through HARQ;
• Priority handling between logical channels of one UE;
• Priority handling between UEs by means of dynamic scheduling;
• Transport format selection;
• Padding.

Air Interface Physical Layer

The LTE air interface physical layer offers data transport services to higher layers. The access to these services is through the use of a transport channel via the MAC sub-layer. The physical layer is expected to perform the following functions in order to provide the data transport service:

• Error detection on the transport channel and indication to higher layers
• FEC encoding/decoding of the transport channel
• Hybrid ARQ soft-combining
• Rate matching of the coded transport channel to physical channels
• Mapping of the coded transport channel onto physical channels
• Power weighting of physical channels
• Modulation and demodulation of physical channels
• Frequency and time synchronisation
• Radio characteristics measurements and indication to higher layers
• Multiple Input Multiple Output (MIMO) antenna processing
• Transmit Diversity (TX diversity)
• Beamforming
• RF processing

S1 Application Protocol (S1AP)

S1AP provides the signalling service between E-UTRAN and the evolved packet core (EPC) and has following functions:

• E-RAB management function
• Initial Context Transfer function
• UE Capability Info Indication function
• Mobility Functions
• S1 interface management functions
• NAS Signalling transport function
• S1 UE context Release function
• UE Context Modification function
• Status Transfer
• Trace function
• Location Reporting
• S1 CDMA2000 Tunneling function
• Warning message transmission function
• RAN Information Management (RIM) function
• Configuration Transfer function

S1 Signalling Transport

S1 signalling bearer provides the following functions:

• Provision of reliable transfer of S1-AP message over S1-MME interface.
• Provision of networking and routeing function
• Provision of redundancy in the signalling network
• Support for flow control and congestion control

L2 - Data link layer

Support of any suitable data link layer protocol, e.g. PPP, Ethernet IP layer

• The eNB and MME support IPv6 and/or IPv4
• The IP layer of S1-MME only supports point-to-point transmission for delivering S1-AP message.
• The eNB and MME support the Diffserv Code Point marking

Transport layer

• SCTP is supported as the transport layer of S1-MME signalling bearer.
• SCTP refers to the Stream Control Transmission Protocol developed by the Sigtran working group of the IETF for the purpose of transporting various signalling protocols over IP network.
• There is only one SCTP association established between one MME and eNB pair.
• The eNB establishes the SCTP association. The SCTP Destination Port number value assigned by IANA to be used for S1AP is 36412.

S1 layer 1

The main functions of S1 interface layer 1 are as following:

• Interface to physical medium;
• Frame delineation;
• Line clock extraction capability;
• Layer 1 alarms extraction and generation;
• Transmission quality control.