Protocol Stack

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The functional split between the EPC and E-UTRAN is shown in Figure 2.7. It represents the layered protocol stacks, which can be divided into user plane and control plane. The user plane is composed by the sub-layers Packet Data Convergence Protocol (PDCP), Radio Link Control (RLC) and Medium Access Control (MAC), which form Layer 2 (data link layer) (L2), and the Physical Layer (PHY), also known as Layer 1 (L1). The control plane additionally includes the Radio Resource Control (RRC), commonly named Layer 3 (network layer) (L3). Together, these layers form a protocol stack known as the Access Stratum (AS). The upper layer in the control plane, which terminates in the UE and in the MME, is referred to as Non- Access Stratum (NAS), whose protocols are completely independent of the access technology. These protocols and their main functionalities are summarized below [Ste11, 3GP14c, 3GP14d, 3GP11b, 3GP17b, 3GP11a, Eri16]:

Figure 2.7: Functional split between the E-UTRAN and EPC (adapted from [Eri16]). ˆ Non-Access Stratum (NAS): the NAS protocol is handled by the MME and includes

features such as EPS bearer management, authentication, security control procedures and different idle-mode procedures (e.g. paging). It is also responsible for assigning an IP address to a device.

ˆ Radio Resource Control (RRC): the RRC layer is responsible for handling the RAN-related procedures, including broadcast of system information necessary for the device to be able to communicate with a cell, transmission of paging messages originating from the MME (to notify the device about incoming connection requests), connection management (including the establishment of radio bearers), mobility functions such as cell (re)selection, measurement configuration and reporting, and handling of UE capabilities. It is also responsible for configuring all the lower layers.

ˆ Packet Data Convergence Protocol (PDCP): the PDCP layer processes RRC messages in the control plane and IP packets in the user plane. The main functions of the PDCP layer are IP header compression, security (integrity protection and ciphering) and support for reordering and retransmission during handover.

ˆ Radio Link Control (RLC): the RLC layer performs segmentation and reassembly of upper layer packets, in order to adapt them to the size which can actually be transmitted over the radio interface, as well as error correction through Automatic Repeat reQuest (ARQ).

ˆ Medium Access Control (MAC): the MAC layer, as illustrated in Figure 2.8, handles the mapping between logical channels (upper layer) and transport channels (lower layer). It also handles error correction through Hybrid Automatic Repeat reQuest (HARQ) and uplink and downlink scheduling information reporting.

ˆ Physical Layer (PHY): the physical layer provides data transport services to the MAC layer through the use of transport channels. It also handles time/frequency re- source mapping, Forward Error Correction (FEC) coding/decoding, modulation/de- modulation of physical channels, multi-antenna processing, amongst others.

To efficiently support various classes of services, LTE adopts a hierarchical channel struc- ture. As previously mentioned, there are three different channel types defined in LTE, each

Figure 2.8: Radio interface protocol architecture around the physical layer (adapted from [3GP11a]).

associated with a Service Access Point (SAP) between different layers (the circles in Figure 2.8). Logical channels provide services at the SAP between the MAC and RLC layers and are characterized by the type of information they carry. Transport channels provide services at the SAP between the MAC and PHY layers and are characterized by how the information is transferred over the radio interface. Physical channels are the actual implementation of transport channels over the radio interface, carrying control messages and user data.

2.3.1 Interface Protocols

Each interface in the network uses standard Internet Engineering Task Force (IETF) transport protocols, which are shown in Figure 2.9. Unlike the air interface, these interfaces use protocols from Layers 1 to 4 of the usual Open Systems Interconnection (OSI) model.

Figure 2.9: Transport protocols used by the network (adapted from [Chr12]).

At the bottom of the protocol stack, the transport network can use any suitable protocols for L1 and L2, such as Ethernet. Every network element is then associated with an IP address and the network uses the IP to route information from one element to another across

the underlying transport network.

Above IP, three transport layer protocols are used. User Datagram Protocol (UDP) trans- mits data packets from one network element to another, being the only transport protocol used in the user plane. Transmission Control Protocol (TCP) re-transmits packets if they arrive incorrectly. Stream Control Transmission Protocol (SCTP) is similar to TCP, but includes additional features that make it more suitable for the delivery of signalling mes- sages. The control plane chooses its transport protocol depending on the overlying signalling protocol.

The LTE user plane contains mechanisms to forward data correctly between the UE and the P-GW, as well as to quickly respond to changes in the UE’s location. These mechanisms are implemented by a 3GPP protocol known as the GPRS Tunelling Protocol-User plane (GTPv1-U) and are used over the S1-U, X2 and S5/S8 interfaces (shown in Figure 2.4). GTPv1-U forwards packets from one network element to another using a technique known as tunnelling.

LTE uses several signalling protocols. On the air interface, the eNB controls an UE’s radio communications by using signalling messages written using the RRC protocol, over the Uu interface. In the RAN, the serving MME controls the eNBs within its pool area using the S1 Application Protocol (S1AP) over the S1-MME interface, while the eNBs communicate with each other using the X2 Application Protocol (X2AP) X2 interface. If the access network is composed of a single eNB, no X2 interface is required.

Inside the EPC, the HSS and MME communicate over the S6a interface using the Diameter protocol, a standard protocol for authentication, authorization and accounting.

The interfaces S5/S8 and S11 utilize a 3GPP protocol known as the GPRS Tunelling Protocol-Control plane (GTPv2-C). This protocol allows for peer-to-peer communications between the different elements of the EPC and is responsible for managing the GTPv1-U tunnels that were previously mentioned in this section [Chr12].

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