02 Air Interface Protocols v04

8/19/2019 02 Air Interface Protocols v04

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1 © Nokia Siemens Networks

Air Interface ProtocolsLTE Air Interface Course



2 © N ok ia S iemens N etw ork s

At the end of this module, you will be able to:

• Explain the protocol stack of the LTE air interface

• Name the functionalities of the single layers of the radio protocolarchitecture

• Discuss the layer 2 data flow

• Give one example for the air interface protocols configuration

Module Objectives




Air Interface Protocols

Radio Protocols Architecture

RRC Tasks and States

Layer 2 Functions and Data Flow

Protocols Configuration Example





MAC

RLC

PDCP

Physical Layer

RRC

L1

L2

L3

Radio Bearer

Logical Channel

Transport Channels

Control Plane User Plane

Physical Channels




Radio Protocols Architecture (1/2)

The EUTRAN radio protocol model specifies the protocols terminated between UE and eNB. The protocol s tack follows

the standard guidelines for radio protocol architectures (ITU-R M1035) and is thus quite similar to the WCDMAprotocol stack of UMTS.

The protocol stack defines three layers: the physical layer (layer 1), data link and access layer (layer 2) and layer 3hosting the access stratum and non-access stratum control protocols as well as the application level software (e.g.IP stack).

physical layer: The physical layer forms the complete layer 1 of the protocol stack and provides the basic bittransmission functionality over air. In LTE the physical layer is driven by OFDMA in the downlink and SC-FDMA inthe uplink. FDD and TDD mode can be combined (depends on UE capabilities) in the same physical layer. Thephysical layer uses physical channels to transmit data over the radio path. Physical channels are dynamicallymapped to the available resources (physical resource blocks and antenna ports). To higher layers the physical layeroffers its data transmission functionality via transport channels. Like in UMTS a transport channel is a block orientedtransmission service with certain characteristics regarding bit rates, delay, collision risk and reliability. Note that incontrast to 3G WCDMA or even 2G GSM there are no dedicated transport or physical channels anymore, as allresource mapping is dynamically driven by the scheduler.

MAC (Medium Access Control): MAC is the lowest layer 2 protocol and its main function is to drive the transportchannels. From higher layers MAC is fed with logical channels which are in one-to-one correspondence with radiobearers. Each logical channel is given a priority and MAC has to multiplex logical channel data onto transportchannels. In the receiving direction obviously demultiplexing of logical channels from transport channels must takeplace. Further functions of MAC will be collision handling and explicit UE identification. An important function for theperformance is the HARQ functionality which is official part of MAC and available for some transport channel types.




Radio Protocols Architecture (2/2)

RLC (Radio Link Control): Each radio bearer possesses one RLC instance working in either of the three modes: UM

(Unacknowledged), AM (Acknowledged) or TM (Transparent). Which mode is chosen depends on the purpose ofthe radio bearer. RLC can thus enhance the radio bearer with ARQ (Automatic Retransmission on reQuest) usingsequence numbered data frames and status reports to trigger retransmission. Note that it shall be possible to triggerretransmissions also via the HARQ entity in MAC. The second functionality of RLC is the segmentation andreassembly that divides higher layer data or concatenates higher layer data into data chunks suitable for transportover transport channels which allow a certain set of transport block sizes.

PDCP (Packet Data Convergence Protocol): Each radio bearer also uses one PDCP instance. PDCP is responsiblefor header compression (ROHC RObust Header Compression; RFC 3095) and ciphering/deciphering. Obviouslyheader compression makes sense for IP datagram's, but not for signaling. Thus the PDCP entities for signalingradio bearers will usually do ciphering/deciphering only.

RRC (Radio Resource Control): RRC is the access stratum specific control protocol for EUTRAN. It will provide therequired messages for channel management, measurement control and reporting, etc.

NAS Protocols: The NAS protocol is running between UE and MME and thus must be transparently transferred viaEUTRAN. It sits on top of RRC, which provides the required carrier messages for NAS transfer.




FDD | TDD - Layer 1

( DL: OFDMA, UL: SC-FDMA )

FDD | TDD - Layer 1


Medium Access Control (MAC)Medium Access Control (MAC)

Physical Channels

Transport Channels

RLC

(Radio Link

Control)

RLC

(Radio Link

Control)

…

PDCP’

(Packet Data

Convergence

Protocol)

PDCP’

(Packet Data

Convergence

Protocol)

…

RLC

(Radio Link

Control)

RLC

(Radio Link

Control)

PDCP’

(Packet Data

Convergence

Protocol)

PDCP’

(Packet Data

Convergence

Protocol)

RLC

(Radio Link

Control)

RLC

(Radio Link

Control)

PDCP

(Packet Data

Convergence

Protocol)

PDCP

(Packet Data

Convergence

Protocol)

RLC

(Radio Link

Control)

RLC

(Radio Link

Control)

PDCP

(Packet Data

Convergence

Protocol)

PDCP

(Packet Data

Convergence

Protocol)

RLC

(Radio Link

Control)

RLC

(Radio Link

Control)

PDCP

(Packet Data

Convergence

Protocol)

PDCP

(Packet Data

Convergence

Protocol)

Logical Channel

(E-)RRC

(Radio Resource Control)

(E-)RRC


IP / TCP | UDP | …IP / TCP | UDP | …

Application LayerApplication Layer

Radio Bearer

ROHC (RFC 3095)

Security

Segment./Reassembly

ARQ

Scheduling /

Priority Handling

HARQ

De/Multiplexing

CRC

Coding/Rate Matching

Interleaving

Modulation

Resource Mapping/MIMO

NAS Protocol(s)

(Attach/TA Update/…)

NAS Protocol(s)





MME

NAS Protocols Transfer

eNBUE MME

NAS NAS

RRC RRC

PDCP PDCP

RLCRLC

MAC MAC

PHY PHY











1 0 © N ok ia S iem en s N et wor ks

RRC Tasks (1/2)

The RRC protocol for EUTRAN is responsible for the basic configuration of the radio protocol stack. But one should

note, that some radio management functions (scheduling, physical resource assignment for physical channels) arehandled by layer 1 and layer 2 autonomously. MAC and layer 1 signaling has usually delays that are within 10 ms,whereas RRC signaling usually takes something around 100 ms and more to complete an operation.

The RRC functional list is of course quite long.

System Information Broadcasting: The NAS and access stratum configuration of the network and the cell must beavailable to any UE camping on a cell. This information is coded as RRC message.

Paging: To locate an LTE_IDLE UE within a tracking area the RRC protocol defines a paging signaling message andthe associated UE behavior.

RRC Connection Management: The UE can have two major radio states: RRC_CONNECTED or RRC_IDLE. Toswitch between the states an RRC connection establishment and release procedure is defined. With the stateRRC_CONNECTED the existence of signaling radio bearers and UE identifiers (C-RNTI) is associated.

EUTRAN Security: Access layer security in EUTRAN consists of ciphering (PDCP) and integrity protection for RRCmessages.

Management of Point-to-Point Radio Bearers: Point-to-point radio bearers are signaling and user data radio bearersfor SAE bearers. RRC is used to create, modify and delete such radio bearers including the associated lower layer

configuration (logical channels, RLC mode, transport channels, multiplexing, …).

(continued on the next slide)




RRC Tasks (2/2)

Mobility Functions: When a UE is in state LTE_ACTIVE, the mobility control is at the eNB. This includes handover from

one EUTRAN cell to another or also inter-system changes. To assist handover decisions in the eNB RRC definesprocedures for measurement control and reporting. In LTE_IDLE mode the UE performs automatic cell re-selection,RRC takes control over this process within the UE.

MBMS (Multimedia Broadcast Multicast Service): RRC is used to inform UEs about available MBMS services in a celland is also used to track UEs that registered for a certain multicast service. This allows the eNB to manage MBMSradio bearers which are usually point-to-multipoint.

QoS Control: The RRC protocol will be QoS aware, allowing implementation of radio bearers with different QoS withinthe UE.

Transfer of NAS Messages: NAS messages are sent and received through the EUTRAN protocol stack. RRC providescarrier services for such messages.




FDD | TDD - Layer 1


FDD | TDD - Layer 1


Medium Access Control (MAC)Medium Access Control (MAC)

Physical Channels

Transport Channels

RLC

(Radio Link

Control)

RLC

(Radio Link

Control)

…

PDCP’

(Packet Data

Convergence

Protocol)

PDCP’

(Packet Data

Convergence

Protocol)

…

RLC

(Radio Link

Control)

RLC

(Radio Link

Control)

PDCP’

(Packet Data

Convergence

Protocol)

PDCP’

(Packet Data

Convergence

Protocol)

RLC

(Radio Link

Control)

RLC

(Radio Link

Control)

PDCP

(Packet Data

Convergence

Protocol)

PDCP

(Packet Data

Convergence

Protocol)

RLC

(Radio Link

Control)

RLC

(Radio Link

Control)

PDCP

(Packet Data

Convergence

Protocol)

PDCP

(Packet Data

Convergence

Protocol)

RLC

(Radio Link

Control)

RLC

(Radio Link

Control)

PDCP

(Packet Data

Convergence

Protocol)

PDCP

(Packet Data

Convergence

Protocol)

Logical Channel

(E-)RRC


(E-)RRC


IP / TCP | UDP | …IP / TCP | UDP | …

Application LayerApplication Layer

Radio Bearer

NAS System Information (BCCH)

E-UTRAN System Info. (BCCH)

Paging (PCCH)

RRC Connection Management

Temporary Identifiers UE-EUTRAN

NAS Protocol(s)


NAS Protocol(s)


Allocation of Sign. Radio Bearers

E-UTRAN Security

Integrity protection for RRC msg.

Mgmt. of ptp radio bearers

Mobility Functions (LTE_ACTIVE)

UE measurement reporting/control

Inter-cell handover

Control of cell (re-)selection

UE context transfer between eNB

MBMS

Notification for MBMS services

Mgmt. of MBMS radio bearers

QoS control

Transfer of NAS messages




RRC States for EUTRANRRC_IDLE RRC_CONNECTED

• UE in DRX (NAS

configuration);• UE receives BCCH;

• UE monitors PCH;

• cell reselection;

• no RRC context in e-NodeB

• UE in DRX/DTX (e-Node B

configuration);• UE uses mainlyDCCH/DTCH;

• UL/DL data transmissionpossible;

• UE monitors PDCCH toget schedulingassignments;

• UE reports channelquality (CQI) to e-NodeB toassist channel dependentscheduling;

• neighbor cellmeasurements by UE(automatic UE detection);

• handover;

• e-NodeB knows UE’s cell;

• RRC context in e-NodeB;

RRC Connection Establishment

(via CCCH)

RRC Connection Release

(via DL-SCH)

RRC will use one or two radio bearers exclusively used for signaling (Signaling RadioBearers). One will be for high, the other for low priority. The PDCP entities of thesesignaling radio bearers will be used for ciphering, but not for header compression.

The RRC protocol in EUTRAN defines two state for a UE: RRC_IDLE andRRC_CONNECTED. In the first state, the UE is not attached to a eNB and does free cellre-selection. In the second state the UE is connected to a eNB and the eNB handles allmobility related aspects of the UE via handovers. There is of course a close relationshipbetween LTE-states and RRC states












Layer 2 Functions and Data Flow (1/2)

For layer 2 let us first take a look into the uplink.

Data transmission is handled through the protocol stack according to the following flow:

1. Data is generated by either signaling control protocols (RRC, NAS) or by some application on the UE’s IP stack. Anassociated chunk of bits is sent to layer 2 within the appropriate radio bearer.

2. The first protocol that handles the data frame is PDCP. For IP datagrams it will compress the IP (or IP/TCP, IP/UDP,IP/UDP/RTP) header according RFC 3095 (ROHC). Note that this is not applicable to signaling radio bearers. Thesecond step within PDCP is encryption of the data packet.

3. Next comes RLC. For all radio bearers the associated RLC instance has to perform segmentation or concatenation orpadding to generate bit frames (RLC PDU) that will fit into the transport channels. If the RLC entity of a radio bearerworks in acknowledged mode (AM), then the data is sent through the ARQ function, which will buffer the packet in aretransmission buffer until the frame has been positively acknowledged. If the RLC entity is not in acknowledgedmode, this step is obviously skipped.

4. RLC PDUs from all logical channels arrive then at the MAC protocol. Here the UE’s uplink scheduler has to decide,which logical channel will be served and multiplexed onto a transport channel. It is possible to combine several dataunits from different logical channels in one transport block, a multiplexer handles this.

(continued on the next slide)




Layer 2 Functions and Data Flow (2/2)

5. The lower part of the MAC entity is the HARQ (Hybrid Automatic Retransmission on reQuest) entity. Note that only

certain transport channel types (UL-SCH) can have this unit. Here the assembled transport block from themultiplexer will be stored in one of the HARQ’s buffers and simultaneously sent to the physical layer. If the eNBreceives the transport block correctly, it will send an ACK indication via a special physical channel. This woulddelete the transport channel from the buffer. If no indication or a NACK indication is received, the HARQ entity willretransmit the transport block. Each retransmission can be done with different encoding in the physical layer.Therefore MAC will tell the physical layer, whether a transport block is new or is the nth retransmission.

6. The physical layer takes the transport block and encodes it (see last part of this register) for transmission on air.




FDD | TDD - Layer 1

( … UL: SC-FDMA )

FDD | TDD - Layer 1

( … UL: SC-FDMA )

Multiplexing

HARQ

Scheduling/Priority

…

Segment-ation

TrCH

Transport Block

(1 per TTI)TB ACK|NACK

MAC

ARQ

Segment-ation

ARQ

Segment-ation

ARQ

…

LogCH LogCH LogCH

RLC

Security

ROHC

Security

ROHC

Security

…

PDCP

RB RB RB

Segment-ation

ARQ

LogCH

ROHC

Security

RB

IP (user plane)IP (user plane)

NASNAS

(E-)RRC(E-)RRC

…

…






DL MAC

Furthermore the MAC layer supports mapping of several logical channels to the transport channel.

RNTI types are defined to handle the mapping of the different types of logical channels.

- C-RNTIs for DCCH and DTCH;

+ C-RNTI

+ temporary C-RNTI

+ semi-persistant C-RNTI

- P-RNTI for PCCH;

- RA-RNTI for Random Access Response on DL-SCH;

- Temporary C-RNTI for CCCH during the random access procedure;

- SI-RNTI for BCCH.

I.e. the MAC scheduling via PDCCH is performed by using the appropriate RNTI.

A MAC PDU consist of a MAC header and there might be MAC control elements, MAC SDUs and Padding. The headeritself is made of subheaders.

The subheader consists of various fields:

R: Reserved bit

LCID: Logical Channel IDE: Extension bit tells if more subheader will follow

F: Indicates the length of the length field (7 or 15 bit)

L: Length field gives the length of the corresponding MAC SDU or control element in byte

SI-RNTI - System Information - Radio Network Temporary Identifier

When the E-UTRA eNB sends broadcast messages on the BCCH it uses a SI-RNTI (System Information - Radio Network Temporary Identifier), which isset to 0xFFFF. This is used to CRC encode the broadcast messages, enabling all UEs to correctly decode/identify.

C-RNTI - Cell Radio Network Temporary Identifier

P-RNTI - Paging - Radio Network Temporary Identifier

Identifies paging groups












Protocols Configuration Example (1/2)

In this example it is assumed that the user is having in parallel 2 downlink applications: an E-Mail download and an FTP

(File Transfer Protocol) download. The target is to show how the air interface protocols could be configured for thisscenario.

It is further assumed that the signaling for the connection setup is already done (i.e. the UE is already in theRRC_CONNECTED state). However, it is assumed that security activation (ciphering) has still to be done.

Configuration Description

Protocol Configuration for the Control Plane: The NAS Signaling it is transferred using the RRC protocol. This isdone with the help of the Signaling Radio Bearers SRBs. In the LTE implementation there are 3 SRBs:

• SRB0 is for RRC messages using the CCCH logical channel (not shown in the example because the assumption isthat the UE is already in RRC_CONNECTED state)

• SRB1 is for RRC messages (which may include a piggybacked NAS message) as well as for NAS messages priorto the establishment of SRB2, all using DCCH logical channel;

• SRB2 is for NAS messages, using DCCH logical channel. SRB2 has a lower-priority than SRB1 and is alwaysconfigured by E-UTRAN after security activation.

The SRB1 is established during the RRC Connection establishment procedure (using the SRB 0). After having initiatedthe initial security activation procedure, E-UTRAN initiates the establishment of SRB2.

Once security is activated, all RRC messages on SRB1 and SRB2, including those containing NAS or non-3GPP

messages, are integrity protected and ciphered by PDCP. NAS independently applies integrity protection andciphering to the NAS messages.

The SRBs are transported using the acknowledged mode RLC. The SRBs will be further mapped to the logical channelDCCH (Dedicated Control Channels).




Protocols Configuration Example (2/2)

Protocol Configuration for the User Plane: The E-Mail application will be transmitted using UDP (connectionless

protocol) and the FTP Application will be sent using the TCP (connection oriented). The reason for this is that thetransmission of the FTP should be more reliable from the QoS point of view. This is also the reason why 2 differentuser plane data radio bearers (DRBs) have to be used for this scenario. Both applications are then using the IP(Internet Protocol). The DRBs are established using the signaling radio bearers SRBs.

The user plane radio bearers are transported further using the acknowledged mode RLC. The radio bearer 1 which iscaring the e-mail will be mapped on the logical channel DTCH1 (Dedicated Traffic Channel) and the radio bearer 2which is caring the FTP download will be mapped on the DCCH2. The logical channels will be further explained inchapter 5.

Common Configuration for control plane and the user plane:

The logical channels belonging to both the user plane and the control plane, i .e. DCCH1, DCCH2, DTCH1 and DTCH2are mapped by the MAC layer to the same transport channel. In downlink the transport channel is DL-SCH(Downlink Shared Channel).

The physical layer is mapping the transport channel DL-SCH to the physical channel PDSCH (Physical Downlink SharedChannel).

The details of the DL-SCH, PDSCH as well as the mapping of the logical channels to the transport channels and to thephysical channels are discussed in chapter 5.




Protocols Configuration Example - Downlink

RRC

SRB1 SRB2 DRB1 DRB2

AM AM AM AM

DCCH1 DCCH2 DTCH1 DTCH2

DL-SCH

PDSCH

NAS

E-mail FTP

UDP TCP

IP IP

RLC

MAC

Physical Layer

Logical Channels

Transport Channels

Physical Channels

PDCP Integrity&ciphering Ciphering&ROHC




Data Flow Example

Header HeaderPayload Payload

PDCP

Header

PDCP

Header

PDCP PDU PDCP PDU

PDCP SDU PDCP SDU

RLC

Header

RLC

Header

RLC

HeaderRLC SDU RLC SDU RLC SDU

MAC

Header

MAC

HeaderRLC PDU RLC PDU

Transport block Transport blockCRC CRC

E-Mail (IP packet) FTP Download (IP packet)

H HPayload PayloadPDCP

RLC

MAC

PHY

PDU = Protocol Data Unit

SDU = Service Data Unit

In this example, two IP packets are transmitted over the air interface:

The first IP packet it is assumed to come from the E-Mail application and the scond IP packet iscomming from the FTP download.

One IP packet is containing the header and the payload. The header length is dependent on the IPversion used: IPv4 or IPv6 (a higher length for IPv6). The payload is containing the user data (E-Mail or FTP transfer) together with UDP/TCP control fields.

PDCP layer: The IP packets are passed through the PDCP layer which is performing IP headercompression and ciphering. Therefore a PDCP header is required. The PDCP SDU (Service DataUnit – data comming from higher layers to the PDCP layer) together with the PDCP header areforming together the PDCP – PDU (Protocol Data Unit). The PDCP PDUs are passed down to theRLC layer.

RLC layer: the RLC configuration is AM (acknowledged mode) for both applications from thisexample. The second functionality of the RLC layer is segmenatation/ reasembly. In the exampleshown the segmentation process is ilustrated for the data comming from the second application(the FTP transfer). An RLC header is needed for both reliable data transfer (acknowledge mode)and to be able to perform reasembly at the receiver side.

MAC layer: the MAC layer is multiplexing together a number of RLC PDUs(Packet Data Units) to

form a Transport Block. How many RLC PDUs are to be multiplexed in one transport bloc isdependent on the transport block size which is sent over the air interface in on TTI (TransmissionTime Interval) = 1 ms. The size of the transport block is decided by the scheduler which shouldtake into account the quality of the radio link (link adaptation mechanism): it depends on thechosen Modulation and Coding Scheme (MCS), the number of resources allocated on the airinterface. Thus, the link adaptation mechanism affects both the RLC and MAC processing(segmentation at RLC and multiplexing at the MAC layer). Thus, the MAC layer should add oneheader to indicate the multiplexing of the RLC PDUs into the transport block.

Physical Layer: the physical layer attaches one CRC (cyclic redundancy coding) for errorcorrection reasons. Details on the transport channel processing at the physical layer are providedin chapter 6.

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02 Air Interface Protocols v04