Air Interface Protocols Ppt

8/10/2019 Air Interface Protocols Ppt

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Air Interface Protocols

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The EUTRAN radio protocol model specifies the protocols terminated between UE andeNB. The protocol stack follows the standard guidelines for radio protocol architectures

(ITU-R M1035) and is thus quite similar to the WCDMA protocol stack of UMTS.The protocol stack defines three layers: the physical layer (layer 1), data link and accesslayer (layer 2) and layer 3 hosting the access stratum and non-access stratum controlprotocols 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 andprovides the basic bit transmission functionality over air. In LTE the physical layer isdriven by OFDMA in the downlink and SC-FDMA in the uplink. FDD and TDD mode canbe combined (depends on UE capabilities) in the same physical layer. The physical layeruses physical channels to transmit data over the radio path. Physical channels aredynamically mapped to the available resources (physical resource blocks and antennaports). To higher layers the physical layer offers its data transmission functionality viatransport channels. Like in UMTS a transport channel is a block oriented transmission

service with certain characteristics regarding bit rates, delay, collision risk and reliability.Note that in contrast to 3G WCDMA or even 2G GSM there are no dedicated transport orphysical channels anymore, as all resource mapping is dynamically driven by thescheduler.

MAC (Medium Access Control):MAC is the lowest layer 2 protocol and its mainfunction is to drive the transport channels. From higher layers MAC is fed with logicalchannels which are in one-to-one correspondence with radio bearers. Each logicalchannel is given a priority and MAC has to multiplex logical channel data onto transportchannels. In the receiving direction obviously demultiplexing of logical channels fromtransport channels must take place. Further functions of MAC will be collision handlingand explicit UE identification. An important function for the performance is the HARQfunctionality which is official part of MAC and available for some transport channel types.


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RLC (Radio Link Control):Each radio bearer possesses one RLC instance working ineither of the three modes: UM (Unacknowledged), AM (Acknowledged) or TM

(Transparent). Which mode is chosen depends on the purpose of the radio bearer. RLCcan thus enhance the radio bearer with ARQ (Automatic Retransmission on reQuest)using sequence numbered data frames and status reports to trigger retransmission. Notethat it shall be possible to trigger retransmissions also via the HARQ entity in MAC. Thesecond functionality of RLC is the segmentation and reassembly that divides higher layerdata or concatenates higher layer data into data chunks suitable for transport overtransport channels which allow a certain set of transport block sizes.

PDCP (Packet Data Convergence Protocol):Each radio bearer also uses one PDCPinstance. PDCP is responsible for header compression (ROHC RObust HeaderCompression; RFC 3095) and ciphering/deciphering. Obviously header compressionmakes 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 forEUTRAN. It will provide the required messages for channel management, measurementcontrol and reporting, etc.

NAS Protocols:The NAS protocol is running between UE and MME and thus must betransparently transferred via EUTRAN. It sits on top of RRC, which provides the requiredcarrier messages for NAS transfer.


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The RRC protocol for EUTRAN is responsible for the basic configuration of the radioprotocol stack. But one should note, that some radio management functions (scheduling,

physical resource assignment for physical channels) are handled by layer 1 and layer 2autonomously. 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 anoperation.

The RRC functional list is of course quite long.

System Information Broadcasting: The NAS and access stratum configuration of thenetwork and the cell must be available to any UE camping on a cell. This information iscoded as RRC message.

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

RRC Connection Management:The UE can have two major radio states:RRC_CONNECTED or RRC_IDLE. To switch between the states an RRC connectionestablishment and release procedure is defined. With the state RRC_CONNECTED theexistence of signaling radio bearers and UE identifiers (C-RNTI) is associated.

EUTRAN Security:Access layer security in EUTRAN consists of ciphering (PDCP) andintegrity protection for RRC messages.

Management of Point-to-Point Radio Bearers:Point-to-point radio bearers aresignaling and user data radio bearers for SAE bearers. RRC is used to create, modify anddelete such radio bearers including the associated lower layer configuration (logicalchannels, RLC mode, transport channels, multiplexing, ).

(continued on the next slide)


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Mobility Functions:When a UE is in state LTE_ACTIVE, the mobility control is at theeNB. This includes handover from one EUTRAN cell to another or also inter-system

changes. To assist handover decisions in the eNB RRC defines procedures formeasurement control and reporting. In LTE_IDLE mode the UE performs automatic cellre-selection, RRC takes control over this process within the UE.

MBMS (Multimedia Broadcast Multicast Service):RRC is used to inform UEs aboutavailable MBMS services in a cell and is also used to track UEs that registered for acertain multicast service. This allows the eNB to manage MBMS radio bearers which areusually point-to-multipoint.

QoS Control: The RRC protocol will be QoS aware, allowing implementation of radiobearers with different QoS within the UE.

Transfer of NAS Messages:NAS messages are sent and received through theEUTRAN protocol stack. RRC provides carrier services for such messages.


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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 these

signaling 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


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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 someapplication on the UEs IP stack. An associated chunk of bits is sent to layer 2 within theappropriate radio bearer.

2. The first protocol that handles the data frame is PDCP. For IP datagrams it willcompress 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. The second step within PDCP isencryption of the data packet.

3. Next comes RLC. For all radio bearers the associated RLC instance has to performsegmentation or concatenation or padding to generate bit frames (RLC PDU) that will fitinto the transport channels. If the RLC entity of a radio bearer works in acknowledgedmode (AM), then the data is sent through the ARQ function, which will buffer the packet ina retransmission buffer until the frame has been positively acknowledged. If the RLCentity is not in acknowledged mode, this step is obviously skipped.

4. RLC PDUs from all logical channels arrive then at the MAC protocol. Here the UEsuplink scheduler has to decide, which logical channel will be served and multiplexed ontoa transport channel. It is possible to combine several data units from different logicalchannels in one transport block, a multiplexer handles this.

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5. The lower part of the MAC entity is the HARQ (Hybrid Automatic Retransmission onreQuest) entity. Note that only certain transport channel types (UL-SCH) can have this

unit. Here the assembled transport block from the multiplexer will be stored in one of theHARQs buffers and simultaneously sent to the physical layer. If the eNB receives thetransport block correctly, it will send an ACK indication via a special physical channel.This would delete the transport channel from the buffer. If no indication or a NACKindication is received, the HARQ entity will retransmit the transport block. Eachretransmission can be done with different encoding in the physical layer. Therefore MACwill 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 thisregister) for transmission on air.


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DL Data Flow:

Of course the eNB has to process the radio bearers of several UE.

Thus the scheduler in the eNB has to balance the traffic between different users. This isdone by taking each radio bearer as individual quality of service instance into account.


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In this example it is assumed that the user is having in parallel 2 downlink applications: anE-Mail download and an FTP (File Transfer Protocol) download. The target is to show

how the air interface protocols could be configured for this scenario.It is further assumed that the signaling for the connection setup is already done (i.e. theUE is already in the RRC_CONNECTED state). However, it is assumed that securityactivation (ciphering) has still to be done.

Configuration Description

Protocol Configuration for the Control Plane: The NAS Signaling it is transferredusing the RRC protocol. This is done 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 examplebecause the assumption is that 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 prior to the establishment of SRB2, all using DCCH logicalchannel;

SRB2 is for NAS messages, using DCCH logical channel. SRB2 has a lower-prioritythan SRB1 and is always configured by E-UTRAN after security activation.

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

Once security is activated, all RRC messages on SRB1 and SRB2, including thosecontaining NAS or non-3GPP messages, are integrity protected and ciphered by PDCP.NAS independently applies integrity protection and ciphering to the NAS messages.

The SRBs are transported using the acknowledged mode RLC. The SRBs will be further

mapped to the logical channel DCCH (Dedicated Control Channels).


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Protocol Configuration for the User Plane: The E-Mail application will be transmittedusing UDP (connectionless protocol) and the FTP Application will be sent using the TCP

(connection oriented). The reason for this is that the transmission of the FTP should bemore reliable from the QoS point of view. This is also the reason why 2 different userplane data radio bearers (DRBs) have to be used for this scenario. Both applications arethen using the IP (Internet Protocol). The DRBs are established using the signaling radiobearers SRBs.

The user plane radio bearers are transported further using the acknowledged mode RLC.The radio bearer 1 which is caring the e-mail will be mapped on the logical channelDTCH1 (Dedicated Traffic Channel) and the radio bearer 2 which is caring the FTPdownload will be mapped on the DTCH2. 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 DTCH2 are mapped by the MAC layer to the same transportchannel. In downlink the transport channel is DL-SCH (Downlink Shared Channel).

The physical layer is mapping the transport channel DL-SCH to the physical channelPDSCH (Physical Downlink Shared Channel).

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


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Air Interface Protocols Ppt