Layer 2 Overview . . . .3.1 MAC General Architecture . . . .3.2 MAC Scheduling Functions . . . .3.3 RACH Procedure for MAC . . . .3.4 RNTI Types . . . .3.5 Transmission Requirement Indications . . . .3.6 L2/L1 Channel Mapping . . . .3.7 RLC General Functions . . . .3.8 RLC Transparent Mode . . . .3.9 RLC Unacknowledged Mode . . . .3.10 RLC Acknowledged Mode . . . .3.11 PDCP Functional Architecture . . . .3.12 RRC Functions . . . .3.13 RRC States . . . .3.14 Signalling Radio Bearers . . . .3.15 System Information Broadcasting . . . .3.16 RRC Connection Structure . . . .3.17 RRC Connection Establishment . . . .3.18 Data Radio Bearer Establishment . . . .3.19 NAS Information Transfer . . . .3.20
CONTENTS
At the end of this section you will be able to:
identify the functions of the RRC protocol
define the RRC protocol connected mode and idle mode states for a UE
explain the use of signalling radio bearers for the transfer of RRC signalling
describe the procedures for the broadcasting of system information by RRC
explain the relationship between signalling radio bearers, data radio bearers and EPS bearers
describe the operation of RRC connection establishment
describe how data radio bearers and EPS bearers are established, modified or removed
explain the measurement configuration and reporting procedures
explain how RRC carries NAS signalling over the air interface
identify the three sublayers: PDCP, RLC and MAC within layer 2 for E-UTRA
explain the key functions of each sublayer within layer 2
list the logical and transport channels defined for information interchange in layers 2 and 1
explain the function and multiplexing options for logical and transport channels
describe the functional architecture of PDCP
describe the functional architecture of RLC
list and explain the three modes of operation for RLC: transparent mode, unacknowledged mode and acknowledged mode
describe the MAC functional architecture
explain MAC functions in respect of logical channel prioritization and scheduling
explain the general operation of the random access process
describe how HARQ is implemented between the MAC layer and the physical layer
OBJECTIVES
PDCP PDCP SAPs
RLC SAPs
System information Paging
RRC dedicated control and NAS direct transfer
SRB0 SRB1 SRB2 DRB1 DRB2 DRB3
Integrity and
BCCH PCCH CCCH DCCH1 DCCH2 DTCH1 DTCH2 DTCH3
RLC PDU and ARQ RLC PDU
and ARQ RLC PDU
and ARQ RLC PDU RLC PDU
and ARQ
Scheduling and priority handling
Physical layer
Multiplexing and HARQ control
Layer 2 Overview
There are three sublayers within the E-UTRA layer 2, PDCP, RLC and MAC (Medium Access Control).
All the sublayers, including PDCP, span both the control and user planes of the protocol stack, although in most cases the functions provided in each plane differ.
PDCP provides SAP (Service Access Point) access to protocol functionality through SRB (Signalling Radio Bearer) provision in the control plane and DRB (Data Radio Bearer) provision in the user plane. At the eNB end a set of SRBs and DRBs is created on a per-UE basis as required. For system information and paging, PDCP has a null function. PDCP provides sequencing of higher-layer PDUs and implements the integrity and ciphering security functions as required.
RLC provides three levels of service through three SAP types, TM (Transparent Mode), UM (Unacknowledged Mode) and AM (Acknowledged Mode). TM is only applicable to system information broadcasting, paging and RRC connection establishment in SRB 0. AM is used for all dedicated signalling functions and packet traffic transfer, providing retransmission and sequencing. For real-time traffic, when AM would not be desirable in achieving the delay requirements UM can be used for sequencing only.
MAC SAPs are known as logical channels. The MAC layer is responsible for mapping and multiplexing logical channels to transport channels at the physical layer. MAC also controls scheduling for resource allocation at the physical layer as well and control for a number of physical layer processes.
Further Reading: 3GPP TS 36.300
PCCH DTCH
BCH
PCH DL-SCH UL-SCH RACH
Logical channels
Transport channels
Logical channel prioritization (UL only)
Multiplexing/demultiplexing
HARQ Random
access control
Control
BCCH CCCH DCCH
MAC control
Grant and HARQ signalling MAC
MAC General Architecture
The MAC layer is defined as part of layer 2. However, many of its functions are closely related to physical layer behaviour, so the architecture indicated in the standards should be treated as informative.
Manufacturers are left to determine an efficient implementation for the realization of MAC and physical layer interaction.
The MAC layer is accessed through logical channels as well as a control SAP. It maps information flows into the physical layer through transport channels. The mapping of logical channels to transport channels is a key function of the MAC layer.
In addition to channel mapping, the MAC layer has important control functionality including management of multiple HARQ processes for each information flow and the random access process.
Most significantly, the MAC layer is responsible for channel prioritization and scheduling of resources on the physical layer.
The MAC layer has a null function for paging and for system information that will be transmitted in the BCH (Broadcast Channel) transport channel.
Further Reading: 3GPP TS 36.321:4.2.1
MAC Downlink Assignment (PDCCH)
The main function of the MAC is to manage the shared access to a common transmission medium by multiple devices. This is achieved through the eNB’s scheduling function. Resource allocation will be performed on the basis of a scheduling algorithm, the specifics of which are not defined by the standards.
However, channel performance, data buffer fill, UE power capability and traffic priority are likely to be considered.
When a UE establishes an RRC relationship with an eNB it is assigned a C-RNTI (Cell Radio Network Temporary Identifier), which will uniquely identify that UE in that cell. The C-RNTI will be used to address any control and scheduling messages to or from the UE. Each UE is capable of establishing multiple EPS bearers, which are the NAS traffic and signalling connections that travel from the UE to the core network.
Resource allocations are defined in terms of one or more PRB (Physical Resource Block), which will be populated using a specified MCS (Modulation and Coding Scheme). The allocations can be made for one or more TTI periods. LTE offers three scheduling modes. The first, known as dynamic scheduling, involves the use of MAC downlink assignment messages and uplink grant messages in the PDCCH to allocate resources as required. Dynamic scheduling is intended for typical bursty packet data traffic.
For VoIP (Voice over IP) traffic where regular and reliable allocation of resources is required to meet more demanding QoS requirements, LTE offers persistent scheduling. This is achieved through a combination of RRC signalling in the DL-SCH (Downlink Shared Channel), for the initial specification of the resource allocation interval, and MAC signalling in the PDCCH for more specific PRB and MCS information. The result is a lower overhead in the PDCCH for these regular resource allocations. The third scheduling option, known as semi-persistent scheduling, is used specifically for the purpose of resource allocation for the establishment or reconfiguration of a persistent scheduled resource, i.e. for the transport of RRC messages relating to the persistent scheduled resource. In this case an SPS-C-RNTI (Semi Persistent Scheduling) will be used to address the UE, which is different from the UE’s C-RNTI.
Further Reading: 3GPP TS 36.321, 36.331
MAC
The random access procedure is handled by the MAC and the physical layer and operates using a combination of the PRACH on the uplink and the PDCCH on the downlink. UEs are informed of the range of random access preambles available in system information, as are the contention management parameters. When a random access event is required, the UE will perform the following functions:
review and randomly select a preamble
check the BCCH for the current PRACH configuration; this will indicate the location and periodicity of PRACH resources in uplink subframes
calculate open loop power control parameters – initial transmit power, maximum transmit power and power step
discover contention management parameters
Once the UE transmits an initial preamble it will wait a specified period of time for a response before backing off and retrying. Open loop power control ensures that each successive retry will be at a higher power level.
Upon receipt of a successful uplink PRACH preamble, the eNB will calculate power adjustment and timing advance parameters for the UE based on the strength and delay of the received signal and schedule an uplink capacity grant to enable the UE to send further details of its request. This will take the form of the initial layer 3 message. If necessary, the eNB will also assign a Temporary C-RNTI for the UE to use for ongoing communication.
Once received, the eNB reflects the initial layer 3 message back to the UE in a subsequent downlink scheduled resource to enable unambiguous contention resolution. After this point further resource allocations may be required for signalling or traffic exchange and these will be addressed to the
C-RNTI.
Further Reading: 3GPP TS 36.321:5.1, 36.213:6
Note: RNTI values falling in the RA-RNTI number range corresponding to a cell’s PRACH configuration cannot be reused for other RNTI types.
RNTI Usage Logical channel Transport channel Value range
RA-RNTI MAC random access response --- DL-SCH 0001–003C
Contention resolution when
no C-RNTI is available CCCH DL-SCH 0001–FFF3
Initial L3 message transmission CCCH/DCCH/DTCH UL-SCH 0001–FFF3 Dynamically scheduled unicast
transmission DCCH/DTCH UL-SCH/DL-SCH 0001–FFF3
Triggering of PDCCH ordered
random access --- --- 0001–FFF3
Semi-persistent scheduled
unicast transmission DCCH/DTCH UL-SCH/DL-SCH 0001–FFF3 Deactivation of semi-persistent
scheduled unicast transmission --- --- 0001–FFF3
SI-RNTI Broadcast of system information BCCH DL-SCH FFFF
P-RNTI Paging and system information
change notification PCCH PCH FFFE
TPC-PUCCH-RNTI Uplink power control --- --- 0001–FFF3
TPC-PUSCH-RNTI Uplink power control --- --- 0001–FFF3
Temporary
The table summarizes the RNTI types defined for E-UTRA. In all cases they have a length of 2 octets and for some RNTI types there is a limited number range or specific reserved values. Outside of these reserved values there is no structure to the RNTI.
A SPS-C-RNTI is allocated to a UE when Semi-Persistent scheduling is used and indicates resources for higher-layer signalling that relates the UE's current persistently scheduled resource. The range of potential values will therefore be dependent on the PRACH configuration used in a cell. Any number in this range cannot be allocated for use as any other RNTI type.
An Temporary C-RNTI is allocated to a UE on initial access as part of the random access procedure. On successful completion of the random access procedure the Temporary C-RNTI becomes the
C-RNTI. This is cell specific and is the main identity for the UE within the cell.
A SPS-C-RNTI is allocated to a UE when persistent scheduling is used and indicates resources for higher-layer signalling that relates the UE’s current persistently scheduled resource.
The fixed SI-RNTI (System Information RNTI) and P-RNTI (Paging RNTI) are used to indicate the allocation of resources in the PDSCH containing system information or paging respectively.
TPC-PUCCH-RNTI (Transmit Power Control PUCCH RNTI) and TPC-PUSCH-RNTI are used to indicate power control information for the PUCCH and PUSCH respectively.
Further Reading: 3GPP TS 36.321:7.1
Logical channels
eNB
Scheduling
Multiplexing and prioritization
Transmission Requirement Indications
The UE will neither receive nor transmit information unless it is scheduled to do so because there is no dedicated radio resource in E-UTRA. Therefore, for every signalling message or data packet some signalling activity must be performed and this must be preceded by a resource request.
Downlink resource allocation is triggered by need in the eNB. All resource allocations are indicated in the PDCCH.
For uplink transmission the UE must first indicate its need to the eNB. There are a number of mechanisms that can result in a scheduled resource being indicated for a UE in the PDCCH. For initial access, or where the UE has not been active for some time, the random access procedure can be used for resource requests. When a mobile is continuously active it may be allocated a resource in the PUCCH to use for resource requests needed for further data or signalling transfer. Additionally, the eNB can request buffer status reports from UE that are currently active. Based on this information the eNB makes scheduling decisions.
In the uplink direction it is the MAC layer within the UE that determines how an allocated transmission resource should be demarcated between a number of different logical channels. This is based on channel priority and channel PBR (Prioritized Bit Rate).
Further Reading: 3GPP TS 36.321, 36.331
RRC
PDCP
RLC
MAC Physical layer
BCCH PCCH CCCH DCCH DTCH
BCH PCH RACH DL-SCH UL-SCH
PBCH PRACH PDSCH PUSCH
Logical channels
Transport channels
Physical channels
L2/L1 Channel Mapping
Logical channels are mapped by the MAC layer to transport channels on entry to the physical layer, and then ultimately to physical channels within the physical layer.
The BCCH is used for system information broadcasting and carries three RRC message types. The MasterInformationBlock message is mapped to the BCH transport channel and then to the PBCH. All other system information messages are mapped to the DL-SCH and PDSCH.
The PCCH (Paging Control Channel) carries paging messages and is mapped to the PCH (Paging Channel) and PDSCH.
The CCCH (Common Control Channel), DCCH (Dedicated Control Channel) and DTCH (Dedicated Traffic Channel) are all bidirectional channels and will be mapped to the DL-SCH and PDSCH for downlink flows and UL-SCH (Uplink Shared Channel) and PUSCH for uplink flows.
The PRACH and RACH are used only in the uplink for initiating RRC connectivity. The random access process involves an interaction at the physical layer under the control of MAC. There is no higher layer information in the random access channels but the process will result in the allocation of resources for higher-layer message exchange.
Further Reading: 3GPP TS 36.300, 36.212, 36.321
RLC
Transmit transparent mode entity
Transmit unacknowledged
mode entity
Receive transparent mode entity Receive
unacknowledged mode entity Acknowledged
mode entity
Transmit side Receive side
Logical channels in MAC
RLC General Functions
RLC provides three levels of service: acknowledged mode, unacknowledged mode and transparent mode. Radio bearers are mapped through RLC to logical channels and an RLC entity is created for each active radio bearer.
For the transparent mode and the unacknowledged mode RLC entities are configured as either transmitting or receiving entities. For acknowledged mode a single entity provides both transmit and receive functionality for one side of the link. This configuration facilitates retransmission of failed RLC PDUs.
Further Reading: 3GPP TS 36.322:4.2.1
Transmitting
TM-RLC entity PDCP PDUs
TM-SAP PDCP PDUs
BCCH/PCCH/CCCH BCCH/PCCH/CCCH
RLC SDUs
RLC PDUs Transmission
buffer TM-SAP
TM-RLC entity
Receiving
RLC Transparent Mode
The transparent mode has no functions, only providing a buffer for higher-layer packets that are to be transmitted over the air interface. Transparent mode entities are accessed via a TM-SAP.
The application of transparent mode is limited to the downlink transmission of system information and paging messages as well as the exchange of RRC connection establishment messages associated with the CCCH (Broadcast Control Channel).
Further Reading: 3GPP TS 36.322:4.2.1.1
Transmitting
Unacknowledged mode entities are accessed through a UM-SAP. Unacknowledged mode reorganizes RLC SDUs into a size requested by the MAC layer. Unacknowledged mode also provides sequence numbering for in-order delivery to higher layers at the receiving end. Reordering in the RLC layer is used in support of the HARQ functions provided by the MAC layer.
Reorganization of RLC SDUs is provided by the segmentation and concatenation function. As shown in the diagram, higher-layer SDUs can be fragmented and reassembled into the RLC PDU payload area to produce a packet size suitable for scheduling by the MAC layer for transmission over the air interface.
The RLC header enables the receiving entity to reassemble the higher-layer SDU in the correct order.
The application of unacknowledged mode is limited to the user plane, where it would be utilized for packet traffic flows with low tolerance to delay. The most common example would be VoIP connections.
Further Reading: 3GPP TS 36.322:4.2.1.2
Transmission
The acknowledged mode of RLC is applicable in the control plane for RRC signalling messages carried in DCCH and for user plane traffic carried in DTCH. Acknowledged mode entities are accessed through an AM-SAP.
General transmission and reception functionality in terms of segmentation, concatenation, buffering and HARQ reordering for AM mode are similar to those for UM mode. However, AM mode also provides retransmission of failed RLC PDUs. In this respect a number of enhancements in functional architecture are provided. Firstly, a single entity for transmission and reception is required for interaction between the transmitting and receiving side. Secondly a retransmission buffer is required in the transmit side. All transmitted RLC PDUs are retained in the transmission buffer until acknowledgement is received.
Additionally, control (status) PDUs are required in addition to data PDUs in order to manage the retransmission process. These must be multiplexed with data PDUs at the transmission end and demultiplexed (routed) from data PDUs at the reception end.
Further Reading: 3GPP TS 36.322:4.2.1.3
PDCP
A PDCP entity is created for each SRB and/or DRB on a per-UE basis. All PDCP entities are bidirectional, thus when the AM mode of RLC is being used there is a one-to-one mapping between a PDCP entity and AM SAP in RLC. However, for the UM mode of RLC one PDCP entity will be associated with two UM SAPs, one configured for transmit functions and the other configured for receive functions.
Within a PDCP entity sequence numbering is applied for higher layer PDUs. This ensures in-order delivery at the receiving end. In the user plane PDCP control PDUs can be used to indicate missing PDUs.
In the user plane, only IETF-defined ROHC (Robust Header Compression) is provided. Support for this is only mandatory for UEs that have VoIP (Voice over IP) capability.
In the control plane, integrity protection is provided for RRC signalling messages.
Ciphering is then applied in both control and user planes, although separate cipher keys are applied for a given UE in the two planes.
Further Reading: 3GPP TS 36.323:4.2.2
RRC
As with other E-UTRA protocols, the RRC layer, which previously resided in the RNC, has been relocated to the eNB. In addition, the functionality and complexity of RRC has been significantly reduced relative to that in UMTS. The main RRC functions for LTE include creation of BCH system information; creation and management of the PCH (Paging Channel); RRC connection management between eNB and UEs, including generating temporary identifiers such as the C-RNTI; mobility-related functions such as measurement reporting, inter-cell handover and inter-eNB UE context handover; QoS management; and direct transfer of messages from the NAS to the UE.
The RRC is in overall control of radio resources in each cell and is responsible for collating and managing all relevant information related to the active UEs in its area.
System information provides the main means of advertising the services available in a cell and the means by which those services can be accessed. For E-UTRA the BCH carries only basic information and acts as a pointer for broader system information related to the NAS, such as PLMN (Public Land Mobile
System information provides the main means of advertising the services available in a cell and the means by which those services can be accessed. For E-UTRA the BCH carries only basic information and acts as a pointer for broader system information related to the NAS, such as PLMN (Public Land Mobile