UMTS Radio Interface Protocols

127
RADIO INTERFACE PROTOCOLS

description

Description of UMTS layers

Transcript of UMTS Radio Interface Protocols

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RADIO INTERFACE PROTOCOLS

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7.1 Introduction7.2 Protocol Architecture7.3 The Medium Access Control Protocol7.4 The Radio Link Control Protocol7.5 The Packet Data Convergence Protocol7.6 The Broadcast/Multicast Control Protocol7.7 Multimedia Broadcast Multicast Service7.8 The Radio Resource Control Protocol

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7.1 INTRODUCTION The protocol layers above

physical layer Data Link Layer (Layer

2) Network Layer (Layer 3)

In UTRA FDD radio interface, Layer 2 is split into sublayers in control plane, Layer 2

contains two sub-layers Medium Access

Control (MAC) protocol Radio Link Control

(RLC) protocol

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7.1 INTRODUCTION The protocol layers above

physical layer Data Link Layer (Layer

2) Network Layer (Layer 3)

In UTRA FDD radio interface, Layer 2 is split into sublayers in control plane, Layer 2

contains two sub-layers Medium Access

Control (MAC) protocol Radio Link Control

(RLC) protocol

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in user plane, in addition to MAC and RLC, two additional service-dependent protocols Packet Data Convergence

Protocol (PDCP) Broadcast/Multicast

Control Protocol (BMC) In UTRA FDD radio interface,

Layer 3 consists of one protocol Radio Resource Control

(RRC), which belongs to control plane

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7.2 PROTOCOL ARCHITECTURE

Figure 7.1 shows the overall radio interface protocol

architecture contains only the protocols that are visible in

UTRAN

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Physical layer offers services to MAC

layer via transport channels

transport channels are characterized by how and with what

characteristics data is transferred

MAC layer offers services to RLC

layer by means of logical channels

logical channels are characterized by what type of data is

transmitted

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RLC layer offers services to

higher layers via service access points (SAPs)

SAPs describe how the RLC layer handles the data packets and if, for example, the automatic repeat request (ARQ) function is used

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註: Automatic Repeat Request (ARQ)

protocol for dealing with data words that are corrupted by errors whereby the receiving system requests a re-transmission of the word(s) in error

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on the control plane RLC services are

used by RRC layer for signaling transport

on the user plane RLC services are

used by either of the following the service-specific

protocol layers PDCP or BMC

other higher-layer u-plane functions (e.g. speech codec)

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RLC services called Signaling

Radio Bearers (SRB) in the control plane

called Radio Bearers in the user plane

RLC protocol operates in three modes transparent mode unacknowledged

mode acknowledged mode

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Packet Data Convergence Protocol (PDCP) exists only for PS

domain services main function is

header compression the services offered

by PDCP are called Radio Bearers

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Broadcast Multicast Control protocol (BMC) used to convey over

the radio interface messages originating from Cell Broadcast Centre

Release’99, the only specified broadcasting service is SMS Cell Broadcast service, which is derived from GSM

the service offered by BMC protocol is also called a Radio Bearer

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RRC layer offers services to higher

layers via service access points (SAP), which are used by the higher layer

protocols in the UE side the Iu RANAP protocol

in the UTRAN side all higher layer signaling

(mobility management, call control, session management, etc.) is encapsulated into RRC messages for transmission over radio interface

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SIGNALING AND TRAFFIC CONNECTIONS BETWEEN MOBILE

AND SGSN

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The control interfaces between RRC and all the lower layer protocols are used by RRC layer to configure characteristics of

the lower layer protocol entities including parameters for

the physical, transport and logical channels

command the lower layers to perform certain types of

measurement report measurement

results and errors to RRC

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7.3 THE MEDIUM ACCESS CONTROL PROTOCOL In Medium Access Control (MAC) layer

logical channels are mapped to transport channels

MAC layer responsible for selecting an appropriate

transport format (TF) for each transport channel depending on the instantaneous source rate(s) of logical channels

the transport format is selected w.r.t the transport format combination set (TFCS) which is defined by the admission control for each connection

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7.3.1 MAC Layer Architecture7.3.2 MAC Functions7.3.3 Logical Channels7.3.4 Mapping Between Logical Channels and

Transport Channels

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7.3.1 MAC LAYER ARCHITECTURE

Figure 7.2 shows the MAC layer logical architecture

MAC layer consists of three logical entities MAC-b MAC-c/sh MAC-d

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MAC-b handles the broadcast channel (BCH) one MAC-b entity in each UE one MAC-b in the UTRAN (located in Node B) for

each cell

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MAC-c/sh handles the common channels and shared channels

Paging Channel (PCH) Forward Link Access Channel (FACH) Random Access Channel (RACH) Uplink Common Packet Channel (CPCH) Downlink Shared Channel (DSCH)

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one MAC-c/sh entity in each UE that is using shared channel(s)

one MAC-c/sh in the UTRAN (located in the controlling RNC) for each cell

BCCH logical channel can be mapped to either BCH or FACH transport channel

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MAC-d responsible for handling dedicated channels

(DCH) allocated to a UE in connected mode one MAC-d entity in UE one MAC-d entity in UTRAN (in the serving RNC)

for each UE

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7.3.2 MAC FUNCTIONS

Mapping mapping between logical channels and

transport channels Transport Format selection

selection of appropriate Transport Format (from the Transport Format Combination Set, TFCS) for each Transport Channel, depending on the instantaneous source rate

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Priority handling priority handling between data flows of one UE

achieved by selecting ‘high bit rate’ and ‘low bit rate’ transport formats for different data flows

priority handling between UEs by means of dynamic scheduling

a dynamic scheduling function may be applied for common and shared downlink transport channels FACH and DSCH

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Identification of UEs on common transport channels when a common transport channel (RACH,

FACH or CPCH) carries data from dedicated-type logical channels (DCCH, DTCH), the identification of the UE (Cell Radio Network Temporary Identity (C-RNTI) or UTRAN Radio Network Temporary Identity (U-RNTI)) is included in the MAC header

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Multiplexing/demultiplexing of higher layer PDUs into/from transport blocks delivered to/from the physical layer on common transport channels MAC handles service multiplexing for common

transport channels (RACH/FACH/CPCH)

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Multiplexing / demultiplexing of higher layer PDUs into/from transport block sets delivered to/from the physical layer on dedicated transport channels MAC allows service multiplexing also for

dedicated transport channels MAC multiplexing is possible only for services

with the same QoS parameters physical layer multiplexing is possible to

multiplex any type of service, including services with different QoS parameters

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Traffic volume monitoring MAC receives RLC PDUs together with status

information on the amount of data in the RLC transmission buffer

MAC compares the amount of data corresponding to a transport channel with the thresholds set by RRC

if the amount of data is too high or too low, MAC sends a measurement report on traffic volume status to RRC

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RRC can also request MAC to send these measurements periodically

RRC uses these reports for triggering reconfiguration of Radio Bearers and/or Transport Channels

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Dynamic Transport Channel type switching execution of the switching between common

and dedicated transport channels is based on a switching decision derived by RRC

Ciphering if a radio bearer is using transparent RLC

mode, ciphering is performed in the MAC sub-layer (MAC-d entity)

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Ciphering is a XOR operation (as in GSM and GPRS) where data is XORed with a ciphering mask produced by a ciphering algorithm

in MAC ciphering, the time-varying input parameter (COUNT-C) for the ciphering algorithm is incremented at each transmission time interval (TTI), that is, once every 10, 20, 40 or 80 ms depending on the transport channel configuration

each radio bearer is ciphered separately the ciphering details are described in 3GPP

specification TS 33.102

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Access Service Class (ASC) selection for RACH transmission PRACH resources (i.e. access slots and

preamble signatures [ 前導簽章,用於區別用戶 ] for FDD) may be divided between different Access Service Classes in order to provide different priorities of RACH usage

the maximum number of ASCs is eight MAC indicates the ASC associated with a PDU

to the physical layer

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7.3.3 LOGICAL CHANNELS

The data transfer services of the MAC layer are provided on logical channels

Logical channels are classified into two groups control channels

used to transfer control plane information traffic channels

used to transfer user plane information

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Control Channels Broadcast Control Channel (BCCH)

a downlink channel for broadcasting system control information

Paging Control Channel (PCCH) a downlink channel that transfers paging

information

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Dedicated Control Channel (DCCH) a point-to-point bidirectional channel that

transmits dedicated control information between a UE and RNC

this channel is established during RRC connection establishment procedure

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Common Control Channel (CCCH) a bidirectional channel for

transmitting control information between the network and UEs

this logical channel is always mapped onto RACH/FACH transport channels

a long UTRAN UE identity is required (U-RNTI, which includes SRNC address) so that the uplink messages

can be routed to the correct serving RNC even if the RNC receiving the message is not the serving RNC of this UE

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Traffic Channels Dedicated Traffic Channel (DTCH)

a point-to point channel, dedicated to one UE, for the transfer of user information

can exist in both uplink and downlink Common Traffic Channel (CTCH)

a point-to-multipoint downlink channel for transfer of dedicated user information for all, or a group of specified, UEs

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7.3.4 MAPPING BETWEEN LOGICAL CHANNELS AND TRANSPORT CHANNELS Figure 7.3 shows the mapping between logical

channels and transport channels Connections between logical channels and

transport channels PCCH is connected to PCH [down]

BCCH is connected to BCH [down] and may also be connected to FACH [down]

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DCCH and DTCH can be connected to RACH [up] and FACH [down] CPCH [up] and FACH [down] RACH [up] and DSCH [down] DCH [up] and DSCH [down] DCH [up] and DCH [down]

CCCH is connected to RACH [down] and FACH [down] CTCH is connected to FACH [down]

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7.4 THE RADIO LINK CONTROL PROTOCOL

RLC protocol provides segmentation and retransmission services for both user and control data

Each RLC instance is configured by RRC to operate in one of three modes Transparent mode (Tr) Unacknowledged Mode (UM) Acknowledged Mode (AM)

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The service the RLC layer provides in the control plane

called Signaling Radio Bearer (SRB) in the user plane

called Radio Bearer (RB) service provided by PDCP, BMC protocols, or

else

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7.4.1 RLC LAYER ARCHITECTURE

Figure 7.5 shows the RLC layer architecture all three RLC entity types and their connection to

RLC-SAPs and to logical channels (MAC-SAPs) are shown

the transparent and unacknowledged mode RLC entities are defined to be unidirectional

the acknowledged mode entities are described as bidirectional

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For all RLC modes CRC (Cyclic Redundancy Check) error detection is

performed on physical layer the result of CRC check is delivered to RLC,

together with the actual data

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In transparent mode no protocol overhead is added to higher layer

data erroneous protocol data units (PDUs) can be

discarded or marked erroneous example

transmission can be of the streaming type

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In unacknowledged mode no retransmission protocol is in use and data

delivery is not guaranteed received erroneous data is either marked or

discarded depending on the configuration on the sender side, a timer-based discard

without explicit signaling function is applied RLC SDUs which are not transmitted within a

specified time are simply removed from the transmission buffer

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an RLC entity in unacknowledged mode is defined as unidirectional because no association between uplink and

downlink is needed example of user services using unacknowledged

mode RLC cell broadcast service Voice over IP (VoIP)

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In acknowledged mode an automatic repeat request (ARQ) mechanism is

used for error correction the quality vs. delay performance of RLC can be

controlled by RRC through configuration of the number of retransmissions provided by RLC

in case RLC is unable to deliver the data correctly (max number of retransmissions reached or the transmission time exceeded) the upper layer is notified and the RLC SDU

(Service Data Unit) is discarded

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RLC can be configured for both in-sequence and out-of-sequence delivery in-sequence delivery

the order of higher layer PDUs is maintained out-of-sequence delivery

forwards higher layer PDUs as soon as they are completely received

the acknowledged mode is the normal RLC mode for packet-type services, examples Internet browsing email downloading

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7.4.2 RLC FUNCTIONS

Segmentation and reassembly performs segmentation/reassembly of variable-

length higher layer PDUs into/from smaller RLC Payload Units (PUs)

one RLC PDU carries one PU the RLC PDU size is set according to the

smallest possible bit rate for the service using RLC entity

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註: Protocol Data Unit (PDU)

in layered systems, a unit of data that is specified in a protocol of a given layer and that consists of protocol-control information of the given layer

possibly user data of that layer

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for variable rate services, several RLC PDUs need to be transmitted during one transmission time interval when any bit rate higher than the lowest one is used

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Concatenation if the contents of an RLC SDU do not fill an

integral number of RLC PUs, the first segment of the next RLC SDU may be put into the RLC PU in concatenation with the last segment of the previous RLC SDU

SDU1 SDU2

PU1 PU2 PU3 PU4 PU5 PU6

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Padding when concatenation is not applicable and the

remaining data to be transmitted does not fill an entire RLC PDU of given size, the remainder of the data field is filled with padding bits

Transfer of user data RLC supports acknowledged, unacknowledged

and transparent data transfer transfer of user data is controlled by QoS

setting

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Error correction provides error correction by retransmission in

the acknowledged data transfer mode In-sequence delivery of higher layer PDUs

preserves the order of higher layer PDUs that were submitted for transfer by RLC using the acknowledged data transfer service

if this function is not used, out-of-sequence delivery is provided

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Duplicate detection detects duplicated received RLC PDUs and

ensures that the resultant higher layer PDU is delivered only once to the upper layer

Flow control allows an RLC receiver to control the rate at

which the peer RLC transmitting entity may send information

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Sequence number check guarantees the integrity of reassembled PDUs provides a means of detecting corrupted RLC

SDUs through checking the sequence number in RLC PDUs when they are reassembled into an RLC SDU

a corrupted RLC SDU is discarded

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Protocol error detection and recovery detects and recovers from errors in the

operation of RLC protocol Ciphering

performed in the RLC layer for acknowledged and unacknowledged RLC modes

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the same ciphering algorithm is used as for MAC layer ciphering the only difference being the time-varying

input parameter (COUNT-C) for the algorithm, which for RLC is incremented together with the RLC PDU numbers

for retransmission the same ciphering COUNT-C is used as for the

original transmission; this would not be so if ciphering were on the MAC layer

ciphering details are described in 3GPP specification TS 33.102

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Suspend/resume function for data transfer suspension is needed during the security

mode control procedure so that the same ciphering keys are always used by the peer entities

suspensions and resumptions are local operations commanded by RRC via control interface

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7.5 THE PACKET DATA CONVERGENCE PROTOCOL PDCP exists only in the user plane

and only for services from PS domain

PDCP contains compression methods which are needed to get better

spectral efficiency for services requiring IP packets to be transmitted over the radio

For 3GPP Release ’99 standards a header compression method

is defined, for which several header compression algorithms can be used

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Example of why header compression is valuable the size of the combined RTP/UDP/IP headers

at least 40 bytes for IPv4 at least 60 bytes for IPv6

the size of the payload for example, for IP voice service, can be about

20 bytes or less

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7.5.1 PDCP Layer Architecture7.5.2 PDCP Functions

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7.5.1 PDCP LAYER ARCHITECTURE

Figure 7.7 shows an example of PDCP layer architecture

Multiplexing of Radio Bearers in PDCP layer is illustrated in Figure 7.7 with two PDCP SAPs (one with dashed lines)

provided by one PDCP entity using AM RLC

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Every PDCP entity uses zero, one or several header compression algorithm types with a set of configurable parameters

Several PDCP entities may use the same algorithm types

The algorithm types and their parameters negotiated during RRC Radio Bearer

establishment or reconfiguration procedures indicated to PDCP through PDCP Control Service

Access Point (SAP)

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7.5.2 PDCP FUNCTIONS

Compression of redundant protocol control information (e.g. TCP/IP and RTP/UDP/IP headers) at the transmitting entity, and decompression at the receiving entity the header compression method is specific to

the particular network layer, transport layer or upper layer protocol combinations, for example TCP/IP and RTP/UDP/IP

the only compression method mentioned in PDCP Release’99 specification is RFC2507

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Transfer of user data PDCP receives a PDCP SDU and forwards it to

the appropriate RLC entity and vice versa Support for lossless SRNS relocation

those PDCP entities which are configured to support lossless SRNS relocation have PDU sequence numbers, which, together with unconfirmed PDCP packets are forwarded to the new SRNC during relocation

only applicable when PDCP is using acknowledged mode RLC with in sequence delivery

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7.6 THE BROADCAST/MULTICAST CONTROL PROTOCOL Broadcast/Multicast Control

(BMC) protocol service-specific Layer 2

protocol exists only in the user plane

This protocol is designed to adapt broadcast and multicast services, originating from the Broadcast domain, on the radio interface

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In Release’99 of the standard, the only service utilizing this protocol is SMS Cell Broadcast service

This service is directly taken from GSM It utilizes UM RLC using CTCH (Common Traffic

Channel) logical channel which is mapped into FACH transport channel

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7.6.1 BMC Layer Architecture7.6.2 BMC Functions

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7.6.1 BMC LAYER ARCHITECTURE

The BMC protocol, shown in Figure 7.8, does not have any special logical architecture

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7.6.2 BMC FUNCTIONS

Storage of Cell Broadcast messages the BMC in RNC stores the Cell Broadcast

messages received over the CBC–RNC interface for scheduled transmission

CBC: Cell Broadcast Centre

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Traffic volume monitoring and radio resource request for CBS on the UTRAN side, BMC

calculates the required transmission rate for Cell Broadcast Service based on the messages received over CBC–RNC interface

requests appropriate CTCH/FACH resources from RRC

CTCH: Common Traffic Channel

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Scheduling of BMC messages the BMC receives scheduling information

together with each Cell Broadcast message over the CBC–RNC interface

based on this scheduling information on the UTRAN side, BMC

generates schedule messages schedules BMC message sequences

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on the UE side, BMC evaluates the schedule messages indicates scheduling parameters to RRC,

which are used by RRC to configure the lower layers for CBS reception

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Transmission of BMC messages to UE this function transmits the BMC messages

(Scheduling and Cell Broadcast messages) according to the schedule

Delivery of Cell Broadcast messages to the upper layer this UE function delivers the received non-

corrupted Cell Broadcast messages to the upper layer

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7.7 MULTIMEDIA BROADCAST MULTICAST SERVICE Multimedia Broadcast Multicast Service

(MBMS) is being added to the standard in Release 6

The principle is similar to the CBS it enables transmission of content to multiple

users in a point-to-multipoint manner The difference from CBS

MBMS enables UTRAN also to control and monitor the users receiving the data, and thus enables charging for the content being delivered via MBMS

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Applicability CBS

has been used for low rate information, like sending cell location name etc.

MBMS the mostly quoted data rate has been 64 kbps,

which enables more sophisticated content to be distributed

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Depending on the number of users that have joined to receive the content via the MBMS, the network can select to use point-to-point transmission

DCH is used as the transport channel point-to-multipoint transmission

FACH is used as the transport channel in a particular cell for the MBMS content

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On the physical layer DCH is mapped to DPDCH (Dedicated Physical

Data channel) FACH is mapped to SCCPCH (Secondary Common

Control Physical Channel) In the case of point-to-point connection

the logical channel can be DCCH or DTCH with all the mapping in Release ’99 possible

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In the case of point-to-multipoint case, there are two new logical channels MBMS point-to-multipoint Control Channel

(MCCH), which carries the related control information

MBMS point-to-multipoint Traffic Channel (MTCH), which carries the actual user data

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UTRAN shall decide, based on the number of UEs in a particular cell which mode of MBMS operation to use, and if the

situation changes, the network can transfer the UEs between different states of MBMS reception (Figure 7.9)

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Figure 7.10 shows an example scenario one cell uses point-to-multipoint while another

cell has only one joined UE which is kept in the point-to-point state

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7.8 THE RADIO RESOURCE CONTROL PROTOCOL

Radio Resource Control (RRC) messages the major part of the control signaling between

UE and UTRAN carry all parameters required to set up, modify

and release Layer 1 and Layer 2 protocol entities carry in their payload also all higher layer

signaling MM (Mobility Management) CM (Connection Management) SM (Session Management)

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The mobility of user equipment in the connected mode is controlled by RRC signaling, which are measurements handovers cell updates

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7.8.1 RRC Layer Logical Architecture7.8.2 RRC Service States7.8.3 RRC Functions

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7.8.1 RRC LAYER LOGICAL ARCHITECTURE

Figure 7.11 shows the RRC layer logical architecture

RRC layer has four functional entities1. Dedicated Control Function Entity (DCFE)

handles all functions and signaling specific to one UE

in the SRNC there is one DCFE entity for each UE having an RRC connection with this RNC

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DCFE uses mostly acknowledged mode RLC (AM-SAP), but some messages are sent using unacknowledged mode SAP (e.g. RRC Connection Release) or transparent SAP (e.g. Cell Update)

DCFE can utilize services from all Signaling Radio Bearers

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2. Paging and Notification control Function Entity (PNFE) handles paging of idle mode UE(s) there is at least one PNFE in the RNC for

each cell controlled by this RNC PNFE uses PCCH logical channel normally

via transparent SAP of RLC PNFE could utilize also UM-SAP

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In this example architecture the PNFE in RNC when receiving a paging message from an Iu

interface needs to check with the DCFE whether or not

this UE already has an RRC connection (signaling connection with another CN domain)

if it does, the paging message is sent (by the DCFE) using the existing RRC connection

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3. Broadcast Control Function Entity (BCFE) handles the system information

broadcasting there is at least one BCFE for each cell in

the RNC BCFE uses either BCCH or FACH logical

channels, normally via transparent SAP BFCE could utilize also UM-SAP

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4. Routing Function Entity (RFE) normally drawn outside of the RRC protocol

and ‘logically’ belonging to the RRC layer, since the information required by this entity is part of RRC messages

its task is the routing of higher layer messages to

different MM/CM entities (UE side), or different core network domains (UTRAN

side)

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7.8.2 RRC SERVICE STATES

Two basic operational modes of a UE idle mode connected mode

Connected mode can be further divided into service states, which define what kind of physical channels a UE is using

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Figure 7.12 shows the main RRC service states in the connected

mode the transitions between idle mode and

connected mode the possible transitions within the connected

mode

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In the idle mode after a UE is switched on, it selects (either

automatically or manually) a PLMN to contact the UE looks for a suitable cell of the chosen

PLMN chooses that cell to provide available services,

and tunes to its control channel this choosing is known as ‘camping on a cell’

after camping on a cell in idle mode the UE is able to receive system information

and cell broadcast messages

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the UE stays in idle mode until it transmits a request to establish an RRC connection

in idle mode the UE is identified by identities such as IMSI, TMSI and P-TMSI (Packet-TMSI)

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In the Cell_DCH state a dedicated physical channel is allocated to the

UE the UE is known by its serving RNC on a cell or

active set level the UE performs measurements and sends

measurement reports according to measurement control information received from RNC

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In the Cell_FACH state no dedicated physical channel is allocated for

the UE, but RACH and FACH channels are used instead, for transmitting both signaling messages and small amounts of user plane data

the UE is also capable of listening to the broadcast channel (BCH) to acquire system information

the UE performs cell reselections, and after a reselection always sends a Cell Update message to the RNC, so that the RNC knows the UE location on a cell level

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for identification, a C-RNTI (Cell-Radio Network Temporary Identity) in the MAC PDU header separates UEs from each other in a cell

when the UE performs cell reselection it uses the U-RNTI (UTRAN-Radio Network Temporary Identity) when sending the Cell Update message so that UTRAN can route the Cell Update

message to the current serving RNC of the UE

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In the Cell_PCH state the UE is still known on a cell level in SRNC, but

it can be reached only via the paging channel (PCH)

in this state the UE battery consumption is less than in the Cell-FACH state since the monitoring of the paging channel

includes a discontinuous reception (DRX) functionality

the UE also listens to system information on BCH

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a UE supporting Cell Broadcast Service (CBS) is also capable of receiving BMC (Broadcast/ multicast control protocol) messages in this state

if the UE performs a cell reselection it moves autonomously to the Cell-FACH state

to execute the Cell Update procedure after which it re-enters the Cell-PCH state if no

other activity is triggered during the Cell Update procedure

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The URA_PCH state very similar to the Cell_PCH except that the UE does not execute Cell Update

after each cell reselection, but instead reads UTRAN Registration Area (URA) identities from the broadcast channel only if the URA changes does UE inform its

location to the SRNC this is achieved with the URA Update

procedure, which is similar to the Cell Update procedure

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When RRC connection is released or at RRC connection failure the UE leaves the connected mode and returns

to idle mode

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7.8.2.1 ENHANCED STATE MODEL FOR MULTIMODE TERMINALS Figure 7.13

an overview of the possible state transitions of a multimode (UTRA FDD–GSM/GPRS) terminal

With these terminal types it is possible to perform inter-system handover between UTRA FDD and

GSM inter-system cell reselection from UTRA FDD to

GPRS

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7.8.2.2 EXAMPLE STATE TRANSITION CASES WITH PACKET DATA

When sending or receiving reasonable amounts of data UE will stay in Cell_DCH state

Once the data runs out and timers have elapsed UE will be moved away from Cell_DCH state

Moving back to the Cell_DCH state always requires signaling between UE and SRNC the network sets up the necessary links to Node

B

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Use of Cell_DCH or Cell_FACH state is always a trade-off between terminal power consumption service delay signaling load network resource utilization

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Figure 7.14 shows the signaling flow of UE-initiated RRC state transition an application has created data to be

transmitted to the network UE goes to Cell_FACH state sending data on RACH is not sufficient, a DCH

needs to be set up

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once in Cell_FACH state the UE initiates signaling on the

RACH (1) after the network has received

the measurement report on RACH (4) and a radio link has been set up between Node B and RNC (5)

the reconfiguration message is sent on FACH to inform of the DCH parameters to be used (6)

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Figure 7.15 shows the signaling flow of Network-initiated RRC state transition the network-initiated RRC state change occurs

when there is too much downlink data to be transmitted, and using FACH is not enough

the network first transmits the paging message in the cell where the terminal is located (as the terminal location is known at cell level in Cell_PCH state) (1)

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upon reception of the paging message, the terminal moves to Cell_FACH state and initiates signaling on the RACH (2)

now there is no need for any measurement report as transition is initiated by the network

the response from the terminal in the example case is a reconfiguration complete message (5), assuming the DCH parameters have been altered in connection to the state transition

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7.8.3 RRC FUNCTIONS

Establishment, maintenance and release of an RRC connection between UE and UTRAN

Control of Radio Bearers, transport channels and physical channels

Control of security functions (ciphering and integrity protection)

Broadcast of system information, related to access stratum and non-access stratum

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註: Access stratum

the set of protocols and capabilities relevant to radio technique

Non access stratum those technique which are independent of radio

access network capabilities

call and session control (set up, modify and release the transmission logic resources relevant to the required service)

mobility control (enable the user to communicate regardless of his location)

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Paging Initial cell selection and reselection in idle

mode Integrity protection of signaling messages UE measurement reporting and control of the

reporting RRC connection mobility functions Support of SRNS relocation

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Support for downlink outer loop power control in the UE

Open loop power control Cell broadcast service related functions Support for UE positioning functions