WCMDA 02 Radio Access Bearer

COMIT Training Course 1

Oct - 2015

At the end of this module, you will be able to • Describe the purpose of Radio Access Bearers (RABs) • Explain the attributes of the RABs • Explain how the CN selects a RAB based on its attributes • Explain the procedure for establishing a RAB • Explain how RABs are mapped onto physical channels

Objectives

3G RPLS / AA / 10/2008

The purpose of a Radio Access Bearer (RAB) is to provide a connection

segment using the WCDMA Radio Access Network (RAN) for support of a

UMTS bearer service. The WCDMA RAN can provide Radio Access Bearer

connections with different characteristics in order to match requirements for

different UMTS bearers

Radio Access Bearer

TE TE CN

Gateway MT UTRAN CN Iu

edge node

End-to-End Service

TE/MT Local Bearer Service

External Bearer Service

UMTS Bearer Service = UMTS QoS

CN Bearer Service

Radio Access Bearer Service

Backbone Bearer Service

Radio Bearer Service

Iu Bearer Service

UTRA FDD/TDD Service

Physical Bearer Service

(adopted from TS 23.107)

RABs

• In UMTS, four QoS classes have been defined: • Conversational class

•is the QoS class for delay sensitive real time services such as speech telephony. • Streaming class

•is also regarded as real-time QoS class. It is also sensitive to delays; it carries traffic, which looks real time to a human user. An application for streaming class QoS is audio streaming, where music files are downloaded to the receiver. There may be an interruption in the transmission, which is not relevant for the user of the application, as long as there are still enough data left in the buffer of the receiving equipment for seamless application provision to gap the transmission time break.

• Interactive class •is a non-real time QoS class, i.e. it is used for applications with limited delay sensitivity (so-called interactive applications). But many applications in the internet still have timing constraints, such as http, ftp, telnet, and smtp. A response to a request is expected within a specific period of time. This is the QoS offered by the interactive class.

• Background class •is a non-real time QoS class for background applications, which are not delay sensitive. Example applications are email and file downloading.

• A set of UMTS bearer attributes have been defined to specify the UMTS service. They are listed on the right hand side. When a UMTS bearer is established, modified or released, aspects such as the UE capabilities, subscription profiles and network specific QoS profiles have to be taken under consideration.

UMTS QoS Architecture

Background class

Interactive class

Streaming class

Traffic class

Maximum bit rate

SDU format information

SDU error ratio

Residual bit error ratio Delivery of

erroneous SDUs

Transfer delay

Guaranteed bit rate

Traffic handling priority

Allocation/Retention priority

Delivery order

Maximum SDU size

Conversational class

(adopted from TS 23.107 chap. 6.4.3.3)

UMTS Bearer Attributes

UMTS Bearer Attributes

Conversational Bearer (CS 12.2) Notes: 1. The value of the

supported “Transfer delay ” for each RAB type is configured. 80ms is considered an appropriate provisional value for this RAB, assuming a minimum allowance of 25 ms for all “external delays ”and 20 ms interleaving period.

2. The presence of this attribute is optional. If present, it is ignored.

Conversational Bearer (CS64) Notes: 1. By TSG_N3 recommended

value for “Transparent data, including Multimedia ”

2. This parameter was not required in P1 but is the result of CR245 to the December version (v3.4.0) of RANAP.

3. Provisional value, assuming a minimum allowance of 20 ms for all “external delays ” and 20 ms interleaving period.

4. The presence of this attribute is optional. If present, it is ignored.

Interactive/Background Bearer (PS) Notes: 1. The maximum bit rate is interpreted as

the maximum dedicated channel rate, which is allowed to be assigned to the RAB by the UTRAN. Acceptance does not imply that UTRAN is capable of providing a connection with the requested maximum rate.

2. Specifies a maximum size of N-PDU (Network PDU) of 1502 octets for PDP type PPP and 1500 for any other PDP type. Although in-sequence delivery is required for PDP type PPP and it is not supported for PS RABs in P2, a length of 1502 octets should be supported to ease migration (1502 octets is already supported in P1).

3. Only present when traffic class =‘interactive’.

4. The presence of this attribute is optional. If present, it is ignored

RAN Connection handling Before the RAB can be established, a signaling connection must be set up between the Core Network

(CN) and User Equipment (UE). RAN Connection Handling comprises procedures for establishment, supervision and release of both signaling and user plane connections inside the WCDMA RAN. It also includes security handling of an established connection in terms of integrity protection and ciphering

Signalling connection handling Signaling Connection Handling provides the ability to establish the control plane

connection and to release both the control plane and user plane. A UE can

simultaneously have one signaling connection towards each Core Network (CN)

domain, although never using more then one RRC connection. A Signaling connection

consists of a RRC connection over the Uu interface and an Iu control plane connection

over the Iu interface

• The RRC Connection is a dedicated connection used for control signaling between WCDMA RAN and one UE. The establishment is always initiated by the UE, due to some signaling procedure to be run towards the CN (e.g. Periodic Location Area update), or user action (e.g. service request) or paging from the CN. The WCDMA RAN always establishes the RRC Connection on a dedicated channel (DCH). A RRC connection between the UE and RNC consists of three or four Signaling Radio Bearers (SRB1, SRB2, SRB3, and SRB4. SRB4 is not established at handover from GSM to UMTS),

RADIO RESOURCE CONTROL (RRC)

Radio Resource Control •The Radio Link Control (RLC) layer will segment the user data packets, known as Service Data Unit (SDU), so that they fit into the Protocol Data Unit (PDU) length. In the receiver the RLC will reassemble the SDUs from the PDUs. •In Radio Link Control (RLC) Unacknowledged Mode (UM) no retransmission protocol is in use and data delivery is not guaranteed. Received erroneous data is either marked or discarded depending on the configuration. UM is used, for example, for certain RRC signaling procedures, where acknowledgement and retransmissions are part of the RRC procedure. The cell broadcast service is an example of a user service that could utilize UM. •Acknowledged Mode (AM) an Automatic Retransmission reQuest (ARQ) mechanism is used for error correction. This is the normal mode for packet-type services such as Internet browsing and email. •Transparent Mode (TM) where no RLC protocol overhead is added to higher layer data. An example is the signaling Radio Bearer (RB) for Broadcast Control Channel (BCCH). Two separate Non Access Stratum Direct Transfer (NAS_DT) signaling bearers (SRBs) are reserved for Dedicated Control Channels (DCCH) between the CN and UE. This allows prioritization of the UE-UTRAN signaling without the need for extra functionality. There are two priorities of NAS_DT bearers: normal and low. NAS_DT low priority can be used for Short Message Service (SMS). •The Medium Access Layer (MAC) will multiplex these PDUs onto either common or dedicated transport channels, which are mapped respectively onto common or dedicated physical channels

Step 1: After the NAS event in the UE (for example a page

or user action) a RRC Connection Request is sent from the UE to the CN using a physical Random Access channel (RACH), whose physical structure is depicted in the next page.

RRC CONNECTION SETUP OVER THE AIR INTERFACE

This message is send using the logical Common Control Channel (CCCH) in Transparent Mode (TM)

RRC CONNECTION SETUP OVER THE AIR INTERFACE (cont.)

RACH structure as below:


Step2:

Upon reception of this message the following

algorithms will take action (in RNC)

• The Power Control algorithm in the SRNC

sets the initial downlink (DL) Dedicated

Channel (DCH) transmission power.

• The Iub and Uu Timing scheduling

algorithms calculate the channel timing

parameters.

• The Admission Control algorithm checks if

the new Radio Link can be allowed in the

cell.

• The Code Control algorithms allocate the

UL scrambling code and DL channelization

code.


Step3:

The SRNC will send a Radio Link Setup Request

message to the RBS/NodeB with the required

resources. The binding identifier and transport

layer address for the AAL2 connection are allocated.

Step4:

The RBS calculates link characteristic parameters

from the received UL/DL Transport Format

combination Set (TFCI)/Transport Format Set

(TFS) information.


Step3:

The SRNC will send a Radio Link Setup Request

message to the RBS/NodeB with the required

resources. The binding identifier and transport

layer address for the AAL2 connection are allocated.

Step4:

The RBS calculates link characteristic parameters

from the received UL/DL Transport Format

combination Set (TFCI)/Transport Format Set

(TFS) information.



Step5:

The RBS/NodeB will return a Radio Link Setup

Response to the SRNC.

Step6:

If the response is positive, the transport bearer

(AAL2 connection) needed for signaling is set up

over Iub interface and synchronized.

Step7:

Once the transport channel is synchronized the

RBS will start to transmit the downlink (DL)

Dedicated Physical Channel (DPCH) and start to

receive the uplink (UL) DPCH. Supervision of the

Radio Link Set (RLS) will also start.

Step8: The SRNC will send a RRC Connection Setup message to request the UE to change to connected state

(Cell_DCH) using the Secondary Common Physical Channel (S_CCPCH)



Step9:

After reception of DL DPCH the UE will start to

transmit the UL DPCH. When the RBS can receive

the UL DPCH, the Layer 1 (L1) connection is

synchronized.

Step10:

The RBS will indicate this to the SRNC by sending a

Radio link restore indication message.


Step11:

The UE will send a RRC connection setup complete

message, which contains the UE capabilities to the

SRNC using the newly established Signaling Radio

Bearer SRB. This information is used by the RAB

establishment procedure, UE Security Handling

and by the Channel Switching function.

The RRC connection is now established on the air

interface and the UE is now said to be in SRB

CELL_DCH state and will be using an uplink and

downlink Dedicated Physical control (DPCCH) and

data (DPDCH) Channel.

The physical structure of the uplink DPDCH and

DPCCH conforms to the 3GPP standard shown in

the next page


THE IU CONTROL PLANE CONNECTION SETTING UP


Step1:

The establishment is initiated by the reception of a RRC message that contains the NAS message that

originally triggered the Signaling Connection setup.

Step2:

Upon reception, the SRNC registers the connection and allocates an identifier for the Iu control plane

connection.

Step3:

The SRNC uses the Signaling Connection Control Part (SCCP) service to set up an Iu control plane

connection to the CN and sends the Radio Access Network Application Part (RANAP) message to the CN.

This RANAP message includes both the UE NAS message, current location information of the UE and the

Iu control plane connection identifier.

Step4:

After the SRNC has received a confirmation from CN that the connection has been setup, both nodes are

ready to send and receive data in SCCP-connection oriented mode.


Step5:

If a RANAP DIRECT TRANSFER message was included in the SCCP Connection Confirm messages, the

SRNC forwards the NAS message to the UE. At this stage the UE has both a RRC Connection to WCDMA

RAN and an Iu control plane connection between WCDMA RAN and CN, thus a Signaling Connection is

established and the UE will now be in ‘CELL_DCH’ state.

Step6:

Soon after the establishment of the Signaling Connection, the CN sends a RANAP Common Id messages

(if not already included in the SCCP connection confirmation messages) to inform the SRNC about the

permanent NAS UE Identity (i.e. IMSI).

Step7: The SRNC creates a reference between the permanent UE identity and the RRC connection, which is

used by the SRNC for WCDMA RAN paging coordination

• The existing Signaling Connection is used to carry the Non Access Stratum (NAS) call setup message

between the CN and UE.

• The RAB establishment is done in two parts, the first is the setup of an Iu bearer and the second is the

setup of the Radio Bearer (RB).

• The establishment and the mapping onto physical channels for each of the

different supported RABs will be explained in this chapter

RAB ESTABLISHMENT

ESTABLISHMENT OF AMR, CS64 AND STR57.6 RADIO ACCESS BEARERS

Step1:

A ‘RAB ASSIGNMENT REQUEST’ is received from the CN indicating the RAB ID to be ‘speech’. The

SRNC determines the new RRC configuration (taking into account the existing one) and also check the

UE capabilities (to check if the new configuration can be supported by the UE).

Step2:

The SRNC initiates the setup of an AAL2 connection for the Iu bearer.

Step3:

The UE is ordered to setup a new Radio Bearer (RB) and activate the new configuration at the desired

Current Frame Number (CFN). In the SRNC the MAC shall be reconfigured without waiting for the RLC

acknowledge. When finished the UE acknowledges the successful RB setup and the UE will be in

CELL_DCH state.

ESTABLISHMENT OF AMR, CS64 AND STR57.6 RADIO ACCESS BEARERS

INTERACTIVE AND STREAMING PS RAB ESTABLISHMENT The interactive PS RAB can be set up on either a common or a dedicated channel. The streaming RAB must be setup on dedicated channel.

Step1:

A ‘RAB ASSIGNMENT REQUEST’ is received from the CN indicating the RAB ID to be ‘interactive’. The

SRNC determines the new RC configuration (taking into account the existing one) and also check the

UE capabilities (to check if the new configuration can be supported by the UE).

Step2:

The existing AAL5 signaling Iu bearer is used to carry the user plane traffic.

Step3:

The UE is ordered by the SRNC to set up a new RB. In the case of Cell_FACH the UE will also perform a

Cell Update after the RB is set up.

Data buffers are used for retransmissions and packet data

INTERACTIVE AND STREAMING PS RAB ESTABLISHMENT

• In this state the UE is able to send packets of control information using the logical Dedicated Control

Channel (DCCH) mapped onto the Random Access Channel (RACH) according to the RB mapping

• User data from the logical Dedicated Traffic Channel (DTCH) is also mapped onto the RACH according

to the RB mapping

• In the downlink the DCCH is mapped onto the Forward Access Channel (FACH) according to the RB

mapping

• The user plane data from the DTCH is mapped onto the FACH according to the RB mapping

• In the uplink up to four Transport Blocks (TBs) may be mapped onto thevDPDCH according to the

mapping. This equates to the maximum uplink data rate of 64 kbps but since less than four blocks may

be sent, the uplink data rate (and hence power) is variable. The combination of TBs is indicated to the

receiver in the Transport Format Combination Indicator (TFCI) bits in the DPCCH.

• In the downlink we can have 64 kbps, 128 kbps and 384 kbps.

INTERACTIVE AND STREAMING PS RAB ESTABLISHMENT

The procedure for this is similar to what is already described when a speech RAB and Interactive RAB is

established. One signaling connection is needed consisting of one RRC and two Iu connections

(towards the MSC and the SGSN)

MULTI-RABs ESTABLISHMENT

MULTI-RABs ESTABLISHMENT

RAB MAINTENANCE Once the RAB is established various Radio Network Functionality algorithms must maintain the required quality of service

For this course module, following 3GPP specifications were used:

• TR 21.902, Evolution of 3GPP system • TS 23.002, Network architecture • TS 23.101, General UMTS Architecture • TS 23.107, Quality of Service (QoS) concept and architecture • TS 25.301, Radio interface protocol architecture • TS 25.308, UTRA HSDPA; Overall description; Stage 2 • TS 25.401, UTRAN overall description • TR 25.876, MIMO in UTRA • TS 25.308, HSDPA – Overall Description • TS 25.309, FDD Enhanced Uplink (HSUPA) – Overall Description • TS 36.300, E-UTRA and E-UTRAN Overall Description • TR 36.913, LTE-Advanced

TS Technical Specification TR Technical Report

Remark: Most of these Specifications are available in different versions, mainly depending on the 3GPP Release. HSDPA is

only available starting with Release 5; therefore, HSDPA is only contained in Version 5 or later specifications. Release 99 is referred to as Version 3. Modifications within one release are possible, resulting in running numbers.

Example: TS 25.401 V3.10.0 gives an overall description of UTRAN based on Release 99. 10.0 refers to 10 (by 3GPP) approved versions with minor corrections.

References

Documents

WCMDA 02 Radio Access Bearer