umts basic refresher

7/28/2019 UMTS Basic Refresher

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Background

Signalling & interfaces

Interface

Application protocol

Service related signalling

MSC/SGSN

node BRNC

UE

RNC

RRC

NBAP

RNSAP

RANAPNAS/layer 3

Uu

Iub

Iur

Iu

service user service access point


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Allow communication (signalling: control plane) between different network elements

Technology specific (e.g. UMTS)

UMTS application protocols:

RRC (Radio Resource Control)

Runs over Uu and Iub interface, between UE and RNC

NBAP (Node B Application Part)

Used on Iub interface between node B and RNC

RANAP (Radio Access Network Application Part)

Used on Iucs/Iups interface between RNC and MSC/SGSN

RNSAP (Radio Network Subsystem Application Part)

Used on Iur interface between different RNCs

Remark: only RRC application protocol will be studied in detail

Can be captured on air interface during drive testing

Background

Application protocols


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Used for service setup and release

Between service user (UE) and service access point (MSC/SGSN)

Service specific, but not technology specific

UMTS service related signalling is different for voice call and PS data session

2G and 3G service related signalling can be the same for the same service type (e.g. voice

call)

Background

Service related signalling


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Only one RRC Connection in connected mode, no RRC Connection in idle mode

RRC Connection consists of typically four signalling Radio Bearers (sRB)

sRB1, sRB2, sRB3 and sRB4

Used for transport of RRC application protocol and NAS/layer 3 messages

Up to two Iu signalling connections in connected mode (to MSC and/or SGSN), none in idle mode

Background

Control plane

MSC/SGSN

node BRNC

UE

RNC

RRC (signalling) connection

Uu

Iub

Iur

Iu

Iu signalling connection


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One RAB for each service type in connected mode (e.g. voice call, PS data service), no RAB in

idle mode

RAB consists of one or three RBs (depending on service)

Voice call three RBs, all other services only one RB

One Iu bearer for every RAB used

Background

User plane

MSC/SGSN

node BRNC

UE

RNC

Radio Bearer (RB)

Uu

Iub

Iur

Iu

Iu bearerRadio Access Bearer (RAB)


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Background

Control/user plane flow during soft handover

MSC/SGSN

node BRNC

UE

Iub

Iur

Iu

RNC

RNC

Iur

node B

node B

Iub

Iub

Serving RNC (SRNC)

Drift RNC (DRNC)

Drift RNC (DRNC)

One logical RRC Connection

Signalling/data flow

Radio Link (RL)


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BackgroundControl plane vs. user plane bit pipe

Control plane bit pipe (RRC Connection):

Signalling transport (RRC, NAS)

Typically 4 sRBs setup during RRC Connection Setup procedure

User plane bit pipe:

Transport of user data

1 or 3 RBs setup during RB Setup procedure

UE

SRNC

C-plane U-plane

sRB 14 RB 13

RRC Connection Setup

procedure

RB Setup procedure


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BackgroundControl plane vs. user plane RRC procedures

RRC Connection Setup procedure

RB Setup procedure

UE SRNC

RB Release procedure

RRC Connection Release procedure

Setup control plane

bit pipe

Release control plane

bit pipe

Setup user planebit pipe

Release user plane

bit pipe


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BackgroundRRC Connection Setup procedure

RRC Connection Setup procedure:

First step to go from idle mode to connected mode

Establishment of RRC signalling connection

Setup dedicated resources control plane (CCCH

DCCH) RRC Connection Request:

Used by UE to get access to network

Retransmitted if no reaction from network

Maximum # retransmissions: N300 (default: 3)

Time between retransmissions: T300 (default: 1000ms) Parameters in System Information

UE SRNC

RRC Connection Request (UL-CCCH)

RRC Connection Setup (DL-CCCH)

RRC Connection Setup Complete (UL-DCCH)


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BackgroundRRC Connection Setup procedure

RRC Connection Setup:

Contains downlink instructions regarding setup control plane bit pipe

RRC Connection Setup Complete:

Confirmation from UE indicating successful setup of control plane bit pipe Contains information on UE capabilities (e.g. HSDPA UE category)

UE SRNC


RRC Connection Setup (DL-CCCH)

RRC Connection Setup Complete (DL-DCCH)


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BackgroundRRC Connection Release procedure

UE SRNC

RRC Connection Release Complete (UL-DCCH)

RRC Connection Release (DL-DCCH)

RRC Connection Release Complete (UL-DCCH)

Used to release RRC signalling connection

All sRBs released

To go from connected mode to idle mode

RRC Connection Release complete: typically retransmitted several times

# retransmissions: N308

Time between retransmissions: T308


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BackgroundRRC Connection Setup failure

UE SRNC


RRC Connection Reject (DL-CCCH)

Indication of unsuccessful RRC Connection Setup procedure

Reject cause, e.g. congestion

Wait time: time needed before retry

Optionally UE redirected to other carrier/RAT


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BackgroundRB Setup procedure

UE SRNC

RB Setup

RB Setup Complete

Setup dedicated resources/bit pipe user plane

Consists of 1 or 3 RBs depending on requested service

Triggered by RAB setup procedure (RANAP on Iu interface)


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BackgroundRB Release procedure

UE SRNC

RB Release

RB Release Complete

Release dedicated resources/bit pipe user plane

Used for PS procedures

Not used for CS procedures

RBs released with RRC Connection Release procedure


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BackgroundCS vs. PS service

CS service PS service

CS service

No RB release procedure

All RBs released implicitly with RRC Connection Release procedure


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BackgroundInitial Direct Transfer

UE SRNC

Initial Direct Transfer (initial NAS message)

Used to setup Iu signalling connection between SRNC and MSC/SGSN

Carries the very first NAS message of the session from UE to SRNC

Contains CN domain:

CS: Iu signalling connection needed to MSC

PS: Iu signalling connection needed to SGSN

Last step to go from idle mode to connected mode:

1) RRC Connection Setup procedure (signalling connection UE-SRNC)

2) Initial Direct Transfer (signalling connection UE-MSC/SGSN)


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BackgroundUplink/Downlink Direct Transfer

UE SRNC

Uplink Direct Transfer (NAS message)

Used to carry NAS messages between UE and SRNC, once the Iu signalling connection exists

Carries all NAS messages of the session except the initial NAS message

Downlink Direct Transfer (NAS message)


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BackgroundMobile Originating Call part 1

Downlink Direct Transfer (CC Call Proceeding)

UE SRNC

RRC Connection Setup Complete

RRC Connection Request

RRC Connection Setup

Initial Direct Transfer (MM CM Service Request)

Security Mode Command

Security Mode Complete

Uplink Direct Transfer (CC Setup)

RB Setup

RB Setup Complete

RRC Connection Setup procedure

RB Setup procedure

Security Mode procedure: ciphering

NAS signalling: service setup part 1

First NAS messageDownlink Direct Transfer (MMAuth Req)

Uplink Direct Transfer (MMAuth Resp) Authentication procedure: double check UE id


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BackgroundMobile Originating Call part 2

UE SRNC

RRC Connection Release

RRC Connection Release Complete

Uplink Direct Transfer (CC Disconnect)

Downlink Direct Transfer (CC Release)

Uplink Direct Transfer (CC Release Complete)

RRC Connection Release Complete

NAS signalling: service release

RRC Connection Release procedure

Downlink Direct Transfer (CC Alerting)

Downlink Direct Transfer (CC Connect)

Uplink Direct Transfer (CC Connect Ack)

NAS signalling: service setup part 2

ACTIVE CALL


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BackgroundIntra-frequency soft handover

3G coverage f1 3G coverage f1

Soft handover area

Handover between cells with same carrier

Soft handover:

UE connected simultaneously to multiple cells

Iur interface needed if several RNCs involved

Soft handover area:

Overlapping cells with same quality

RNCRNC

Iur


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BackgroundIntra-frequency soft handover area

2-cell soft handover area


Active Set (AS): all cells the UE is connected to in connected mode

Typically Active Set is limited to maximum three cells

Several possibilities for soft handover area:



Soft handover causes redundancy, but results in reduction of transmitted power (SHO Gain)


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BackgroundRRC state model main states

Two main states:

Idle mode: no RRC connection

Connected mode: one logical RRC connection

Connected mode

Idle mode


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BackgroundRRC state model - substates

cell-DCH cell-FACH

cell-PCHURA-PCH

Connected mode

Idle mode

Connected mode: four substates

Cell-DCH: UE is allocated dedicated resources (e.g. codes from code tree)

Cell-FACH: UE is using shared resources (common control channels RACH/FACH)

Cell-PCH/URA-PCH

UE in sleep mode, not using shared/dedicated resources from network

Paging needed to wake up UE, resulting in transition to cell-FACH

Location of UE known to network: cell (cell-PCH) or URA (URA-PCH)


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BackgroundRadio Bearer upgrade/downgrade

Switching between different data rates

64kbps 128kbps 384kbps

By changing the uplink/downlink spreading factor

Performed with Transport Channel Reconfiguration


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BackgroundRadio Bearer upgrade/downgrade Spreading Factor

Symmetrical service: SF DL is double of SF UL

Data rate (DL/UL) SF DL SF UL

64/64 32 16

64/128 32 8

64/384 32 4

128/64 16 16

128/128 16 8

128/384 16 4

384/64 8 16

384/128 8 8

384/384 8 4


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BackgroundRadio Bearer upgrade/downgrade logfile example

umts basic refresher

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