UMTS Radio Link Control Protocol (RLC) Overview 25.322

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Radio Interface Architecture

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Why RLC is required?

  • Larger pieces of data are not suitable to be sent over the air interface where bit faults are common.
  • Smaller pieces can be individually retransmitted
  • Retransmissions on higher Protocol level take too long time. Therefore it is better to have the retransmission as close to the biggest trouble source (i.e. The radio interface)

RLC main functionalities

  • Segmentation and reassembly.
  • Concatenation.
  • Padding.
  • Transfer of user data.
  • Error correction.
  • In-sequence delivery of upper layer PDUs.
  • Duplicate detection.
  • Flow control.
  • Sequence number check.
  • Protocol error detection and recovery.
  • Ciphering.
  • SDU discard.
  • Out of sequence SDU delivery.
  • Duplicate avoidance and reordering.

Data flow between layers

Data flow for transparent RLC

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Data flow for non-transparent RLC

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Model of RLC Sublayer

  • TM (Transparent mode)
  • UM (Unacknowledged mode)
  • AM (Acknowledged mode

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Transparent Mode (TM) RLC

RLC-TM

Services provided to upper layer
  1. Segmentation and reassembly
  2. Transfer of user data
  3. SDU discard
  • If segmentation is configured by upper layers and a RLC SDU is larger than the TMD PDU size, the transmitting TM RLC entity segments RLC SDUs to fit the TMD PDU size without adding RLC headers.
  • All the TMD PDUs carrying one RLC SDU are sent in the same TTI and no segment from another RLC SDU are sent in that TTI.

Unacknowledged Mode (UM) RLC Entity

RLC-UM

Services provided to upper layer
  1. Segmentation and reassembly.
  2. Concatenation.
  3. Padding.
  4. Transfer of user data.
  5. Ciphering.
  6. Sequence number check.
  7. SDU discard.

Acknowledged Mode RLC Entity

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Services provided to upper layer
  1. Segmentation and reassembly.
  2. Concatenation.
  3. Padding.
  4. Transfer of user data.
  5. Error correction.
  6. In-sequence delivery of upper layer PDUs.
  7. Duplicate detection.
  8. Flow Control.
  9. Protocol error detection and recovery.
  10. Ciphering.
  11. SDU discard.
RLC Error Correction

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RLC Protocol Data Unit

Data PDUs

  1. TMD PDU (Transparent Mode Data PDU)
  2. UMD PDU (Unacknowledged Mode Data PDU)
  3. AMD PDU (Acknowledged Mode Data PDU)

Control PDUs

  1. Status PDU and Piggybacked STATUS PDU
  2. Reset PDU
  3. Reset-ACK PDU
TMD PDU (Transparent Mode Data PDU)
  • The TMD PDU is used to transfer user data when RLC is operating in transparent mode.
  • No overhead is added to the SDU by RLC.
  • The data length is not constrained to be a multiple of 8 bits.
UMD PDU (Unacknowledged Mode Data PDU)

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  • The UMD PDU is used to transfer user data when RLC is operating in unacknowledged mode.
  • The length of the data part shall be a multiple of 8 bits.
UMD PDU Header Details

Sequence Number

  • Sequence number of UMD PDU encoded in binary.
  • SN is of length 7 bits
  • It is used for reassembly

Length Indicator

  • LI is used to indicate the end of a SDU in a PDU.
  • Lenth is 7 or 15 bits

E (Extension Bit)

E bit tells ”What there in the next field”

  • If E = 0 : Next field is data or status PDU or a complete SDU
  • If E 0 1 : Next filed is Length Indicator (LI) followed by extension bit (E).
AMD PDU (Acknowledged Mode Data PDU)

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  • The length of the data part shall be a multiple of 8 bits.
  • The AMD PDU header consists of the first two octets, which contain the "Sequence Number".
AMD PDU Header Details

D/C

Indicates whether a data or control PDU

  • If D/C = 1 : Its a data PDU
  • If D/C = 0 : Its a control PDU

Sequence Number

  • Length of sequence number is 12 bits i.e after SN reaches 4095 there will be a rollover.
  • SN is used for retransmission and reassembly

P bit

Polling bit is used to request a status report from the receiver.

  • If P = 0 : No status requested
  • If P = 1 : Status requested

HE

Header Extension is of 2 bits and it indicates the next bit is data or ”Length Indicator” and E bit.

  • If HE = 00 : Next octet is data
  • If HE = 01 : Next octet is LI and E bit
  • If HE = 10 : Next value is the data and last octet of the PDU is the last octet of the SDU.
  • If HE = 11 : Reserved, discarded by protocol
Status PDU

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  • The length of the STATUS PDU shall be a multiple of 8 bits.
  • Status PDU is used for transmission of status information.
Status PDU Header Details

PDU Type

PDU type indicates the Control PDU type

  • 000 : Status PDU
  • 001 : Reset PDU
  • 010 : Reset-Ack PDU
  • 011 – 111 : Reserved

SUFI

Super-Field indicates which AMD PDUs are received correctly and which are missing.

SUFi has three sub-fields:

  • Type
  • Length
  • Value
Reset and Reset-Ack PDU

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RESET PDU

The RESET PDU is used in acknowledged mode to reset all protocol states, protocol variables and protocol timers of the peer RLC entity in order to synchronies the two peer entities.

RESET ACK PDU

The RESET ACK PDU is an acknowledgement to the RESET PDU.

Reset/Reset-Ack PDU Header

RSN

  • Reset Sequence Number is of 1 bit length.
  • RSN is the sequence number of the transmitted RESET PDU.
  • Initial value of this field is 0.

R1

  • This field is used to make RESET/RESET-ACK PDU octet aligned.
  • The value of R1 is 000.

HFNI

  • Hyper Frame Number Indicator is of 2o bits.
  • It is used to indicate the HFN to the peer entity.
  • With the help of this HFN in UE and UTRAN can be synchronised.

RLC State Model

RLC TM State Model

TM-Data-Transfer

RLC UM State Model

UM-State-Model

RLC AM State Model

AM-State-Model

Reference

Radio Link Control (RLC) protocol specification: 3GPP TS 25.322

WCDMA Physical Layer: Principles and Features

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Wideband Code Division Multiple Access (WCDMA) is the physical layer used for Universal Terrestrial Radio Access (UTRA) FDD mode. The WCDMA technology provides the UMTS system some of the new functionalities on the air interface.

Physical Layer

 

The main features provided by WCDMA physical layer are:

  1. Spreading and Scrambling
  2. Transport Channel Combining
  3. Soft Handover
  4. Compressed Mode
  5. Power Control

Spreading and Scrambling

Scrambling

The concept of scrambling was introduced in CDMA systems before it was implemented in WCDMA. Scrambling is used to separate the base stations or UEs from each other.

Channelisation Code

Transmissions from a single source are separated by channelisation codes. The examples can be :

- In the downlink it can be used to separate different connections from the same sector.

- In the uplink it can be used to separate different dedicated physical channels from each other, e.g. separating DPDCH from DPCCH.

The spreading and channelisation codes are based on Orthogonal Variable Spreading Factor (OVSF).

More about OVSF can be found in the following link:

http://ieeexplore.ieee.org/Xplore/login.jsp?url=http%3A%2F%2Fieeexplore.ieee.org%2Fiel1%2F2220%2F12040%2F00555056.pdf%3Farnumber%3D555056&authDecision=-203

Transport Channel Combining

Another new feature in WCDMA air interface to combine different services together. This may be Voice service with PS bearer service.

The physical layer receives data from different sources. All these data streams can differ from each other in terms of data rate and reliability. The objective of the transport channel combiner is to bring together all the data streams from higher layers, changing them when appropriate and then multiplex them onto physical channel.

Changing the data rate of individual channels is the job of the rate matcher. The rate matcher works with the instructions from the RNC. The change in data rate can be achieved either through puncturing or repetitions.

Soft Handover

Soft Handover is a handover in which the mobile station starts communication with a new Node-B on a same carrier frequency, or sector of the same site (softer handover), performing utmost a change of code.

There are areas of the UE operation in which the UE is connected to a number of Node-Bs.

Active Set: It is defined as the set of Node-Bs the UE is simultaneously connected to.

The Soft Handover procedure is composed of a number of single functions:

- Measurements;

- Filtering of Measurements;

- Reporting of Measurement results;

- The Soft Handover Algorithm;

- Execution of Handover.

 

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Compressed Mode

If a UE need to perform a handover to a UMTS cell on different frequency or to a different system on a different frequency, it will need to make measurements on the target cells. To do this job compressed mode technique is used in UMTS. Compressed mode inserts an artificial gap in transmission in either the uplink or the downlink. During this gap UE is able to switch to the other frequency, make a measurement and come back to the original frequency.

The UE capabilities define whether a UE requires compressed mode in order to monitor cells on other FDD frequencies and on other modes and radio access technologies. UE capabilities indicate the need for compressed mode separately for the uplink and downlink and for each mode, radio access technology and frequency band.

compressedmode

The following parameters characterise a transmission gap pattern:

TGSN (Transmission Gap Starting Slot Number): A transmission gap pattern begins in a radio frame, henceforward called first radio frame of the transmission gap pattern, containing at least one transmission gap slot. TGSN is the slot number of the first transmission gap slot within the first radio frame of the transmission gap pattern;

TGL1 (Transmission Gap Length 1): This is the duration of the first transmission gap within the transmission gap pattern, expressed in number of slots;

TGL2 (Transmission Gap Length 2): This is the duration of the second transmission gap within the transmission gap pattern, expressed in number of slots. If this parameter is not explicitly set by higher layers, then TGL2 = TGL1;

TGD (Transmission Gap start Distance): This is the duration between the starting slots of two consecutive transmission gaps within a transmission gap pattern, expressed in number of slots.

TGPL1 (Transmission Gap Pattern Length): This is the duration of transmission gap pattern 1, expressed in number of frames;

TGPL2 (Transmission Gap Pattern Length): This is the duration of transmission gap pattern 2, expressed in number of frames. If this parameter is not explicitly set by higher layers, then TGPL2 = TGPL1.

The following parameters control the transmission gap pattern sequence start and repetition:

TGPRC (Transmission Gap Pattern Repetition Count): This is the number of transmission gap patterns within the transmission gap pattern sequence;

TGCFN (Transmission Gap Connection Frame Number): This is the CFN of the first radio frame of the first pattern 1 within the transmission gap pattern sequence.

Power Control

Uplink Power Control

The uplink power control has two parts:

  • Inner loop
  • Outer loop
Inner loop power control

For inner loop power control the UE transmits to the NodeB. The NodeB receives the Signal to Interferance Ration (SIR) and compares it with the target. Based on the target level the NodeB instruct the UE to increase or decrease power. This process occurs for every transmission slot (every 667 micro second).

SIRest > SIRtarget TPC = 0

SIRest < SIRtarget TPC = 1

Outer loop power control

The outer loop power control ensures the QoS for a particular UE. The outer loop power control is between the NodeB and RNC.

To achieve this objective NodeB sends quality information to the RNC. The quality information is based on BER and Block Error Rate (BLER) for a specific UE. Based on this info RNC decides whether to increase or decrease the quality of the radio link.

Downlink Power Control

Power control on the downlink is same as that of the uplink except that the roles of Nodeb and UE are switched.

References

Physical layer procedures (FDD): 3GPP TS 25.214

Physical layer – Measurements (FDD): 3GPP TS 25.215

Radio resource management strategies: 3GPP TR 25.922

UTRAN Interfaces and Protocols

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3G Network

UMTS Architecture

Interfaces

Interfaces Description
B MSC – VLR
C MSC – HLR
D VLR – HLR
E MSC – GMSC
F MSC – EIR
G VLR – VLR
Gc GGSN – HLR
Gd SGSN – SMSC
Gf SGSN – EIR
Gn SGSN – GGSN
Gr SGSN – HLR
Gs SGSN – VLR
H HLR – AuC
IuPS RNC – SGSN
IuCS RNC – MSC/VLR
Iub NodeB – RNC
Iur RNC – RNC
Uu UE – NodeB

Important Interfaces and Protocols

Uu Interface

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Protocols

Protocol Description Standard
MM Mobility Management 3GPP TS 24.008
CC Call Control 3GPP TS 24.008
SMS Short Message Service 3GPP TS 23.040, 3GPP TS 24.011
SS Supplementary Service 3GPP TS 24.080
GMM GPRS Mobility Management 3GPP TS 24.008
SM Session Management 3GPP TS 24.008
GSMS GPRS Short Message Service 3GPP TS 23.040, 24.011
AMR Adaptive Multi Rate 3GPP TS 26.071
IP Internet Protocol IETF RFC 791
PPP Point-to-Point Protocol IETF RFC 1661
RRC Radio Resource Protocol 3GPP TS 25.331
PDCP Packet Data Convergence Protocol 3GPP TS 25.323
RLC Radio Link Control 3GPP TS 25.322
MAC Medium Access Control 3GPP TS 25.321

Iub Interface

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Protocols

Protocol Description Standard
NBAP NodeB Application Part 3GPP TS 25.433
BMC Broadcast Multicast Control 3GPP TS 25.324
TAF Terminal Adaptation Function 3GPP TS 27.001, 27.002
RLP Radio Link Protocol 3GPP TS 24.022
SSCF-UNI Service Specific Co-ordination Function – User Network Interface ITU-T Q.2130
SSCOP Service Specific Connection Oriented Protocol ITU-T Q.2110
ALCAP Access Link Control Application Part ITU-T Q.2630.1, Q.2630.2
STC Signalling Transport Converter ITU-T Q.2150.1
AAL2 ATM Adaptation Layer 2 ITU-T I.363.2
AAL5 ATM Adaptation Layer 5 ITU-T I.363.5
ATM Asynchronous Transmission Mode
NOTE: Other protocols are defined above

Iur Interface

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Protocols

Protocol Description Standard
RNSAP Radio Access Network Application Part 3GPP TS 25.413
MTP3-B Message Transfer Part Level 3 for ATM ITU-T Q.2110
SSCF-NNI Service Specific Co-ordination Function – Network Node Interface ITU-T Q.2140
M3UA MTP3 User Adaptation Layer IETF RFC 3332
SCTP Stream Control Transmission Protocol IETF RFC 2960

NOTE: Other protocols are defined above

IuCS Interface

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Protocols

Protocol Description Standard
AAL2-SAR SSCS Segmentation and Reassembly Service Specific Convergence Sublayer for the AAL type 2 ITU-T I.366.1

NOTE: Other protocols are defined above

IuPS Interface

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Protocols

Protocol Description Standard
UDP User Datagram Protocol IETF RFC 768
GTP-U GTP User Plane ETSI GSM 09.60 / 3GPP TS 29.060

NOTE: Other protocols are defined above

3G Tutorials: Introduction to 3G

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What is 3G?

3G is the third generation cellular system. Here we will discuss about UMTS (Universal Mobile Communication System) which is a 3G system.

Foundation of Cellular Concept

In 1972 Bell Labs registered a patent which was the foundation for 2nd generation and 3rd generation cellular technologies.

The Idea: Instead of base stations that cover large areas, each base station should cover a small area. This way the same frequency can be reused.

The patent can be found at the following link:

http://www.google.com/patents/about?id=7yI1AAAAEBAJ&dq=bell+labs+patent+for+cellular+antenna

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Frequency Reuse

UMTS Requirements

2nd Generation Mobile systems were mainly developed for Voice based services. Later some data services were added to that system. But 3G was developed taking the future into consideration. The main requirements for 3G are listed below:

  • Bit rate up to 2 Mbps.
  • Variable bit rate support.
  • Multi service support. Example: Browsing at the time of voice communication.
  • Delay requirements for delay sensitive real time traffic.
  • Quality requirements from 10% frame error rate to 10 to the power -6.
  • Inter-system Hand Over. Handover between 2nd and 3rd generation systems.
  • High spectrum efficiency.
  • Support of TDD and FDD mode.
  • Support of location based services.

3gpp Specifications

At the high level the 3gpp specifications are structured through different releases. The releases specify a version of system with some particular features. Following are the different releases of UMTS specifications are the most important new features implemented in that release.

Release Date Frozen New Features
99 March 2000 W-CDMA air interface
4 March 2001 Bearer independent CS Architecture
TS-SCDMA
5 June 2002 HSDPA
IMS
6 March 2005 HSUPA
7 September 2007 HSPA+
8 December 2008 Long Term Evolution
9 Enhanced eNodeB
10 Not Updated

The 3gpp specification has document number like: TS 25.331 V 6.22.0.

TS: Technical Specification

25: Series number

331: Specification Number

6: Release number

22: Technical Version Number

0: Editorial Version Number

List of 3gpp series number:

21 Requirements
22 Service aspects (“stage 1″)
23 Technical realization (“stage 2″)
24 Signalling protocols (“stage 3″) – user equipment to network
25 Radio aspects
26 CODECs
27 Data
28 Signalling protocols (“stage 3″) -(RSS-CN)
29 Signalling protocols (“stage 3″) – intra-fixed-network
30 Programme management
31 Subscriber Identity Module (SIM / USIM), IC Cards. Test specs.
32 OAM&P and Charging
33 Security aspects
34 UE and (U)SIM test specifications
35 Security algorithms
36 LTE (Evolved UTRA) and LTE-Advanced radio technology
37 Multiple radio access technology aspects

UMTS Architecture

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Core Network

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The Release 99 Core network contains the circuit switched domain and packet switched domain. The Core Network contains the following entities.

HLR: Home Location Register

The HLR is located in the user’s home system. It contains the user’s service profile.

AuC: Authentication Center

The AuC contains security related information.

EIR: Equipment Identity Register

EIR is an optional component. It contains information such as list of stolen mobiles.

MSC/VLR: Mobile switching center / Visitor location register

The MSC job is to switch the CS transactions and VLR job is to store a copy of the user’s service profile as well as more precise information about UE’s location within the service system.

GMSC: Gateway MSC

This is a switch and this connects the UMTS PLMN to the external CS networks.

SGSN: Serving GPRS Support Node

The SGSN job is similar to MSC/VLR but this is for PS traffic.

GGSN: Gateway GPRS Support Node

This is similar to GMSC but it serves for the PS traffic.

Radio Access Network

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UTRAN has two main nodes NodeB and RNC.

NodeB:

NodeB is a physical unit for radio transmission and reception with cells. NodeB may have one or more cells depending on sectoring. UE is connected to the NodeB through the Uu interface, which is a radio interface.

The main task of the NodeB is conversion of data to and from the Uu interface, including Foreword error correction (FEC), rate adaptation, W-CDMA spreading and dispreading. The NodeB also takes care of the Soft Handover in case of FDD.

RNC: (Radio Network Controller)

The RNC owns and controls the radio resources in its domain. It is an intermediate component between NodeB and the CN.

RNC has three main functions. The RNC can acquire three different names depending on the function it is performing.

CRNC: Controlling RNC

Each NodeB is controlled by a particular RNC. This RNC is called the Controlling RNC (CRNC) for the NodeB.

SRNC: Serving RNC

Each mobile or UE is controlled by a particular RNC. This RNC is called serving RNC. The SRNC exchanges signaling messages with the mobiles it serves, and act as a sole point of contact with the Core Network.

DRNC: Drift RNC

A drift RNC uses the Iur interface to carry UE specific signalling information between the NodeBa and the SRNC.

User Equipment

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The UE consists of two main parts, Mobile Equipment (ME) and Universal Integrated Circuit Card (UICC).

ME: Mobile Equipment

The ME consists of two parts, Terminal Equipment and Mobile Terminal.

Terminal Equipment is the point where all data streams start and end.

Mobile Terminal handles all 3G communication functions.

USIM (UICC)

USIM  is a smartcard that holds the subscriber identity, performs the security algorithm, and stores the authentication and encryption key and subscription information.