8_GPRS Air Interface

8/9/2019 8_GPRS Air Interface

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GPRS Air Interface


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At the end of the module, the participant will be ableto:

Explain the functions of the air interface in thephysical, MAC and RLC layers

Differentiate between physical and logical GPRSchannels

List and describe the GPRS air interface logicalchannels and their functions

Explain the GPRS TDMA frame, multiframe andsuperframe structure

List and compare four different coding schemes andthe puncturing concept

Describe multiple timeslot usage

Describe briefly the process of channel allocation,in the uplink and downlink

without using any references.

Objectives


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Interface between the MS and BSS

Major bottleneck in GPRS performance

MS GSM/GPRS NetworkUm

Uplink Direction

Downlink Direct ion

What is the air interface?


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Modulation and demodulation

Framing

Channel coding

Congestion control

Medium access control Synchronisation

Multiplexing

Multi-access

Timing advance

Power control

Handover Ciphering Segmentation Interleaving Puncturing Signal measurements

Do you still

rememberthese

functionsfrom GSM?

Radio Interface Tasks


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BSC RXCDR

BTSBTS

MSC/VLR

SMS-C

EIR

AA

EE CC

DD

SMS-GMSC

SMS-IWMSC

HLR

Signalling & Data Interface

Signalling Interface

GbGb

SGSN GGSN

SGSNGGSN

PDN

Other PLMN

PCU

GsGs GrGr

GpGpGnGn

GnGn GiGi

GfGf

GbGb

GdGd

GcGc

GbGb

CG

GaGa

ETSI GPRS Reference Model


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MS BSSMS BSS SGSN GGSNSGSN GGSN

RLC /RLC /

MACMAC

RLC /RLC /

MACMAC

GSM RFGSM RF GSM RFGSM RF

LLCLLC LLCLLC

NS \ FRNS \ FR NS \ FRNS \ FR

L1 bisL1 bis L1 bisL1 bis

BSSGPBSSGP BSSGPBSSGP IPIP IPIP

L2L2 L2L2

L1L1 L1L1

TCP /TCP /

UDPUDP

TCP /TCP /

UDPUDP

GTPGTP GTPGTPSNDCPSNDCPSNDCPSNDCP

IPIPIPIP

TCP /TCP /

UDPUDP

TCP /TCP /

UDPUDP

EMAILEMAIL

FTPFTP

WEBWEB

EMAILEMAIL

FTPFTP

WEBWEB

Transmission Plane


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BSS (PCU, CCU)MS

SNDCP

RLCRadio Link

Control

MACMedium Access

Control

GSM RF

phy. link & RF

IP / X.25

LLC

Um

RLCRadio Link

Control

MACMedium Access

Control

GSM RF

phy. link & RF

LLC segmentation/ re-assemblyacknowledged/ unacknowledgedmode

Backward Error Correction BEC

Access signalling proceduresphysical channel bundlingsub-multiplexing

physical channel organisationchannel coding

GSMK

ransm ss on anes e weenand BSS


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SNDCP PDU (SN-PDU)

LLC-PDU

RLC Block

MAC Block

Network PDU (NPDU) e.g. IP-packet

SNDCP

LLC

RLC

MAC

Phys. Link

Phys. RF

Network

LLC-PDU

RLC Block

Burst Burst Burst Burst

channel coding

PDU, radio block, and bursts


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Lowest layer of GPRS protocol stack

Primary function is to provide services for information transfer

over a physical channel What is a physical channel?

Physical Layer is split into two sublayers

Physical RF layer

Physical Link sublayer

Physical Layer


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Modulation of RF signals at the transmitter

GMSK for GPRS (1 symbol per bit)

8 PSK for EGPRS (1 symbol per 3 bits)

Demodulation of RF signals at the receiver

SNDCP

RLCRadio Link

Control

MACMedium Access

Control

LLC

Phy. Link

Phy. RF

0 1 01

Physical RF Sublayer (RFL)


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Functions:

Framing: Placement of data into bursts, frames, radioblocks, etc.

Data coding for maximising the data throughputCS1 CS4

Detection and correction of errorsdue to noise in the medium

Procedures for detecting congestionon the air interface

Procedures for synchronising MS and network

Procedures for monitoring and evaluationof radio link quality

Procedures for cell (re-)selection

Transmitter power control

SNDCP

RLCRadio Link

Control

MACMedium Access

Control

LLC

Phy. RF

Phy. Link

tailbits

3

encrypted bits57

SB1

trainingsequenc

e26

SB1

encrypted bits57

tailbits

3

guardperiod

8,25 bits

allocated time slot

BTS

Physical Link Layer (PLL)


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Function

Uplink and downlink multiplexing of data and controlsignalling

Handling contention resolution, collision detection andrecovery for mobile originated channel access

Scheduling of access attempts, includingqueuing of packet accesses for mobileterminated channel access

Handling priority of data and controlmessages

SNDCP

RLCRadio Link

Control

MACMedium Access

Control

LLC

Phy. RF

Phy. Link

Medium Access Layer


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Transfer of Logical Link Control layer PDUs(LLC-PDU) to the MAC layer

Segmentation and re-assembly of LLC-PDUs into RLCData Blocks

Backward Error Correction (BEC) proceduresfor selective retransmission of uncorrectable

code words in the acknowledged mode oftransmission

Transmission of code words based on channelconditions,i.e link adaptation

Storing soft values of the erroneous RLC Data Blocksand combining them with the retransmitted RLC

Data blocks

SNDCP

RLCRadio Link

Control

MACMedium Access

Control

LLC

Phy. RF

Phy. Link

LLC-PDU

Radio Link Layer


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RLCData

BCS

SNDCP

RLCRadio Link

Control

MACMedium Access

Control

LLC

Phy. RF

Phy. Link

user data

LLC PDU

& segmentation

RLC DC Data

BCS = Block CheckSequence

RLCData

BCS

RLCData

BCS

MACHeader

radio link signalling &

control data

RLC/MAC ControlMessages

MACHeader

RLCHeader

RLCHeader

RLC/MAC GPRS blocks


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UPLINK DOWNLINK

GSM900: 890 MHz - 915 MHz 935 MHz - 960 MHz

GSM1800: 1710 MHz - 1785 MHz 1805 MHz - 1880 MHz

1 2 3 ...

Channel 1 - 124 1 - 374

200 kHz

1 2 3 ...

Duplex frequency 45 MHz / 95 MHz

guard band

GSM Radio Interface Organisation: FDD and FDMA

GSM R di I f O i i TDMA


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TDMA frame= 8 timeslots

01

23

45

7

6

01

23

4 5

76

01

23 4

5

200 kHz

Physcial channel,e.g. allocated to one

subscriber with FR voice &no frequency hopping

frequency

time

TDMA frame

GSM Radio Interface Organisation: TDMA

Wh t i b t?


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Smallest unit of transmission in GSM and GPRS

Normal Burst

Frequency Correction Burst Synchronizing Burst

Access Burst

Dummy Burst

What is a burst?

L i l Ch l i GSM


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BCCH

FCCH Frequency correction

Signallingand Control

Traffic

CCCH

DCCH

SCH Frame synchronisation + BSIC

PCH Paging mobiles

RACH Requesting dedicated channel

AGCH Allocating dedicated/traffic CH

Broadcast of cell information,e.g. channel combination

SDCCHSignalling between MS and BTSe.g. Authentication, SMS, LUP

SACCH Measurements, TA, PC, ...

FACCHExtra signalling within26 TDMA Multiframe

TCH/F full rate traffic channel

TCH/H half rate traffic channel

BCH

DL

UP

DL

DL

DL & UP

DL & UP

Logicalchannels

are usedtotransmita welldefinedcontent

Logical Channels in GSM

M i /F i


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The logical channel informationmust be transmitted on a

physical channels

Multiframes specify, at whichposition within a physicalchannel a specific logical

channel information is

transmitted

TDMA Frame

26 TDMAFrame

e.g.used for

GSMspeech

TS 0 TS 1 TS 2 TS 3 TS 4 TS 5 TS 7TS 6

TCH

TCH

SACCH

idle

Re-memb

erGSM

Mapping/Framing


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Radio block


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Radio

Block

TS0 TS7TS1 TS2 TS3 TS4 TS5 TS6




Frame 0

Frame 1

Frame 2

Frame 3

Radio block

What is a Packet Data Channel (PDCH)?


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Physical channel dedicated to packet data traffic is called a PDCH(Packet Data Channel)

PDCH can use spare traffic channels (TCH) in a cell Each PDCH can be shared by multiple MS and network

Each PDCH may have a number of logical channels

PDCH carry GPRS data and control signalling PDCH classified into (details later)

PCCCH (Packet Common Control Channels) PBCCH (Packet Broadcast Control Channels)

PDTCH (Packet Data Traffic Channels)

What is a Packet Data Channel (PDCH)?

Additional logical channels with GPRS


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PTCCH/D

PTCCH/U

PBCCH

Signalling

and Control

Packet

Traffic Channel

PCCCH

PPCH

PRACH MS initiates uplink transfer

PAGCH Resource assignment to an MS

PNCH Notifying PtM Packet Transfer

Broadcast of packet data

specific information

PDTCH Packet Data Transfer; (multislot)

PACCH

DL

UP

DL

DL

DL

DL & UP

PTCH

Signalling: resource allocation,

acknowledgements, PC, TA, etc.

Paging MSs for packet data

and circuit switched services

Used by MS to send randomburst to BSS for timing advance

Used to send timing advance

Information to MSs of one PDCH

UP & DL

UL

DL

PDCCH

Based

onGSMRel.99

Additional logical channels with GPRS

GPRS channels in a cell


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A GPRS cell uses one or more TCH available in a cell as PDCH

The allocation of TCH for PDCH is done dynamically according tothe demand of GPRS traffic

Common control signalling on GPRS at initial phase of packettransfer can be either on

Dedicated Channel (PCCCH):

All GPRS attached MSs camp on the PCCCH

PCCCH are permanently or dynamically allocated when demandrises

If the network releases PCCCH then MSs move to CCCH

GSM CCCH (when PCCCH is not allocated)

All GPRS attached MSs camp on the CCCH as they do in Idle state

Existence of PDCH does not imply existence of PCCCH

GPRS channels in a cell

Broadcast channels


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GSM

FCCH Frequency Correction ChannelSCH Synchronisation Channel

BCCH Broadcast Control Channel

GPRSPBCCH Packet Broadcast Control Channel

Broadcasts packet data specific System Information messages

MS continuously monitors this

GSM BCCH can also be used

Broadcast channels

Common Control channels


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GSMPCH (Paging Channel) RACH (Random Access Channel) AGCH (Access Grant Channel)

GPRS PPCH (Packet Paging Channel)

Can be used for paging both CS & PS services GSM PCH can also be used

PRACH (Packet Random Access Channel)Used for uplink channel reservation & to obtain

TA GSM RACH can also be used

PAGCH (Packet Access Grant Channel)Used for resource assignment during packettransfer establishment phase

GSM AGCH can also be used

PNCH (Packet Notification Channel)Downlink only channel used for PTM-M

notifications to a group of MS before PTM-Mpacket transfer

Only in GPRS Phase 2

Common Control channels

Dedicated channels


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GSM

SDCCH

(Standalone DedicatedControl Channel)

SACCH(Slow AssociatedControl Channel)

FACCH(Fast Associated

Control Channel) TCH (EFR/FR/HR

Traffic Channel)

GPRS

PACCH (Packet Associated Control Channel) Bi-directional dedicated channel for transferring

ack./power control or resource assignment/re-assignment messages

PDTCH (Packet Data Traffic Channel)

Bi-directional

Corresponds to the resource allocated to a single MS

on one physical channel for user data transmission

PTCCH (Packet Timing advance Control Ch.)

Uplink dedicated (for transmission of random accessbursts)

Downlink common (for transmission of timingadvance information to several MSs).

Dedicated channels

GPRS: 52 TDMA Frame


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TS 0 TS 1 TS 2 TS 3 (Frame 0) TS 4 TS 5 TS 6 TS 7

Radio Block 0

PTCCH

Radio Block 3

Radio Block 4

Radio Block 5

PTCCH

Radio Block 9

Radio Block 10

Radio Block 11

Radio Block 1

Radio Block 2

IDLE

Radio Block 6

Radio Block 7

Radio Block 8

IDLE TS 3 (Frame 51)

TDMA Frame

1 Radio Block= 4 Frames= 456 info. bits

PDCH multiframe= 52 TRMA Frames

GPRS: 52-TDMA-Frame

GPRS: 52-TDMA-Frame for PDCHs


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B0 B1 B2 T B3 B4 B5 i B6 B7 B8 T B9 B10 B11 i

Radio Block

= 4 TS in consecutive

TDMA frames

idle frame

= 1 frame

52 TDMA Frame = PDCH Multiframe

Uplink on one PDCH:

Multiplexing of

PDTCH & PACCH, or PDTCH, PACCH & PRACH

Downlink on one PDCH:

Multiplexing of

PDTCH, PACCH PDTCH, PACCH & PCCCH, incl.

PBCCH (indicated by BCCH) PDTCH, PACCH and PCCCH

(indicated by (P)BCCH)

PTCCH

GPRS: 52 TDMA Frame for PDCHs

GPRS multiframe structure


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GSM uses a 51 & 26-multiframe structure

GPRS uses a 51 & 52-multiframe structure

Important concepts: Timeslots (TS), in which 114 bit bursts are sent (duration)

Frames = Eight timeslots (8 * 114 bits)

Multiframe = 52 frames (52 * 8 * 114 bits)

Radio block is 4 timeslots (or bursts) from 4 consecutiveframes in the same carrier and in the same TS (456 bits)

Each multiframe has 12 blocks + 2 idle frames + 2 PTCCHframes

12 *4 + 2 +2 = 52 frames

Each uplink TS can be used by several MSs for data.

Need for a mechanism to identify

GPRS multiframe structure

GPRS multislot capabilities


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GPRS multislot capabilities

1-slot

2-slot

3-slot

0 1 2 3 4 5 6 7 0 1

0 2 3 4 5 65 6 7 1

0 1 2 3 4 5 6 7 0 1

0 2 3 4 5 65 6 7 1

0 1 2 3 4 5 6 7 0 1

0 2 3 4 5 65 6 7 1

Downlink

Uplink

Monitor

Downlink

Uplink

Monitor

Downlink

Uplink

Monitor

Channel coding


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GPRS: CS 1-4. (Trade-off of throughput vs. error performance)

EGPRS: MCS 1-9

CS-1 CS-2 CS-3 CS-4

Increasing data throughput rates

Increasing protection against errors

Channel coding

Block diagram


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RLC Data Block+ MAC header

Convolutional Code

In: 228bitsOut: 456 bits

Cyclic Coding +

Tail

16 + 4 bits

Fire Code + Tail

In: 184 bits

Out: 228 bits

Reordering,

Partioning,

Adding StealingFlags

Interleaving

CS-1

CS-2, 3, 4 Convolutional Code

And Puncturing

In: x bits

Out: 456 bits

CS-4

Information bits Interleaved bits

Block diagram

Stages in processing


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Rate 1/2 Convolution Coding Stage

Puncturing Stage

456 bits

USF BCS

40/16bits

MAC Header

Tail

(4 bits)

Precoded USF

3/6/12 bits

USF

(3 bits)

MAC

(5 bits)

RLC Data/Control Block

(176/288/307 bits)

Cyclic or Fire Coding

Stages in processing

Summary of coding schemes


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Scheme Code

rate

USF Pre-

coded

USF

Radio Block

excl. USF

and BCS

BCS Tail Coded bits Punctured

bits

Data rate

kb/s

CS-1 1/2 3 3 181 40 4 456=2*(3+181+40+4) 0 9.05

CS-2

2/3 3 6 268 16 4 588=2*(6+268+16+4) 132 13.4

CS-3

3/4 3 6 312 16 4 676=2*(6+312+16+4) 220 15.6

CS-4 1 3 12 428 16 - 456=428+12+16 0 21.4

y g

Coding schemes and multiple timeslots


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Increased data

transmission ratesvia new

Coding Schemes

CS-19.05kbps

CS-213.4

kbps

CS-315.6kbps

CS-4

21.4kbps

Bundlingof up to8 physical channels

Net transmission rate up to171.2kbps

g p



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Increased data


Coding Schemes

CS-19.05kbps

CS-213.4

kbps

CS-315.6kbps

CS-4

21.4kbps



g p



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Increased data


Coding Schemes

CS-19.05kbps

CS-213.4

kbps

CS-315.6kbps

CS-4

21.4kbps



g p



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Channel CodingScheme

CS-1 CS-2 CS-3 CS-4

Single TSL Data

Rate

9.05 kbit/s 13.4 kbit/s 15.6 kbit/s 21.4 kbit/s

3-TSL Data Rate 27.15 kbit/s 40.2 kbit/s 46.8 kbit/s 64.2 kbit/s

8-TSL Data Rate 72.0 kbit/s 107.2 kbit/s 124.8 kbit/s 171.2kbit/s

g p

Simulated throughput of user data


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0

2

4

6

810

12

14

16

0 5 10 15 20 25

C/I

Kbit/s

CS-1

CS-2

CS-3

CS-4

0

10

20

30

40

50

0 5 10 15 20 25

C/I

Kbit/s

CS-1

CS-2

CS-3

CS-

Minimum Average

Typical NW C/I

Min imum Average

Typical NW C/I

1 Tim eslot 3 Tim eslots

g

Radio block allocation to MS


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How does a MS know which PDCH and which radio block on that PDCH it may use?

Assigning radio blocks of a PDCH to different MSs is done by MAC layer

Allocation may be static or dynamic

This process is referred to as content arbitration

Content arbitration occurs only in the uplink direction. Why?

An Uplink Status Flag (USF) is used for content arbitration

USF is transmitted downlink and it tells which MS may use a radio block

In initial PAGCH (AGCH), MS is assigned a USF value for each PDCH

MS monitors USF values in downlink transmission on assigned PDCH MS may transmit in radio blocks that have the same USF value as was allocated to it

in the PAGCH message

USF has 3 bits at the beginning of each radio block on the downlink

So one PDCH can be used by up to 8 MSs at one time (or up to 7 MSs plus 1 PRACH)

Usage of Uplink State Flag (USF)


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USF=

1

USF=

3

USF=

2

USF=1:B0- B4

USF=2:B5- B9

B0 B

1B2

B3

B4

B5

B6

B7

B8B9B1

0B1

1

B9

B8B7

B6

B5

B3

B2

B1

B0

B4

Radio block identification


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Downward multiplexing of radio blocks is done using Temporary Flow Identifier (TFI)

What is TFI? TFI is assigned in a resource assignment message prior to transmission LLC layer frames

between MS and BSS TFI is unique among concurrent processes

TFI is preferable to MS identity which is a very long number

TFI is included in every RLC frame header

Help!Whichonesare

mine?

Radio blocks, but

several subscribers served on on physical channel

up to 8 channels can be used by one phone

Radio resource management


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Mobile originated packet transfer


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MS Network

Packet Channel Request

Packet Immediate Assignment

Packet Resource Request

Packet Resource Assignment

PRACH or RACH

PAGCH or AGCH

PACCH

PACCH

(Optional)

(Optional)

Access & allocation

Mobile terminated packet transfer


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MS Network

Packet Resource AssignmentPACCH orPAGCH orAGCH

Downlink PDTCH assignment in Ready state

Mobile terminated packet transfer


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With resource reassignment

Data Block

temporary Packet Ack/Nack

Access and Assignment

MS Network

PDTCH

PACCH

Packet Resource Reassignment

Packet Resource Reassignment Ack

PACCH

PACCH

PACCHfinal Packet Ack/Nack

Data BlockPDTCH

Data BlockPDTCH

Data Block (polling)PDTCH

Data BlockPDTCH

Data BlockPDTCH

Data BlockPDTCH

Data BlockPDTCH

Data BlockPDTCH

Data Block (last, polling)PACCH

Modulation


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MODULATORMODULATOR

Digital databits/sec

Modulated

datasymbols/sec

PSK modulation scheme


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Digital bits Symbol Phase

(1,1,1) 0 0

(0,1,1) 1 /4

(0,1,0) 2 /2

(0,0,0) 3 3/4

(0,0,1) 4

(1,0,1) 5 -3/4

(1,0,0) 6 -/2

(1,1,0) 7 -/4

(0,0,1)

(1,0,1)

(0,0,0)(0,1,0)

(0,1,1)

(1,1,1)

(1,1,0)

(1,0,0)

I

Q


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Review Questions to

GPRS Air Interface

Review


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1. Which fields are used for medium access control and multiplexingmultiple users on the uplink and downlink PDCH?

2. How many users can share the same Packet Data Channel (PDCH)timeslot in the uplink direction?

For medium access control, the USF field is used

for uplink. There is no need for MAC in the

downlink direction since there is only one

source in that direction.

The TFI is used to identify the data flow in theuplink and downlink direction.

The USF field has 3 bits; consequently, up to 8 subscriberscan share one PDCH physical channel. 111 can be

reserved for PRACH, and then only up seven users can

share a PDCH with USF, using the values 000 to 110.

Review


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3. How many frames, radio blocks, and bursts are there in aPDCH multiframe?

4. What is the purpose of PTCCH?

One PDCH multiframe that is transmitted in a GPRS

timeslot consists of 52 TDMA frames organised into 12

radio blocks of 4 bursts each, 2 idle TDMA frames and 2

PTCCH TDMA frames.

UL: for transmission of RACH bursts to estimate timing

advance.

(up to 16 MSs)

DL: for transmitting timing advance information for up to

16 MSs.

Review


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5. Which layer is responsible for segmentation and reassembly of LLCPDUs and Backward Error Correction (BEC) procedures?

6. Which coding scheme has adopted the same coding as used forSDCCH?

7. Which layer uses the functionality of USF?

8. Which coding scheme does not use FEC?

9. Which logical channels can be used for resource assignment?

RLC layer (Radio Link Control)

CS-1 (9.05 kb/s)

MAC layer (Medium Access Control)

CS-4 (21.4 kbps, but no redundancy added)

AGCH (PAGCH), PACCH

8_GPRS Air Interface

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