6.4 Principles behind packet data transfer in GPRS

Mobile communications

6.4 Principles behind packet data transfer in GPRS

- interaction between a given application and GPRS takes place at a network

layer level – the main task of a GPRS enabled network is to transport IP

datagrams/X.25 packets between a MS and some external network

-> from this point of view, similarly to GSM CSD services, GPRS has the role

of a bearer service

GSM/GPRS

PLMN

SGSN GGSN

GPRS-MS Host

Application

IP IP

Application

IP datagrams IP datagrams

tunneling

1


- in order to be able to use GPRS services a mobile station must previously

attach itself to a SGSN

- a TLLI uniquely identifies a MS (a given IMSI) at SGSN level on

a given routing area

- an attached MS is communicating with the corresponding SGSN using a

logical link that allows addressing a specific mobile station when transmitting

data or signaling. GPRS includes a dedicated protocol that manages the logical

link

MS1TLLI1

MS2 TLLI2

TLLI2<->IMSIMS1

TLLI1<->IMSIMS1

- other identifiers are used subsequently at BSS level for allowing

dinamic or static sharing of the radio resources (TFIs)

SGSN

2


- for transferring data between a MS and an external PDN, another

connection must be created when a data transfer session is initiated i.e.

the activation of a PDP context between MS and GGSN is compulsory

A PDP context is characterized by:

- an Access Point Name (APN) – identifies the external PDN via a reference

to the GGSN in charge of the APN

- a PDP address (IPv4 or X.121 address for X.25 networks) assigned

statically or acquired dynamically to the GPRS-MS.

(static addresses are assigned at subscription or dynamic

allocation may be done by a server running DHCP –Dynamic Host

Configuration Protocol)

- a QoS profile subject to negotiation

- the PDP context can be activated by the GPRS-MS (typically) or by

the network (GGSN) for incoming (MT type) data calls

❑ PDP contexts

3


- as a result of a PDP context activation procedure, another logical link is

established between the current SGSN and the GGSN dedicated for

handling the traffic to/from the network indicated through the APN identifier

GPRS-MS SGSNTLLI

GGSN1

GGSN2

PDN1

PDN2

PDP context 2

PDP2 address, QoS2,

APN2

- data transfer between GGSN and SGSN is done using tunneling – IP

datagrams having destination or origin a given MS are packed into another

network layer format (GTP in GPRS) without altering their content

BSS

PDP context 1 PDP1

address, QoS1, APN1

4


- a given MS might have multiple PDP contexts active simultaneously (ex:

email access, ftp or http browsing with different QoS profiles). Each PDP

context is further distinguished by NSAPI (Network Service Access Point

Identifier). The NSAPI has no other meaning and it is inserted by the MS

(up to 15 different values)

- when receiving a request from a MS the SGSN concatenates the IMSI of

the mobile subscriber with the received NSAPI. The tunnel for a specific

data transfer is simply identified by a special identifier called TID (Tunnel

ID) =IMSI + NSAPI

-the PDP context is activated by the GGSN associated with the APN field

- the serving SGSN maintains tables with mappings between TLLI/IMSI,

NSAPI, TID and the IP address of the GGSN

- the GGSN maintains tables with the IP address of the MS, the IP

address of the SGSN, and TID

5


MS

Internet

host

SGSN

GGSN

BSS (transfer through segmentation, retrasmissions, RRM

procedures etc.)

IPMS IPHost Payload

IPSGSN IPGGSMTID IPMS IPHost Payload

6


-if a PDP context has not been previously activated and if a GGSN has

received data for a MS that has a public IP address, the GGSN can

initiate a PDP context activation procedure through the serving SGSN

(its address is stored in HLR)

- a PDP context activation procedure is not equivalent with a GPRS

attach procedure

- a PDP context can be deactivated if the MS is not transmitting

anything for a predefined amount of time

buffer

buffer

SGSN

BTS1

BTS2

PCU LLC

frames

RLC/MAC

blocks

TS/

ARFCn

TS/

ARFCn

IP datagram

7


Type Group Direction Name Role

Signaling and common

channel control

PCCCH (Packet

Common Control

Channels)

DL PBCCH (Packet Broadcast

Control Channel)

broadcasting of GPRS

related information on

the current cell

DL PPCH(Packet Paging Channel) packet mode paging

UL PRACH (Packet Random Access

Channel)

Initiation of uplink

transfers

DL PAGCH (Packet Access Grant

Channel)

Resource assignment

to an MS

DL PNCH (Packet Notification

Channel)

For broadcast traffic

Packet Traffic and

Dedicated Control

Channels

DL/UL PDTCH (Packet Data Traffic

Channel)

transport of user data

(multislot operation

possible – up to 8)

DL/UL PACCH (Packet Associated

Control Channel)

Signaling: resource

allocation,

Acknowledgements

DL/UL PTCCH(Packet Timing advance

Control Channel)

TA for GPRS (3GPP

TS 45.10)

6.5 Logical and physical channels in GPRSphysical channel – defined similarly to GSM (a timeslot on a given carrier frequency)-physical channels are named in GPRS PDCH (Packet Data Channels). A PDCH is essentially

one or multiple TSs on a given frequency for GPRS operation

8


6.6 Mapping of logical channels onto physical channels

- is done according to a periodic pattern formed of 52 TDMA frames.

Such a structure in further divided in 12 radio blocks (a radio block =

recurrence of a(several) TS(s) on 4 consecutive TDMA frames), inactive

frames and frames carrying TA commands

B0 B1B2 B3 B3 B5

B6 B6 B8 B9 B10 B11

…

Inactive

(measurements )

TA commands/information

9


- the duration of a radio block defines also the minimal time

resolution for data/signaling transfer and resource allocation

Example: a MS has a PDTCH allocated on TS2

…0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 70 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7

Data sent or received by a MS in radio block B0

B0

Frame

TDMAn

Frame

TDMAn+1

Frame

TDMAn+2

Frame

TDMAn+3

B1

Data sent or received by another MS in radio block B1

Logical channels are mapped using some constraints (3GPP TS

03.64)10

http://www.qtc.jp/3GPP/Specs/GSM_GERAN/0364-8c0.pdf


Two possible solutions are implemented in GPRS :

-Static (fixed) allocation : a MS uses for the whole duration of a data call a

dedicated PDTCH; allocation is indicated by the network using a “”bitmap format”

-dynamic allocation :- traffic channels are allocated in single or multiples TSs on the same carrier

frequency, the time resolution for allocation corresponds to the duration of radio

block

TS7

TS6

TS5

TS4

TS3 User 1 User 1 User 1 User 1

TS2 User 1 User 1 User 1 User 1

TS1 User 1 User 1 User 1 User 1 User 2 User 2 User 2 User 2

TS0 User 1 User 1 User 1 User 1 User 2 User 2 User 2 User 2

n n +1 n+2 n+3 … TDMA frames / radio blocks

Example : 2 users with different QoS profiles (set of parameters defining de

priority, delays, mean and maximum data rate, bit error rate)

6.6 Sharing of radio resources

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- distinct solutions are implemented for the uplink and downlink

directions

-downlink – each TBF is labeled with a special 5 bit identifier called TFI -

Temporary Flow Identity, previously allocated during the downlink data

session initialization procedure (sent in the header the packets

associated to RLC/MAC blocks)

- all the MSs sharing the same TS are inspecting the TFI and only the one

recognizing its TFI (its identity for the TBF) will keep data, the others are

discarding it

TFI1 TFI1 TFI1TFI2 TFI2

Data for MS1

- information exchanged corresponding to a LLC frame

segmentation called in GPRS Temporary Block Flow (TBF). Separate

TBFs exist for the uplink and downlink direction

How can a GPRS-MS distinguish if it can emit/receive during a TS shared

with other MSs?

12


- in the uplink direction access is managed by the insertion of another

identifier called USF- Uplink State Flag; the USF is allocated also during

the TBF initiation

- the USF has a 3 bits length, and is carried in each downlink RLC/MAC block

and is indicating which of the GPRS MSs can use the following uplink radio

block

BSS MS1

MS2

USFMS1

On the above example MS1 and MS2 are sharing the same PDTCH

(same TS) and only MS1 will use the following radio block in uplink

MS2 data

MS1

BSS

data MS1 + TFIMS1

TFI2

- the TFI/USF mechanism controls the medium access and is implemented

at the PCU level

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Example: 3 mobile stations sharing the same TS

….

US

F A

TF

I C

US

F C

TF

I A

US

F B

TF

I A

B2B0 B1

US

F B

TF

I B

downlink

uplink

…

TF

I A

TF

I C

B2B0 B1T

FI B

B3

B3

TF

I B

B4

14


6.8 Channel coding in GPRSIncreased data rates

-> multiple TS/ user (up to 8) – multislot operation

-> improved FEC schemes- 4 coding schemes (CS) changed adaptively for PDTCH;

- control channels are encoded with a single CS (CS1)

BTS

distance

CS1

MS MS

CS2

MS

CS3

MS

CS4

redundancy

- attenuation increases with distance -> noise level becomes

comparable to signal level leading to a low SNR-> actual coding

scheme is changed with another one with an increased redundancy

15


Link adaptation = the mechanism of automatically changing the coding

scheme based on signal strength measurements; main purpose: to

increase throughput

CS – is chosen by the network and is indicated to the MS by using signaling

means

FEC is based on similar concepts to GSM: block coding, convolutional code,

puncturing techniques, interleaving (similar to SDCCH) etc.

- besides user data USF is also encoded

Variable

length user

bits

Block code

USF pre-

encodingConvolutional

code

BCS

456 bits

20msPuncturing

Tail bits

(0000)

Interleaving

USF

16


Block code: Fire code (40 redundancy bits) or cyclic code (16 redundancy

bits)

BCS – Block Check Sequence – error correction / detection on the radio

interface (Fire code/cyclic code)

USF – is FEC encoded (“pre-encoded”) using standardized schemes for

CS2-CS4 only for CS1 no USF encoding is used

Convolutional code: – R=1/2, constraint length K=5 (same as GSM)

Puncturing: process of removing some of the parity bits after encoding

Interleaving: 4 consecutive burst (as in GSM for signaling information)

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USF user bits

BCSUSF user bits

BCSUSF user bits

Encoded bits

Encoded block (456/20ms)

Block code

USF pre-encoding

Convolutional code

Puncturing

CS USF Pre-

encoded

USF

BCS bits Tail bits User

bits

Encoded

bits

Punctured

bitsData

rate

[kbps]

CS1 3 3 40 (Fire) 4 181 456 - 9.05

CS2 3 6 16(cyclic) 4 268 588 132 13.4

CS3 3 6 16(cyclic) 4 312 676 220 15.6

CS4 3 12 16(cyclic) - 428 456 - 21.4

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6.9 GPRS transmission protocols

- proper transfer of user packets is achieved using dedicated transmission

protocols on each GPRS interface

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❑ SNDCP – Sub-network Dependent Convergence Protocol- insures transfer of data packets between MS and SGSN by multiplexing several

PDP contexts on the same logical connection (distinction between several contexts

is done through NSAPI – up to 11 distinct values)

- handles headers and user data compression (V.42bis)

- handles segmentation/reassembly of packets

❑ LLC –Logical Link Control- provides a reliable logical link between MS and SGSN; the link is identified by a

TLLI identifier and is established by an initial exchange of special signalling frames

- LLC has data link layer functionality for acknowledged data transfer– frame

segmentation/reassembly, ARQ retransmissions; unacknowledged mode can be used

also

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LLC in also in charge of encryption (performed using a dedicated algorithm GEA –

(GPRS encryption algorithm) – different ciphering keys are used for GSM and

GPRS services

❑ RLC–Radio Link Control – data link layer functionality between MS and PCU

– implements segmentation/reassembly of LLC PDUs to/from fixed size RLC/MAC

blocks; the resulting RLC/MAC blocks are “labelled” with the TFI, numbered and

transmitted in non-acknowledged or acknowledged mode, the later transmission mode

being based on a selective repeat ARQ mechanism between a MS and the

correspondent PCU ( a Block Check Sequence (BCS) is inserted by the RLC protocol

for this purpose)

❑ MAC–Medium access control – controls sharing of radio resources (PDCHs –

packet data channels) between several users; implements the USF mechanism for

multiplexing several users in the uplink direction on the same PDCHs

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InformationFH FCS

InformationBH

LLC – PDU

InformationBH

Normal burst Normal burst Normal burst

RLC/MAC blocks

Normal burst

Physical layerChannel coding, interleaving, burst formatting…

BCS BCS

Bx – radio block(s)

22


❑Example: downlink data transfer

Paging

Resource assignment for the transmission of

an LLC frame in which the MS is identified by

TLLI, the frame is relayed up to SGSN

(ensures the updating of the cell info at SGSN)

Resource assignement of the data transfer

Data transfer

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6.4 Principles behind packet data transfer in GPRS