GPRS AirInteface

GPRS Air Interface

Mohan Rao G.N.S.

Mobile Communication Group,

SISO

GPRS Logical Channels• A new set of logical radio channels have been defined for use with GPRS

• Packet Common Control Channel (PCCCH) is a set of logical channels used for common signaling between dedicated mobiles and the network– Packet Random Access Channel (PRACH)– Packet Paging Channel (PPCH)– Packet Access Grant Channel (PAGCH)– Packet Notification Channel (PNCH)

• Packet Broadcast Control Channel (PBCCH) is used to broadcast packet data system information to all GPRS terminals in a cell

• Packet Traffic Channel (PTCH)– Packet Data Traffic Channel (PDTCH)– Packet Associated Control Channel (PACCH)– Packet Timing Advance Control Channel (PTCCH)

PBCCH

• PBCCH is used to broadcast packet data system information to all GPRS terminals in a cell– If the PBCCH is not allocated, this system information can be

broadcast on the Broadcast Control Channel (BCCH).

– The PBCCH is only found on the downlink

PCCCH

• PRACH (Uplink)– It is used by MS to initiate uplink transfer, e.g. for sending data or

as a response to a paging message

– In addition, access burst is used to obtain timing advance information

• PPCH (Downlink)– It is used to page an MS prior to downlink packet transfer

– PPCH can be used for paging both CS and GPRS services but CS services are only paged if the MS is in network operation mode 1 and is a class A or Class B mobile.

– Additionally, an MS that is currently engaged in packet transfer, can be paged for circuit switched services on a PACCH

PCCH (cont)

• PAGCH (Downlink)– It is used in the packet transfer establishment phase to send

resource assignment messages to an MS prior to packet transfer

– Additional resource assignment messages can be sent on PACCH if the MS is currently involved in packet transfer

• PNCH (Downlink)– It is used to send a Point-to-Multi-point - Multicast (PTM-M)

notification to a group of MSs prior to PTM-M packet transfer

PTCH• PDTCH (Uplink & Downlink)

– It is allocated for data transfer.– It is temporarily dedicated to one MS or a group of MSs in the PTM-M case– In multi-slot operation, one MS may use multiple PDTCHs in parallel for individual packet

transfer

• PACCH (Uplink & Downlink)– It conveys signaling information related to a given MS. Such information include

acknowledgements, power control information and resource reassignments, etc– One PACCH is associated to one or several PDTCHs that are concurrently assigned to one

MS

• PTCCH (Uplink & Downlink)– PTCCH in the uplink direction is used for transmitting random access bursts to allow for

the estimation of the timing advance required for one MS whilst in packet transfer mode– PTCCH in the downlink direction is used to transmit the timing advance information

updates to several MSs

Mapping PDCH onto Physical Channels

• A Packet Data Channel (PDCH) is a physical timeslot that has been allocated to a mobile for use by GPRS. This allocation may be permanent or temporary in nature such that dynamic reconfiguration between GPRS & GSM can take place

• The sharing of the physical channel by the different packet data logical channel is based upon blocks of 4 consecutive bursts except for the PTCCH

• The PCCCH does not have to be allocated permanently in a cell. When the PCCCH is not allocated, the CCCH is used to initiate a packet transfer.

• The mapping of the PCCCH when it does exist onto the physical channels may transmit Circuit Switched information

• The existence & location of the PCCCH is broadcast on the cell

PCCCH on the 52 multiframe

• PCCCH is mapped onto one or several physical channels according to the 52 multiframes. In this case, PCCCH, PBCCH & PDTCH all share the same Physical channel

• Control channel & traffic channels may be combined onto a single physical channel in a variety of fixed & dynamic allocation schemes

• The Idle frames may be used for – BSIC identification for cell selection

– Interference measurements for power control

PCCCH on the 52 multiframe (cont)

B0

0 5 10 15 20 25 30 35 40 45 50

IDLE

PTCCH

PTCCH

B1 B2 B3 B4 B5 B6 B7 B8 B9 B10 B11

PTCCH

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

51

PCCCH on the 52 multiframe (cont)

• DOWNLINK– On the downlink B0 is used as the PBCCH, although 3 more may be allocated

if required, i.e. 4 in total, this being indicated in the first PBCCH block.– If 3 PBCCH channels are required, then Blocks B6,B3,B9 will be allocated– The remaining blocks, i.e. (12-alocated PBCCH blocks) may be used for

PAGCH, PNCH, PDTCH or PACCH– Any blocks not allocated as above may be used for PPCH, PAGCH, PNCH,

PDTCH or PACCH

• UPLINK– On an UPLINK packet data channel containing PCCCH, then all ablocks in the

multiframe can be used as PRACH, PDTCH or PACCH– When PCCCH is not allocated, all blocks can be used for PDTCH and PACCH

Channel Combinations

• PBCCH + PCCCH + PDTCH + PACCH + PTCCH

• PCCCH + PDTCH + PACCCH + PTCCH

• PDTCH + PACCH + PTCCH– where PCCCH = PPCH + PRACH + PAGCH + PNCH

Allocation of Resources to GPRS

• A cell which is supporting GPRS may allocate resources on one or more physical channels. These physical channels are taken from a common pool of channels available within a cell and as such reduce the number of circuit switched channels available. The actual allocation of physical channels to either circuit switched or GPRS is done dynamically according to the “capacity on demand” principle. Common Channel Signaling required by GPRS in initial phase of the packet transfer is conveyed on the PCCCH when it has been allocated or alternatively on the CCCH. Thus, the operator can now allocated resources to GPRS within a cell only when a packet is to be transferred.

• It is envisaged that PCCCH will be allocated either as the result of increased demand for packet data transfer or whenever there is enough available physical channels in a cell to be able to improve the quality of service

Allocation of Resources to GPRS (cont)

• As the number of allocated PDCHs can be increased or decreased according to the demand, it is necessary for the networks to support the following principles– Load Supervision: Carries out the monitoring of load conditions on the

PDCHs and as a result, the number of PDCHs can be increased or decreased

– Dynamic Allocation: Unused channels can be allocated as PDCHs to increase the overall quality of service offered by GPRS

– Fixed Allocation: In the future as more packet users require access then network may allocate permanent slots for GPRS

Allocation of Resources to GPRS (cont)

• The fast release of PDCHs is critical if a dynamic sharing of resources is to take place between CS & PS services. To achieve this, GSM offers 3 possible methodologies– Wait for all the assignments to terminate on the particular PDCH

– Individually notify all users that have an assignment on the particular PDCH

– Broadcast the notification about de-allocation

MS multi-slot capabilities

• A multi-slot consists of multiple traffic channels and associated control channels allocated to one mobile

• A multi-slot configuration could occupy upto 8 physical channels with different timeslots but with the same frequency, i.e. an MS may allocated several PDTCH, both uplink & downlink

• Type 1 mobiles are not allowed to transmit and receive simultaneously

• Type 2 mobiles which are able to transmit and receive simultaneously

• For HSCSD, only multislot classes 1-18 are recognized

MS multi-slot capabilities (cont)Multislot

classMaximum number of slots Minimum number of slots Type

Rx Tx Sum Tta Ttb Tra Trb

1 1 1 2 3 2 4 2 12 2 1 3 3 2 3 1 13 2 2 3 3 2 3 1 14 3 1 4 3 1 3 1 15 2 2 4 3 1 3 1 16 3 2 4 3 1 3 1 17 3 3 4 3 1 3 1 18 4 1 5 3 1 2 1 19 3 2 5 3 1 2 1 1

10 4 2 5 3 1 2 1 111 4 3 5 3 1 2 1 112 4 4 5 2 1 2 1 113 3 3 NA NA a) 3 a) 214 4 4 NA NA a) 3 a) 215 5 5 NA NA a) 3 a) 216 6 6 NA NA a) 2 a) 217 7 7 NA NA a) 1 0 218 8 8 NA NA 0 0 0 219 6 2 NA 3 b) 2 c) 120 6 3 NA 3 b) 2 c) 121 6 4 NA 3 b) 2 c) 122 6 4 NA 2 b) 2 c) 123 6 6 NA 2 b) 2 c) 124 8 2 NA 3 b) 2 c) 125 8 3 NA 3 b) 2 c) 126 8 4 NA 3 b) 2 c) 127 8 4 NA 2 b) 2 c) 128 8 6 NA 2 b) 2 c) 129 8 8 NA 2 b) 2 c) 1

a) = 1 with frequency hopping. = 0 without frequency hopping.b) = 1 with frequency hopping or change from

Rx to Tx. = 0 without frequency hopping

and no change from Rx to Tx.c) = 1 with frequency hopping or change from

Tx to Rx. = 0 without frequency hopping and no change

from Tx to Rx.

MS multi-slot capabilities (cont)

• Rx - indicates the max. number of receive timeslots per frame

• Tx - indicates the max. number of transmit timeslots per frame

• Tta - time required to perform adjacent cell measurements and prepare to transmit

• Ttb - min. time needed for the MS to prepare for transmission but will only be used when adjacent cell measurements are not required

• Tra - min. time needed to perform adjacent cell measurements and prepare to receive

• Trb - mini. Time needed to prepare to receive when adjacent measurements are not required

MS Class Marks

• The radio access class mark comprises the multi-slot capability and the power class as well as all the information required by the BSS in order that it can handle radio resources at that mobile.

• The class mark is sent in MM messages to the SGSN who then inform BSS, although initial optimization allows the BSS to receive a reduced class mark during the initial access from the MS (The class mark is stored in the network until a GPRS detach)

• MS Class Mark 1: Information provides the network with ‘priority’ details which affect how the network handles the mobile. In this case, class mark 1 indicates the power capability of the phone in relation to its power class

MS Class Marks (cont)

• MS Class Mark 2: information element provides the network with general mobile characteristics, such as power capability, whether SMS is supported, which encryption algorithms are supported, etc.

• MS Class Mark 3: indicates the bands available, the power, the encryption algorithms and other information relative to multislot capabilities and SMS details

Power Control

• Power control is used to improve power consumption in the MS and to minimize co-channel interference. The MS calculates the output power (Pch) to be used on each uplink PDCH, the power being expressed in dBm

• The output power on any channel is, in general terms, the minimum that is needed to maintain the quality of service. The exception to this is for access bursts (Packet channel request or polling response) when the maximum power (Pmax) is always used

• On other channels, the power is determined by the specific power control parameters set by the network, which are dependant upon, for example, the maximum allowed output power in the cell, the mobile’s power class, the received signal strength at the mobile and any interference, interference measurements being taken during idle slots

Power Control (cont)

• MS Output power, The uplink RF Output power , Pch, employed on the PDCH is:– Pch = min ( (o - ch - * (C+48)), Pmax)

o = A frequency band dependent constant (39dBm for GSM900 or 36dBm for GSM 1800

ch = an MS channel specific power control parameter sent to the MS in an RLC control message. This value can be modified at any time by the network in 31 steps of 2dB, from 0dB to 62dB

= This is another system parameter broadcast on the PBCCH or sent in an RLC control message. Its value varies from 0.0 to 1.0

• C = received signal level at the MS

• Pmax = the max. power output allowed in the cell

• In the packet idle mode, the MS periodically measures signal levels on the PCCCH or BCCH in order to calculate the value of C


• The C value for each paging block monitored by the MS is given by:– C = SSBlock + Pb

• SSBlock is the mean signal level of the 4 normal bursts comprising the block, and

• Pb is the BTS output power reduction relative to the power on BCCH (for BCCH, Pb = 0)

• In packet transfer mode the MS measures the C value on the BCCH carrier and modifies its output power. The BSS updates the MS specific ch values and informs the MS as necessary, i.e. when the interference level has changed.

• Alternatively, the MS measures the PACCH, especially if the BCCH is on another frequency. Any downlink signal variation is then included in the uplink channel quality report (exception is fixed uplink assignment, where measurements not taken)


• At the BTS, constant power levels are used on the PBCCH and PCCCH but these may be lower than that used on the BCCH, the difference being broadcast on the BCCH. The power reduction used on the PCCCH is given as Pb. On other PDCHs downlink power control may be implemented, the value chosen being determined by assessing channel quality reports

• Two methods of downlink power control exist and is indicated in the initial assignment command– Mode A - used for any allocation

– Mode B - used only for fixed allocation

• In both control modes a parameter PO is used, this being defined as a power reduction relative to the BCCH. PO does not change during packet transfer until a new assignment is implemented


• On the downlink PDTCH the coding of the power reduction field (PR) depends on the downlink power control mode (A or B). The PR field in the RLC header indicates the power level reduction of the current RLC block in the PDCH relative to BCCH level minus PO for mode A, and relative to BCCH for mode B

Power GSM 400 & GSM 900 DCS 1 800 PCS 1 900 Tolerance (dB)class Nominal Maximum

outputNominal Maximum

outputNominal Maximum

outputfor conditions

power power power normal extreme

1 - - - - - - 1 W (30 dBm) 1 W (30 dBm) ±2 ±2.5

2 8 W (39 dBm) 0.25 W (24 dBm) 0.25 W (24 dBm) ±2 ±2.5

3 5 W (37 dBm) 4 W (36 dBm) 2 W (33 dBm) ±2 ±2.5

4 2 W (33 dBm) ±2 ±2.5

5 0.8 W (29 dBm) ±2 ±2.5

NOTE: The lowest nominal output power for all classes of GSM 400 and GSM 900 MS is 5 dBm and for all classes ofDCS 1 800 and PCS 1 900 MS is 0 dBm.

Timing Advance

• Timing advance procedure is used to derive the timing advance that the MS has to use for the uplink transmission of radio blocks. It is in two parts:

– initial timing advance estimation based on the single access burst carrying the packet channel request, and

– continuous timing advance updates carried on the PTCCH allocated to the MS

• The initial timing advance estimation is used by the MS for uplink transmissions until the continuous timing advance update is received, unless

– a packet queuing notification has been used and so the timing advance estimation becomes updated, or

– a packet assignment is sent without prior paging and so no timing advance value is available. This could occur in the ready state

Timing Advance (cont)

• When this happens, the network may– transmit a packet polling request so that the MS responds with an

acknowledgement from which timing can be estimated

– transmit an assignment without timing advance information, this indicating to the MS that it can only begin uplink transmissions upon receipt of the timing advance by using continuous update procedure

– set the poll bit in the assignment message to trigger a acknowledgement

Timing Advance (cont)

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

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

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

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

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

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

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

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

D ownlink(BTS -> M S)

0 1 2 3

3210

TD M A fram e num ber

TD M A fram e num ber

T im eslot num ber

U plink(M S -> BTS)

D elay

RF

channels

Routing Area

• For location purposes a GSM PLMN is divided into a number of location Areas. Each of these areas consisting of a number of cells. The location area and cells are identified by Location Area Identity (LAI) & Global Cell Identity (GCI), respectively– LAI = MCC + MNC + LAC

– GCI = LAI + Cell Identity (CI)

• However, for GPRS, cells are grouped into Routing Areas (RA) which are defined as:– RAI = LAI + Routing Area Code

• Routing Area is a subset of Location Area

GPRS Cell Selection

• Initial cell selection for GPRS is similar to that used in GSM in that at switch-on the MS has to camp on suitable cell, tune to the control channel and receive information on the BCCH

• If an MS in GPRS Idle mode wishes to initiate an attach and the camped on cell does not support GPRS, then cell-reselection is required before attach can take place

• In the standby and ready states cell re-selection is performed by the mobile, the exception being a class A mobile in CS dedicated mode, which resumes normal re-selection when the CS connection has ended

• A GPRS mobile continuously monitors all BCCH carriers indicated in the BA list, broadcast on the BCCH of serving cell and uses this to calculate the RF signal ‘Received level average’ (RLA) for each cell and lists the 6 strongest non-serving carriers from neighboring cells

GPRS Cell Selection (cont)

• The mobile also monitors the BSIC for these 6 cells. It also does this during the idle frames on the PDCH multiframe when engaged in packet transfer

• The presence of a PBCCH in the cell is indicated by a PBCCH description in the SI13 message on the BCCH. If the MS receives a SI13 message without PBCCH description, it will assume that the PBCCH is not present on the cell. In such cases, the mobile will read System Information Messages on the BCCH. Non GPRS MS will ignore SI13 messages

GPRS Cell Selection & Re-selection

• New Cell re-selection criteria C31 & C32 are used within GPRS to complement the existing C1 & C2 parameters

• The parameter C1 used in GSM CS cell selection is used as the minimum signal strength criterion for GPRS Cell re-selection. However, when applied to GPRS– C1 = A - Max(B,0) dB

• where A = RLA_P - GPRS_RxLEV_ACCESS_MIN

• B = GPRS_MS_TxPWR_MAX_CCH - P

• RLA_P = downlink receive level average

• GPRS_RxLEV_ACCESS_MIN = the minimum downlink signal strength required for system access

• GPRS_MS_TxPWR_MAX_CCH = the maximum uplink power required for cell access

• P = Maximum output power of the mobile

GPRS Cell Selection & Re-selection (cont)

• C31 Parameter– A Signal Strength threshold criterion parameter C31 is used to determine

whether Cell re-selection should take place or not

– For the serving cell, C31(s) = RLA_P(s) - HCS_THR(s)• where HCS_THR is the threshold criteria for determining cell re-selection in a

hierarchical structure

– For neighboring cells, C31(n) = RLA_P(n) - HCS_THR(n) - TO(n)*L(n)• Where TO is a temporary offset which can be applied (or not) for a network

determined period dependent upon whether L = 0 or 1, this being the priority class dependent upon cell loading


• C32 Parameter– The C2 criterion used in GSM is a re-selection parameter, having a temporary offset in

order to prevent fast-moving mobiles passing through a micro-cell from selecting that cell

– C32 is an improvement on C2 in that it applies an individual offset and hysteresis. C32 is used therefore to select cells where a number of cells have the same priority class

– For the serving cell, C32(s) = C1 (s)

– For the neighboring cells, C32(n) = C1(n) + GPRS_Reselect_Offset(n) - TO(n)*(1-L(n))

• GPRS_Reselect_Offset applies an offset and hysteresis value to each cel• TO is the temporary offset applied (or not) for a network determined period TO(n) =

GPRS_Temporary_offset(n)*H(GPRS_Penalty_Time(n) - T(n))• L(n) = 0 if Priority_Class(n) = Priority_Class(s) 1 if

Priority_Class(n) Priority_Class(s)• H(x) = 0 for x < 0 1 for x 0


• The MS will Reselect if:– The path loss criterion C1 on the serving cell falls below zero, or

– if another neighboring cell is evaluated to be better than the serving cell i.e. the one with the highest C32 value

– Hysteresis is also taken into account in that within a routing area the mobile should use Hysteresis when re-selecting in the ready state. Hysteresis is applied to the C32 criteria. The network can apply hysteresis to C31

Coding Schemes

• 4 coding schemes CS-1 to CS-4 are defined for packet data traffic which offer differing degrees of error protection over the air interface

• For CS-2 to CS-4, the USF field present in the MAC header must be pre-coded to either 6 bits for CS-2 & CS-3 and to 12 bits for CS-4

• In addition, for CS-2 to CS-4, the USF bits of the data block are encoded such that the first 12 coded bits represent the same bit pattern, irrespective of the coding scheme, depending solely upon the USF bits.

• To also aid in the detection of the USF, the stealing bits of a block are also used to indicate the coding scheme used

• Steps for the Coding Schemes– Add Block Check Sequence (BCS) for the error protection– For CS-1 to CS-3, pre-coding of USF (except for CS-1), adding 4 tail bits and

convolutional coding for error correction that is punctured to give the desired coding rate. For CS-4, there is no coding for error correction.

Coding Schemes (cont)

Scheme Code rate USF Pre-codedUSF

Radio Block excl.USF and BCS

BCS Tail Codedbits

Puncturedbits

Data Rate

(kbps)

CS-1 1/2 3 3 181 40 4 456 0 9.05

CS-2 2/3 3 6 268 16 4 588 132 13.4

CS-3 3/4 3 6 312 16 4 676 220 15.6

CS-4 1 3 12 428 16 - 456 - 21.4

BlockEncoder

Pre-codingof USF

Add Tail bits

ConvolutionalEncoder

Puncturing


D D D D

u(D)

u(D)g(1)(D)

u(D)g(2)(D)

g(1)(D) = 1 + D3 + D4

g(2)(D) = 1 + D + D3 + D4


• Coding Scheme - 1– For CS-1, 40 bits are used for the Block Check Sequence (BCS) to

increase protection

– BCS & USF (3 bits), Header & Data (181 bits) & 4 tail bits are passed though a 1/2 rate convolutional encoder resulting in a “protocol” 456 bit payload

– When Using CS-1, the data rate is equal to 181 bits / 20ms = 9.05 kbps

USF(3)

Header & Data (181)BCS(40)

6

+

456 bits

224+4 bits

TAIL(4)

+

Convolutional coding1/2 rate


• Coding Scheme 2– For CS-2 only 16 bits are used for the BCS but in this case, a CRC code is

used.

– Differences between CS-2 & CS-1 include the use of a 6 bit USF to increase robustness during transmission over the air interface

– 4 tail bits are added to the sequence prior to passing through a 1/2 rate convolutional encoder.

– The result of this is a bit stream of 588 bits/20ms but this needs to be reduced to 456 bits to bring it in line with GSM burst structures. 132 bits should be punctured (USF bits should not be punctured)

– When using CS-2 data rate is equal to 268 bits/20ms = 13.4 kbps


USF(6)


12

+

588 bits

290+4 bits

TAIL(4)

+


12456 bits

Puncturing132 bits

1 2 15 16 17 18 19 20 21 22 23 587 588……. …….

First Last


• Coding Scheme - 3– The process used in CS-3 is almost identical to that of CS-2 other than degree

of puncturing.

– In this case, the Header & data is made up of 312 bits and after encoding (with USF, BCS & Tail bits) the result is 676 bits. This is punctured once again to the 456 bits required by the GSM burst structure

– The data rate will be equal to 312 bits/20ms = 15.6 kbps

• Coding Scheme - 4– There is no FEC applied to the data and as such, thee is more capacity for user

information.

– In this case, BCS is made up of 16 bits, using a CRC code and the USF has been extended to 12 bits for robustness

– It is possible to carry 428 bits of header and data

– Data rate of CS-4 is equal to 428 bits/20ms = 21.4 kbps


USF(6)


12

+

676 bits

334+4 bits

TAIL(4)

+


12456 bits

Puncturing220 bits

1 2 15 16 17 18 19 20 21 22 23 587 672 673 674 675 676

LastFirst

…... …...

Coding Scheme - 3


USF(12)

Header & Data (428)BCS(16)+

456 bits

456 bits

No Coding

Coding Scheme - 4

USF(12)

Transmission & Reception of data flow

Normal burst Normal burst Normal burst Normal burst

BH

FH

LLClayer

RLC/MAClayer

Physicallayer

Information field FCS

Info fieldBH BCSRLCblocks

LLCframe

Primaryblock

Followingblock

Info field BCS Info field BH BCS

FH = Frame HeaderFCS =BH =BCS = Block Check Sequence

Frame Check SequenceBlock Header

Radio Resource Operating modes

• Packet Idle mode– In packet idle mode no TBF exists. The MS listens to the PBCCH

– A class A in packet idle mode may enter a different RR service mode

– A class B or C must leave packet idle and packet transfer before entering dedicated mode, group receive mode or group transmit mode

• Packet Transfer Mode– The MS is allocated radio resources providing a TBF on one or more

physical channels

– When selecting a new cell the MS leaves packet transfer mode and enter idle mode before changing to new cell, reading system information and resuming packet transfer mode

GPRS Attach

BTSMS

PAGCH Packet Uplink Assignment (Page mode, Referenced address (Global TFI,TQI, Packet Req. Ref), Channel_coding_cmd, TLLI_Block_ch_coding,

Packet Timing Advance, Frequency parameters (FH or ARFCN),Dynamic Alloc (USF+TN) or Single Block alloc (TN+Start frame) or

Fixed Alloc (TN+blocks alloc))

FCCH

SCH

BCCH (SI 3 & SI 13)

PBCCH

PCCCH USF (USF = FREE)

PRACH Packet Channel Request (MM Procedures, random reference, multislot class, radio priority, no. of RLC blocks requried)



GPRS Attach (cont)BTS

MS

PACCH Attach Accept Result (GPRS only, combined), Radio Priority (SMS), RAI, P-TMSI Signature, GPRS timer, P-TMSI allocation

PACCH Authentication & Ciphering Request(Ciphering Alg, Force to Standby, cksn USF

PACCH Attach Complete

PACCH Authentication & CIphering Response SRES

PACCH Identity Response IMSI, IMEI

PACCH Identity Request (IMSI, IMEI) USF

PACCH RLC/MAC Block USF

PACCH Attach Request MS n/w capability (SMS on GPRS & CS, SS screening,GPRS encryption alg), Attach type (GPRS only, IMSI, Combined),GPRS cksn, DRX parametr, P-TMSI or IMSI, Old RAI, MS RA capability, GPRS timer

PDP Context Activation

BTSMS

PDTCH TFI UA (RLC/MAC Block)

PDTCH LLC SABM (RLC/MAC Block)

PACCH USF

PACCH Activate PDP Context Accept

PACCH Activate PDP Context Request (NSAPI, PDP Type, PDP Address,QoS Requested)

PACCH USF

Uplink Data Transfer

• The process begins with the MS sending a Packet Channel Request on the PRACH requesting uplink resources. The network responds to this with a Packet Immediate Assignment message at the same time as reserving suitable resources. The GPRS specification allows for two different packet access methods

• One Phase Access– Packet Channel Request is responded to by the network with the Packet Uplink

Assignment reserving the resources on the PDCH(s) for uplink transfer of a number of radio blocks and also the corresponding USF values.

– When accessing the system via the RACH, there is only two cause values available for denoting GPRS and the network can assign limited uplink resources or two phase access

– However, when accessing via a PRACH, the request message may contain more adequate information about the requested resources and consequently, uplink resources on one or several PDCHs can be assigned by using the packet uplink assignment message

Uplink Data Transfer (cont)

• Two Phase Access– Packet Channel Request is responded with Packet Uplink Assignment

which reserves the uplink resources for transmitting the Packet Resource Request. This message carries the complete description of the requested resources for the uplink transfer. Thereafter, the network responds with the Packet Uplink Assignment reserving resources for the uplink transfer

BTSMS

Packet Uplink Assignment

Packet Resource Request (PACCH)

Packet Uplink Assignment (AGCH)

Packet Channel Request (RACH)


• Dynamic Allocation– Packet Uplink Assignment message includes the list of PDCHs and the

corresponding USF value. A unique TFI is also allocated and is included in each RLC data and control block relating to that TBF. The MS monitors the USFs on the allocated PDCHs and transmits Radio Blocks on those which currently bear the USF value reserved for use by that particular MS

– When a mobile detects the USF on a downlink, it transmits either a single RLC/MAC block or a sequence of 4 blocks on the same PDCH. The number of blocks the MS is allowed to use is controlled by the USF Granularity parameter: 0 for one uplink block or 1 for 4 blocks, the USF on the last 3 being set to ‘unused’

– The channel reservation algorithm can also be implemented on an assignment basis allowing individual MSs to transmit for a predetermined amount of time without interruptions


BTSMS

PACCH (Packet Resource Reassignment Ack)

Packet Uplink Ack/Nack

PDTCH (Data Block , last)

PACCH (USF)

PACCH (Packet Resource Reassignment)

PDTCH (Data Block)

PACCH (USF)

PACCH (USF)

PDTCH (Data Block, last in send window)

Packet Uplink Ack/Nack

PDTCH (Data Block, last in send window)

PACCH (USF)

PACCH (USF)

PDTCH (Data Block)

Packet Uplink Assignment (Dynamic, TFI, USF, open ended)

Packet Channel Request (PRACH)


• Release of Resources– The release of resources is normally initiated from the MS by indicating

the last RLC data block to be sent and await the final ACK message. In the case of successful transmission, the network will send final ACK message on the downlink PACCH and will await the mobile to acknowledge. Alternatively, a time is set in the transmission of the final ACK message and the channel will release upon the timer hitting its pre-defined limit

– The pre-mature release or change of assignment for one MS can be initiated by the network. In the case of release, the MS is ordered to interrupt the TBF and back-off. The MS will then reorganize the uplink buffer and issues a new packet channel request to continue the uplink transfer with the data blocks carrying “un-transferred” LLC frames


• Contention Resolution– Two phase access is immune to the possibility of two MSs perceiving the

same channel allocation as their own. This is because the second phase access uses the Packet Resource Request which uniquely identifies the MS by its TLLI. The same TLLI is also included in the Packet Uplink Assignment and thus no mistake is possible

– The one phase access method is however less secure and as such an efficient contention mechanism has been introduced. This is achieved by adding the TLLI in the RLC headers for the RLC data blocks in the uplink until the first temporary acknowledgement is received. Following on from this, the network notifies the users of who actually owns the allocation. This is achieved by including the TLLI in the temporary Packet Ack/Nack which can be sent early and even before the receive window for the RLC/MAC window is full. By introducing this mechanism, the contention is resolved by the first Packet Ack/Nack


• Extended USF– If the resource assignment by the network does not allow the multi-slot

MS to monitor the USF on all the assigned PDCHs, the MS must carry out a modified “dynamic” allocation procedure.

– Whenever the MS receives its USF in one downlink PDCH e.g. timeslot 1 when 1,2,3 & 4 were assigned, the MS may consider the proceeding uplink block and all subsequent ones from the list of assigned PDCHs as allocated e.g. 2,3 & 4

– Thus, if the network allocates a block to a MS on an assigned PDCH, it shall also allocate blocks to this MS on all subsequent PDCHs in the list

– For each allocated block, the network sets the value of the USF to the value reserved for usage by that particular MS. In addition, it should be noted that these rules apply on a block period basis


• Fixed Allocation– Fixed Allocation uses the Packet Uplink Assignment message to

communicate a detailed fixed uplink allocation to the MS. The fixed allocation consists of a start frame, slot assignment and block assignment bitmap representing the assigned blocks per timeslot. The MS waits until the start frame indicated and then transmits radio blocks on those blocks indicated in the block assignment bitmap. As such, a MS receiving this form of allocation is free to transmit on the uplink without monitoring the downlink for the USF

– If the current allocation is not sufficient to transfer the LLC frames, the MS may request additional resource in one of the assigned uplink blocks. A unique TFI is allocated and is therefore included in each RLC data and control block related to that TBF. Since each radio block includes an identifier (TFI), all received radio blocks are correctly associated with a particular LLC frame and a particular MS

Paging Procedures

• An MS in the standby state must be paged by the SGSN before downlink transfer can occur. The result of the page is that the mobile moves into the ready state.

• Network can send Packet paging request either on downlink PPCH or PCH according to its mode of operation

• The MS will respond to the Packet Paging Request by initiating a procedure for paging response. This RLC/MAC paging response contains the TLLI as well as the complete LLC frame

• Transmission of a packet to an MS in the Ready state is initiated by the network using the Packet Downlink Assignment message transmitted on either PAGCH or AGCH. This message includes the list of PDCHs assigned, the TFI in addition to Timing Advance and Power control information

Downlink Packet Data Transfer

• The sending of Packet Ack/Nack is initiated by the network polling the MS. The Ack/Nack message is sent in a reserved radio block which is allocated to the mobile at the time of polling

• The network transmits the downlink blocks within a window, whilst the Packet Ack/Nack acknowledges all correctly received RLC data blocks up to an indicated Block Sequence Number (BSN). This action moves the sending window and also initiates the request for retransmission of erroneously received blocks, the network allocating additional resources as required.

Downlink Packet Data Transfer (cont)

MSBTS

Packet Downlink Ack/Nack(final)

PDTCH (last Data Block, poll)

PACCH (Packet Control Ack)

PACCH (Packet Downlink Assignment)

PDTCH (Data Block)

PDTCH (Data Block)

Packet Downlink Ack/Nack

PDTCH (Data Block, Polling)

PDTCH (Data Block)

PACCH (Packet Pagign Response , LLC Frame)

PAGCH or AGCH (Packet Downlink Assignment)

PRACH or RACH (Packet Channel Request)

PPCH or PCH (Packet Paging Request)

Downlink Packet Data Transfer (cont)

• Release of Resources– The release of the resources is initiated by the network by terminating the

downlink transfer and polling the MS for a final Packet Downlink Ack/Nack

– It is also possible for the network to modify the current downlink assignment by terminating the current data TBF and initiating a new one after the MS returns to start monitoring the PCCCH. An alternative method to achieve this would be by using Packet Downlink Assignment message or Packet Time slot Reconfiguration message

Simultaneous uplink & downlink

• During ongoing uplink TBF, the MS continuously monitors one downlink PDCH for possible Packet Downlink Assignment message or Packet Timeslot Reconfiguration messages on the PACCH

• If the MS wants to send packets to the network during ongoing downlink TBF, it will indicate this during an acknowledgement message. As such, no explicit Packet Channel Request need be sent to the network.

Thank You Very Much

GPRS AirInteface

Documents

uplink packet data channel

packet data channel

downlink packet transfer

packet transfer pnch

ptcch pccch

cell pccch

packet transfer mode

packet data system information