GPRS Chap9

GPRS-Radio Interface 19

Rohde & Schwarz Trainingcenter, V 1.1

8. GPRS Radio Interface

The packet switched GPRS-network uses on the air interface the same two accessprinciples as known from a GSM network: TDMA and FDMA. Besides, the physicalparameters describing channel configuration, bit period duration and modulationscheme stay absolutely identical and therefore these principles will not be discussedin the further pages. The transportation unit on the physical layer for user data via theair interface will be the Normal Burst, as well as known in a GSM network. Thefollowing picture reminds of used Burst structures and types in GSM as well asGPRS networks:

TailBits 3

Sync.Sequence 41

Encrypt. Bits 36

Ext.Tail Bits 8

Normal Burst

Frequency Correction Burst

Synchronisation Burst

Dummy Burst

Access Burst68,25 Bit Periods

Guard Period

8,25Bit Periods

Timeslot

Mixed Bits 58

TailBits 3

TailBits 3

TrainingSequence 26

Mixed Bits 58

TrainingSequence 26

Encrypted Bits 58

TailBits 3

Encrypted Bits 58

TailBits 3

TailBits 3

TailBits 3

Fixed Bits 142

Encrypted Bits 39

Encrypted Bits 39

TailBits 3

TailBits 3

Ext. Training Sequence 64

Figure: Burst types for GSM and GPRS

GPRS-Radio Interface Page 120


GPRS uses the Access Burst for Access to the network on the uplink-path and lateron it uses the Normal Burst for any kind of data transport. The next picture shallresolve again, that nothing changes concerning the introduction of GPRS in a GSMnetwork in what concerns the used Burst types. The Normal Burst stays at it isdefined for GSM. The higher data rate reached by GPRS is due to channel bundlingand due to channel coding, i.e. the ratio between information bits and redundancybits in the information field of the Normal Burst. Which of the actual Coding SchemesCS1-4 is actually used, is indicated by the stealing flag bits. The maximum data rateof 2x57 Bit per Burst is not touched. As well as the function of the other Bits of thisBurst type, e.g. the Training Sequence or the Guard Period.

TInformation F GInformationFTrainings- sequenz

3 57 1 26 31 57 8,25

T

T = Tailbits F = Flagbits G = GuardtimeFigure: Normal Burst, used in GPRS in an unmodified mode

The three Burst types: Frequency Correction Burst, Synchronisation Burst andDummy Burst are not explicitly mentioned in the GPRS-Specifications, but they areindispensable for the use of GPRS. They are essential for the mobile station torecognise a BCCH carrier channel of a BTS and to synchronise to the frequency andtime of this basestation. Because this should also happen in the Packet TransferMode, i.e. more or less in the Transmitting or Running mode with allocated Trafficchannel, we can see the hybrid structure: The packet switched services of GPRS arebased on the circuit switched signalling o GSM. Some Signalling information still iscontinued transmitting in circuit switched mode.

8.1. The Channel – expression in GPRS

The expression „channel“ is used in GPRS in different meanings and contexts. Thefollowing chapter will derive the differences between: frequency channel, physicalchannel and logical channel.

The frequency channel:

The expression frequency channel in GPRS defines a pair of two times a nominal200 kHz wide spectrum in the GSM – frequency bands (GSM-900, GSM-1800, GSM-1900 und GSM-400).



The physical channel in GPRS:

A physical channel in GSM consists of the assignment of a timeslot on a frequencychannel. At this position it shall not be responsive to the separated modes of aphysical channel: Full-Rate, Half-Rate or Frequency Hopping. It is referred to thedocumentation of GSM.

Firstly it shall be described the ratio between circuit switched and packet switchedinside a cell. The allocation and separation of all possible physical channels intopacket oriented and circuit switched channels is effectuated in a dynamic manner bythe concept: Capacity on demand. The name of a packet oriented channel is: PacketData Channel, PDCH.

Figure: Capacity on demand: Example of 1 radio cell with assignment of 4*CircuitSwitched, 3*Packet Switched and 1* Signalling.

In GPRS a physical means that there is allocated physical resource to transfer anamount of data and that comprises one or more combined timeslots. In other words,there is physical resource allocated to transmit N Radio Blocks within one or moretimeslots. The allocated physical resource is named with the expression TemporaryBlock Flow, TBF, i.e. a physical connection between two endpoints on one or morePDCH.

Concerning their assignment, physical channels can be distinguished in GPRS in thefollowing way:

Fixed Allocation:Similar to GSM, physical resource is assigned in a fixed manner. One or moretimeslots are assigned and the mobile station is allowed to use a certain number ofRadio Blocks within a certain time period. Concrete, there is allocated a fixedphysical resource to be used to transfer the amount of data.

Dynamic Allocation:

Capacity on demand: Provides Circuit Switched Channelsand Packet Switched Channels in a dynamic manner

0 1 2 3 4 5 6 7

Signalling: BCCH + FCCH + SCH + CCCH

Circuit switched channels Packet switched channelsPDCH



Packet switched services allow the possibility of channel sharing, i.e. moresubscribers are using the same physical channel. This is achieved by the assignmentof this physical resource (e.g. ARFCN + TN) not in a continuos mode and also not forthe complete duration during a connection, but the physical resource is assigned onlyfor a short period of time. The procedure in its concrete way looks like the following: 4Normal bursts in 4 consecutive frames are forming a Radio Block. The permission touse the next Radio Block on the uplink path is indicated by the Uplink State Flag,USF. The allocation of this USF value is temporarily and only valid for the duration ofthe packet data transfer on one PDCH. This way allows the network to switch in adynamic manner between different users. It is possible to generate by this a quasi –permanent connection, the Uplink State Flag will always point to the same user.

Extended dynamic Allocation:The extended dynamic allocation is in principle the same as the dynamic allocationwith only one small difference. In its case, the Uplink State Flag, USF does not onlypoint to the next 1 Radio Block but to the following 4 Radio Blocks, the receiver ofthis USF is allowed to use on the uplink path. In one job, more physical resource isassigned. The use of the extended dynamic allocation by a network operator isoptional, the fixed and dynamic allocation must be provided by the network operatorthat offers GPRS services.

The asymmetric ratio between Up- and Downlink is given, because the assignment ofdownlink and uplink physical resource is done in a separated manner.

Figure: Asymmetric and Symmetric Allocation

DownlinkUplink Downlink Uplink

Symmetric data transfer Asymmetric data transfer

or



Figure: Different physical channels: circuit switched and packet switched.

In the figure above, once more it is mentioned the difference between a circuitswitched physical channel in the 26-Multiframe-Mode and the packet switchedphysical channel, consisting of different Radio Blocks.

0 1 2 3 4 5 6 7TN

FN = Frame number TN = Timeslot number

}

= 1 Frame

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

Mapping of TCH and PDCH on a frequency channel

GSM TCH: Traffic channel in circuit switched mode

1 2 x T C H 1 2 x T C H S I

0 1 2 3 4 5 6 7 8 9 1 0 1 1 1 2 1 3 1 4 1 5 1 6 1 7 1 8 1 9 2 0 2 1 2 2 2 3 2 4 2 5

Frequency channel

GPRS PDCH:1 Radio Block =4 Bursts

GPRS PDCH:1 Radio Block =4 Bursts

GPRS PDCH: Traffic channel in packet switched mode



Logical channels in GPRS:

Logical channels are defined and named by its function and its content. There arelogical channels with the task of user traffic transport, (TCH) and other logicalchannels, used to exchange signalling information between mobile station andnetwork. The logical channels known in GSM are still used, for packet switched modethere will be additional logical channels. These new logical channels are alsomapped onto physical channels belonging to a multiframe structure. In GSM we knowthe two multiframe structures: 26-Multiframe for traffic channels and 51-Multiframe forsignalling, in GPRS all PDCH’s are configured into a 52-Multiframe structure,independent if its content is a packet switched traffic channel or a packet switchedcontrol channel.

Figure: Used Multiframes in GSM and GPRS

460

54 6 7 1 2 53 4 60 7 1 2 53 4 60 7

0

1 2 3 4

1 2 3 40

47 48 49 50

48 49 50 5147

51-Multiframe: GSM- Signalling

26-Multiframe: GSM- Traffic

52-Multiframe: GPRS -Traffic and Signalling

1 2 3 4 21 22 23 24 25

0 1 2 3



The following figures are summarising the used logical channels in GSM and GPRSplus their function.

Figure: Logical channels in GSM

Figure: Logical Channels in GPRS

BCCH

FCCH Frequency correct ion

Signallingand Control

Traffic

C C C H

D C C H

S C H Frame synchronisat ion + BSIC

PCH Paging mobi les

RACH Request ing dedicated channel

AGCH Allocating dedicated/traff ic CH

NCH N otifying MSs (for V G C S & V B C)

Broadcast of cel l information,e.g. channel combination

SDCCH Signal l ing between MS and B T Se.g. Authentication, S M S, L U P

SACCH Measurements, TA, PC, . . .

FACCH Extra signall ing within 26 TDMA Mult i frame

TCH/F full rate traffic channel

TCH/H half rate traff ic channel

B C HDL

U P

DL

DL

DL

DL & UP

DL & UP

PBCCH

Signalling

and Control

Packet

Traffic Channels

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

UL

DL

DL

DL

DL & UL

PTCHSignalling: resource allocation,

acknowledgements, PC, TA, etc.

Paging MSs for packet dataand circuit switched services



Packet Common Control Channel PCCCH

The logical channels being contained in the PCCCH are taking the responsibility forsignalling and are mapped onto single physical channels within a 52-Multiframe.Their assignment is not permanent over one PDCH, they are co-sharing the PDCHtogether with the PBCCH and the PDTCH. The first indication about the existenceand position of the PCCCH is performed on the circuit switched CCCH’s. Concrete,the PCCCH consists of: PRACH, PNCH, PPCH and PAGCH:

Packet Random Access Channel, PRACH:

This logical channel is used by the MS to initiate a packet date transfer. The PRACHis mapped on one or more physical channels, its position is indicated on the BCCHor PBCCH. There are two possibilities to enable the access over a PRACH to a MS:One is to send an Uplink State Flag, USF with the content that the next Radio Blockon the uplink is to be used as PRACH. The other possibility could be to reserve ablock on a physical channel to be used by the PRACH. This would save thepermanent observation of the USF by the Mobile Station.

Packet Paging Channel, PPCH:

These logical channels are used to distribute a paging call to a MS. Hereby areformed the known paging groups, derived out of the IMSI, to avoid the continuosobservation of the paging channels, the so called Discontinuos Reception, DRX. Onespeciality of the PPCH is, that also circuit switched calls can be initiated via thispaging channel, if the MS provides the simultaneous use.

Packet Access Grant Channel, PAGCH:

Before the actual data transfer there must be exchanged some signalling informationon a dedicated control channel. This will be assigned by the PAGCH. Just like all theother logical channels, the PAGCH is mapped on a certain part of a PDCH. Theinformation about its position is received from the Broadcast information on BCCH orPBCCH.

Packet Notification Channel, PNCH:

This logical channel is used for further features of the data transfer, e.g. to transmitmessages to more than one MS. It enables group call abilities of the system andpoint-to-multipoint data transfer in GSM and GPRS.



Packet Broadcast Control Channel PBCCH:

Same as the known BCCH in circuit switched mode, this logical channel transportssome relevant signalling information to all mobile stations already listening. It isforming the information board of the radio cell. The use of the PBCCH is optional andit contains only relevant information for the GPRS services. If it is not used in thecertain radio cell, all relevant information should be distributed on the BCCH. As wecan see, the circuit switched signalling is absolutely necessary for the GPRSservices. At least to indicate the existence of a PBCCH.

Packet Timing Advance Control Channel, PTCCH:

This logical channel does not posses a direct counterpart to the circuit switchedmode, it forms a novelty. In the circuit switched mode of GSM, the Timing AdvanceCorrection Value can be assigned in a permanent manner, because there is asymmetric assignment of physical resources, on uplink as well as downlink. In apacket switched data transfer mode, between the transmission of two packet dataunits, there can pass a certain period of time. But in all parts, the timing advancevalue has to be updated if the MS persists in Ready State, because the MS does nothave any information about the distance to the BTS. For every data transfer there isassigned a PTCCH combination. PTCCH/uplink, a logical channel on which the MS isallowed to send an Access Burst and the corresponding PTCCH/downlink, where theBTS will respond with the Timing Advance Value to be set by the MS.

Figure: Usage of the PTCCH on Uplink and Downlink

Packet Data Traffic Channel, PDTCH:

The logical channel used for data transfer. The PDTCH is assigned to the MS for acertain period of time, in the case of channel bundling, the MS will be assigned somemore PDTCH’s on different PCH’s. As already mentioned, the assignment canhappen in a fixed or dynamic manner and is performed independently on uplink anddownlink.

PTCCH / Uplink

PTCCH / Downlink

= 1 Access Burst

= 1 Radio Block = 4 Normal Bursts



Packet Associated Control Channels, PACCH:

Specific or dedicated signalling during every connection is conveyed on a PACCH,similar to the dedicated control channels in GSM, SDCCH, SACCH or FACCH. Thereis no longer a distinction between Slow Associated and Fast Associated in GPRS,the signalling channel PACCH will be used together with a PDTCH on the samephysical channel. The usage of the PACCH happens dynamically, i.e. instead ofsignalling there will be send signalling information. This is possible in a packetswitched network, the data flow will be omitted for this moment, no data will get lost.

Figure: Comparison of the logical channel types used in GSM and GPRS

Logical Channel Comparison

CIRCUIT SWITCHED GSM GPRS

FCCH No Direct EquivalentSCH No Direct EquivalentBCCH PBCCHPCH PPCHRACH PRACHAGCH PAGCHNCH PNCHSDCCH PCCCH/PACCHTCH PDTCHSACCH PACCHSACCH PTCCHFACCH PACCH

CCCH PCCCH

PDCH



8.2. Channel combinations in GSM and GPRS

Channel combination describes the mapping of different logical channesl onto onephysical channel. In GSM we know 10 different combinations, indicated in thefollowing figure. Additionally, in GPRS there are defined 3 more channelcombinations:

Figure: Channel combinations in GSM and GPRS

8.3. Multiframe Structure for packed data channels, PDCH:

As known from GSM, the logical channels are mapped onto a physical channelbelonging a certain multiframe structure. The used multiframe structure for packetswitched data is the 52-Multiframe like described in the figure below. It consists of 12Radio Blocks (each Radio Block à 4 Bursts), 2 Idle Frames and 2 PTCCH – Frames.c52 TDMA Frames

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

X = Idle frameT = Frame used for PTCCHB0 - B11 = Radio blocks

Figure: Multiframe Structure for PDCH‘s

Channel Combinations

CIRCUIT SWITCHED (51 MFM)i ) TCH/F + FACCH/F + SACCH/TFii) TCH/H(0,1) + FACCH/H(0,1) + SACCH/TH(0,1)iii) TCH/H(0,0) + FACCH/H(0,1) + SACCH/TH(0,1) + TCH/H(1,1)iv) FCCH + SCH + BCCH + CCCHv) FCCH + SCH + BCCH + CCCH + SDCCH/4(0..3) + SACCH/C4(0..3)vi) BCCH + CCCHvii) SDCCH/8(0 .7) + SACCH/C8(0 . 7)viii) TCH/F + FACCH/F + SACCH/Mix) TCH/F + SACCH/Mx) TCH/FD + SACCH/MD

GPRS (52 MFM)xi) PBCCH+PCCCH+PDTCH+PACCH+PTCCHxii) PCCCH+PDTCH+PACCH+PTCCHxiii) PDTCH+PACCH+PTCCH

where CCCH = PCH + RACH + AGCH + NCH and PCCCH=PPCH+PRACH+PAGCH+PNCH.



As long as already mentioned, every packet switched connection starts with thecircuit switched signalling. The circuit switched signalling channels, like the BCCH,received and examined by the MS are the first information, the MS will get. Thesecircuit switched channels contain if necessary a pointer to packet switched signalling.The pointers in the following picture are more or less optional, hierarchically readfrom the left to the right.

Figure: Hierarchy in the signalling, from Circuit Switched to Packet Switched

If the network operator decides to use a PBCCH, than its position will always be inthe Radio Block B0, within the 52-Multiframe structure of the PDCH carrying thisPBCCH. In case of his existence, it carries all relevant information for the MS, it doesno longer need to survey the BCCH.

8.4. Operation Mode of the Radio Resource Management:

In all known instances, GPRS Mobility Management, GMM, SMS Management,Logical Link Control and GPRS Radio Resource, GRR, the GRR is the one thatperforms the complete control, management and survey of the radio link. Thereby wecan distinguish between two operation modes:

Packet Idle Mode:

In this operation mode, there does not exist a TBF, corresponding to the fact, that nophysical resource has been assigned on the air interface. In this state, the RR-Management will remain until it gets from a higher layer the request to establish aradio connection, synonymous with the establishment of a TBF. For example, a MSobserves the Paging channel and is searching for incoming paging calls or it waitsuntil higher layer indicate mobile originated data transfer.

Packet Transfer Mode:

If the RR-Management is in the Packet Transfer Mode, it is synonymous with theestablishment of a TBF, or the assignment of physical radio resource. A MS isallowed to use the assigned physical channels. One exception happens in the caseof a cell reselection in the mode of established TBF. The MS will change from PacketTransfer Mode into Packet Idle Mode, receives and evaluates the cell information andinitiates the continuation of the connection by sending once again an Access Burst.

Circuit switched

BCCH

PDCH additional PDCH

PBCCH PCCCH

GPRS-Radio Interface 1


The following figure shows the context between the operation modes of theRadio Resource Management and the different modes of the MobilityManagement.

Radio Resource MS Packettransfer mode

Packet idle mode Packet idle mode

Mobility ManagementNSS and MS

Ready Standby

Figure: Combination of RR Operating Modes and MM states

It is shown, that the MM State „Ready“ does not signify the assignment of physicalresource, it only indicates, that the location information of the MS is known the mostexact way. A Paging call does not need to happen before every data transfer. Butbefore every data transfer there must be the establishment of a TBF and thedescription of the physical resource.

8.5. Protocol layers of the Radio Interface:

The air interface of GPRS is subdivided into several layers. Each layer is defined bya logical layer with specific functions. The complete structure of the protocol layers isgiven in the following figure:

Figure: Logical Layers of the air interface

It shall be remarked, that the protocol layers belong this figure are distributed overdifferent protocol layers.

Um Network

SNDCP

LLC(Note)

RLC

MAC

Phys. Link

Phys. RF

SNDCP

LLC

RLC

MAC

Phys. Link

Phys. RF

MS



Physical Layer:

For better understanding, the Layer 1 is subdivided into 2 different layers.

Physical RF-Layer:

The physical RF-layer modulates the bit sequence, coming from the higher layersand generates the carrier frequency. It is also responsible to synchronise the timing,the sending of the burst in the right time. On the receiver side, this process runs inthe opposite way, the demodulation of the received frequency into a binarysequence.

Physical Link-Layer:

This layer provides means and functions to transport the bit sequence over the airinterface. These are: Burst generation, data encoding, ciphering and error protection.In GPRS or GSM these are the known mechanisms of Interleaving and ConvolutionalCoding.

RLC/MAC-Layer, Radio Link Control / Medium Access Control-Layer:

The RLC/MAC-Layer guarantees the data transport over the physical interface, itprovides control functions for signalling and protects against loss or corruption due toBEC (Backward Error Correction), i.e. it guarantees the retransmission of packet dataunits. The MAC – Layer is controlling the access to the air interface, it manages theaccess of multiple users on one physical channel and its dynamic allocation.

LLC-Layer, Logical Link Control Layer:

This layer provides a data transport layer between MS and SGSN, independently ofthe used methods on the air interface. The LLC-Layer possesses separate interfacesor Access Points to higher layers, like GMM, SMS or the layer for user data transport,SNDCP.

SNDCP-Layer:

The indicated SNDCP-Layer in the presented figure shall demonstrate the higherlayer instances. It does not influence the air interface, furthermore it forms a datatransport mechanism for user data instead of signalling.



8.6. Delay Correction due to Timing Advance:

Timing Advance and Guard Period in GSMAs a repetition, it shall remind of the timing advance adaptation and delay correctionin GSM.

Each Burst used in GSM contains the Guard Period, a period of the length of 8,25 bitperiods (≈ 30 µs). During this time, the output level of the burst must go down from itslevel to a certain minimum, reduced by up to minus 70 dB. In the same time, thesuccessor burst shall ramp up his output power level for that he reaches its desiredvalue after the passing of this guard period. The time interval, not useful for datatransfer is used in that way twice, to ramp down the power of the predecessor burstand to ramp up the power of the successor burst.

timeslot n timeslot n+1

guard period8.25 bit periods

burst in timeslot n

burst in timeslot n+1

powerramping

Figure: Guard Period used for power ramping

The integrity of every timeslot depends on the ability of all partners to transmit only inthe assigned timeslot. During the remaining time, they shall keep their transmittersinactive. Absolute necessity and prerequisite to this is, that all participants arestrongly time-synchronised to the main time. In GSM this is organised in the way, thatthe BTS provides the reference time, broadcasts its value in the synchronisationburst and all listening MS shall synchronise to that main time. Synchronisationproblems only appear in uplink, there, multiple MS’s share the channel due to theTDMA principle. In downlink are addressed multiple Mobile Stations, but there is onlyone partner transmitting, the BTS.



The guard period at the end of the burst has a length of only ~30 µsec and is nearlycompletely necessary for the normal power ramping. But the further a MS is awayfrom the BTS, the longer is the delay time of the signal transmitted from the MS andreceived by the BTS. This occurs to the fact, that the MS, that does not know in whatdistance it is away from the BTS sends a burst, that collides with it successor burstdue to the long propagation or delay time. The strategy to avoid that problem is, thatthe MS shall start transmitting the burst earlier, corrected by the value of the delaytime, for that its signal arrives at the right time at the BTS. Prerequisite for this, is theknowledge about the distance between MS and BTS. Time measurement can only betaken, if a radio link is established. The BTS is responsible to ascertain the delay timeof the MS’s signals and to determine the correction value. This correction value iscalled timing advance, a 6-bit coded decimal value that range from 0 up to 63 anddescribes how many bit periods the signal is delayed. The MS uses this value to setits transmitting time to an earlier value. By this way, the MS is synchronised again toall other MS’s.This delay compensation, called "Timing Advance (TA)" is performed in continuosmode in that way, that every 480 msec a new TA-value is transmitted.

BTSFigure: Delay correction

Up to now, the initiation problem seems unresolved. If the MS wants to access thenetwork, it has neither information about the delay time nor about the distance to theBTS. The MS shall transmit a burst to receive in the succeeding message the timingadvance value to be set. If the MS would use a normal burst, this could stick out overinto the following burst. To eliminate every potential perturbation, the MS uses aspecial burst in this first access or every time, when there is no timing advance valueknown. This special burst, called Access Burst is much shorter then the NormalBurst, its guard period has a length over 63,25 bit periods, much longer then thenormal guard period to avoid all kind of collision with following burst from other MS’s.

Delay correction and guard period in GPRS:

After this short reminder how the timing advance adaptation is done in GSM, it shallbe explained, how this value has to be adapted and transmitted in GPRS. The basicproblem stays the same, we have to correct the delay time on the radio link, causedby the different distances between the MS and the BTS. Main problem additional in apacket switched transmission is, that compared to a circuit switched connection, theMS does not transmit continuously. Between the transfer of two packet data units canpass a time delay. During this, the MS may be moved and changed its location andthe before determined timing advance value is no longer valid. This impedes the BTSto correct this value in a continuos mode, associated to the established radio link.



The procedure used to adapt the timing advance value and to perform the delaycorrection can be structured into two phases:

- Initial Timing Advance

- Continuous Timing Advance Update

Initial Timing Advance:

This delay correction is based on the receipt of a single Access Bursts, contained inthe Channel request message. The basestation must derived the correction value forthe timing advance adaptation and transmit this value in the Packet Downlink/UplinkAssignment–message. This case describes the practised procedure of GSM how todetermine the TA-value. One exception in a special case shall be mentioned. InGPRS a connection establishment without prior paging call is possible. In the case,that the MS is still in Ready State (Mobility Management state of the MS), there is noprior paging call. The network has the possibility to “poll” the MS. This “polling”procedure orders the MS to send 4 successive Access Bursts on the assignedphysical resource, instead of a radio block, consisting of 4 Normal Bursts. By thisway, the BTS can determine the TA value.

Continuous Timing Advance:

This procedure is new in GPRS, it shall resolve the problem to determine the TA-value in a continuous mode, but for the case, that the MS is not transmittingcontinuously. First it shall remind at the 52-Multiframe-Structure. Between thismultiframe, there are two frames, carrying the PTCCH.

52-Multiframe Structure

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

PTCCH

Figure: Position of PTCCH within the 52-Multiframe-Structure



Together with the assignment of a physical resource (Packet Downlink/UplinkAssignment), the MS gets allocated a Timing Advance Index, TAI and a PTCCH. Inthe uplink the MS sends on the allocated PTCCH a single Access Burst, that is usedby the network to determine the Timing Advance value. The calculated TA-value willbe transfered on a Downlink-message on the PTCCH/D.

Figure: PTCCH-Organisation on the 52 Multiframe

The figure above describes the organisation of the PTCCH on the 52-Multiframe inUplink and Downlink. To distribute and structure the PTCCH-messages, 8succeeding 52-Multiframes are considered in a combined mode. All the even Idle-frames in Uplink are forming the PTCCH/U, i.e. in the figure all the frames indicatedwith TAI=0, 2, 4, 6, .. 30. In Downlink the message is send on the PTCCH/D in 4succeeding PTCCH-fames. These frames are indicated with a TA Message in thefigure above. The TA-value to adjust is in that way transmitted in a 4 Burst message.The TA-value shall be transmitted to the MS in the TA-message following the TAI. Toavoid misunderstandings cause to the unequal ratio between PTCCH downlink anduplink messages, the MS is only allowed to send an Access Burst in its assignedPTCCH/U frame, but this is not done in every 8*52-multiframe conglomeration.Finally it shall be remarked, that the actual TA-value can also be transmitted in anormal PACCH-message on the downlink.

Idle Slot Operation on 52 MFM (PTCCH)

B0 B1 B2 0 B3 B4 B5 1 B6 B7 B8 2 B9 B10 B11 3

Uplink TAI = 1TAI = 0

Downlink TA Message 1 TA Message 1

52-multiframe number n:

B0 B1 B2 4 B3 B4 B5 5 B6 B7 B8 6 B9 B10 B11 7



52-multiframe number n+1:

B0 B1 B2 28

B3 B4 B5 29

B6 B7 B8 30

B9 B10 B11 31



52-multiframe number n+7:

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