packet core Interfaces

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Contents Contents 1

1 OOvveerrvviieeww: : IInntteerrffaaccees s & & PPrroottooccoollss 33 1

1..11 SSiiggnnaalliinng g iin n GSGSM M PPhhaassee11//22 44 1

1..22 TTrraannssmmiissssiioon n iin n tthhe e GGSSMM//GGPPRRSS--PPLLMMNN 66 1

1..33 GGPPRRSS TTrraannssmmiissssiioonn PPllaannee 88 1

1..44 GGPPRRS S ((SSiiggnnaalliinng g PPllaannee) ) iin n tthhe e GGPPRRSS 1166 2

2 TThhe e RRaaddiio o IInntteerrffaacce e ((LLaayyeer r 11)) 2211 2

2..11 LLaayyeer r 1 1 oof f tthhe e GGSSMM--//GGPPRRSS--RRaaddiio o IInntteerrffaacce e UUmm 2222 2

2..22 CChhaannnneel l BBuunnddlliinngg, , SShhaarriinng g oof f CChhaannnneellss 2244 2

2..33 CChhaannnneell CCooddiinngg 2266

2

2..44 LLooggiiccaal l GGPPRRS S RaRaddiio o CChhaannnneellss 3300 2

2..55 MMuullttiiffrraammeess iinn GGPPRRSS 3344 3

3 AAccttiivvaattiioon n oof f GGPPRRS S SSeerrvviicceess 3377 3

3..11 MMoobbiilliitty y MMaannaaggeemmeennt t SSttaatteess 4400 3

3..22 PPaacckkeet t DDaatta a PPrroottooccool l PPDDP P SSttaatteess 4422

Interfaces, Protocols, Procedures

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TM2110EU01TM_0003 TM2110EU01TM_0003

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TM2110EU01TM_0003 33

1

1 Overview: Interfaces & Protocols

Overview: Interfaces & Protocols

Overview:

Interfaces & Protocols

GPRS:

Interfaces,

Protocols & Procedures

Fig. 1 Fig. 1

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1.1

1.1 Signaling in GSM Phase1/2

Signaling in GSM Phase1/2

In GSM-PLMN phase 1/2 the Signaling

In GSM-PLMN phase 1/2 the Signaling System No. 7 SS7 is used for System No. 7 SS7 is used for the transmis-the transmis-sion of signaling information between the components of the network switching sion of signaling information between the components of the network switching sub-system NSS (interfaces B-G), as well as between MSC and BSC (A-interface) and in system NSS (interfaces B-G), as well as between MSC and BSC (A-interface) and in direction of the external ISDN networks.

direction of the external ISDN networks. SS7 comprises 4 levels, of which

SS7 comprises 4 levels, of which the lowest 3 layers the lowest 3 layers are combined to form the mes-are combined to form the mes-sage transfer part MTP whereas level

sage transfer part MTP whereas level 4 contains different user parts depending on4 contains different user parts depending on the tasks to be performed. Level 1

the tasks to be performed. Level 1 serves for the physical transmission (physicalserves for the physical transmission (physical layer) of data and f

layer) of data and for the provision of the requested equipment. (e.g. cable or the provision of the requested equipment. (e.g. cable connec- connec-tion, radio relay links,

tion, radio relay links, ...). In the GSM-PLMN PCM30/PCM24 (E1/T1) is used for the...). In the GSM-PLMN PCM30/PCM24 (E1/T1) is used for the realization of level 1

realization of level 1 functions.functions. Level 2 serves for

Level 2 serves for the safe transmission of signaling information (link layer). Its func-the safe transmission of signaling information (link layer). Its func-tions include fault location and clearance across a sub-part of the

tions include fault location and clearance across a sub-part of the transport.transport. Level 3 determines the entire transport link (network layer)

Level 3 determines the entire transport link (network layer) including the transport of including the transport of information in the event of faults in

information in the event of faults in individual signaling points (e.g. overload).individual signaling points (e.g. overload). The Mobile Application Part M

The Mobile Application Part MAP AP is the most importis the most important User Part UP (layer 4). Itant User Part UP (layer 4). It regulates the mobility aspects in the GSM-PLMN between the

regulates the mobility aspects in the GSM-PLMN between the MSCs as well as MSCs as well as be- be-tween MSCs and registers. Its functions include amongst others: updating and tween MSCs and registers. Its functions include amongst others: updating and clear-ance of location information in the VLR, storing of

ance of location information in the VLR, storing of routing information in the HLR, up-routing information in the HLR, up-dating and supplementing of user profiles in

dating and supplementing of user profiles in the HLR&VLR, Inter-MSC handover, ...the HLR&VLR, Inter-MSC handover, ... The ISDN user part ISUP handles the connection-oriented signaling between MSCs The ISDN user part ISUP handles the connection-oriented signaling between MSCs and external networks.

and external networks.

GSM-specific signaling between MSC and BSC

GSM-specific signaling between MSC and BSC is defined in the BSS Applicationis defined in the BSS Application Part BSSAP. The BSSAP is subdivided into the

Part BSSAP. The BSSAP is subdivided into the Direct Transfer Application PartDirect Transfer Application Part DTAP used for the BSC-transparent transport of signaling (call

DTAP used for the BSC-transparent transport of signaling (call control CC and mobil-control CC and mobil-ity management MM) between MS and MSC,

ity management MM) between MS and MSC, and the BSS Management Applicationand the BSS Management Application Part BSSMAP used for radio

Part BSSMAP used for radio resource management RR.resource management RR. The signaling connection control

The signaling connection control part SCCP and transaction capabilities applicationpart SCCP and transaction capabilities application part TCAP are user-neutral user parts which

part TCAP are user-neutral user parts which serve for the support of complex MAPserve for the support of complex MAP applications. SCCP can be used also for

applications. SCCP can be used also for the support of ISUP and BSSAP.the support of ISUP and BSSAP. Layer1 and layer 2

Layer1 and layer 2 tasks (Link access Protocol for D-channel) on the Asub and tasks (Link access Protocol for D-channel) on the Asub and AbisAbis interfaces have been slightly modified as compared to SS7. The radio

interfaces have been slightly modified as compared to SS7. The radio interface Um ininterface Um in the GSM-PLMN is set up of three layers.

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TM2110EU01TM_0003 55

Layer 3 on the

Layer 3 on the Um radio interface is subdivided in Um radio interface is subdivided in three sublayers: radio resourcethree sublayers: radio resource management RR (channel administration, cell selection, power control

management RR (channel administration, cell selection, power control and hando-and hando-ver), mobility management MM

ver), mobility management MM and connection management CM (set-up, operationand connection management CM (set-up, operation and clear-down of

and clear-down of services). The connection management consists of three phases:services). The connection management consists of three phases: call control CC, supplementary services support SS and short message services call control CC, supplementary services support SS and short message services SMS support. SMS support.

Signaling in the GSM-PLMN:

(Phase 1 / 2) (Phase 1 / 2)

H

HL

LR

R

A

AC

C

V

VL

LR

R

E

EIIR

R

ISDN

BTS

MS

OMC-B

MSC

OMC-B

L1 L1 L2 L2 L3 L3 MTP MTP SCCP SCCP TCAP TCAP MAP MAP

MTP

ISUP ISUP SCCP SCCP

MTP

SCCP SCCP BSSAP BSSAP DTAP DTAP BSSMAP BSSMAP L1 L1 LAPDm LAPDm RR RR MM MM CM CM CC SS SMS CC SS SMS

x

LAPDLAPD_L1_L1

Signalling System

No. 7

SS7

RR RR

LAPD(m)

BSC

Fig. 2 Signaling in the GSM-PLMN based on LAPD(m) and SS7 Fig. 2 Signaling in the GSM-PLMN based on LAPD(m) and SS7

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1.2 Transmission in the GSM/GPRS-PLMN

Beside the interfaces in the “classical” GSM PLMN, a number of new interfaces are defined for the implementation of GPRS services based on the introduction of the new network elements SGSN and GGSN.

The interfaces Gi (external PDN-GGSN), Gn (GSN-GSN), Gb (SGSN-BSS) and Gd (SGSN-SMS/IWMSC) serve for the transport of both signaling data and of user data. Interfaces Gp (GSN-GSN in external PLMNs), Gf (SGSN-EIR), Gc (GGSN-HLR), Gs (SGSN-MSC/VLR) and Gr (SGSN-HLR) serve exclusively for the transfer of signaling data.

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TM2110EU01TM_0003 7 MS BSS

_SGSN

_GGSN

PDN TE GGSN other _PLMNs SMS-GMSC SMS-IWMSC MSC/VLR HLR/(GR) SMS-SC EIR

SGSN

Gp Gf Gi Gr Gc Gn Gb Um Gs Gd E C A D

Transmission in

the GPRS-PLMN

_Signalling Signaling & user data Gn Packet switched Also for user data transmission Protocols above Layer 1 !!

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1.3 GPRS Transmission Plane

The transmission plane has a layered protocol structure for the transfer of user infor-mation. It includes the control procedures associated with the information transfer, e.g. flow control, fault detection and fault clearance.

The bird's-eye view on the protocols reveals the intention of this structure:

If the application is internet access for example the GPRS MS (WWW client) and the PDN (WWW server) will exchange IP packets. This is the IP protocol below the appli-cation in the stack of the MS and the IP on top of the stack of the GGSN. The rec-ommendations have defined that X.25 protocol is possible too. In case of IP the MS has to be part of the IP world and needs to be identified by an IP address which can be either temporary or static. This IP address has to remain the same as long as the PDP which is related to this application is active. This is necessary because the PDN is not able to handle the mobility of the subscriber. If the GPRS MS is moving to cell in the service area of another SGSN the GPRS network has to solve the problem by the IP layer on the Gn interface above the L2 layers. In consequence the fact that the GPRS user is a mobile user is not to be seen by the PDN, the user data is tunneled transparently.

The air interface makes it necessary to introduce protocols which adopt the size of the packets. They perform segmentation/re-assembly depending on the direction of the packets to be able to send IP packets via an air interface which consists of bursts which a fixed bit structure.

One of the main advantages of GPRS compared to HSCSD is that it is packet switched. This can only be done by introducing new network elements using new hardware/protocols and by changes in the protocol structure on Um to enable packet switching. The latter is done by the MAC protocol.

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TM2110EU01TM_0003 9

GPRS transmission plane

MAC GSM RF RLC LLC SNDCP IP / X.25 Application MAC GSM RF RLC FR L1bis BSSGP Relay FR L1bis BSSGP LLC SNDCP L1 L2 L2 IP IP UDP / TCP UDP / TCP GTP GTP Relay IP / X.25 MS Um BSS Gb SGSN Gn GGSN Gi L1

SNDCP: SubNetwork Depentent Protocol LLC: Logical Link Control

RLC: Radio Link Control MAC: Medium Access Control

BSSGP: BSS GPRS Protocol FR: Frame Relayl

GTP: GPRS Tunnelling Protocol UDP: User Datagrm Protocol TCP: Transmission Control Protocol IP: Internet Protocol

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Protocol Structure for the interfaces Gi and Gn

The following protocols are needed to pass the user data from the PDN to the SGSN (or vice versa) during GPRS transmission in the GSM-PLMN:

L2‘, L1‘: L2‘ and L1‘ are the link layer and physical layer of the external networks connected via the Gi-interface to the GSM-GPRS-PLMN. As such, L2‘ and L1‘ are situated outside the GPRS definition area. However, there has to be an agreement in terms of these layers functions between the different network operators (GSM-PLMN and PDN) interconnected via the Gi-interface, or between the GSM network operator and a transit network.

GTP (GPRS Tunneling Protocol)

The GTP task is to tunnel user data and user signaling between the GPRS support nodes GSN of the GPRS backbone network. The data packets (protocol data units PDUs) supplied by different packet data protocols PDPs, e.g. X.25 or IP, have to be encapsulated / de-capsulated by the GTP prior to tunneling. GTP is specified in Rec.09.60.

UDP / TCP (User Datagram Protocol / Transmission Control Protocol) : UDP and TCP respectively are used for the transfer of data packets encapsulated by the GTP across the GPRS backbone network. The protocol needed for this is called UDP. It has to be supported by all GSNs as minimum solution since it transports data pack-ets (GTP PDUs) of protocols which require a safe data connection (e.g. IP). UDP also protects transmission against data corruption/mutilation. TCPs have to be sup-ported in the GSNs whenever data packets of protocols have to be transsup-ported, re-quiring safe data connections (e.g. X.25). TCP ensures the flow control and provides protection against loss of data and data corruption.

IP (Internet Protocol): is used in the GPRS backbone network for the routing of user data and network information. At the beginning, the GPRS backbone network can be based on the IP version 4. However, the objective envisaged is IP version 6.

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TM2110EU01TM_0003 11

GGSN

Gn Gi

SGSN PDN

(e.g. X.25, IP)

• En-/De-capsulation PDUs (IP,X.25) • tunneling of user data & signalling

data between GSNs

• transmit encapsulated GTP data packets • protect against data corruption

• UDP / TCP Protocols for unreliable /

reliable data link (z.B. IP / X.25) • UDP: minimum solution for each GSN • TCP includes flow control & data protection

Operator specific L2 Link Layer IP Internet Protocol UDP User Datagram Protocol TCP Transmission Control Protocol GTP GPRS Tunnelling Protocol L1 Physical Layer L2‘ Link Layer L1‘ Physical Layer IP / X.25 Relay GPRS backbone network IP V4 / V6 • extern • arrangement PLMN -PDN necessary

Protocols

via

Gi, Gn

GPRS-transmission plane

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Protocol Structure for the interface(s) Gb (and Um)

SNDCP (SubNetwork Dependent Convergence Protocol):

The SNDCP supports the following functions: compression/segmentation and joining, multiplexing and de-multiplexing of data packets onto one or several LLC SAPs

(service access points). The compression function is applied to the user data of the data packet and (if applicable) to the packet header. Segmentation is required to limit the size of the data packets which is transferred by the LLC as one single unit via the radio interface. The SNDCP is specified in the GSM Rec. 04.65.

LLC (Logical Link Control): The LLC layer realizes a highly reliable ciphered logical connection and thus provides the basis for maintaining communication between the SGSN and the MS. From the point of view of the LLC layer, there is a complete con-nection between SGSN and MS, even if the RLC/MAC do not support a physical connection, i.e. even if no data packets are transferred at that point in time. A physi-cal connection is set-up by the RLC/MAC layer only if the LLC layer supplies the data required for transmission. LLC layer has several access points to be able to transport various types of data; also, it distinguishes between several “quality of service QoS” classes. The LLC layer is also responsible for carrying out the ciphering function in the GPRS network. LLC is specified in GSM Rec. 04.64.

BSSGP (BSS GPRS Protocol): The BSSGP transports the LLC frames as well as routing and QoS-related information between the BSS (PCU) and the SGSN. The BSSGP does not carry out fault correction. It is specified in GSM Rec. 08.18.

FR (Frame Relay): The Network Service (NS) layer transports the BSSGP data packets. NS is based on frame relay, which thus represents the link layer protocol for the connection between SGSN and PCU (Gb interface). NS is specified in GSM Rec. 08.16.

L1bis: Physical Layer of the Gb-interface. L1bis is realized through E1/T1 (PCM30/PCM24)technology.

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TM2110EU01TM_0003 13 RLC MAC SNDCP SubNetwork Dependent Convergence Protocol

SGSN

Gb _Gn • transmit LLC frames • & Routing & QoS - Infos • no error correction

Unreliable transport BSSGP PDUs FR Frame Relay IP UDP / TCP GTP L1bis Physical Layer L2 L1 Relay BSSGP BSS GPRS Protocol LLC Logical Link Control

BSS (PCU)

L1bis Physical Layer FR Frame Relay GSM RF BSSGP BSS GPRS Protocol Relay Um E1 / T1 (PCM30/24)

• logical connection (even

without physical connection)

• different SAPs (SNDCP,

GMM/SM, SMS), QoS,..

• Ciphering • Compression

(user data + maybe header)

• Segmentation / Re-assembly • Multiplexing / De-Multiplexing different PDPs

Protocols

via

Gb, (Um)

GPRS-transmission plane

SAP: Service Access Point

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Protocol Structure for Um

RLC (Radio Link Control) / MAC (Medium Access Control) : RLC and MAC are the layers used for the implementation of a reliable physical connection via the radio in-terface on which data packets are transported. RLC and MAC are closely associated with each other and are defined in GSM Rec. 04.60.

RLC (Radio Link Control): The RLC function supplies a reliable connection (pro-vides BEC) via the radio interface. The physical connection depends on how the ra-dio transmission is realized in each case (L1-dependency). RLC segments LLC frames and re-assembles them respectively. In addition, the RLC carries out sub-multiplexing in order to place more than one MS on a physical channel and to bundle up to 8 physical channels for one MS.

MAC (Medium Access Control): The MAC function controls the signaling dures via Um which are required to obtain network access (access signaling proce-dures), e.g. request and grant of radio resources (packet data channel PDCH). Fur-thermore, the MAC function controls the mapping of LLC frames to the physical channels of the radio interface. The identifiers (TFI "Temporary Flow Identifier, USF "Uplink State Flag") which are used by the MAC protocol enable the sharing of physi-cal channels by several MSs. Different mechanisms of allocation of radio resources may be used, dynamic or fixed allocation (to be explained in the next chapter).

GSM RF (Radio Frequency): GSM RF is the physical channel used to transfer packet data via the GSM radio interface Um.

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TM2110EU01TM_0003 15 RLC Radio Link Control MAC Medium Access Control SNDCP SGSN Gb LLC

BSS (PCU)

L1bis FR BSSGP Relay • Segmentation / Re-assembling

LLC-frames RLC radio blocks

• Backward Error Correction BEC

Protocols

via

Um

GPRS transmissions plane GSM RF Um

MS

RLC Radio Link Control MAC Medium Access Control GSM RF IP / X.25 Application GGSN TE

• Access Signalling Procedures

(Requests, Grants)

• Sub-Multiplexing:

different MSs 1 physical channel

channel combining 1 MS (1..8 TS) RLC/MAC: enable reliable physical connection via Um Physical RF-channel

for packet data transmission

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1.4 GPRS (Signaling Plane) in the GPRS

The signaling plane consists of protocols for the control and support of transmission plane functions:

Control of GPRS network access, e.g. „attaching“ and „detaching“

Control of the data elements (attributes) of an established network connection and activation of the packet data protocol PDP (e.g. X.25 / IP) addresses.

Control of the routing path of an established connection in terms of subscriber mobil-ity support.

Support of the network resource allocation to account for various user requests. Supplementary services implementation

Signaling Plane MS-SGSN:

In addition to the protocols of the transmission plane a further plane, based on the functions GSM FR, RLC/MAC and LLC, is required:

GMM/SM (GPRS Mobility Management and Session Management): The GMM/SM protocol supports mobility management functions such as GPRS attach, GPRS de-tach, safeguarding functions, routing area & location update), and session manage-ment functions as PDP context activation & deactivation & modification.

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TM2110EU01TM_0003 17 MAC GSM RF RLC LLC GMM/SM GPRS Mobility Management & Session Management MAC GSM RF RLC FR L1bis BSSGP Relay FR L1bis BSSGP LLC GMM/SM GPRS Mobility Management & Session Management MS Um BSS Gb SGSN

GPRS signaling plane

MS-SGSN

Mobility Management functions • GPRS attach / detach

• security functions

• Update Location (CGI, RAI)

• PDP context (de-) activation / modification

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Signaling SGSN - HLR / EIR / SMS-GMSC, GGSN - HLR:

For signaling via Gr-, Gf-, Gd- and Gc-interface, i.e. between SGSN and HLR, EIR, SMS-GSMC and between GGSN and HLR the same protocols of Signaling System No. 7 (SS7) are used as in the NSS of GSM-PLMN (Phase1/2). The realization of the Message Transfer Parts MTP (L1 – L3), of the Signaling Connection Control Part SCCP as well as of the Transaction Capabilities Application Parts TCAP are identi-cal.

MAP (Mobile Application Part): The MAP used in GSM (Phase1/2) needs to be panded by mobility management functions particularly in view of the information ex-change between SGSN and GGSN and between SGSN and HLR respectively (GSM Rec. 09.02.)

The information flow between GGSN and HLR can also flow across further GSNs and is tunneled in this case by using the GPRS tunneling protocol GTP between the GSNs (Gn-interface).

Signaling plane SGSN – MSC/VLR

Signaling via the Gs interface, i.e. between SGSN and MSC/VLR, uses the same protocols of the SS7 as the ones used via the A-interface of the GSM-PLMN (GSM Rec. 09.16).

BSSAP+ (BSS Application Part+): Signaling is performed via a subset of the BSSAP functions used on the A-interface (GSM Rec. 09.18).

Signaling plane GSN-GSN:

The exchange of signaling information between the different GPRS Support Nodes GSN (Gn-interface), i.e. via the IP-based GPRS backbone uses the corresponding transmission plane protocols: L1, L2 (operator-specific), IP (V4, later V6), UDP (User Datagram Protocol) and GTP (GPRS Tunneling Protocol). The GTP tunnels both user and signaling data between the various SGSN and GGSN.

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TM2110EU01TM_0003 19 L1 SCCP Signalling Connection Control Part TCAP Transaction Capabilities Application Part MAP Mobile Application Part MTP L2 MTP L3 L1 SCCP TCAP MAP Mobile Application Part MTP L2 MTP L3 L1 SCCP MTP L2 MTP L3 L1 SCCP MTP L2 MTP L3 BSSAP+ BSS Application Part + BSSAP+ BSS Application Part + SGSN Gr,f,d HLR, EIR, SMS-GMSC

GPRS Signaling plane

SGSN Gs MSC/VLR GGSN Gc HLR

MAP enhanced for GPRS Subset of BSSAP functions

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TM2110EU01TM_0003 21

2 The Radio Interface (Layer 1)

The Radio Interface Um

(Layer 1)

GPRS:

Interfaces,

Protocols & Procedures

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2.1 Layer 1 of the GSM-/GPRS-Radio Interface Um

By introducing GPRS services into the GSM-PLMN, world-wide modifications are necessary also in the area of physical transmission (layer1) via the air or radio inter-face Um. The tasks of layer 1 radio interinter-face relate to the transmission of user and signaling data as well as to the measuring of receiver performance, cell selection, determination and updating of the delayed MS transmission (timing advance TA), power control PC and channel coding.

In the GPRS, a decisive difference to the realization of the connection-oriented serv-ices (circuit-switched servserv-ices) relates to the fact that a physical channel and a so-called packet data channel can be used by several mobile stations at the same time. One packet data channel is allocated per radio block, i.e. for four consecutive TDMA frames and not for a specific time interval. This means that signaling and the packet data traffic of several mobile stations can be statistically multiplexed into one packet data channel. Furthermore, the packet data channel can be seized asymmetrically. On the other hand it is also possible for a mobile station to use more than one packet data channel at the same time, i.e. to combine several physical channels of one radio carrier. In principle, up to 8 packet data channels can be seized simultaneously. The number of channels that are combined for reception (DL) and transmission (UL) can be different to achieve asymmetric data rates for certain applications (e.g. file transfer protocol FTP, internet surfing).

The assignment of radio resources can be done dynamically or in a fixed allocation. In case of the fixed allocation a message with a bit pattern is sent downlink to indi-cate which channels can be used by this MS for UL transmission.

If dynamic allocation is applied the MS will be receive a temporary flow identifier (TFI) and an uplink state flag (USF) for each of the time slots it is allowed to use. The TFI is part of the control information in the DL packet and identifies the "owner" of the packet. Each packet also includes an USF that indicates which of the MSs (that has been assigned to use this time slot UL) is allowed to transmit the next radio block UL.

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TM2110EU01TM_0003 23

GSM RF:

GPRS Layer 1 (Um)

L1-tasks Transmission of user & signaling data determinate & actualise Timing Advance Cell Selection Measure signal strength Power Control

functions _{1 physical channel to be used}Resource optimisation: by many MSs simultaneously !!

asymmetrical traffic UL / DL possible !! High data rate traffic

up to 171.2 kbit/s:

combining 1..8 PDCH for 1 MS !! Allocation of physical channel

(Packet Data Channel PDCH) dynamically: 1 or 4 Radio Blocks

(1 Radio Block = 4 Normal Burst in 4 consecutive TDMA-frames)

  

 User & signalling data of several MSs

statistically to be multiplexed into 1 PDCH (also fixed allocation possible)

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2.2 Channel Bundling, Sharing of Channels

Sharing of Resources in a Cell: GSM circuit switched (CS) users will share the time slots in a BTS with the GPRS packet switched (PS)users. A physical channel can ei-ther be used for GSM CS or GPRS PS traffic but not for both at the same time. De-pending on the traffic load in the cell there will be more or less channels available for GPRS, CS connections are dealt with priority.

Sharing of Physical Channels: It is a characteristic of a CS connection that the physical resource (the time slot) is reserved for one subscriber. Therefore the

GSM CS users cannot share their channels with others. In contrast GPRS PS sub-scribers can share physical channels. The handling of the channels, the multiplexing of subscribers onto the same time slots is done by software (protocol, MAC) and hardware (PCU). Packet oriented connections are not only carried out through the core network by usage of an appropriate hardware (ATM switches) and software (protocols) but also on the air interface. This is an important feature of GPRS with re-gard to an optimized usage of resources on Um which is the limiting bottleneck in the PLMN.

Multislot Class: The subscribers for GPRS will have different needs (applications, data rates) and therefore the MS will have more or less capabilities. The network (PCU) will have to identify these different MSs by their multislot class which indicates how many time slots (channels) can be bundled by the MS uplink and downlink. A cheap GPRS mobile will be a GSM mobile that is able to handle the protocols and coding schemes of GPRS. This will be multislot class 1: one time slot UL and one time slot downlink can be "bundled". The other extreme is multislot class 29 which will be able to receive and to transmit in eight time slots UL and DL simultaneously. In

consequence such a MS has to have two synthesizers, and a high battery capacity because this is more or less continuous transmission and reception. The MS will

send its multislot class and the PCU will only assign time slot combinations which can be handled by this equipment.

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TM2110EU01TM_0003 25

Channel Bundling, Sharing of Channels

e.g. BTS with 2 RFCs => 2 TRX = 16 channels

RFC 1 RFC 2 UL RFC 1 RFC 2 DL _{sharing of physical} channels => packet switching on Um

TDMA frame = 8 time slots

multislot classes • 29 classes defined • assignment of channels by PCU according to capabilities of MS • identifying “high

end“ and “low cost“ MS e.g. 6 GSM CS users e.g. 4 GPRS PS users Signaling: BCCH, SDCCHs,... time slot

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2.3 Channel Coding

Channel coding was modified substantially for GPRS purposes (GSM Rec. 03.64). Channel coding starts with the division of digital information into transferable blocks. These radio blocks, i.e. the data to be transferred (prior to encoding) comprise:

 a header for the Medium Access Control MAC (MAC Header)

 signaling information (RLC/MAC Signaling Block) or user information (RLC Data

Block) and

 a Block Check Sequence BCS.

The functional blocks (radio blocks) are protected in the framework of convolutional coding against loss of data. Usually, this means inserting redundancy.

Furthermore, channel coding includes a process of interleaving, i.e. different ar-rangement in time. The convolutional radio blocks are interleaved to a specific num-ber of bursts/burst blocks. In the case of GPRS, interleaving is carried out across four normal bursts NB in consecutive TDMA frames and, respectively, to 8 burst blocks with 57 bit each.

Four new coding schemes were introduced for GPRS (Rec. 03.64): CS-1 to CS-4. These can be used alternatively depending on the information to be transferred and on the radio interface’s quality.

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TM2110EU01TM_0003 27

Channel Coding

collect user data signaling RLC Data Block BCS MAC Header

RLC/MAC Control Block BCS MAC Header Convolutional coding (not CS-4) Radio Block Radio Block

BCS: Block Code Sequence (for error recognition)

Radio Block

(Redundancy !) rate 1/2 convolutional coding

Radio Block (456 Bits) puncturing

Puncturing (only CS-2, CS-3)

4 new Coding Schemes:

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

MAC: Medium Access Control RLC: Radio Link Control

Interleaving 57 Bit 8

Burst-blocks 57 Bit 57 Bit

_•••

57 Bit 57 Bit

Um: Allocation of PDCH for 1 / 4 Radio Blocks = 4 / 16 Normal Bursts

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Coding Schemes:

CS-1: CS-1 uses the same coding scheme as specified by Rec. 05.03 for the

SDCCH. It comprises a half rate convolutional code for FEC forward error correction. CS-1 corresponds to a data rate of 9.05 kbit/s.

CS-2 and CS-3 are punctured version of the same half rate convolutional code as CS-1. The coded bits are numbered starting from 0 and certain punctured bits are removed.

CS-2: With CS-2 the punctured bits have numbers 4  i + 3 with i = 3,...,146

(excep-tion: i = 9, 21, 33, 45, 57, 69, 81, 93, 105, 117, 129, 141). This means that none of the first 12 bits is punctured. CS-2 corresponds to a data rate of 13.4 kbit/s. Remark: For CS-2 the puncturing pattern must be adapted to the future new TRAU frame for-mat in order to be used via the Abis interface (e.g. more bits must be punctured to make space for RLC signaling).

CS-3: With CS-3 the punctured bits have numbers 6  i + 3 and 6  i + 5 with i =

2,...,111. CS-3 correspond to a data rate of 15.6 kbit/s.

CS-4: CS-4 has no redundancy (no FEC) and corresponds to a data rate of 21.4 kbit/s.

By bundling up to 8 packet data channels of one carrier into one MS, transmission rates up to 171.2 kbit/s are possible.

(30)

TM2110EU01TM_0003 29

9,05 kbit/s 13,4 kbit/s 15,6 kbit/s 21,4 kbit/s

CS-1

CS-2

CS-3

CS-4

different redundancy (FEC)     Quality Um Coding Scheme Code Rate Radio Block* Coded Bits Punctured Bits Data Rate kbit/s CS-1 1 / 2 181 456 0 9,05 CS-2  2 / 3 268 588 132 13,4 CS-3  3 / 4 312 676 220 15,6 CS-4 1 428 456 0 21,4 common

coding & interleaving for 4 Normal Bursts: 456 Bit coded user

data

bundling 1..8 TS

max. 171,2 kbit/s

Channel Coding: Coding Schemes

* Radio Block without Uplink State Flag USF & Block Check Sequence BCS

(31)

2.4 Logical GPRS Radio Channels

Use of "classical" logical channels for GSM-CS

A Logical channel is used for a special purpose/contents. For example the MSs have to find out if this cell is a suitable one (operated by the "right" network operator),

which features are offered (e.g. HR/FR/EFR, GPRS, ...), what is the structure of Um (channel combination), ... This is provided by the BCCH which is naturally only

transmitted in the downlink. Some resources have to be given for initial access for the MS (RACH). For these reasons logical channels have been defined to fulfil all tasks which are necessary in a GSM network on the air interface (see figure 13).

The GPRS subscribers will share the air interface with the circuit switched users. On the other hand the protocol structure of GPRS is different from "classical" GSM-CS. Therefore the user traffic and (part of) the signaling will have to be separated. Before this separation can take place the different MS (GPRS/non-GPRS) have to be han-dled by signaling procedures for access (channel assignment. There are two solution of this problem. The first one is to use (some of) the logical channels for GSM-CS: The GPRS-MS detects the BCCH of this particular cell and looks for the system in-formation to find out if GPRS is available. If this is a cell belonging to the same rout-ing area the MS can choose this cell and wait for pagrout-ing or for the user to use the RACH for activating a PDP. In case that the user wants to run an PS application the GPRS MS will use an access burst (RACH) which indicates that this is a GPRS MS and the request will be answered by the PCU assigning resources for packet

switched traffic (time slots reserved for GPRS). Signaling (e.g. for authentication) will then take place using these resources indicated by the message in the AGCH.

So GPRS uses some of the logical channels of GSM-CS. On one hand this can be an advantage if the resources are sufficient. On the other hand if in the f uture more and more GPRS traffic has to be handled, separate logical channels reserved for GPRS MS will have to be given. This is the second solution. In any case the GPRS MS will have to look for the BCCH of the cell to find out if GPRS is available. If the second solution has been chosen the GPRS MS will also read information where a PBCCH (Packet Broadcast Control Channel) is to be found (which time slot). This second solution will be explained in figure 14.

(32)

TM2110EU01TM_0003 31

Allocation of dedicated signaling channel

Dedicated signaling MS BTSE (Call

Setup, LUP, Security, SMS, CBCH,...) Signaling Traffic User Data DL DL UL UL + DL DL UL + BCCH FCCH SCH PCH AGCH RACH SDCCH SACCH FACCH TCH/F TCH/H

CGI, FR/EFR/HR, GPRS available

frequency hopping, channel combination,...) frequency synchronization

Time synchronization + BSIC, TDMA-No. Paging / Searching (MTC)

Request for access

Measurement Report, TA, PC, cell parameters,...

Signaling instead of TCH BCH

CCCH

DCCH

User traffic (Full Rate)

User traffic (Half Rate)

Logical channel

(for GSM Circuit Switched)

Synchronisation Channel Frequency Correction Channel

Access Grant Channel

Random Access Channel Paging Channel Broadcast Control Channel

Stand Alone Dedicated Control Channel Broadcast Channel Slow Associated Control Channel Fast Associated Control Channel Traffic Channe/Fl Traffic Channel/H Dedicated Control Channel

Common Control Channel

(33)

Use of new logical channels for GPRS

In addition to the nine existing logical radio channels used for signaling (BCCH, SCH, FCCH, PCH, RACH, AGCH as well as SDCCH, SACCH and FACCH) and the Traffic Channel (TCH) for circuit switched user information, a new set of logical channels was defined for GPRS.

Packet traffic is realized by means of the Packet Traffic Channel (PTCH) which in-cludes the following :

Packet Data Traffic Channel PDTCH.

Packet Associated Control Channel PACCH

The PDTCH is temporarily assigned to the mobile stations MS. Via the PDTCH, user data (point-to-point or point-to-multipoint) or GPRS mobility management and session management GMM/SM information is transmitted.

The PACCH was defined for the transmission of signaling (low level signaling) to a dedicated GPRS-MS. It carries information relating to data confirmation, resource al-location and exchange of power control information.

New GPRS signaling channels are mainly specified analogously to GSM Phase1/2. The Packet Common Control Channel PCCCH has been newly defined. It consists of a set of logical channels which are used for common control signaling to start the connection set-up:

Packet Random Access Channel PRACH Packet Paging Channel PPCH

Packet Access Grant Channel PAGCH Packet Notification Channel PNCH

PRACH and PAGCH fulfil GPRS-MS functions which are analogue to the “classical” logical channels RACH and AGCH for non-GPRS-users. The PNCH is used for the initiation of point-to-multipoint multicast (PtM multicast).

For the transmission of system information to the GPRS mobile stations, the

Packet Broadcast Control Channel PBCCH was defined analogue to the “classical” BCCH.

(34)

TM2110EU01TM_0003 33 Packet-Signaling Packet Traffic Channel PTCH DL DL UL UL & DL PDTCH Packet Data Traffic Channel PACCH Packet Associated Control Channel System information for GPRS-MS Packet Common Control Channels PCCCH

New logical channel

for GPRS

PBCCH Packet Broadcast Control Channel PPCH Packet Paging Channel PAGCH Packet Access Grant Channel PNCH Packet Notification Channel PRACH Packet Random Access Channel Paging GPRS-MS Resource Allocation for Setup of Packet-Transfer

Notification for GPRS-MS in

PtM Multicast access request for UL packet data transmission Dedicated signaling MS-network

e.g.: Acknowledgements, Power Control, Resource

(Re-)Assignment

Transmission of user packet data; Multislot operation:

1 MS - many PDTCH simultaneously

(35)

2.5 Multiframes in GPRS

The GPRS packet data traffic is arranged in 52-type multiframes (GSM Rec. 03.64). 52 TDMA frames in each case are combined to form one GPRS traffic channel multi-frame which is subdivided into 12 blocks with 4 TDMA multi-frames each. One block

(B0-B11) contains one radio block each (4 normal bursts, which are related to each other by means of convolutional coding). Every thirteenth TDMA frame is idle. The idles frames are used by the MS to be able to determine the various base station identity codes BSIC, to carry out timing advance updates procedures or interference measurements for the realization of power control.

For packet common control channels PCCH, conventional 51-type multiframes can be used for signaling or 52-type multiframes. The GPRS users can use "classical" common control channels of GSM before they will be directed onto their PTCHs. The BCCH will be read by all mobiles anyway. Either in case of GSM mobiles to fulfil the same tasks as before and for GPRS equipment this logical channel will indicate weather GPRS service is available and if extra logical channels (PBCCH, PPCH, ...)

are used.

GSM CS traffic and GPRS subscribers are clearly separated so that there is no con-flict due to different signaling or multiframe structure.

It is important that there are no "visible" changes for "GSM only mobiles" due to the introduction of GPRS. GSM CS connections will use for example the same 26 multi-frame structure for TCH an the 51 multimulti-frame structure for signaling.

(36)

TM2110EU01TM_0003 35

i

B0 B1 B2

B3 B4 B5

i

B6 B7 B8

i

B9 B10 B11

i

52 TDMA Frames = PDCH Multiframe

4 Frames _{1 Frame}

New multiframe

for GPRS

• PDCH follows 52 multiframe structure

• 52 Multiframe: 12 Blocks à 4 TDMA-frames • PCCCHs: „classical“ 51er Multiframes

or 52er Multiframes

B0 - B11 = Radio Blocks (Data / Signaling) i = Idle frame

• BCCH indicates PDCH with PBCCH (in B0) • DL: this PDCH bears PDCCH & PBCCH

PBCCH in B0 (+ max. 3 further blocks; indicated in B0)

PBCCH indicates PCCCH blocks & further PDCHs with PCCCH

• UL: PDCH with PCCCH: all blocks to be used for PRACH, PDTCH, PACCH PDCH without PCCCH: PDTCH & PACCH only

Idle frame:

• Identification of BSICs

• Timing Advance Update Procedure • Interference measurements

for Power Control

(37)

(38)

TM2110EU01TM_0003 37

3 Activation of GPRS Services

Activation of

GPRS services

GPRS:

Interfaces,

Protocols & Procedures

(39)

States of the GPRS services

With regard to point-to-point PtP packet data transmission the GPRS service oper-ates in two independent state models/circles. One circle describes the mobility man-agement behavior whereas the other is assigned to the activation of a packet data protocol PDP.

The circle related to mobility management states in the MS and the associated SGSN consist of the :

 "Idle" state  "Standby" state  "Ready" state

The circle related to a specific packet data protocol has the:

 "Inactive" state  "Active" state

(40)

TM2110EU01TM_0003 39 Packet Data Protocol PDP

States of

GPRS services

2 circles regarding: Inactive State Active State Idle State Ready State Standby State Mobility Management

(41)

3.1 Mobility Management States

"Idle" state

A mobile station MS in the idle state is detached from the GPRS. Only GPRS sub-scription data is available in the HLR. No further information exists in other network units such as SGSN and GGSN. It is not possible to activate a packet data protocol PDP or to maintain a PDP in its active state. The GPRS MS must monitor the BCCH to determine the availability of cells which support GPRS services. Accordingly, the GPRS MS can carry out PLMN and cell selection procedures. To exit idle state, the MS must execute the “attach” procedure. Upon successful completion of this proce-dure, the MS changes to ready state.

"Standby" state

In the standby state the GPRS MS is attached to the GPRS network. The GPRS and the SGSN have a mobility management context comparable to the circuit switched connections. The MS monitors the broadcast channel to determine the availability of cells offering GPRS services and also the paging channel PCH, to be informed about paging requests. The SGSN recognizes/stores the routing area RA of the GPRS-MS. The routing area is a sub-unit of the location area LA, in other words a more detailed determination of the GPRS-MS location. The GPRS-MS informs the SGSN about changes of the routing area and answers paging requests.

"Ready" state

In the ready state, the SGSN detects the current cell of the GPRS-MS beyond the routing area RA of the GPRS-MS. If the GPRS-MS changes cells, it informs the SGSN. Paging is thus superfluous in the ready state. The DL packet data transfer can be performed any time. Ready state does not mean that a physical connection is established between SGSN and MS. Only in the ready state, SGSN and MS can transfer data packets. MS and SGSN exit ready state upon expiry of a ready timer or in case of a faulty packet data transmission and change to standby state. Upon log-off, i.e. execution of a detach procedure, MS and SGSN exit ready state and change to idle state.

(42)

TM2110EU01TM_0003 41

Mobility Management

States

IDLE

state

READY

state

STANDBY

state

GPRS detach expire STANDBY Timer

expire READY Timer / Transmission errors GPRS attach SGSN: Paging / MS: initiates Transfer • SGSN & GGSN without MS information

• only HLR contains subscription data • no PDP context can be activated

• MS observes BCCH • PLMN- & Cell Selection

• SGSN knows Routing Area & cell !! • UL & DL packet transmission possible

• SGSN_{MS: MM-Context}

• SGSN knows Routing Area

• MS observes BCCH, PCH • initiates RA-Update • reacts to Paging Request

• MS initiates Cell Update

(43)

3.2 Packet Data Protocol PDP States

There are separate state circles for every authorized PDP of a GPRS-MS

"Inactive" State

The inactive state of a PDP means that this PDP is not operating at that moment. There is no routing context in the MS, SGSN and GGSN. A transition in the active state is only possible if there is a mobility management connection and if MS and SGSN are in the standby or ready state.

No data transfer is possible in the inactive state. Data packets which reach the GPRS network are either rejected or ignored.

"Active" State

In the active state the MS, GGSN and SGSN are in a routing context. Data can be transmitted or received by the MS. The active state is ended explicitly if the MS de-activates a certain PDP. With GPRS detach and expiry of the standby timer, all the activated PDP are deactivated, too.

(44)

TM2110EU01TM_0003 43

PDP States

INACTIVE

state

ACTIVE

state

De-activation PDP context / GPRS detach

expire STANDBY timer

Activation PDP context • PDP not activated

• no Routing-context for MS, SGSN & GGSN

• no data transmission possible !

Transition to „Active“ State only if MM-context exists ( MS & SGSN: STANDBY / READY)

• Routing context

for MS, SGSN & GGSN

• Data transmission possible !

(45)

3.3 GPRS Packet Data Transmission

The transmission of GPRS packet data presupposes the execution of

 GPRS Attach Procedure as well as of the  PDP Context Activation Procedure.

In the case of a mobile packet data transfer, a one or two phase packet access is added. This access procedure is necessary for packet data transfer.

Common Mobility Management / MS-Location

To reduce the signaling load via the radio interface during GPRS and non-GPRS op-eration, important mobility management MM procedures are carried out jointly (com-mon MM). This regards the procedures for : attachment / detachment, location & routing area update and paging.

The result of a GPRS routing area update procedure is stored in the SGSN. The routing area represents a more exact indication of the MS location, than is actually needed for non-GPRS services. Triggered by the MS ( in the framework of a RA up-date) the SGSN informs the MSC/VLR via the Gs interface of a change in the loca-tion areas which has taken place simultaneously.

Further mobility management procedures are also executed via GPRS procedures. If possible , all messages containing mobility management information, are transferred through signaling data packets. The MM procedures are defined in the GGM/SM (GPRS Mobility Management & Session Management).

(46)

TM2110EU01TM_0003 45

GPRS Packet data transfer

GPRS Attach Procedure

PDP Context Activation Procedure

One / Two Phase Packet Access

RLC Data Transmission

Common Mobility Management

MS:

GPRS & Non-GPRS

operation

Þ Reduce Signaling load via Um Attachment

Detachment Location Update Routing Area Update

CS-Paging GPRS Procedures MS, « SGSN (GMM/SM) adjust with HLR, MSC/VLR

(47)

3.4 Combined GPRS & IMSI Attach

A first prerequisite for the GPRS data transfer to be carried out is the registration of the MS in the GPRS network, i.e. the execution of an attach procedure. If an MS is designed for common GPRS & non-GPRS operation, a common procedure for IMSI and GPRS attach is carried out.

Attach Request (1)

Start of the combined attach procedure. In the attach request transmission the MS indicates, which attach (only GPRS or also IMSI) is requested. The MS indicates its identities (IMSI or packet TMSI: P-TMSI) and also the routing area identity RAI. Identification Request / Response (2)

This is needed in case of SGSN change. The old SGSN (determined by the new SGSN via P-TMSI) hands over to the new SGSN the IMSI as well as existing authen-tication triples.

Security procedures (3)

The attach procedure can also contain security functions. The SGSN may request an authentication of the MS and then initiate ciphering. Furthermore, the MS equipment number in the EIR can be checked, too (IMEI check).

Update Location / Cancel Location & Ack / Insert Subscriber Data & Ack / Up-date Location Ack (4)

In the event of SGSN change or first attach, routing area update procedures are performed. The HLR is updated, the old SGSN released. The HLR delivers the GPRS subscriber profile to the new SGSN which stores this profile for future PDP context activation. The SGSN can then establish a mobility management context for the MS.

Location Updating Request & Accept (5)

The IMSI attach is effected via SGSN in the framework of a combined routing area RA /location area LA procedure (location updating request / accept). The SGSN ne-gotiates the IMSI attach procedure with the MSC/VLR. From this, MSC/VLR derive that the MS is also GPRS-attached and marks the MS accordingly.

(48)

TM2110EU01TM_0003 47 MS new _SGSN old _SGSN _MSC/VLR _HLR Attach Request Identification Request Identification Response Security functions (if necessary)

Update Location (SGSN-Id.) Cancel Location

Cancel Location Acknowledge Insert Subscriber Data

Insert Subscriber Data Acknowledge Update Location Acknowledge Location Updating Request

Location Updating Accept

normal LUP with HLR, maybe with MSC/VLR change

Attach Accept Attach Complete

only if P-TMSI re-allocation

TMSI Reallocation Complete

Common GPRS &

IMSI Attach

Signaling message transferred by conventional signaling channel

by Packet Data Channel PDCH only for TMSI Reallocation

MS & SGSN: Ready State 1 2 3 4 5 6

(GPRS/IMSI, P-TMSI, RAI,..)

(49)

3.5 PDP Context Activation Procedure

For packet data to be transferred, an attach procedure must be followed by a PDP context activation.

Activate PDP Context Request (1)

Normally, the mobile station starts a request to the network in order to activate the desired PDP context (PDP type, e.g. Internet Protocol, PDP address) with the needed Quality of Service QoS.

This request can also be started by a network and is then called network request. Security functions (2)

Here, too, authentication / ciphering / IMEI can be carried out. Create PDP Context Request & Response (3)

The SGSN checks the authorization of the data delivered in the activate PDP context request, i.e. their agreement with the subscription data stored in the SGSN. The QoS requests can be limited (network capacity, current load state). The routing context is activated in the GGSN. Hereby, it is possible to transmit (tunneled) packet data be-tween SGSN and GGSN.

Activate PDP Context Accept (4)

The complete routing context from MS until GGSN is established, activation is com-pleted successfully. Packet data can now be transmitted.

Network Requested PDP Context Activation Procedure PDP context activation can also be initiated by the network.

(A) If a new PDP Packet Data Unit PDU reaches the GGSN, the latter checks whether a PDP context is activated.

(B) If there is no PDP context, the GGSN needs routing information (IMSI, SGSN address, Mobile Station Not Reachable Reason) from the HLR.

(50)

TM2110EU01TM_0003 49

MS SGSN GGSN

Activate PDP Context Request

Security functions (if necessary)

Create PDP Context Request

Activate PDP Context Accept

PDP Context Activation

1 2

3

4

Create PDP Context Response

HLR

PDP PDU Send Routing

Info for GPRS

Send Routing Info for GPRS Ack(SGSN Address, IMSI, MS Not Reachable Reason)

PDU Notification Request PDU Notification Response Request PDP Context Activation Network Requested Normal / MS Requested A B C D

(51)

3.6 Start of Mobile Originated Packet Transfer

An MS initiates a packet data transfer in a „One or Two Phase Packet Access“ method. If the GPRS subscriber uses a RACH to send a channel request only 8 bit are usable to indicate which service is requested. In case of a random access carried out on a PDCH 11 bit are reserved for the identification of the request. This speeds up the call set-up by avoiding two phase packet accesses.

One Phase Packet Access

(1) Packet Channel Request: this serves as a starting point of the packet data trans-fer in an activated MS and is realized via random access channel RACH or via

packet RACH. The message contains a brief information about the resources needed for the transfer.

(2) Packet Immediate Assignment: is used for allocation of UL resources to one / several packet data channel(s) PDCH(s) for a number of radio blocks. The packet immediate assignment message is realized via an access grant channel AGCH or packet AGCH (PAGCH). The timing advance derived from the received PRACH and RACH respectively and an information about the MS power control are communi-cated to the MS as well.

Two Phase Packet Access

If the allocated resources do not have the requested quality of service, (e.g. only 1 time slot allocated), the MS can end the one phase access and initiate a two phase access. Upon (1) and (2) the MS sends to the network message a complete descrip-tion of the resources needed for the UL transfer with the

(3) packet resource request.

(4) packet resource assignment: The network confirms the resource request. The message contains information about the resources allocated in each case. (3) and (4) are realized via the packet associated control channels PACCH.

(52)

real-TM2110EU01TM_0003 51

MS Network

Start Packet data transfer

1

2

Packet Channel Request

Packet Immediate Assignment (TA, PC)

3 4

Packet Resource Request

Packet Resource Assignment

One Phase Packet Access Two Phase Packet Access (optional) RLC Data Transfer 4 RLC block RLC block    PRACH or RACH PAGCH or AGCH PACCH PACCH PDTCH