Subscriber information

When GPRS messages are sent, there is a need to give information about the GPRS user and other parameters within the message. Information about a GPRS user in the different network elements falls into four categories:

• Identity: How do we identify subscribers?For example, IMSI, TMSI, P-TMSI, TLLI, and IP address.

• Location: How do we identify the location of a subscriber?For example, location area, routing area, SGSN serving subscriber, and serving MSC.

• Services: What sort of services is the subscriber allowed to access and where will this information be maintained in the network?

• Authentication data: What algorithms and keys are used to authenticate the subscriber? What encryption techniques are used for data transfer between SGSN and MS? Where is this information held?

Subscriber information, information elements, and location

Type of info What info Where

Identity IMSITMSIIP address

SIM, HLR, SGSNSGSN, MS

MS, SGSN, GGSN Location VLR address

Location areaServing SGSN

Routing area

HLRSGSNHLR

SGSN Services Basic services, supplementary

servicesGPRS service information

HLRSGSN

Authentication data

Ki, algorithms

Triplets SIM

SGSN

GPRS attach and detach

• (1) The MS requests GPRS attach.

• The MS is not known in the PLMN (for example first time registration), so:

• (2a) The SGSN requests subscriber identity.

• (2b) The MS sends its IMSI.RA-1

HLR

SGSN-1

BSC

LA-1

1

2a 2b

GPRS attach, first time in PLMN: request for GPRS attach

The SGSN authenticates the subscriber

• There are no valid authentication triplets for the new subscriber in the SGSN; so the following steps occur next:

• (3a) SGSN requests triplets from HLR.

• (3b) The HLR generates the triplets (RAND, SRES, and Kc), which is similar to the GSM network and hands them to the SGSN.

• (3c) The SGSN sends an authentication request to the MS (along with RAND).

• (3f) The SIM calculates a SRES, and sends this to the SGSN.

The SGSN verifies the authentication (SRES=SRES’) since it is only the subscriber SIM and the HLR that are aware of the security key Kc and the algorithm that generates SRES.

RA-1

HLR/AC

SGSN-1

BSC

LA-1

3a

3b

3f

3c

GPRS attach, first time in PLMN: The SGSN authenticates the subscriber

IMEI checking

The GPRS attach continues with IMEI checking.

• (4a) SGSN asks for the MS IMEI.

• (4b) MS sends the IMEI.

• (4c) SGSN sends a Check IMEI message to the EIR.

• (4d) EIR replies with a Check IMEI Ack that will include the list type where the IMEI was found (unknown, white, grey, or black).

• Network operators may choose to ignore this functionality.

RA-1

EIR

SGSN-1

BSC

LA-1

4c

4d

4a

4b

RA-1

HLR

SGSN-1

BSC

LA-1

5a

5b

5c

5d

6a

6b

HLR interaction

GPRS attach continue:

• (5a) The SGSN sends an Update location message to the HLR with the subscriber IMSI.

• (5b) The HLR responds by giving subscriber data to the SGSN.

• (5c) The SGSN acknowledges that it has received the subscriber data ok.

• (5d) The HLR ends the transaction with an Update Location Ack.

• (6a) The SGSN accepts the GPRS attach and sends the MS a new P-TMSI. P-TMSI is an alias for the GPRS MS just like the TMSI.

• (6b) The MS acknowledges that it has received the new P-TMSI. The TLLI (Temporary Logical Link Identity) is derived from the P-TMSI.The TLLI is used as an identifier for the connection between the MS and the SGSN.

PDP Context Activation

MS SGSN GGSN

2. SecurityFunctions

1. Activate PDPContext Request

DNSServer

DNSinquiry

3. Create PDPContext Request

4. Create PDPContext Response

Connection establishment

with PDN

5. Activate PDPContext Accept

PDP context activation is sent from the MS to the SGSN

(1) MS sends an Activate PDP Context Request.(2) The MS may be authenticated and the IMEI checked.(3) The SGSN checks that the request is valid and sends a Create PDP Context

message to the GGSN which includes the tunnel ID (TID). (4) The GGSN returns a Create PDP Context Response.

The response message includes confirmation of the TID, IP address, and charging ID. The IP address is included if the GGSN allocates an IP address. The TID is used to identify the GTP tunnel used to transfer subscriber packets between the GGSN and the SGSN. The charging ID is used as an identifier for charging the subscriber.

(5) The SGSN returns Activate PDP Context Accept message to the MS. This message includes important details, for example, the IP address the MS should use.

PDP context activation

• MS requests context activation:• (1)The MS sends an 'Activate PDP

Context Request' to the SGSN.The parameters sent in this message include:

• PDP type, that is, IP connection or X25 connection.

• APN: a symbolic name for a network interface to an external network in the GGSN. One GGSN could have several different access points to access different networks.

• IP address (empty = dynamic), which is also referred to as PDP address.

• QoS parameters (optional).• (2)The SGSN checks the subscription

data. The subscriber data which is held in the HLR contains a list of allowed access point names and information whether dynamic or static IP address should be used. This information is usually downloaded and kept in the SGSN visiting GPRS subscriber database known as GSBASE.

• The GSBASE is to the VLR in GSM networks.

PDP context activation: Finding the GGSN

BTS BSC

SGSN

GGSN

GPRS Core

Network

DNS

Intranet

Internet

1.

2.

AccessPoint

Finding the requested GGSN and sending it the request for context creation:

(1) The SGSN gets the GGSN IP address from the Domain Name Server (DNS). The DNS finds the correct GGSN IP address based on the access point name supplied by the MS. The DNS maintains a mapping between APNs and the IP address.

• The SGSN sends a 'Create PDP Context Request' message to the GGSN. The request includes:

• IP address (if empty means a request for a dynamic address)

• Access point name (APN)• Proposed TID.

GGSN Response:

(1) The GGSN sends a 'Create PDP Context Response' back to the SGSN, which includes given IP address, TID confirmation, and charging ID.

(2) The SGSN sends an 'Activate PDP Context Accept' to the MS. The SGSN is now ready to route user traffic between the MS and the GGSN.

BTS BSC

SGSN

GGSN

GPRS Core

Network

Intranet

Internet

1.

2.

Reliance GPRS Network

GRX Architecture

GRX Peering Point:

Point of interconnection of all GRX. For example AMSIX, where 15 GRX operators interconnect: Belgacom, BT, Deutsche Telekom, France Télécom, Sonera (withEquant), Telecom Italia, Telefonica Data, Telenor, Telia International Carrier, Cable & Wireless, UUNet, Equant (with Sonera), Aicent, Comfone and TSI.

GPRS Quality of Service

Quality of Service (QoS) information for a user is contained in the HLR (subscribed QoS). The user may also request a specific QoS profile (requested QoS) which is associated with a PDP context. During the establishment of a PDP context, the GPRS network and the MS must negotiate a QoS profile (negotiated QoS profile).

The QoS profile is based in terms of the following attributes:

• Precedence class: Priority to be given to user packets

• Delay class: Delay associated with packets

• Reliability class: Amount of error control to be provided

• Peak throughput class

• Mean throughput class

Precedence class (priority)

• The service precedence indicates the priority of maintaining a service under abnormal conditions such as network congestion. Packets may be discarded according to precedence level. The following precedence levels are defined:

• Precedence Class 1 (High precedence):Service commitments will be maintained ahead of all other precedence levels.

• Precedence Class 2 (Normal precedence):Service commitments will be maintained ahead of low priority users.

• Precedence Class 3 (Low precedence):Service commitments will be maintained after the high and normal priority commitments have been fulfilled.

Delay class

Size 128 octets 1024 octets

Class Mean Delay

95% Mean Delay

95%

1 (Predictive)

0.5 s 1.5 s 2 s 7 s

2 (Predictive)

5 s 25 s 15 s 75 s

3 (Predictive)

50 s 250 s 75 s 375 s

4 (Best Effort)

Not specified

The delay parameter defines the maximum values for the mean delay and 95% delay to be incurred by data passing through the GPRS network.

The delay parameter defines the delay incurred by data packets within the GPRS network.

The ETSI standards state:

The GSM standards define four delay classes (1 to 4). A PLMN may support only a subset of the delay classes. As a minimum, the PLMN shall support the best effort delay class (4)."

Reliability class

The reliability class defines error rates for data loss, out-of-sequence delivery and corrupted data. There are five reliability classes.

• The reliability classes define the probability of:

• Lost data• Duplication of data• Data arriving out of sequence• Corruption of data.• The reliability class specifies the

requirements of the various network protocol layers. The combinations of the GTP, LLC, and RLC transmission modes support the reliability class performance requirements.

Throughput classes

Max. Throughput

Class

Max. Throughput in octets per second

1 Up to 1000 (8 kbit/s)

2 Up to 2000 (16 kbit/s)

3 Up to 4000 (32 kbit/s)

4 Up to 8000 (64 kbit/s)

5 Up to 16 000 (128 kbit/s)

6 Up to 32 000 (256 kbit/s)

7 Up to 64 000 (512 kbit/s)

8 Up to 128 000 (1024 kbit/s)

9 Up to 256 000 (2048 kbit/s)

The throughput class indicates the data throughput requested by the user. Throughput is defined by two negotiable parameters:• Peak throughput defines the maximum bit rate• Mean bit rate. This includes, for example for "bursty" transmissions, the periods in which no data is transmitted.• The maximum and mean bit rates can be represented by a parameter known as the Information Transfer Rate.• It is possible for the network to re-negotiate the throughput parameters at any time during a session. User data throughput is

specified in terms of a set of throughput classes that characterise the expected bandwidth required for a PDP context.

Maximum bit rate classes

Mean Throughput Class

Mean Throughput in octets per hour

1 Best effort.

2 100 (~0.22 bit/s)

3 200 (~0.44 bit/s)

4 500 (~1.11 bit/s)

5 1000 (~2.2 bit/s)

6 2000 (~4.4 bit/s)

7 5000 (~11.1 bit/s)

8 10 000 (~22 bit/s)

9 20 000 (~44 bit/s)

10 50 000 (~111 bit/s)

11 100 000 (~0.22 kbit/s)

12 200 000 (~0.44 kbit/s)

13 500 000 (~1.11 kbit/s)

14 1 000 000 (~2.2 kbit/s)

15 2 000 000 (~4.4 kbit/s)

16 5 000 000 (~11.1 kbit/s)

17 10 000 000 (~22 kbit/s).

18 20 000 000 (~44 kbit/s).

19 50 000 000 (~111 kbit/s).

Mean bit rate classes

Quality of Service negotiation

QoS negotiation

QoS negotiation is a procedure where a mobile station (MS) requests the SGSN for a certain QoS and the SGSN, after negotiating with the HLR and the GGSN, sends a negotiated QoS to the MS

• PDP-context activation request from MS

• SGSN compares to the user profile PDP-context subscription record it has received from the HLR. Based on this the SGSN may downgrade the QoS request. Users may not request higher quality than the profile allows.

• The create PDP-Context request is sent with the SGSN negotiated QoS setting

• The GGSN checks the Access Point QoS settings and may further reduce the QoS.

• Finally the GGSN returns the negotiated QoS settings which are forwarded to the MS