1 Cigalegprssw Basics v51 Ats Ra

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Cigale GPRS

GPRS BasicGPRS Basic

• At the end of the lesson the student will be able to:

- Describe the GPRS architecture

- Describe the main GPRS network elements and functionalities

Objectives

Astellia confidential

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- Describe the Gb-Interface protocol

- Understand the different basic procedures

GPRS architecture

BG

HLR

EIR

VLR GGSN

SGSN GaGs

Gf

Gc

Gp

GiNetwork (IP/X25)

Packet Data

Network (IP/X25)

Gn

Gr


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Foreign

PLMN

BG

GbPCU

BSC

BTS

BGSGSN

SGSN

Ga

Ga

Abis

Gd

SMS-G-MSC / SMS-IW-MSC

Air Interface

CG

Gn

GPRS : GSM Network update

• GPRS : General Packet Radio Service

• HW and SW update for Packet Switched services :- No changes of the architecture for voice part and circuit switched services

- Integration of PCU (Packet Control Unit) on BSS side (co-located with BSC)

- Integration of 2 new GPRS nodes (SGSN and GGSN) and associated interfaces

(Gb, Gn, Gp, Gi, Gr, Gd,…)


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(Gb, Gn, Gp, Gi, Gr, Gd,…)

• Opening and Merge toward Internet world for data traffic

GPRS Nodes

• SGSN (Serving GPRS Support Node) :

- Packet routing and transfer to and from MS within a geographical area

- Mobility management of subscribers

- Users profile and location storage

• GGSN (Gateway GPRS Support Node) :


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• GGSN (Gateway GPRS Support Node) :

- Logical point of one or more Packet Data Networks

- IP allocation management and IP routing function

- Charging data collection

Mobile Station

• GPRS Mobile Station :

- 29 Multi-slots Class in GPRS and EGPRS (GPRS over 8-PSK modulation) according to radio TS capabilities of transfer

- 3 types of mobiles :

• Class A : allows to lay out one GPRS service and one GSM communication simultaneously.


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simultaneously.

• Class B : listens to GSM and GPRS Paging channels, but can activate only one type of service.

• Class C : Mobile station works either in traditional GSM mode or in GPRS mode.

Location Subsets for mobility

• Location Area : With identifier LAC (Location Area Code as in GSM)

• Routing Area : With identifier RAC (Routing Area Code)- Subset lower or equal to LAC


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• Cells : With identifier CI (Cell Id as in GSM)- Only one CI per LAC/RAC

- Possibility to have the same CI in two different RAC

• MS States & Transitions

Mobility on GPRS network

• 3 mobility states for MS (to limit signalization)

- Idle : MS off or detached

- Standby : Routing Area localization of MS

- Ready : Cell localization of MS

• Transitions between mobile states

- ATTACH procedure : IDLE � READY

- Uplink Transfer : STANDBY � READY

DETACH procedure : READY / STANDBY IDLE


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READY

IDLE

STANDBY

GPRS

AttachGPRS

Detach

UL Transfer

Expiry of Ready Timer (44s)

GPRS

Detach

- DETACH procedure : READY / STANDBY �IDLE

- Expiry of Ready Timer : STANDBY � READY

(Default value : 44s)

Packet switched mode over the air

• Data Transmission in packet mode

- Mobile stations able to receive and transmit on several Time Slots

(ex : 4 TS Rx / 1TS Tx for class 8 MS)

- Load share on Packet Data channels

(Idle periods of MS1 allocated to MS2 or radio bandwidth share between MS1 and MS2 in case of transfer)

- User throughput increase compared to Circuit Switched mode


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• Several Coding Schemes used

- Acts on the compression of the data

- 4 types for GPRS (CS1 to CS4) and 9 types for EGPRS (MCS1 to MCS9)

- CS1 offers the best data protection and a lower throughput,

- MCS9 offers less protection but a higher throughput.

GPRS Throughput

9,06GMSKCS-1

21,4GMSKCS-4

15,6GMSKCS-3

13,4GMSKCS-2

Throughput (kbits/s)

ModulationCoding Scheme

GPRS


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59,28-PSKMCS-9

44,88-PSKMCS-7

54,48-PSKMCS-8

29,68-PSKMCS-6

22,48-PSKMCS-5

21,4GMSKCS-4

EDGE

PDP Context

• PDP : Packet Data Protocol

• Information stored in MS, SGSN, GGSN to allow data exchanged with PDP network

- Type of PDP network (IP/X25)

- Address of user mobile (IP/X25)

- IP address of current SGSN


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- IP address of current SGSN

- Network Service Access Point Identifier (NSAPI) and LLC Service Access Point Identifier (SAPI)

- Access Point Name (APN) : logical name of a PDP network

(ex : www.internet.com)

- Quality of service

PDP Context

• Quality of Service parameters

- Reliability : defines 5 level of data protection and acknowledgment

- Precedence (3 classes) : defines priorities between sessions (low, normal & high). In case of overload, service commitments of sessions with high priority shall be maintained ahead of other ones

- Delay (4 classes) : indicates the transmission duration of one packet through


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- Delay (4 classes) : indicates the transmission duration of one packet through GPRS network. “Best effort” is often used.

- Peak Throughput : max throughput in octets/s

- Mean Throughput : in octets/h

→ Release99 introduces additional parameters as Traffic class

Possible Applications

• Web browsing

• Mail access

• WAP

• File transfer via FTP


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• MMS

• Streaming

• Push To Talk

• Others…

Signaling/transmission plane


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Gb Interface

(capture)

Signaling Plane

GMM/SM

LLC

BSSGP

Network Service

L1 bis

GMM/SM

LLC

RLC

MAC

GSM RF

RLC BSSGP

MACNetwork Service

GSM RF L1 bis

Relay

MS SGSNBSS


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- L1 : Physical Layer

- NS : Network Service

- BSSGP : Base Station System GPRS Protocol

- LLC : Logical Link Control

- GMM : GPRS Mobility Management

- SM : Session Management

• Several layers of protocols for the management of the exchanges between mobile and

network on the Gb interface:

MS SGSNBSS

Interface Gb protocols

• Protocol NS (Network Service)

- This protocol is divided into 2 entities

• The control entity

• The protocol « Frame Relay »

- Communication between 2 entities NS is made over a Virtual Connection (NS-VC)

- Generally carried by N*64Kbits transmission resources FR


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- Generally carried by N*64Kbits transmission resources FR

- Optimizes the distribution of the traffic with load sharing function

- Logical link management is made by:

• Blocking and Unblocking of the logical link :

« NS BLOCK » and « NS UNBLOCK » procedures

• Testing the link state : « NS ALIVE » procedure

• Reseting the equipments : « NS RESET » procedures

- Packet information is carried out in a « NS UNITDATA » message


• Protocol NS

BSSIntermediate

Transmission

network

SGSN

End-to-end NS-VC

NS-VL at the BSS side NS-VL at the SGSN side


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DLCI : Data Link Connection Identifier NS-VL : Network Service Virtual Link

- A NS-VC is an end-to-end connection between SGSN and BSS

- One « NS-VC group » gathers all NS-VCs between the same NSEI (PCU) and SGSN

NS-VL at the BSS side NS-VL at the SGSN side


• Protocol NS : MultiPoint FR

FR

DLCI DLCI DLCI DLCI DLCI

NSVL1 NSVL2 NSVL3 NSVL4

NSVC

BSS SGSN


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- A NS-VL is a FR DLCI on one physical link

- Each NS-VC is made of several NS-VL identifying the different links and DLCI used across the FR network

- At each side, a NS-VC is identified by the first NS-VL (port-DLCI)

- In case of Point To Point : 1 NSVC = 1 NSVL = DLCI

NSVC


• Protocol BSSGP (BSS GPRS Protocol)

- Provides a logical link without connection between the SGSN and the BSS to ensure the data transfer with layer LLC

- BVC (BSSGP Virtual Connection)


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- BVC (BSSGP Virtual Connection)

• 1 BVC per cell of the BSS (identified by a BVCI)

• 1 BVC « 0 » for non dedicated signalling information for a particular BVC (As Paging PS, Suspend, Flush LL ACK messages)

- Contains routing information and quality of service mandatory for data transfer


• Protocol BSSGP

- One group of NS-VCs supports one or several BVCs for data transfer

- Each BVC is supported by one group of NS-VCs

Group of NS-VCs


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BVCs, 1 per cell


• Protocol BSSGP

- Manages BVC connections by :• Blocking or unblocking link with « BVC BLOCK » and « BVC UNBLOCK » messages

• Reseting link with « BVC RESET » message

- Manages BVC traffic thanks to :• Flow control procedure including « BVC FLOW CONTROL » messages


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• Flow control procedure including « BVC FLOW CONTROL » messages

- Manages MS traffic thanks to :• Flow control procedure including « MS FLOW CONTROL » messages• « FLUSH » mechanism in case of Cell Update procedure detected by SGSN in order to transfer (or not) data from the old BVC to the current one

• « LLC DISCARDED » message due to packet time expiry• « SUSPEND » & « RESUME » procedures to stop and restart ongoing data transfer

• « RADIO STATUS » message if the BSS can't send frames to MS


• Protocol BSSGP

GP_FL_C_MS (or _BVC)

GP_FL_C_MS_ACK (or _BVC_ACK)

Leak RateUser Throughput = LeakRate

kbits/s Bytes

PCU SGSN

MS

1 Gbit/s2 Mbit/s128 kbit/s

User

Throughput

BTS


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• Flow Control message information :

- Leak Rate : Throughput requested by the PCU to the SGSN

- Bucket Size : Size of the buffer inside the PCU (for one MS)

• Beginning of transfer : the PCU buffer is empty, the SGSN sends data

• When PCU buffer is full, the leak rate is equal to the user throughput

Leak Rate

Bucket Size

PCU Buffer

PCU1st UL data frame from Cell B

Flush Message (Old BVCI, New BVCI)

Flush Ack Message (Erased or Transferred, bytes affected)

Transfered data

Erased data

SGSN


• Protocol BSSGP: Flush procedure- Intra-PCU Cell Reselection

- MS from Cell A to Cell B


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Cell A Cell B

Flush Ack Message (Erased or Transferred, bytes affected)

BvcBBvcA

BvcA

Erased dataBvcB


• Protocol BSSGP: Flush procedure- Inter-PCU Cell Reselection

- MS from Cell A (PCU 1) to Cell B (PCU 2)

SGSN1st UL data frame from Cell B

Flush Message (Old BVCI, New BVCI)

Flush Ack Message (Deleted, bytes affected)

PCU 2PCU 1


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Flush Ack Message (Deleted, bytes affected)

Cell ACell B


• Protocol BSSGP: Discard procedure

BufferDiscard Message (Bytes affected)

DL Data Frames

Radio Problem

SGSNPCU 1


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Radio Problem or Congestion

Cell A

- Because of radio problems or cell congestion, the PCU can’t send the DL data frames to the MS

- When PCU buffer is full, if the SGSN is sending data frames to PCU, it is forced to discard oldest data stored in buffer

- In this case a BSSGP discard message is sent to inform SGSN that some frames were not sent to MS


• Protocol LLC (Logical Link Control)

- Provides a end-to end connection between the MS and the SGSN for data or signalling transfer

- One LLC connection is identified by :

• SAPI (Service Access Point Identifier) : Access point identifier at the SGSN

side and always associated to a QoS (4 choices: 3, 5, 9, 11).


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side and always associated to a QoS (4 choices: 3, 5, 9, 11).

» SAPI1 is dedicated to Signalling data.

» SAPI7 is reserved for SMS traffic.

» SAPI2 & SAPI8 are reserved for TOM message

• TLLI (Temporary Logical Link Identity) :

» Temporary Identifier of a mobile in Routing Area

» Derived from P-TMSI

» A new TLLI is allocated at each change of Routing Area


• Protocol LLC- Use of TLLI Temporary Logical Link Identity (derived from P-TMSI)

• Local: (TLLI=PTMSI, for a mobile always under the same RA, the same TLLI is always used)

• Foreign: (in case of RA mobility, the MS takes one foreign TLLI, if a valid PTMSI is already present)

• Random: (in case of no valid PTMSI, a specific range of TLLI is used for example a first GPRS Attach a TLLI random is used)

• Auxiliary: the SGSN provides a TLLI (not implemented)


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• Auxiliary: the SGSN provides a TLLI (not implemented)

31 30 29 28 27 26 to 0 TLLI

1 1 T T T T Local

1 0 T T T T Foreign

0 1 1 1 1 R Random

0 1 1 1 0 A Auxiliary


• Protocol LLC


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• Protocol LLC

- 4 types of messages :

• LLC_UI : Unconfirmed Information transfer between layer-3 entities

• LLC_I : Information transfer between Layer-3 entities

• LLC_S : Logical link Supervisory control functions (acknowledge I frames)

• LLC_U : Unnumbered additional logical link control functions

- 3 options :


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- 3 options :

• Acknowledged or not

• Protected or Unprotected according to data protection used with FCS (Frame Check Sequence)

• Ciphered or Unciphered according to operator choice

- Information : In case of ciphering, Astellia proposes a Gb deciphering solution to decipher Gb frames


• Protocol GMM (GPRS Mobility Management)

- Manages the GPRS users mobility thanks to main procedures

• GPRS Attach procedure

• GPRS Detach procedure

• Routing Area Update procedure in case of RAI mobility


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• Routing Area Update procedure in case of RAI mobility

• Periodic RA Update

• Security procedure (Authentication, Identity, Ciphering)

• PTMSI Reallocation procedure to update temporary identity

(at least at each RAI change)

• Identity Request (IMSI or IMEI)


• Protocol SM (Session Management)

- Manages the session activity thanks to

• PDP Context Activation procedure (to open session)

• PDP Context Deactivation procedure (to close session)

• PDP Context Modification procedure (to change QoS) (3G → 2G)


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• PDP Context Modification procedure (to change QoS) (3G → 2G)

• Secondary PDP Context procedures associated with a main one (same PDP address, APName, different QoS)


• Protocol SNDCP (SubNetwork Dependant Convergence Protocol)

- Dedicated to user plane, manages the exchanges between MS and SGSN

- Segmentation and re-assembly of IP frames

- Provides compression algorithms applicable to upper layer


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- SNDCP packet header is made of :

• Network Service Access Point Identifier (NSAPI) (to identify a PDP Context)

• Segmented, First, Last packet indicators

• Compression algorithm reference if used


• Protocol SNDCP : example for 1 user with 3 PDP Contexts

. . .

SNDCP

Packet Data Protocol

NSAPI

N-PDU



75 6


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- The NSAPI identifies the PDP context using the services of the SNDCP layer

- NSAPI 5 is associated with SAPI 3

(QoS negotiated at SM layer during the session activation)

- NSAPI 6 and NSAPI 7 are associated with the same SAPI 5 (same QoS)

- 11 possible NSAPI values (5-15) and only 4 available QoS (SAPI 3,5,9,11)

SNDCP

LLC

SAPI

SN-PDU

3 5 7


• Protocol IP (Internet Protocol)- IP protocol encapsulated in SNDCP frame contains information about transport (and

application) protocols :

• The UDP (User Datagram Protocol)

- Used to send short datagrams between applications

- Functions of Flow and reliability control not supported

- Information used : IP addresses/ports (source and destination)

ex : Push-To-Talk, WAP, Streaming, MMS


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ex : Push-To-Talk, WAP, Streaming, MMS

• The TCP (Transport Control Protocol)

- Use of flow control based on acknowledgment of received data

- Information used : IP addresses/ports (source and destination), TCP connection bits, sequence number and acknowledgment number

ex : Mail, POP3, HTTP, SMTP, MMS

• The ICMP (Internet Control Message Protocol) indicates to an IP datagram source that a delivery failure occures during its routing.


• Protocol IP (Internet Protocol)

MMS

DNSWTP

WSP

RTPRTCP FTP SMTP POP3HTTP RTSP


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DNSWTPRTPRTCP

UDP TCP

FTP SMTP POP3HTTP RTSP

L1

NS/FR

BSSGP

LLC

SNDCP

IP ICMP

Recommendations

• Rec. 23.060 GPRS Service Description, Stage 2

• Rec. 24.008 MS-SGSN, Specification Core Network Protocols

• Rec. 04.65 MS-SGSN, Sub Network Convergence Dependant Protocol : SNDCP

• Rec. 04.64 MS-SGSN, Logical Link Control layer : LLC


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• Rec. 04.64 MS-SGSN, Logical Link Control layer : LLC

• Rec. 08.18 BSS-SGSN, BSS GPRS Protocol : BSSGP

• Rec. 08.16 BSS-SGSN, Network Service : NS

• Rec. 08.14 BSS-SGSN, Gb layer 1

Main GPRS procedures

• GPRS Attach procedure


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• Routing Area Update procedure (with authentication)


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• Routing Area Update procedure (without authentication)


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• Routing Area Update procedure (inter-SGSN mobility)


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• PDP Context procedure


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Documents

1 Cigalegprssw Basics v51 Ats Ra