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• 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
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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 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
Interface Gb protocols
• 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
Interface Gb protocols
• 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
Interface Gb protocols
• 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
Interface Gb protocols
• 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
Interface Gb protocols
• 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
Interface Gb protocols
• 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
Interface Gb protocols
• 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
Interface Gb protocols
• 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
Interface Gb protocols
• 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
Interface Gb protocols
• 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
Interface Gb protocols
• 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
Interface Gb protocols
• 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
Interface Gb protocols
• 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)
Interface Gb protocols
• 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)
Interface Gb protocols
• 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
Interface Gb protocols
• Protocol SNDCP : example for 1 user with 3 PDP Contexts
. . .
SNDCP
Packet Data Protocol
NSAPI
N-PDU
Packet Data Protocol
Packet Data Protocol
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
Interface Gb protocols
• 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.
Interface Gb protocols
• 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
• Routing Area Update procedure (without authentication)
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