GPRS Tunneling Protocol GTP

7/14/2019 GPRS Tunneling Protocol GTP

1/57

GPRS architecture and GTP protocol

GPRS/UMTS Overview

GTP Overview

APNs

GTP Details


2/57

ContentsGPRS and its Features

GPRS Network Architecture -

Role of SGSN & GGSN

GPRS Protocol Architecture -

Definition and functions of various protocol layers i.e. SNDCP, RLC-MAC, LLC, PHY

GPRS Session Management -

PDP context establishment, accept, negotiation

GPRS Mobility Management -

Network Attach\detach

Routing area update

GPRS Applications


3/57

GSM networks Constraints

Data Rates too slowabout 9.6 kbps

Connection setup time too long

Inefficient resource utilization for bursty traffic

Proves expensive for bursty traffic utilization

No efficient method for packet transfers


4/57

GSM GPRS

Data Rates 9.6 Kbps 14.4 to 115.2 Kbps

Billing Duration of connection Amount of data transferred

Access to HLR every MT call causes query to HLR no need to access HLR for everyGPRS packet

Flow of data Data transmission burstyContinuous flow of data in both

direction

Type of Connection

Circuit Switched =>

connection with circuit switchednetworks typically one call per hour,

average call 2 minutes

Packet - Switched

=>connection with external packetdata networks and can last for

several hours

PR o ers s gn cant

performance improvement over

GSM


5/57


6/57

What is GPRS ?

General Packet Radio Service (GPRS) is a new bearer service forGSM that greatly improves and simplifies wireless access to packet

data networks

GPRS applies packet radio principal to transfer user data packets in

an efficient way b/w MS & external packet data network


7/57

GSM network architecture

Base Station Controller (BSC):It controls a group of BTSs and manages their radioressources. A BSC is principally in charge of handoffs, frequency hopping,exchange functions and power control over each managed BTSs.

Base Transceiver Station (BTS) :it maps to transceivers and antennas used ineach cell of the network. It is usually placed in the center of a cell. Its transmitting

power defines the size of a cell. Each BTS has between 1-16 transceiversdepending on the density of users in the cell.

MS consists of :The Subscriber Identity Module (SIM) and Mobileequipment/terminal.

Network and switching subsystem (NSS) :Its main role is to manage thecommunications between the mobile users and other users, such as mobile users,ISDN users, fixed telephony users, etc. It also includes data bases needed in orderto store information about the subscribers and to manage their mobility. The

different components of the NSS: MSC, HLR,VLR, AuC, EIR.

MSC: the central component of the NSS. The MSC performs the switchingfunctions of the network. It also provides connection to other networks.


8/57

Benefits of GPRS New Data Services

High Speed (Data Rate 14.4115 kbps)

Efficient use of radio bandwidth (Statistical Multiplexing)

Circuit switching & Packet Switching can be used in parallel

Constant connectivity

Billing based on volume of data transferred

Utilizes existing GSM authentication and privacy procedures.


9/57

High Data Rate

GPRS uses radio channel i.e. 200 kHz wide

Radio channel carries digital data stream of 271 kbps

This rate is divided into 8 time slots each carrying 34 kbps per timeslot

Data rate 14 kbps per time slot achieved after corrections GPRS can combine upto 8 time slots giving data rate of 114 kbps


10/57

GPRS Network Elements GPRS Architecture same as GSM except few hardware modifications

GPRS includes GSNs

SGSN : Serving GPRS Support Node

GGSN : Gateway GPRS Support Node

GPRS Register


11/57

BSCBTS

GSM/GPRS AccessNetwork

Serving

GSN

PSTNMSC

BSC

BTS

BTS

BTS

Internet

Gateway

GSN

GPRS Network Architecture


12/57

SGSNServing GPRS Support

Node

Delivers data packets to mobile stations & vice-versa

Detect and Register new GPRS MS in its serving area

Packet Routing, Transfer & Mobility Management

Authentication, Maintaining user profiles

Its location register stores location info. & user profiles


13/57

GGSNGateway GPRS Support Node

Interfaces GPRS backbone network & external packet datanetworks

Converts the GPRS packets from SGSN to the PDP format

Converts PDP addresses change to GSM addresses of thedestination user

Stores the current SGSN address and profile of the user in its

location register

Performs authentication

Many-to- many relations among SGSNs & GGSNs


14/57

GPRS Register

GPRS Register is integrated with GSM-HLR.

Maintains the GPRS subscriber data and Routing information.

Stores current SGSN address


15/57

MS and SGSN Signaling messages protocol stack

GSM RF

GMM\SM

Um Gb

MAC

RLC

LLC LLC

GSM RF

Frame

Relay

MAC

RLC BSSGP

L1bis

BSSGPFrame

Relay

LLC

GMM\SM

L1bis

MS BSS (PCU) SGSN

BSSGP-Base station system GPRS protocol


16/57

User Data Protocol Stack

GSM RF

SNDCP

IP / X.25

Application

Um GnGb

MAC

RLC

LLC

IP / X.25

GTP

L1

L2

LLC

GSM RF

Frame

Relay

MAC

RLC BSSGP

L1bis

BSSGP

Frame

Relay

LLC

SNDCP GTP

L1bis L1

L2

IP

MS BSS (PCU) SGSN GGSN Gi

IP

UDP/

TCP

UDP/

TCP


17/57

SNDCP - Sub Network Dependent Convergence Protocol

Used to transfer data packets (TCP/IP) between SGSN and MS.

Converting and segmenting external network formats (N-PDUs) into sub-

network formats (SN-PDUs). In the opposite direction, SNDCP layer

receives SN-PDUs from LLC and converts them back into network data

packets.

Managing multiple PDP context sessions ensuring that N-PDUs fromeach PDP context are txed to LLC layer in sufficient time to maintain

QOS for each PDP context.

Compression and decompression of user data and header information

Negotiating the compression parameters to be used between MS and

network.


18/57

GiGn

Gc

Gp

Signalling and Data Transfer

Signalling Interface

TEPDN

Iu

UTRANTE MT

Gr or Gc

HLR

Other PLMN

SGSN GGSN

SGSN

GTP-MAPprotocolconverting

GSN

Gn

IuTE MT UTRAN

SGSN

Gn

BSSGb

TE MT

Gn

GPRS/UMTS Logical Architecture

U Pl f GPRS/UMTS


19/57

User Plane for GPRS/UMTS

Relay

Network

Service

GTP-U

Application

IP / X.25

SNDCP

LLC

RLC

MAC

GSM RF

SNDCP

LLC

BSSGP

L1bis

RLC

MAC

GSM RF

BSSGP

L1bis

Relay

L2

L1

IP

L2

L1

IP

GTP-U

IP / X.25

Um Gb Gn GiMS BSS SGSN GGSN

Network

Service

UDP /

TCP

UDP /

TCP

L1

RLC

PDCP

MAC

IP,

PPP,

OSP

Application

L1

RLC

PDCP

MAC

ATM

UDP/IP

GTP-U

AAL5

Relay

L1

UDP/IP

L2

GTP-U

IP,

PPP,

OSP

3G-SGSNUTRANMS

Iu-PSUu Gn Gi

3G-GGSN

ATM

UDP/IP

GTP-U

AAL5

L1

UDP/IP

GTP-U

L2

Relay


20/57

GPRS Higher Layer Signalling

OverviewApplication Layer

SM LayerSet up data calls,

negotiates QoS

GMM- Registration, authentication, attach,

location management

LLCProvides Highly reliable data link

RLC- MAC

PHY

AM radio link UM radio link


21/57

GPRS Identities

IMSI=> International Mobile Subscriber Identity-is the identity number of the SIM. MS send to NWduring attach.

(not more than 15 digits)

3 digits 2-3 digits

P-TMSI =>Packet Temporary Mobile Subscriber Identity

P-TMSI is sent during GMM attach , if the network does not recognize the P-TMSI (incase of roaming) ,it will ask MS to send IMSI for authentication purposes. SGSN selects a P-TMSI from a store of randomnumbers and sent to MS in ciphered mode which uses P-TMSI to generate TLLI.

PDP (Packet Data Protocol) addresses=> MS is identified with IMSI. IMSI will have related PDPaddresses for the services subscribed to. These addresses can be temporary or permanent and mustconfirm to the standards of external networks.

MCC- mobile country code , MNC- PLMN network code , MSIN- mobile subscriber identity code

MCC MNC MSIN


22/57

GTP

GTP specifies a tunnel control and management protocolwhich allows the SGSN toprovide GPRS network access for an MS. In the transmission plane, GTP uses a tunnellingmechanism to provide a service for carrying user data packets. The choice of path isdependent on whether the user data to be tunnelled requires a reliable link or not.

The GTP protocol is implemented only by SGSNs and GGSNs. No other systems need tobe aware of GTP. GPRS MSs are connected to a SGSN without being aware of GTP.

It is assumed that there will be a many-to-many relationship between SGSNs and GGSNs.An SGSN may provide service to many GGSNs. A single GGSN may associate with manySGSNs to deliver traffic to a large number of geographically diverse mobile stations.


23/57

GPRS Tunneling Protocol is

The protocol between GPRS Support Nodes

Includes both the GTP signaling (GTP-C) & data transfer

(GTP-U) proceduresIs defined for the Gn interface, i.e. the interface between GSNs

within a PLMN &

For the Gp interface between GSNs in different PLMNs

Allows multi-protocol packets to be tunneled through theUMTS/GPRS backbone

GTP Overview


24/57

GTP Overview (Cont.)

GTP-C is

the signaling plane and specifies a tunnel control and

management protocol

is used to create, delete modify tunnels.

GTP-U is

the transmission plan & uses a tunneling mechanism

(GTP-U) to provide a service for carrying user data packets

Definitions

T-PDUOriginal data packet

G-PDUOriginal data packet (T-PDU) + GTP header


25/57

GTP Header

Version PT (*) E S PN

Message Type

Length (1stOctet)

Length (2ndOctet)

Tunnel Endpoint Identifier (1stOctet)

Tunnel Endpoint Identifier (2ndOctet)

Tunnel Endpoint Identifier (3rdOctet)

Tunnel Endpoint Identifier (4thOctet)

Sequence Number (1st

Octet)

Sequence Number (2ndOctet)

N-PDU number

Next Extension Header Type

8 7 6 5 4 3 2 1Octets1

2

3

4

56

7

8

910

11

12SGSN

specific


26/57

GTP Messages

GTP defines a set of messages. These are classified into

Path Management messages

Tunnel Management messages

Location Management messages

Mobility Management messages


27/57

Call set up and tear down

Create PDP Context req.

Create PDP Context resp.

G-PDU (data transfer)

Delete PDP Context req.

Delete PDP Context resp.

SGSN GGSN


28/57

Tunnel Management Messages

Create PDP Context RequestCreate PDP Context Response

Delete PDP Context Request

Delete PDP Context Response

Update PDP Context request

Update PDP Context Response

PDU Notification request

PDU Notification response

PDU Notification reject request

PDU Notification reject response

Used for n/w initiated PDP context

Used for SGSN handoff

DHCP relay of PDP address


29/57

Access Point Name

An APN identifies a PDN that is configured on and accessible from

A GGSN. APN has two parts.

APN Network Identifier

APN Operator Identifier

The APN network identifier must correspond to a fully qualified

name in DNS e.g. cisco.com

The access Points that are supported by the GGSN are preconfigured

on the GGSN.

APN is sent in the create PDP context request message to GGSN.

APN has several attributes associated with its configuration

which specify how the user can access the network at the entry point.


30/57

APN Attributes

Some of the attributes are

- APN Network Identifier- Access ModeTransparent (no authentication) / Non-transparent

- IP address pool

- Primary DHCP server

- Secondary DHCP server- DHCP gateway address

- RADIUS server

- Backup RADIUS server

- IP address of the next hop for the Gi interface

Attributes to get logs

- Current allocated IP address count

- Current PDP context count


31/57

Path Management Messages

Path management messages can be sent between any type of GSNs.

Echo RequestNo elements

Echo ResponseRecovery

Version not supportedNo elements

Supported Extension Header NotificationsExtension header

type list


32/57

Echo Request/Response Message

Echo request is send on a path to another GSN to find out if the Peer

GSN is alive.

The periodicity is implementation dependent but is > 60 secs.

The Echo response is sent with the local restart counter of the GSN.

The GSN receiving the response would compare the restart countervalue to the previous stored value to check if the GSN has restarted.

The receiving GSN clears off all the PDP context belonging to the

restarted GSN.

Every GSN maintains its own restart counter in non-volatile memoryand its peers restart counters in volatile memory.

Tunnel Management Messages (cont )


33/57

Tunnel Management Messages (cont.)Create PDP Context Request

IMSI(identify a subscriber by the operator. ) Conditional

Recovery Optional

Selection Mode Conditional

Tunnel End Point Identifies Data 1 Mandatory

Tunnel Endpoint Identifier Control Plan Conditional

NSAPI Mandatory

Linked NSAPI Conditional

Charging Characteristics Optional

Trace Reference Optional

Trace Type Optional

End User Address Conditional

Access Point Name Conditional

Protocol Configuration Options Conditional

SGSN Address for signaling Mandatory

SGSN Address for user traffic Mandatory

MSISDN( for routing call , number to dialin) Conditional

Quality of service Profile Mandatory

TFT Conditional

Trigger ID Optional

OMC identity OptionalPrivate Extension Optional


34/57

Tunnel Management Messages (cont.)

Create PDP Context response

Information Element Presence required

Cause Mandatory

Reordering required Conditional

Recovery Optional

Tunnel Endpoint Identifier Data 1 Conditional

Tunnel Endpoint Id. control Plane Conditional

Changing ID Conditional

End User Address Conditional

Protocol Configuration Options Optional

GGSN Address for Control Plane Conditional

GGSN Address for Use Traffic Conditional

Quality of Service profile Conditional

Charging Gateway Address Optional

Private Extension Optional

* Private extension contains vendor or operator specific info


35/57

Tunnel Management Messages (cont.)

Cause: Indicates if the PDP context was created. Some of the

values are

- Request accepted

- No resource available

- All dynamic PDP addresses are occupied- No memory available

- Missing or unknown APN etc.

If the PDP context request fails the SGSN tries to create the PDPcontext with the next GGSN in its GGSN list.


36/57

Tunnel Management Messages(cont)

Error Indicationmsg.

Information Element Presence

Tunnel Endpoint Identifier data Mandatory

GSN Address Mandatory

Private extension Optional

Sent from GSN if no active PDP context exists for the received

G-PDU.

When an error indication is received, the receiving GSN deletes the

corresponding PDP context.

The elements TEID and GSN address are derived from the data

message and uniquely specify the PDP context.


37/57

Location Management Messages

Used for network initiated PDP context request when there is no

SS7 MAP interface at the GGSN

Gn+

UDP

L2IP

GGSN

L1L2IP

UDP

SGSN

GTP

L1MTP2MTP3SCCPTCAPMAP

L1MTP2MTP3SCCP

HLR

TCAP

MAP

L1

GTP

Gc

GTP-MAP Interworking


38/57

Location Management Messages (cont)

Send Routing Information for GPRS Request

Send Routing Information for GPRS Response

Failure Report Request

Failure Report Response

Note MS GPRS Present Request

Note MS GPRS Present Response

Mobility Management Messages:Sent between SGSNs at the GPRS attach and Inter SGSN routing

update procedures.


39/57

Reliable Delivery of Signaling Messages

Each path maintains a queue with Signaling msg. to be sent

A request (with a response) shall be sent with a sequence number

and shall be held in a path list till the response comes.

Each path has its own listThe sequence number is unique for each outstanding req. msg.

T3-Response timerMax time a requesting entity would wait before retransmission

N3-Requests timesMax number of retransmissions allowed.

these two are configurable parameters on the stack.


40/57

Handling of Sequence Number (optional)

For GTP-C

Used in request-response type of signaling messages as transactionIdentity. In the response message the seq. no. of request message is

copied.

For GTP-U

An increasing number of T-PDUs are transmitted via GTP-U tunnel

when transmission order must be preserved.

The receiving GGSN would reorder the incoming T-PDUs in seq. if

the reordering required flag in the PDP context is set. In this case if

required the GGSN would maintain a window and a timers to makesure that a G-PDU was lost.

The sequence number is kept unchanged irrespective of the tunnels it

travels before the destination.


41/57

Session Management SM Layer is sandwiched between Network Layer and GMM layer. It provides service

by establishing a PDP context when asked to do so by network layer

To set up session for transfer of SN-PDUs and to de-activate the session wheninstructed

To modify QOS and Radio priority given to a PDP context when the network instructs itto do so.

It relies upon the services provided by GMM layer and will check that it is GPRSattached and then send PDP context activation request asking the GMM layer to deliver itto SGSN SM layer.

MS will receive from the network PDP context activation accept (or reject!) . This is theinformation SNDCP is waiting for and now accepts the N-PDUs from application layer.


42/57

Cont.PDP context activation is a data call set-up for GPRS operations. Unlike circuitswitched call where set-up is removed when call is completed but PDP contextactivations remains valid when the call or session which caused the Context iscompleted.

For each session PDP context following parameters are exchanged :

PDP type

NSAPI APN Access Point Name MS may use this to specify which external network

it wishes to access

Requested PDP address- may be X.121, IPv4 or IPv6 address

QOS

Radio priority

Protocol configuration Options


43/57

PDP Context Activation


44/57

PDP Context DeactivationInitiated by :

Network- if PDP context is not used for some time

Mobile- QOS not accepted or Insufficient resources


45/57

GMM GPRS M bilit


46/57

GMM - GPRS Mobility

Management

Attaching to the GPRS network when requested by SM or

application layer. This attach process alerts the network to MS

presence and allows network to authenticate it and give

information required for encryption.

Perform Routing area updates

Perform cell updates when MS is in ready state and reselects a

cell

Detaching from the network

Authentication


47/57

GMM states

Idle MS is not attached to GPRS

Standby

Subscriber is attached to GPRS mobility management

MS performs RA and cell selection locally, reports RA changes Data, signaling or page response move the MS to READY

Detach procedures moves the state to Idle

Ready

Information on cell selection is reported

Cell selection may be done locally or by network control

State supervised by a timer


48/57

Flow for GPRS attach procedure GMM is requested by SM layer to attach. Then GMM sendsAttach Request

(PTMSI, RAI,TLLI, cksn, QOS, Cause, Radio priority) down to LLC layer. From the PTMSI and cksn , the network operator will have a decision making processto authenticate MS or to go immediately in encryption mode, by passing authentication.

MS receives Authentication and Encryption commandfrom network, it extractsRAND, passes it to SIM card which produces SRES, and encryption key Kc .

GMM layer puts SRES into Authentication and Encryption response and passesthis Kc key to LLC for encryption procedures.

When GMM layer received Attach Accept(Ra update timer, RAi, PTMSi, PMTSIsig, new TLLI ), the authentication process is completed.

The GMM accepts the PTMSI and other information and send an Attach completemessage to the network.

RAND-Random number related to IMSI in the SGSN.

SRES-signed response


49/57

Routing Area Update

RAU is performed when MS is GPRS attached and RLC indicates achange in routing area. The RA reported to GMM layer is continously

compared to RA stored in SIM card. When these differ an RA update to

GPRS network is initiated by GMM layer.

GSM Location Area(LA) is divided into several Routing Areas(RA)

RA consists of several cells SGSN is informed when MS moves to a new RA

MS sends a Routing Area Update Request to its assigned SGSN


50/57

Routing Area Updatecontd.

Whenever an MS moves to a new RA, it sends a routing area update request to itsassigned SGSN. The message contains the routing area identity (RAI) of its old RA. Thebase station subsystem (BSS) adds the cell identifier (CI) of the new cell, from which theSGSN can derive the new RAI. Two different scenarios are possible:

Intra-SGSN routing area update: The MS has moved to an RA that is assigned to thesame SGSN as the old RA. In this case, the SGSN has already stored the necessaryuser profile and can assign a new packet temporary mobile subscriber identity (P-TMSI)

to the user (routing area update accept). Since the routing context does not change,there is no need to inform other network elements, such as GGSN or HLR.

Inter-SGSN routing area update: The new RA is administered by a different SGSN thanthe old RA. The new SGSN realizes that the MS has changed to its area and requeststhe old SGSN to send the PDP contexts of the user. Afterward, the new SGSN informsthe involved GGSNs about the users new routing context. In addition, the HLR and (i fneeded) the MSC/VLR are informed about the users new SGSN.


51/57

Routing Area Update


52/57

Applications of GPRS Web browsing

Corporate & Internet Email

Vehicle Positioning

Remote LAN Access

Home Automation

Document Sharing/Collaborative working

Virtual private network (VPN):With VPN, companies will be able to let their remote workers wirelessly access to

corporate resources and stay in touch with their work teams.


53/57

ADDITIONAL SLIDES


54/57

Data Link layer Divided into two sub layers :

LLC layer (between MS-SGSN)

RLC/MAC (between MS-BSS)


55/57

LLC-Logical Link Control Establishes highly reliable logical link between MS & its assigned SGSN using

TLLI as the mobile station address.

Works either in connection mode (acknowledged) which is high reliability link

or connectionless mode (unacknowledged) for high delivery of SN-PDUs .

Data confidentiality is ensured by ciphering and deciphering functions


56/57

Radio Link Control(RLC) Establish a reliable link between MS & BSS

Segmentation and reassembly of LLC frames into RLC data blocks

To allow ordered transfer of LLC PDUs between MAC and LLC.

Medium Access Control(MAC)

To allow shared use of radio medium (physical channels) betweenmultiple mobile stations or incase of a single MS between differentPDP contexts.

Allow contention resolution

To allow queuing and scheduling for MS terminating calls.

To control TBF for carrying data packets

RLC/MAC Layer


57/57

Physical Layer

Divided into two sub layers :

Physical Link Layer (PLL)

Physical RF Layer (RFL)

PLLProvides a physical channel between MS and BSS

Channel coding, interleaving, detection ofphysical link congestion, neighboring cellmeasurements, Handovers (Cell changeorders), Measurement Reports, Cell re-selection.

RFL - Operates below PLL

Documents

GPRS Tunneling Protocol GTP