LTE University

Very helpful site for the LTE aspirants!
in reference to:

"LTE University"
- Knowledge Center - LTE University (view on Google Sidewiki)

GTP session establishment.

In this post I will try to touch briefly in every aspects of GTPv1 session setup.

I am not going to alot deep in the Radio network side, But covering the MS attach.

When a MS or UE(mobile station or User equipment) first comes up/powered on it has to go through the GPRS initial attach.

In this procedure

1. The UE first send the GMM attach message to the nearest SGSN. This message contains the following stuffs

TMSI - Temporary Mobile subscriber Identity, MNC - Mobile Network Code  , MCC - Mobile Country code, LAC - Location Area Code and RAC -Router area code.

2. The Nearest SGSN search for the Temporary Mobile subscriber ID in its database which may or may not be the old SGSN serving UE. If it was not found then the older SGSN ID is being used for the next message.

Here SGSN send the Identity request to the older SGSN with TMSI.

3. The Older SGSN resolve the TMSI and send back the GPRS IMSI (international mobile subscriber Id) to the nearest SGSN.

4. The SGSN send the Identity request to the UE.

5. The UE has to respond back with Identity response message.

6. SGSN then authenticate to UE with some random number.

7. UE uses this number and some GSM algorithm to generate a few other keys for the sesion and send back in Authentication response.

8. Till now the The mobile identity is not been checked so SGSN ask for mobile Identity to the UE,

9. UE reply back with the self IMEI number (International Mobile equipment Identity), One can see their IMEI in Nokia handsets by dialing *#06#

10. The SGSN then check for the supplied IMEI is valid and unique from the GSM EIR Database (Equipment Identity Register).

11. On successful IMEI check the SGSN get reply from the GSM database.

12. The SGSN then try to locate the UE and the same is done using Location update request and response, all the GSM data bases are updated with the newer location of the UE.

13. On successful update of UE location, SGSN sends the "UE attach accept" message to UE.

14. UE sends the "Attach complete" message. same is also updated with the visitor location register (VLR).

This completes the Radio side initial attah of the UE.

Now Lets begin with the Packet data Network communication!

PDP: Packet data protocol is designed for the Packet switched external networks in the GPRS core network. 

Both of our Serving and Gateway nodes contain the PDP data structure for activation, updation and deactivation of the GPRS sessions!

PDP context is the message format for the understanding of GPRS support nodes for GTPv1 session establishment, updation, termination, quality of services and other specific requirements.

The Session activation goes on with the following set of messages.

1. Create PDP context request from the SGSN to GGSN

2. Create PDP context response from GGSN to SGSN. 

                                                                                                         The UDP port number 3386 mentioned here was used for all the GTP traffics in GTPv0, from GTPv1 this port is used only for charging (GTP').

While GTP-c uses UDP port 2123 and GTP-u uses UDP port 2152. 

                                                                                                             


1. Create PDP context request( Packet data protocol context).

Below here is detailed information elements of the GTPv1 header with the wireshark!

I will touch upon QoS separately when we deal with the topic Secondary PDP context! Rest all fields are clearly visible! 

download capture

                                                                                                            2. Create PDP context response. 

                                                                                                       Feeling really sleepy,

I will show the modify and delete pdp context messages with elaborations of few of the mandatory IEs used here in the next post. 

Then we will see GTPv1 configurations on Cisco 7200 router! 

All your comments and suggestions are highly appreciated! 

GTP Demystified!

K I decided to write here! 
Actually I got so inspired from my frnd Santosh, that I thought to join him!


If you wish to go there please visit,
http://wired-n-wireless.blogspot.com/


This is extremely helpful for the LTE beginners, I congratulate him on the first anniversary his blog! keep up the good work!


He has started from the LTE, and GTPv2 and that is pretty new and modern technology, so I decided to go back few years ago.
As GTPv2 ie. EGTP (Evolved core) actually came from its earlier versions and understanding GGSN and SGSN is really important to reach the level of MME, SGW and PDN Gateway


Here I am starting from the GTPv0 and v1.
first of all Abbreviations! 


GPRS General Packet Radio Service.
GTP GPRS Tunnelling Protocol

GTP-C GTP Control
GTP-U GTP User
IE Information Element
IGMP Internet Group Management Protocol
IP Internet Protocol
IPv4 Internet Protocol version 4
IPv6 Internet Protocol version 6
QoS Quality of Service
RANAP Radio Access Network Application Part
RNC Radio Network Controller
TEID Tunnel Endpoint IDentifier
UDP User Datagram Protocol
UTRAN UMTS Terrestrial Radio Access Network
GGSN Gateway GPRS Support Node
SGSN Serving GPRS support Node
PLMN Public Land Mobile Network

No need to worry I will try to use Long names, in the beginning, there are so many and really confusing for a new being.

Interfaces

Gb

Interface between the base station subsystem and the SGSN the transmission protocol could be Frame Relay or IP.

Gn

IP Based interface between SGSN and other SGSNs and (internal) GGSNs. DNS also shares this interface. Uses the GTP Protocol.

Gp

IP based interface between internal SGSN and external GGSNs. Between the SGSN and the external GGSN, there is the border gateway (which is essentially a firewall). Also uses the GTP Protocol.

Ga

The interface servers the CDRs (accounting records) which are written in the GSN and sent to the charging gateway (CG). This interface uses a GTP-based protocol, with modifications that supports CDRs (Called GTP' or GTP prime).

Gr

Interface between the SGSN and the HLR. Messages going through this interface uses the MAP3 protocol.

Gd

Interface between the SGSN and the SMS Gateway. Can use MAP1, MAP2 or MAP3.

Gs

Interface between the SGSN and the MSC (VLR). Uses the BSSAP+ protocol. This interface allows paging and station availability when it performs data transfer. When the station is attached to the GPRS network, the SGSN keeps track of which routing area (RA) the station is attached to. An RA is a part of a larger location area (LA). When a station is paged this information is used to conserve network resources. When the station performs a PDP context, the SGSN has the exact BTS the station is using.

Gi

IP based interface between the GGSN and a public data network (PDN) either directly to the Internet or through a WAP gateway.

Ge

The interface between the SGSN and the service control point (SCP); uses the CAP protocol.

Gx

The on-line policy interface between the GGSN and the charging rules function (CRF). It is used for provisioning service data flow based charging rules. Uses the diameter protocol.

Gy

The on-line charging interface between the GGSN and the online charging system (OCS). Uses the diameter protocol (DCCA application).

Gz

The off-line (CDR-based) charging interface between the GSN and the CG. Uses GTP'.

Gmb

The interface between the GGSN and the broadcast-multicast service center (BM-SC), used for controlling MBMS bearers..

Here I am highly concerned about the interfaces Gn and Gi marked bold.
oh I forgot to say about what is interface, In GPRS core network an interface is a logical point of connection which shares same connection.

According to my knowledge an interface is a point in the GPRS core network where two completely or partially different protocol layer merges.
Well we will see it later, I will show some nice diagram for that!



As shown here in the image, I got it from Wikipedia. I choose this image because this image contain lots of interface mentioned above for the understanding!

K lets check with a somewhat simpler image!
The GPRS Tunneling Protocol (GTP) allows multiprotocol packets to be tunneled through a GPRS backbone.


The GGSN acts as a Gateway to the external packets through Gi interface, while it provides the Tunnel end point to the Serving node through Gn interface.
Here the tunneled packet travel through the Gn interface while the Gi interface flows Plain text.


Later we will see the complete signaling for GTPv0 and GTPv1. with the Cisco DUT configuration, some more facts and then we will try to co-relate with the GTPv1 to the GTPv2(EGTP).


Your suggestions are extremely important, I am also a beginner for Long Term Evolution!