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!