FortiCarrier Introduction

GPRS authentication

GPRS authentication is handled by the SGSN to prevent unauthorized GPRS calls from reaching the GSM network beyond the SGSN (the base station system, and mobile station). Authentication is accomplished using some of the customer’s information with a random number and uses two algorithms to create ciphers that then allow authentication for that customer.

User identity confidentiality ensures that customer information stays between the mobile station and         SGSN — no identifying information goes past the SGSN. Past that point other numbers are used to identify the customer and their connection on the network.

Periodically the SGSN may request identity information from the mobile station to compare to what is on record, using the IMEI number.

Call confidentiality is achieved through the use of a cipher, similar to the GPRS authentication described earlier. The cipher is applied between the mobile station and the SGSN. Essentially a cipher mask is XORd with each outgoing frame, and the receiving side XORs with its own cipher to result in the original frame and data.

Parts of a GTPv1 network

A sample GTP network consists of the end handset sender, the sender’s mobile station, the carrier’s network including the SGSN and GGSN, the receiver’s mobile station, and the receiver handset.

When a handset moves from one mobile station and SGSN to another, the handset’s connection to the Internet is preserved because the tunnel the handset has to the Internet using GTP tracks the user’s location and information. For example, the handset could move from one cell to another, or between countries.

The parts of a GPRS network can be separated into the following groups according to the roles of the devices:

  • Radio access to the GPRS network is accomplished by mobile phones and mobile stations (MS).
  • Transport the GPRS packets across the GPRS network is accomplished by SGSNs and GGSNs, both local and remote, by delivering packets to the external services. l Billing and records are handled by CDF, CFR, HLR, and VLR devices.

GPRS networks also rely on access points and PDP contexts as central parts of the communication structure. These are not actual devices, but they are still critical .

These devices, their roles, neighboring devices, the interfaces and protocols they use are outlined in the following table.

Carrier network showing the interfaces used (GTPv1)


Devices on a GTPv1 network

Device role Neighboring Devices Interfaces used Protocols used
Mobile Users Mobile Stations (MS) Radio Access

Technology (RAT)

Mobile Stations (MS) Mobile Users, SGSN Gb IP, Frame Relay
SGSN (local) MS, SGSN (local or remote),

GGSN (local and remote),


Ga, Gb, Gn, Gp, Gz IP, Frame Relay, GTP, GTP’
SGSN (remote) SGSN (local) Gn GTP
GGSN (local) SGSN (local or remote),

GGSN (local and remote),


Ga, Gi, Gn, Gp, Gz IP, GTP, GTP’
GGSN (remote) SGSN (local), WAP gateway,

Internet, other external services

Gi, Gp IP, GTPv1
CDR, CFR SGSN (local), GGSN (local) Ga, Gz GTP’
HLR, VLR SGSN (local), GGSN (local) Ga, Gz GTP’

Radio access

For a mobile phone to access the GPRS core network, it must first connect to a mobile station. This is a cellular tower that is connected to the carrier network.

How the mobile phone connects to the mobile station (MS) is determined by what Radio Access Technologies (RATs) are supported by the MS.


Transport protocols move data along the carrier network between radio access and the Internet or other carrier networks.

FortiOS Carrier should be present where information enters the Carrier network, to ensure the information entering is correct and not malicious. This means a Carrier-enabled FortiGate unit intercepts the data coming from the SGSN or foreign networks destined for the SSGN or GGSN onto the network, and after the GGSN as the data is leaving the network.


GPRS Tunnelling Protocol (GTP) is a group of IP-based communications protocols used to carry General Packet

Radio Service (GPRS) within Global System for Mobile Communications (GSM) and Universal Mobile Telecommunications System (UMTS) networks. It allows carriers to transport actual cellular packets over their network via tunneling. This tunneling allows users to move between SGSNs and still maintain connection to the the Internet through the GGSN.

GTP has three versions version 0, 1, and 2. GTP1 and GTP2 are supported by FortiOS Carrier. The only GTP commands that are common to all forms of GTP are the echo request/response commands that allow GSNs to verify up to once every 60 seconds that neighboring GSNs are alive.


There have been three versions of GTP to date. The original version of GTP (version 0) has the following differences from version GTPv1.

l the tunnel identification is not random l there are options for transporting X.25 l the fixed port number 3386 is used for all functions, not just charging l optionally TCP is allowed as a transport instead of UDP l not all message types are supported in version 0


On a GPRS network, Packet Data Protocol (PDP) context is a data structure used by both the Serving GPRS Support Node (SGSN) and the Gateway GPRS Support Node (GGSN). The PDP context contains the subscribers information including their access point, IP address, IMSI number, and their tunnel endpoint ID for each of the


The Serving GPRS Support Node (SGSN) is responsible for the delivery of data packets from and to the mobile stations within its geographical service area. Its tasks include packet routing and transfer, mobility management (attach/detach and location management), logical link management, and authentication and charging functions. The location register of the SGSN stores location information (e.g., current cell, current VLR) and user profiles (e.g., IMSI, address(es) used in the packet data network) of all GPRS users registered with this SGSN.


GTPv1-C refers to the control layer of the GPRS Transmission network. This part of the protocol deals with network related traffic.

FortiOS Carrier handles GTPv1-C in GTPv1 by using the Tunnel Endpoint IDentifier (TEID), IP address and a Network layer Service Access Point Identifier (NSAPI), sometimes called the application identifier, as an integer value that is part of the PDP context header information used to identify a unique PDP context in a mobile station, and SGSN.

For more information on GTPv1-C, see GTP-C messages.

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