Struktur Jaringan Inti GPRS

Sistem GPRS dipaké ku telepon mobil atawa telepon sélulér GSM, sistem telepon mobil anu paling ilahar di sakuliah dunya (mimiti taun 2004), pikeun nganteurkeun pakét-pakét IP. **Jaringan inti GPRS** mangrupa bagian nu jadi puseur dina sistem GPRS sarta ogé narima jaringan 3G anu dumasar kana WCDMA. Jaringan inti GPRS mangrupa bagian anu teu bisa dipisahkeun tina jaringan inti GSM.

## Jaringan inti GPRS sacara umum
: *main article Packet Data Protocol#GPRS Tunnelling Protocol (GTP)*

**Jaringan Inti GPRS** ngandung manajemén mobilitas, manajemén sési sarta 
pangangkut pikeun layanan pakét Internet Protocol (IP) dina jaringan GSM jeung jaringan WCDMA. Jaringan inti ieu ogé bisa nyadiakeun fungsi tambahan lianna saperti tagihan (charging) jeung panyadapan anu diwenangkeun sacara hukum legal interception.

Saperti GSM umumna, GPRS mangrupa sistem anu digerakkeun ku standar nu *open (terbuka)* sarta badan standardisasina nyaéta 3GPP.

## GPRS Tunnelling Protocol (GTP)
:*main article GPRS Tunnelling Protocol*

*GPRS Tunnelling Protocol* ngamungkinkeun pamaké jaringan GSM atawa WCDMA pindah ti hiji tempat ka tempat lianna bari terus nyambung kana Internét saolah-olah pakét-pakét Internét asalna tina hiji tempat dina *Gateway GPRS Support Node* (GGSN). GPRS ngalakukeun hal ieu ku cara mawa data langganan ti *Serving GPRS Support Node* (SGSN) kiwari ka GGSN anu *ngahéndel* sési langganan. Aya tilu wangunan GTP anu digunakeun ku jaringan inti GPRS.

* **GTP-U**: pikeun mindahkeun data pamaké dina kanal-kanal nu misah ka tiap PDP context
* **GTP-C**: pikeun ngadalikeun alesan-alesan kaasup:
** panyusunan jeung panghapusan PDP context
** verifikasi *reachability* GSN
** *update*, contona pindahna langganan ti hiji SGSN kanu lianna.
* '''GTP' '*: pikeun mindahkeun data tagihan ti GSN ka fungsi panagihan.

GGSNs and SGSNs (collectively known as GSNs) listen for GTP-C messages on UDP port 2123 and for GTP-U messages on port 2152.  This communication happens within a single network or may, in the case of international roaming, happen internationally, probably across a GPRS Roaming Exchange (GRX).

The *"Charging Gateway Function" (CGF)'' listens to GTP' messages sent from the GSNs on UDP port 3386.  The core network sends charging information to the CGF, typically including PDP context activation times and the quantity of data which the end user has transferred.  However, this communication which occurs within one network is less standardised and may, depending on the vendor and configuration options, use proprietary encoding or even an entirely proprietary system.

## GPRS Support Nodes (GSN)
A GSN is a network node which supports the use of GPRS in the GSM core
network.  All GSNs should have a *Gn* interface and support the
GPRS Tunnelling Protocol.  There are two key variants of the GSN;
the GGSN and the SGSN defined below.

### GGSN - Gateway GPRS Support Node
A gateway GPRS support node (GGSN) acts as an interface between the GPRS backbone network and the external packet data networks (radio network and the IP network). It converts the GPRS packets coming from the SGSN into the appropriate packet data protocol (PDP) format (e.g. IP or X.25) and sends them out on the corresponding packet data network. In the other direction, PDP addresses of incoming data packets are converted to the GSM address of the destination user. The réaddressed packets are sent to the responsible SGSN. For this purpose, the GGSN stores the current SGSN address of the user and his or her profile in its location régister. 
The GGSN is responsible for IP address assignment and is the default router for the connected UE (User Equipment).The GGSN also performs authentication and charging functions

### SGSN - Serving GPRS Support Node
A Serving GPRS Support Node (SGSN) is responsible for the delivery of data packets from and to the mobile stations within its géographical service aréa. Its tasks include packet routing and transfer, mobility management (attach/detach and location management), logical link management, and authentication and charging functions. The location régister 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 régistered with this SGSN.

### Common SGSN Functions
* Detunnel GTP packets from the GGSN (downlink)
* Tunnel IP packets toward the GGSN (uplink)
* Carry out mobility management as Standby mode mobile moves from Routing Aréa to Routing Aréa.
* Billing user data

### GSM/EDGE Specific SGSN functions
* Carry up to about 60  kbit/s (150 kbit/s for Edge) traffic per subscriber
* Connect via frame relay or IP to the PCU using the Gb protocol stack
* Accept uplink data to form IP packets
* Encrypt downlink data, decrypt uplink data 
* Carry out mobility management to the level of a cell for connected mode mobiles;

### WCDMA Specific SGSN functions
* Carry up to about 300 kbit/s traffic per subscriber (R99)
* Carry up to about 7.2 mbit/s traffic downlink & 2.0 mbit/s traffic uplink (HSPA)
* Tunnel/detunnel downlink/uplink packets toward the RNC
* Carry out mobility management to the level of an RNC for connected mode mobiles.

These differences in functionality have led some manufacturers to
créate specialist SGSNs for éach of WCDMA and GSM which do not support
the other networks, whilst other manufacturers have succeeded in
créating both together, but with a performance cost due to the compromises
required.

## Access Point
:*Main article  Access Point Name*

An access point is:
* An IP network to which a mobile can be connected
* A set of settings which are used for that connection
* A particular option in a set of settings in a mobile phone

When a GPRS mobile phone sets up a **PDP context**, the access point
is selected.  At this point an Access Point Name (APN) is
determined

:Example: flextronics.mnc012.mcc345.gprs.
:Example: internet
:Example: mywap.

This access point is then used in a DNS query to a private DNS
network.  This process (called APN resolution) finally gives the IP
address of the GGSN which should serve the access point.  At this
point a **PDP context** can be activated..

## PDP Context
The PDP(Packet Data Protocol, e.g. IP, X.25, FrameRelay) context is a data structure present on both the SGSN and
the GGSN which contains the subscriber's session information when the
subscriber has an active session.  When a mobile wants to use GPRS, it
must first attach and then **activate a PDP context**.  This
allocates a PDP context data structure in the SGSN that the subscriber
is currently visiting and the GGSN serving the subscribers 
**access point**.   The data recorded includes.

* Subscriber's IP address
* Subscriber's IMSI
* Subscriber's 
** Tunnel ID (TEID) at the GGSN
** Tunnel ID (TEID) at the SGSN

The tunnel ID (TEID) is a number allocated by the GSN which identifies
the tunnelled data related to a particular PDP context.

There are two kinds of PDP contexts. 
* Primary PDP Context
** Has a unique IP address associated with it
* Secondary PDP Context
** Shares an IP address with another PDP context
** Is créated based on an existing PDP context (to share the IP address)
** Secondary PDP contexts may have different Quality Of Service settings

A total of 11 PDP contexts (with any combination of Primary and Secondary) can co-exist.

## Reference Points and Interfaces
Within the GPRS core network standards there are a number of
interfaces and
reference points (logical points of connection which probably share a
common physical connection with other reference points).  Some of these names can be seen in the network structure diagram on this page.

### Interfaces in the GPRS network
* **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 CG (Charging Gateway).  This interface uses an GTP Protocol, with extensions 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 RA (Routing Aréa) the station is attached to. An RA is a part of a larger LA (Location Aréa). 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** - The interface between the GGSN and a PDN (Public Data Network) either directly to the Internet or through a WAP gateway. Uses the IP protocol.
* **Ge** - The interface between the SGSN and the SCP (Service Control Point). Uses the CAP Protocol.
* **Gx** - The on-line policy interface between the GGSN and the CRF (Charging Rules Function). 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 OCS (Online Charging System). Uses the Diameter Protocol (DCCA application).
* **Gz** - The off-line charging interface between the GSN and the CG (Charging Gateway). Uses the CDRs (Accounting records).
* **Gmb** - The interface between the GGSN and the BM-SC (Broadcast-Multicast Service Center), used for controlling MBMS béarers.

## External links
* 3GPP web page including standards
* easy access to different specs
* GSM World's introduction to GPRS
* GPRS Attach and PDP Context Activation Sequence Diagrams