GPRS Core Network
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The GPRS system is used by GSM Mobile phones, the most common mobile phone system in the world (as of 2004), for transmitting IP packets. The GPRS Core Network is the centralised part of the GPRS system and also provides support for WCDMA based 3G networks. The GPRS core network is an integrated part of the GSM core network.
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[edit] GPRS Core Network in General
- main article Packet Data Protocol#GPRS Tunnelling Protocol (GTP)
The GPRS Core Network (GPRS stands for Generic Packet Radio Services) provides mobility management, session management and transport for Internet Protocol packet services in GSM and WCDMA networks. The core network also provides support for other additional functions such as charging and lawful interception. It was also proposed, at one stage, to support packet radio services in the US TDMA system, however, in practice, most of these networks are being converted to GSM so this option is becoming largely irrelevant.
Like GSM in general, GPRS is an open standards driven system and the standardization body is the 3GPP.
[edit] GPRS Tunnelling Protocol (GTP)
- main article GPRS Tunnelling Protocol
GPRS Tunnelling Protocol is the defining IP protocol of the GPRS core network. Primarily it is the protocol which allows end users of a GSM or WCDMA network to move from place to place whilst continuing to connect to the internet as if from one location at the Gateway GPRS Support Node (GGSN). It does this by carrying the subscriber's data from the subscriber's current Serving GPRS Support Node (SGSN) to the GGSN which is handling the subscriber's session. Three forms of GTP are used by the GPRS core network.
- GTP-U: for transfer of user data in separated tunnels for each PDP context
- GTP-C: for control reasons including:
- setup and deletion of PDP contexts
- verification of GSN reachability
- updates, e.g. as subscribers move from one SGSN to another.
- GTP' : for transfer of charging data from GSNs to the charging function.
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 TCP or 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.
[edit] 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.
[edit] 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 readdressed 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 register. 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
[edit] 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 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.
[edit] 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 Area to Routing Area.
- Billing user data
[edit] 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;
[edit] 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 create specialist SGSNs for each of WCDMA and GSM which do not support the other networks, whilst other manufacturers have succeeded in creating both together, but with a performance cost due to the compromises required.
[edit] 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..
[edit] 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 Endpoint ID (TEID) at the GGSN
- Tunnel Endpoint ID (TEID) at the SGSN
The Tunnel Endpoint 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 created 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.
[edit] 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.
[edit] 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 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 RA (Routing Area) the station is attached to. An RA is a part of a larger LA (Location Area). 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 (CDR-based) charging interface between the GSN and the CG (Charging Gateway). Uses GTP'.
- Gmb - The interface between the GGSN and the BM-SC (Broadcast-Multicast Service Center), used for controlling MBMS bearers.
