http://www.3g4g.co.uk/Tutorial/ZG/zg_pdp
A look at PDP Context in UMTS networks
By Zahid Ghadialy
Last Updated: 04/11/2007
Packet Data Protocol (PDP) A Packet Data Protocol (PDP) context offers a packet data connection over which the UE and the network can exchange IP packets. Usage of these packet data connections is restricted to specific services. These services can be accessed via so-called access points. Packet Data Protocol Context is one of the most important concepts for the UMTS Packet Data Architecture. The PDP Context has a record of parameters, which consists of all the required information for establishing an end-to-end connection:
The PDP Context is mainly designed for two purposes for the terminal.
Multiple PDP Context As mobile phones develop, there will be a need to serve parallel applications running on them with simultaneous PS calls. These PS calls can differ in their QoS (Quality of Service) parameters, and/or in the target network (PDN � Packet Data Network) to which they provide connection. Multiple PDP Contexts means that one mobile terminal can have multiple PDP contexts. Each of the Multiple PDP Contexts can at the same time have different QoS profiles. The primary PDP Context is a normal PDP Context with default QoS profile attributes and it is always activated first. For the multiple primary PDP Contexts, each context has different PDP Address and different APN Multiple PDP contexts will have special significance when IMS is introduced and all the services will be PS (IP) based. In an IMS based network the MS can (and will) activate separate PDP contexts for SIP based signaling and for all the sessions of different, eventually parallel services (e.g. parallel VoIP call and PS data call, etc.). A different QoS � which matches the application - will be used for each connection. The data flow (user plane) of a particular PDP context can terminate either in the Mobile Terminal (MT) itself or in the connected Terminal Equipment (TE) as shown in Figure below. The application for which the connection is provided is running either on the MT or on the TE respectively. An example for the first possibility is a video telephony client running on the mobile, for the second possibility a web browser running on the connected notebook. In IMS based systems it is expected that several embedded applications will run on the MT, requiring multiple PDP contexts. For the TE (e.g. connected PC) one additional PDP context may be also active. Multiple PDP contexts have two sub-categories:
Multiple Primary PDP Contexts Multiple primary PDP contexts are two or more PDP contexts independent from one another, each of them using one unique PDP address. They give the possibility to have simultaneous connections to different PDNs � e.g. to the internet for one application, while to a private network for another one. Beside the unique PDP address, each PDP context has its own QoS and NSAPI (Network Layer Service Access Point Identifier, see later) assigned. Each PDP context has a separate RAB (Radio Access Bearer) and GTP tunnel to transfer user plane data. The PDP contexts typically terminate in different access points on the network side (although it is allowed that they terminate in the same access point). The terminating access points can be located in the same or in different GGSNs. The example in Figure below shows the user plane path for three primary PDP contexts providing connections to three different PDNs: Primary PDP contexts can be activated or deactivated independently from one another. QoS of any of the active PDP contexts can be modified with the PDP context modification procedure initiated by the MS or by the network. (See Below for details) Secondary PDP Contexts A secondary PDP context is always associated with a primary PDP context. PDP address (IP address) and access point (AP) is re-used from the primary context. Hence the primary and the associated secondary PDP context provide connection to the same PDN with different guaranteed QoS. One primary PDP context might have multiple secondary contexts assigned. Each PDP context (i.e. the primary and all secondary) has its own RAB and GTP tunnel to transfer user plane data. Also, each context is identified by a unique NSAPI (Network Layer Service Access Point Identifier). The primary PDP context has to be active prior to activating an associated secondary PDP context. Any secondary PDP context can be deactivated while keeping the associated primary context (and eventual other secondary PDP contexts) active. If a primary PDP context is deactivated, this will also deactivate all the assigned secondary PDP contexts. QoS of any active primary or secondary PDP context can be modified with the PDP context modification procedure initiated by the MS or by the network. (See below for details) As the PDP address (IP address) is common for the primary and for (all) the associated secondary PDP contexts, the TFT (Traffic Flow Template) is introduced to route downlink user plane data into the correct GTP tunnel and hence into the correct RAB for each context. The example in Figure below shows the user plane for a primary and two associated secondary PDP contexts:
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