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:

  • PDP Type
  • PDP address type
  • QoS profile request (QoS parameters requested by user)
  • QoS profile negotiated (QoS parameters negotiated by network)
  • Authentication type (PAP or CHAP)
  • DNS type (Dynamic DNS or Static DNS)

The PDP Context is mainly designed for two purposes for the terminal.

  • Firstly PDP Context is designed to allocate a Packet Data Protocol (PDP) address, either IP version 4 or IP version 6 type of address, to the mobile terminal.
  • Secondly it is used to make a logical connection with QoS profiles, the set of QoS attributes negotiated for and utilized by one PDP context, through the UMTS network.

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:

  1. multiple primary PDP contexts: they provide connections to different PDNs
  2. secondary PDP contexts: they provide connections to the same PDN but with different QoS

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:



Combination of multiple primary PDP contexts and secondary PDP contexts is also possible. For example, two primaries with one secondary context for each will result in four active PDP contexts in total. The maximum number of supported PDP contexts is terminal dependent.

Traffic Flow Template (TFT)

The Traffic Flow Template (TFT) is used by GGSN to discriminate between different user payloads. The TFT incorporates from one to eight packet filters; a unique packet filter identifier identifies each filter. Filtering can be based on one or more of the following filter attributes:

  • Source address (with subnet mask)
  • IPv4 protocol number
  • Destination port range
  • Source port range
  • IPSec Security Parameter Index (SPI)
  • Type of Service (TOS) (IPv4)

The TFT is provided by the MS in the Activate Secondary PDP Context Request message, it is stored by the GGSN, and is examined when routing downlink user plane data. The TFT can be modified or deleted with the MS initiated PDP context modification procedure. A TFT may be also assigned to a primary PDP context by means of the MS initiated PDP context modification procedure.

A TFT is built up from Packet Filters (minimum 1, maximum 8 of them) to provide flexibility in filtering. The relationship between PDP contexts, TFTs and Packet Filters is illustrated in Figure below:



PDP context procedures

Primary PDP context activation

This procedure is used to establish a logical connection with the Quality of Service (QoS) functionality through the network from the UE to the GGSN. PDP context activation is initiated by the UE and changes the session management state to active, creates the PDP context, receives the IP address and reserves radio resources. After a PDP context activation the UE is able to send IP packets over the air interface. The UE can have up to 11 PDP contexts active concurrently.

Secondary PDP context activation

A secondary PDP context activation allows the subscriber to establish a second PDP context with the same IP address as the primary PDP context. The two contexts may have different QoS profiles, which makes the feature useful for applications that have different QoS requirements (e.g., IP multimedia). The access point name, though, will be the same for the primary and secondary PDP contexts.

PDP context modification

The UE, the SGSN or the GGSN initiate this procedure for updating the corresponding PDP context. Additionally, the radio access network is able to request a PDP context modification from the SGSN (e.g., when coverage to the UE has been lost). The procedures modify parameters that were negotiated during an activation procedure for one or several PDP contexts.

PDP context deactivation

This procedure is used to delete a particular logical connection between the UE and the GGSN. The initiative to deactivate a PDP context can come from the UE, the SGSN, the Home Location Register (HLR) or the GGSN.



Access points

Access points can be understood as IP routers that provide the connection between the UE and the selected service. Examples of such services are:

  • Multimedia Messaging Service (MMS);
  • Wireless Application Protocol (WAP);
  • direct Internet access;
  • IP Multimedia Subsystem (IMS).

Depending on the operator of the network, more than one of these services might be provided by the same access point. The UE needs to be aware of an Access Point Name (APN) – the address of a GGSN – which gives access to the service-providing entity (e.g., an MMSC, the Internet or the P-CSCF). One GGSN may provide different services that can be accessed by different APNs.

When establishing a primary PDP context with an APN the UE receives an IP address or – in the case of IPv6 – an IPv6 prefix that it has to use when communicating over that PDP context. This means that when a UE has established several connections to different APNs the UE will have different IP addresses for each of the provided services.

REFERENCES


[1] The IMS: IP Multimedia Concepts and Services, Second Edition Miikka Poikselkä, Georg Mayer, Hisham Khartabil and Aki Niemi
[2] Multiple PDP Contexts in UMTS - ESG Group, Qualcomm
[3] 3GPP TS 23.060: "General Packet Radio Service (GPRS); Service description"
[4] 3GPP TS 24.008: "Mobile radio interface layer 3 specification; Core Network Protocols"
[5] What are Secondary PDP Contexts Good For? - Martins Mobile Technology Blog
[6] Using Traffic Flow Templates (TFTs) on BGAN - Inmarsat








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