PDP Context in UMTS networks
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:
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:
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 can be understood as
IP routers that provide the connection between the UE and the selected
service. Examples of such services are:
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 |