Hola Jose Daniel
Es un gusto recibir tus preguntas y comentarios.
Respecto a tu pregunta, la sesion PDP (Packet Data Protocol) es una asociacion unica entre el usuario y la red GPRS usando la informacion del MSISDN, APN, la direccion IP asi como la informacion del TEID (Tunnel Endpoint ID) entre el SGSN-GGSN. Se usa para establecer un canal de datos y poder realizar el envio/recepcion de paquetes entre el usuario y el destino final.
Una sesion PDP puede consistir de uno o mas PDP context, (un primario y varios secundarios, hasta un maximo de 11 PDP context).
La sesion de PDP esta presente en ambos nodos SGSN (serving GPRS support node) y el GGSN (gateway GPRS support node).
Te anexo un ejemplo del call flow donde se involucra el usuario y la red GPRS.
Espero esta informacion te sea de utilidad, y quedo a tus ordenes si tienes alguna pregunta adicional sobre este u otros temas relacionados.
1. The Mobile Station (MS) goes through the process of attaching itself to the GPRS/UMTS network.
2. The MS sends an Activate PDP Context Request message that is received by an SGSN. The message contains information about the subscriber such as the Network layer Service Access Point Identifier (NSAPI), PDP Type, PDP Address, Access Point Name (APN), Quality of Service (QoS) requested, and PDP configuration options.
3. The SGSN authenticates the request message and sends a Create PDP Context Request message to a GGSN using the GPRS Tunneling Protocol (GTPC, “C” indicates the control signaling aspect of the protocol). The recipient GGSN is selected based on either the request of the MS or is automatically selected by the SGSN. The message consists of various information elements including: PDP Type, PDP Address, APN, charging characteristics, and Tunnel Endpoint Identifier (TEID, if the PDP Address was static).
4. The GGSN determines if it can facilitate the session (in terms of memory or CPU resources, configuration, etc.) and creates a new entry in its PDP context list and provides a Charging ID for the session.
If the MS required the dynamic assignment of an IP address (i.e., the PDP Address received from the mobile was null), the GGSN will assign one. The IP address assignment methods supported by the system GGSN are described in the Dynamic IP Address Assignment section of this guide.
The GGSN replies with an affirmative Create PDP Context Response using GTPC. The response will contain information elements such as the PDP Address representing either the static address requested by the MS or the address assigned by the GGSN, the TEID used to reference PDP Address, and PDP configuration options specified by the GGSN.
5. The SGSN returns an Activate PDP Context Accept response to the MS.
The MS can now send and receive data to or from the PDN until the session is closed or times out. The MS can initiate multiple PDP contexts if desired and supported by the system. Each additional PDP context can share the same IP address or use alternatives.
6. The MS can terminate the data session at any time. To terminate the session, the MS sends a Deactivate PDP Context Request message that is received by the SGSN.
7. The SGSN sends a Delete PDP Context Request message to the GGSN facilitating the data session. The message includes the information elements necessary to identify the PDP context (i.e., TEID, and NSAPI).
8. The GGSN removes the PDP context from memory and returns a Delete PDP Context Response message to the SGSN. If the PDP context was the last associated with a particular dynamically assigned PDP Address, the GGSN will re-claim the IP address for use by subsequent PDP contexts.
9. The SGSN returns a Deactivate PDP Context Accept message to the MS.
10. The GGSN delivers the GGSN Charging Detail Records (G-CDRs) to a Charging Gateway (CG) using GTP Prime (GTPP). Note that, though not shown in this example, the GGSN could optionally be configured to send partial CDRs while the PDP context is active.
11. For each accounting message received from the GGSN, the CG responds with an acknowledgement.