Packet Data Network Connection

Description

A PDN Connection is a fundamental concept in the Evolved Packet System (EPS) that defines the logical association between a User Equipment (UE) and a Packet Data Network (PDN). It is identified by the combination of the UE's IP address (IPv4, IPv6, or both) and the Access Point Name (APN) that identifies the external network. Each PDN Connection provides an IP pipe through which the UE can exchange data with services in that PDN, such as the Internet, an IMS core, or a corporate intranet. The PDN Connection is established during the initial attach procedure or via a separate PDN Connectivity Request, and it remains active until explicitly deactivated or released due to inactivity or network policy.

Architecturally, a PDN Connection spans multiple network elements: the UE, the evolved NodeB (eNB), the Mobility Management Entity (MME), the Serving Gateway (SGW), and the Packet Data Network Gateway (PDN GW). The MME manages the control plane signaling for establishing, modifying, and releasing the connection, while the SGW and PDN GW handle the user plane data forwarding. The PDN GW is the anchor point, assigning the IP address and enforcing policies. The connection is realized as one or more EPS Bearers, which are tunnels that carry user data with specific Quality of Service (QoS) parameters. The default bearer is established with the PDN Connection and provides basic connectivity, while additional dedicated bearers can be added for specialized services like VoLTE.

The establishment procedure begins when the UE sends a PDN Connectivity Request to the MME, including the APN and the requested PDN type (IPv4, IPv6, IPv4v6). The MME selects an SGW and a PDN GW based on the APN and subscription data, then sends a Create Session Request to the SGW, which forwards it to the PDN GW. The PDN GW allocates an IP address (the PDN Address) and returns a Create Session Response. This triggers the setup of radio bearers and S1-U tunnels between the eNB and SGW. The UE receives the assigned IP address and can start sending data. The PDN Connection is identified in the core network by the EPS Bearer Identity and in the UE by the APN-IP address pair.

Mobility management is a key aspect of PDN Connections. When the UE moves and changes eNB or even MME/SGW, the PDN Connection is maintained through handover procedures. The PDN GW remains the same to preserve the IP address, ensuring session continuity. If the UE moves to a different access technology, such as UTRAN or GERAN, interworking procedures allow the PDN Connection to be transferred to a Serving GPRS Support Node (SGSN) and Gateway GPRS Support Node (GGSN) in the legacy core, though this may involve a change in anchor node. In 5G, the equivalent concept is the PDU Session, which is managed by the Session Management Function (SMF) and anchored at the User Plane Function (UPF).

PDN Connections support multiple simultaneous connections, allowing a UE to access different PDNs at the same time. For example, a smartphone might have one PDN Connection to the Internet and another to the IMS for voice services. Each connection can have its own QoS profile, security policies, and charging rules. The network can also apply Policy and Charging Control (PCC) rules per PDN Connection, enabling differentiated services. The PDN Connection is identified in charging records, allowing operators to bill based on usage per APN. Overall, the PDN Connection is the cornerstone of IP connectivity in EPS, providing a flexible and scalable framework for mobile data services.

Purpose & Motivation

The PDN Connection concept was introduced in 3GPP Release 8 as part of the System Architecture Evolution (SAE) to support always-on IP connectivity, higher data rates, and multiple simultaneous data sessions. It replaced the older PDP Context mechanism from GPRS/UMTS, which was limited to a single PDP address per context and did not efficiently support dual-stack or multiple PDN access. The PDN Connection provides a standardized way to manage IP sessions in the Evolved Packet Core (EPC), enabling operators to offer differentiated services based on APN, QoS, and charging.

One of the main problems it solves is the need for session continuity during mobility. By anchoring the PDN Connection at the PDN GW, the IP address remains stable even as the UE moves across eNBs, MMEs, and SGWs. This is essential for maintaining TCP connections, VoIP calls, and other real-time applications. Additionally, the PDN Connection allows a UE to connect to multiple PDNs simultaneously, each with its own IP address and QoS, which is not possible with a single PDP context. This supports use cases like split billing, where personal and corporate traffic go through different APNs.

Furthermore, the PDN Connection framework integrates with Policy and Charging Control (PCC) to enforce dynamic QoS and charging rules. It also facilitates interworking with legacy networks (GERAN/UTRAN) and non-3GPP access (e.g., Wi-Fi), providing a unified session management approach. The concept has evolved into the PDU Session in 5G, but the principles of anchoring, multiple sessions, and QoS differentiation remain. In summary, the PDN Connection exists to provide reliable, flexible, and scalable IP connectivity for mobile devices, forming the backbone of modern mobile broadband services.