Local IP Access

Description

Local IP Access (LIPA) is a 3GPP standardized functionality that allows a mobile device connected via a femtocell (Home NodeB or Home eNodeB) to directly access other IP-capable devices within the same local premises network. The architecture centers on the Local Gateway (L-GW), a logical function that is typically collocated with the femtocell. When a UE initiates a Packet Data Network (PDN) connection for LIPA, the femtocell and its associated L-GW establish a direct IP path between the UE and the local network. The L-GW acts as the default IP router for the UE for this specific PDN connection, performing functions like IP address allocation (using DHCP) and traffic forwarding. The mobile operator's core network, specifically the Serving Gateway (S-GW) and Packet Data Network Gateway (P-GW), is bypassed for this local traffic, though the core network remains involved in control plane aspects like connection establishment and mobility management for the cellular link.

The LIPA functionality is tightly integrated with the femtocell subsystem and its security gateway (SeGW). The L-GW can be implemented as a separate physical entity or, more commonly, as a logical function integrated into the femtocell hardware. For a UE to use LIPA, it must be within the coverage area of a femtocell that supports LIPA and is authorized for such access. The UE requests a PDN connection for LIPA by using a specific Access Point Name (APN) configured for local access. The femtocell, upon receiving this request, interacts with the core network's Mobility Management Entity (MME) for authorization. Once authorized, the data path is set up directly between the UE and the L-GW. All IP packets destined for the local network are routed by the L-GW, while packets destined for the public internet would typically require a separate, non-LIPA PDN connection that routes through the operator's core P-GW.

LIPA's role is significant in offloading localized, data-intensive traffic. It enables use cases like accessing a local network-attached storage (NAS), printing to a local printer, or streaming from a local media server without consuming the femtocell's often-limited backhaul capacity to the operator's core. This architecture preserves the user experience for local services even if the wide-area internet connection is congested or unavailable. Security for the local access is managed at the IP layer within the local network, as the cellular air interface security terminates at the femtocell. LIPA represents a convergence of cellular and local area networking, extending the concept of a trusted local network to devices using cellular radio technology.

Purpose & Motivation

LIPA was created to address the growing use of femtocells and small cells in residential and enterprise environments. A key problem was that all UE data traffic, even traffic destined for a printer in the same room, was traditionally routed through the mobile operator's core network gateway (P-GW), often located hundreds of miles away. This 'tromboning' of local traffic increased latency, consumed valuable backhaul bandwidth (which can be a cost bottleneck for femtocell deployments), and provided no performance benefit for purely local communications. It also limited the utility of femtocells for creating effective local area networks for cellular devices.

Historically, before LIPA, a UE could only access services through the operator's core network. This was inefficient for the emerging small cell deployment model. The motivation for LIPA was to enable a more efficient architecture where localized communication could be kept local, mirroring the behavior of Wi-Fi in a local network. It solves the problem of backhaul congestion for small cells, reduces latency for local services, and allows operators to offer new service bundles that include seamless access to customer premises equipment. It addressed the limitation of previous cellular architectures that were designed primarily for wide-area network access, not for optimized local area access via operator-deployed infrastructure.