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Giving a Little Life to LIPA - Data Offload in the Home and Office


In my last couple of posts (, ) I looked at the different dimensions of ‘offload’, with possibilities of making best use of radio spectrum, either by using small cells or combining with WiFi. In this post I’ll look at getting new services from cellular and offloading data from an operator’s core network as well.

From a Service Provider’s perspective there is a desire to be able to offer comparable local services in the home and the enterprise to those that a nomadic WiFi user with additional software can achieve in the same place, but with the added advantage of doing it from a cellular handset and the ability to keep a connection whilst roaming. This means that services in the home, such as uploading photos from your phone to your PC or media server, streaming music or movies from a home media system or printing out documents and pictures should be available (some early demonstrations of femtocell connected home concepts that we put together at ip.access can be seen here) In a business premises, access the corporate intranet, printing, and short-code dialling the extension of a colleague are also handy. Some of the business services are already available from operators as a value added service, but at some cost and complexity, as Figure 1 below helps show. It shows the traditional core network layout for a 3G system that I have used earlier but with a home or enterprise network added near the local broadband modem. The traditional route for the data is shown with a thin red line, and the ideal Direct Route from the Broadband Modem to an enterprise or home router is a thicker line. The key point to note is that data has to traverse the operator’s network to the anchoring routers (GGSN) before it can be sent to an external network such as the home or an enterprise. If you are already attached to a cell in the home or enterprise where you want to access data or make a call, this is clearly highly inefficient. For serving large businesses, the price premium an operator can charge may justify this inefficiency and the business service may be provided by a particular GGSN router which then securely tunnels the data back into the enterprise, but the economics as they stand may not make sense for the normal home user or small business.

current_vs_direct_data_route.pngFigure 1: Traditional tunnelled access to business premises, and optmized offload route (click to enlarge)

To achieve this simplification, what we have done in 3GPP is to specify a new user service known as LIPA (Local IP Access). Normally, what happens when a mobile requests a data service is that the SGSN that handles all the control decisions (shown towards the right of the figure) is asked to arrange for a router (GGSN) to provide the requested service and once the details are confirmed and addresses of the small cell and GGSN exchanged as endpoints, user data can flow to and from external networks. So far what has been specified is that a cut-down GGSN, called a Local Gateway, can be co-located with the femtocell and when a mobile device asks for a connected service, the femtocell offers the SGSN the address of the Local Gateway to act as the GGSN. Depending on permissions and the nature of the requested service, the SGSN may accept and then allocate the Local Gateway for the service. The radio bearers that support the service are indicated to the femtocell so that it can directly send the user data to the Local Gateway without sending to the Core Network at all. We arrive at the diagram in Figure 2 below, with the local user data taking the direct route and not going near the core network.

LIPA.pngFigure 2: Femtocell enhanced with Local Gateway to provide local offload (click to enlarge)

This is clearly a benefit for both service provider and user as long as the user experience matches up to the alternatives. In fact, to enable users to get the full connected home experience, the Local Gateway is boosted to make sure that it forwards all the necessary broadcast messages from the local network that enable seamless service discovery of printers and other UPnP devices.

What next in this area? There are still refinements being worked on, such as having the Local Gateway separate from the femtocell; this will give more flexibility and also enable continued service between more than one femtocell as the user walks around the office or a large home. In a slightly different direction, there is the natural question that if a home or enterprise network also has external internet access, once you've got a Local Gateway can it be used to route all the internet traffic from the mobile rather than just local traffic? in 3GPP terms, this is known as SIPTO (Selective IP Traffic Offload) and is being discussed at the moment. It's feasible but needs a bit more work to handle making the choices between when a core network GGSN is needed to provide or monitor the service, and when Local Gateway is possible. Perhaps more another time.

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