Femtocell Gateway location and Femtocell Re-Parenting

ReparentingWhere should a network operator located their femtocell gateways and how would they distribute femtocells between them? The term re-parenting may not mean much to you – it sounds more like something associated with child adoption than telecoms - but for network operations departments, this can be quite a common task required as the network grows and evolves. Re-parenting cellsites to new controllers can be time-consuming or complex to do and has consequences if not implemented accurately. Do these issues also occur in the femtocell landscape and what are the implications?

A definition of cellsite reparenting

Mobile radio networks are connected in a hierarchical manner. Groups of cellsites are connected to a cellsite controller (called a BSC in 2G networks, an RNC in 3G networks). These would typically handle some 100 to 200 sites each. This compares with femtocell gateways which support hundreds of thousands of femtocells.

In turn, the BSCs and RNCs are connected up to an MSC which handles all the voice calls and text messages, and to a GSN which manages all the packet data sessions. The ratios at this level in the hierarchy are not so dramatic, perhaps only 5 or 10 BSC/RNC per MSC. This varies with network design and products used.

Re-parenting (also sometimes called rehoming) is the network management operation where a cellsite is switched across to a different controller (BSC or RNC) and is done as a managed task under the planning and direction of the network plannning and operations department.

Why does re-parenting occur?

Organic Network Growth: As networks grow, additional cellsites are added and connected to the nearest controller. At some point, the capacity of the controller may near its full capacity. A new controller is installed and some cellsites swapped across to balance the load.

Swanky New Products: Vendors bring out new products with increased capacity or more cost effective technology. Several controllers can be replaced by a single, larger product. Once installed, all the cellsites must be transferred across to the new one.

Grooming: Strategic planners may identify the need for new switching centres or to consolidate equipment in fewer sites. The existing transmission and location of controllers may then be organised to be more efficient. Since the network grew to meet demand in areas that might not have been predicted when it was first launched, the shape and location of equipment and resources may not match today's demands. This tidying up is sometimes called “grooming”, because it straightens out these niggles.

Commercial changes: From time to time network operators make some commercial agreements which significantly impact the way they run their operations. This includes deals with fixed network providers for leased lines (e.g. British Telecom provide high capacity Ethernet links to urban cellsites for several mobile operators, where the capacity can be increased on demand). There have also been several deals when Radio Access Networks are shared, including the transmission and cellsite equipment.

Why is this a difficult or complex problem?

Many network operators don't have a full technical map of their tranmission circuits, so when they make changes to it, these are done directly on the equipment itself. For 3G mobile networks, each cellsite requires up to 15 separate ATM virtual circuits (known as VCI/VPI pairs), each of which must be reconfigured on the ATM switches through which they transit.

Multiply this by the 20,000 cellsites and 50,000 transmission links for a large network and you can see that this can easily become a full time job for a large team of engineers. Fortunately, there are software tools which handle these problems and automate the common processes and procedures used.

Cellsite controllers are also moved between MSC/GSNs, but on a much less frequent basis.

All of these operations are normally done on a managed and carefully controlled basis, but occasionally it may be necessary to reparent a controller quickly due to a major outage (e.g. an MSC or site going offline). This can be done in 20 minutes or less where manually implemented. Some recent products have a hot-standby architecture, which effectively allows MSCs on other sites to share the load and/or takeover all calls in the case of faults. This substantially increases the network reliability and uptime, improving customer satisfaction and keeping the revenue generating network alive more of the time.

So how would this work in a femtocell environment?

Femtocells are connected to the operators network via broadband internet. This is typically routed via one or more of the few large interconnect points in the country. These are geographically independent of the customer's location, and more determined by which ISP they use.

When femtocell services are first launched, an operator may choose to install only one or two femtocell gateways. It's likely that two would be installed to cater for major outages and equipment failures. These would operate as separate devices.

In addition to the femtocell gateways, femtocells also talk to a management server. This uses the TR-069 protocol common in DSL modems. The management server is used to set the configuration of the femtocell, download any software updates and run diagnostics on demand.

Femtocells will initially contact their TR-069 server (known as an ACS), which would have been pre-programmed into the unit when shipped. This can then download any software updates and configuration, which would include assigning a femto gateway.

As the network grows, the ACS can be used to change the femtocell gateway assigned to any femtocell access point. When the femtocell is updated, it will re-register with a new controller, moving the resources required to support it across to a new device.

Another possibile implementation is for the IP address used to access a femtocell gateway to be spread across several femtocell gateways. This would be much the same as a large web server farm hides behind a common shared IP address. So although a large number of femtocells are accessing the same gateway, the load is distributed across several femtocell gateways.

With the typical capacity of a femtocell gateway being around 300,000 femtocells and being able to scale up to 1,000,000, perhaps operators would prefer to have separate devices to limit the impact of any individual equipment failure.

Why would femtocell gateway reparenting be required?

As for todays BSCs and RNCs, the capacity of individual femtocell gateways will be limited. When the number of femtocells increases dramatically, then there will be a need to install additional gateways which will continue to be located next the major ISP interconnect points.

This will increase the load on the co-located MSC and GSN equipment, which will need to scale to match the millions of calls and data sessions arriving through these gateways. Additional femtocell gateways will be installed in line with the number of femtocells sold.

As these grow, some femtocells will be migrated across to other gateways to balance the load. This can be done easily through provisioning commands on the ACS and should be much easier to manage than the reparenting operations on the outdoor macrocellular network.

A knock on effect is that the major capacity for femtocell gateways and supporting core network (MSC, GSN) equipment will be at those sites closest to the Internet Exchange points around the country.

Emergency Re-Routing

In the case of sudden and major outages, it would probably be easiest to redirect the IP address for a femtocell gateway across to another location. Recovery procedures would cause all the attached mobile phones to be re-registered in a matter of hours, but in these usual circumstances their might be periods where incoming calls are directed to voicemail because the location of mobile handsets is unknown.

Summary

  • Femtocell Gateways have much greater capacity than their RNC/BSC counterparts
  • Femtocell Gateways are likely to be co-located near to national internet exchange points rather than being distributed around the country
  • Reparenting of femtocells to different femtocell gateways can be easily managed through the ACS configuration servers and does not require any complex planning or reconfiguration. This is much easier (and more scalable) than for outdoor macrocellular networks today.

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