Femtocell solutions: Key questions to ask before you invest

Femtocells are all the rage, and these tiny cellular radios promise to improve in-building cellular coverage and are primarily targeted at the home-user. However, some vendors are now building enterprise femtocell solutions. Should you consider femtocell deployment in order to improve your cellular coverage? This article identifies the key questions you should ask before you buy a femtocell solution.

 Femtocells are all the rage, and these tiny cellular radios promise to improve in-building cellular coverage and are primarily targeted at the home-user. However, some vendors are now building enterprise femtocell solutions. Should you consider femtocell deployment in order to improve your cellular coverage? This article identifies the key questions you should ask before you buy a femtocell solution.

Femtocell's value proposition
Network operators now offer femtocell solutions such as the AT&T 3G MicroCell, Sprint AIRAVE, and Verizon Wireless Network Extender. The devices provide in-building cellular coverage and look like a Wi-Fi access point (see Figure 1).

AT&T 3G MicroCell Verizon Wireless Network Extender
Figure 1: AT&T 3G MicroCell and Verizon Wireless Network Extender

The devices communicate with the cellular phone and convert voice calls into voice over IP (VoIP) packets. The packets are then transmitted over your Ethernet network, through your firewall, and over your broadband connection to the network operator's servers.

At first glance, the femtocell value proposition seems like a win-win for the customer and the network operator. The customer receives better in-building coverage. The network operators not only avoid the cost and hassle of building a macrocell, they also use the customer's power, internal network and broadband connection, saving themselves more money. What's not to love? Well, let me count the ways.

Key questions to consider before investing in a femtocell solution
Before you add a femtocell solution to your telecommunication/mobility strategy, you should consider the following questions.

  • Which cellular operator's signal needs improvement? If yours is a typical enterprise, your employees probably use cellular service from several network operators. But a femtocell supports only one operator (e.g., AT&T or Verizon Wireless). So you may need to install femtocells from many operators throughout your enterprise.
  • Where do you need coverage? Cellular signals from a macrocell are often unable to penetrate into the innermost regions of a building (e.g., the basement). Unfortunately, femtocells must be installed near a window so that the attached  GPS antenna has a line-of-sight view to orbiting satellites. For instance, AT&T recommends that its microcell be located within three feet of a window and not in the basement or a closet. So a femtocell might not be able to provide coverage where you need it.
  • What type of phone is supported? The AT&T 3G Microcell supports 3G phones only (sorry, no EDGE support). The Verizon Wireless Network Extender supports 2G phones but no 3G phones (yup, no EV-DO support). So it is very likely that many employees will not reap the femtocell benefits, since their phones are not supported.
  • How will a femtocell affect battery life? A femtocell could save battery life because less power is required to transmit a signal over the short distance to the femtocell rather than over the long distance to a macrocell. On the other hand, lots of handoffs from femtocell to femtocell may drain the battery because the phone needs to continually scan for a new femtocell. So be sure to test phone battery life in your environment.
  • How will you identify rogue femtocells? A rogue femtocell is an unauthorized femtocell connected to your network. In the case of Wi-Fi, vendors provide rogue access point detection software to help find the  rogue AP. However, what if an intruder configured a rogue femtocell with a valid AT&T account ID (but not the enterprise account ID) and then connected the femtocell to the enterprise network? How would you detect this rogue femtocell?
  • How will you isolate femtocell traffic? Femtocell traffic is backhauled across the enterprise network. Enterprises may want to isolate the femtocell traffic from all other enterprise traffic by using a virtual LAN (VLAN) connection or a separate physical Ethernet connection.
  • Is the femtocell solution scalable? Most femtocells limit the number of simultaneous calls to three or four. Does this provide sufficient capacity for your enterprise?
  • How will you block access to non-employees? A femtocell signal may spill into an adjacent building or floor. Most femtocells provide a "white list" or "black list" mechanism to explicitly include/exclude phone numbers. This is akin to configuring a Wi-Fi AP with MAC addresses for allowed/disallowed laptop connections. Is this mechanism a manageable solution?
  • How will you manage interference? As with any radio, femtocells are subject to interference. Unlike Wi-Fi, which uses unlicensed spectrum, femtocells use licensed spectrum that is controlled by the network operator. So if your femtocell is subjected to interference from another femtocell or macrocell, how will you alleviate the interference? (Note that your ability to move the femtocell may be restricted by the need to maintain the GPS connection.)

Femtocells promise to improve in-building cellular coverage dramatically, but they raise many questions. This article identified the key questions you should answer before investing in a femtocell solution.


This was first published in October 2009

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