What to ask before you buy a femtocell

Posted by: Paul DeBeasi

Introduction

Femtocells are all the rage.   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 femtocells.  Should you consider femtocell deployment in order to improve your cellular coverage?  This post identifies the key questions you should ask before you buy a femtocell.

Value proposition

Network operators now offer femtocells 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.

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 operator not only avoids the cost and hassle of building a macrocell, they also use the customer’s power, internal network, and broadband connection, further saving themselves money.  What’s not to love?

Well, let me count the ways.

Key questions

Before you add femtocells to your telecommunication strategy, you should consider the following questions.

• Which cellular operator’s signal needs improvement?  If you are like most enterprises, your employees probably use cellular service from several network operators.   But a femtocell only supports 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? Oftentimes cellular signals from a macrocell can’t 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 their 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 because the underlying technology for their phone is 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 detect 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.

Conclusion

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

Comments

[Chris Bird]

I have this stunning visual of an unauthorized red colored femtocell. In the article in various places you describe rouge femtocells when you meant rogue femtocells.

[Van]

Bought 3g Microcell (AT&T). Brought home, eventually got it working on two iPhones in the house (with AT&T help). Noticed a day later that the gps on the phones was now erroneous (off by 220 miles). Turned off Microcell, gps errors went away. Clearly interference, but not sure how AT&T will resolve when I get them on the phone again tomorrow.

[Van]

AT&T said today that when the 3G Microcell is active, the location capabilities of the iPhone are non-functional. So finding locations, such as restaurants, and associated directions requires disconnecting from Microcell. Of course, reconnecting can take 10 to 15 minutes while the 3G and GPS signals are reacquired. Does this seem like a design that still needs some work?