Wi-Fi and Femto Offload Challenges

Dean Bubley over at Disruptive Wireless has recently published an interesting post in which he wonders whether he has or will have a problem with Wi-Fi and femto offload in the future.

He describes how he is walking from his home to the tube station and his mobile device in the pocket picking up several known Wi-Fi hotspots from BT and connecting to them for a couple of seconds just to loose them again. This wreaks havoc on applications that are actively exchanging information over the network even when the phone is not actively used as the connection is dropped every time. An interesting point he makes there and I don't have a single and simple solution for it but some things come to my mind on how to deal with this:

On the Wi-Fi side:

  • The connection manager could wait for a couple of seconds checking the signal strength and only connecting when it stays relatively constant, i.e. the user is not moving and hence, the Wi-Fi hotspot is worth using.
  • Distinguish between public Wi-Fi hotspots where such rules should be applied and encrypted home / office hotspots for which it could apply different rules.
  • A good Wi-Fi offload solution would be not to get a new IP address in every new hotspot and to break the 3G connectivity. Ideally, an IP tunnel is used so no matter whether connected over 3G or Wi-Fi the device would always use the same IP address. That would also deal with the security issues of unencrypted public Wi-Fi hotspots.

On the Femto side:

  • I think there would be less of a problem here because GSM, UMTS and LTE offer myriads of possibilities to ensure a mobile only uses a femto if it is not moving.
  • There is no need to assign a new IP address when hopping between femtos so connectivity would not be broken when jumping into or out of a femto.
  • The connection can be handed over from and to a femto so continued connectivity with little or no outage can be ensured.
  • Access to a femto can be restricted. This book goes into the details of this.

So, yes, small cells are a challenge for mobility but there are options to deal with this.

4 thoughts on “Wi-Fi and Femto Offload Challenges”

  1. Hi Martin,

    I would like to ask you a question about traffic offload from 3G to WiFi networks.

    Do you think the reason for this offload is to reduce the load on the 3G radio access network or on the mobile operator’s core network? In other words, do you think the bottleneck is the 3G RAN or the CN?

    Thank you very much in advance.

    Best regards,


  2. Hi Jamie,

    well, any part of a network can of course be under-dimensioned 🙂 I see the benefit of a macro network offload mainly in reducing the overall cost of the RAN. The core seems to be less of a problem to me. After all, these are just boxes that are used for normal IP routing as well with some software on top to handle the peculiarities of moving subscribers. That takes of course capacity and processing power but I think that is manageable at a reasonable cost.


  3. An advantage that Wi-Fi offload has over femtocells is that there are separate receivers for the Wi-Fi and cellular signals, so phone can simultaneously search for (and use) both Wi-Fi and cellular.

    Since few if any phones have two UMTS receivers, in order for phones operating in UMTS mode to move back and forth between femtocells and the macro network, Compressed Mode has to be permanently enabled on both cell types. The result is a waste of capacity on both layers.

    Also, the subtending femtocells have to be made neighbors of the host macros, which can be burdensome to Net Ops if the number of femtos becomes large.

  4. 3G / Wi-Fi offload solutions based on the Mobile IP protocol, such as SafeMove from Birdstep Technology (www.birdstep.com), have been specifically designed to allow devices to roam across wireless networks whilst maintaining their IP address so users get a reliable, seamless experience.

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