Since femtocells launched a few years back, I have been truly amazed by them. To me they offer a glimpse into the future, where wireless communications will be low power, self-regulating & self-optimising. Vodafone in the UK (and in other markets) has been offering a femto product for a while now, mainly aimed at the consumer segment and especially for those people that don't get macro coverage at home. I happen to have one at home (they only cost 50GBP) and have been looking at it a bit more closely lately.
For this post, I "hubbed out" of the femto and monitored the bandwidth utilisation CS calls have on my ADSL. The femto product that Vodafone use (a.k.a. Sure Signal) can support a maximum of 4 simultaneous users, so I tested it up to this limit.
As can be seen from the Wireshark graph below, a single CS call generates about 60kbps of throughput (in one direction). Adding more calls linearly increases this to approx. 250kbps when the maximum of 4 CS calls are taking place. Considering a UMTS CS call using the standard AMR codec runs at 12.2kbps, this means the overhead is approx. 48kbps or 400%!
Whether this is an issue or not very much depends on someones ADSL package. With a low end ADSL package where the uplink can be as low as 512kbps, this represents a considerable portion of it. With better ADSL packages things obviously get better and I guess the probability that 4 people in your home will be on the phone at the same time is rather slim..
For those that are wondering, the uplink vs downlink utilisation is almost the same and the actual payload from the femto is encapsulated using IPSec.
More femto insights will follow in future posts..
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