One of the common concerns about femtocells is that they may use up a lot of broadband internet service, either exceeding a basic allowance or causing additional charges. Let's analyse this with some rough calculations to bring some reality to the debate.
Mobile phones have always had to be frugal in their use of bandwidth. Engineers had to make tradeoffs between:
- excellent voice quality
- good performance in poor radio conditions
- minimising the number of bits sent in order to save cost.
A lot of serious mathematical research has been done to improve the coding of voice into digital form, which is done using codecs (Coder/Decoder). Early GSM systems used a so-called full-rate codec running at 13 kbit/s, the half-rate codec managed to fit two voice calls into the same bandwidth.
The Adaptive Rate Modulation codec takes this further by automatically adjusting coding to balance the quality of voice against poor radio conditions. In bad RF areas, more bits are used to duplicate/protect what's being sent, in good RF areas more bits are used to encode higher quality voice. But in either case, we're talking about 13 kbit/s being a baseline data rate for voice.
You don't talk and listen at the same time
In most phone calls, both parties don't talk at the same time (there may be some countries/families where this isn't the case!). If you include the odd gap for hesitation/thinking and waiting for the other party to respond, a reasonable figure here is about 35% utilisation in each direction. Mobile phone codecs take advantage of this and send only minimal background sound when you are not speaking. This is better than not sending anything at all, but extremely efficient.
Unfortunately, most broadband internet providers measure both uplink and downlink traffic against your usage allowance. So let's assume about 70% (35% each direction) of full rate 13 kbit/s is active during each voice call. We can round this up to 10 kbit/s to make the calculations easier.
How much talking for a GigaByte?
First let's calculate the numbers for the voice itself - we'll add in the extra overheads and protocols afterwards.
Assuming a simple Voice over IP (VoIP) voice call using the standard mobile phone codec.
Lets convert bits to bytes. There are 8 bits in a byte. 10 kbit/s is therefore 1250 bytes/second.
For simplicity, lets round down 1Gbyte to 1 billion bytes.The actual number is a bit larger.
1 Gbyte divided by 1250 bytes = 800,000 seconds
That's more than a week (approx 9.25 days)
or more than 14,000 minutes!
An hour's VoIP phone call would be 3600 seconds = 4.5 MBytes
Usage of hundreds of minutes a month is considered high for mobile phone users.
You could easily squeeze hundreds of VoIP minutes into a GByte.
But there is overhead and other extra data sent
Sure, there are extra layers of data packaging and protocol on top of the basic vocie codec. All data from the femtocell is securely encrytped in a IPsec tunnel to avoid interception. Various signalling protocols are used to setup and tear down the calls.
The actual overhead will vary depending on the traffic scheme used. Verizon Wireless indicate that each call takes about 40 kbit/s - some four times our simple VoIP estimate. That's still about 3 days continous talking per Gbyte.
You can see that letting others use your femtocell for voice calls is negligle for all but the most frugal of broadband packages, and very unlikely to cost you substantial extra money.
Only if you use your laptop for extensive data sessions would traffic be high
Obviously, using femtocells for hgih speed data sessions would ramp up the traffic much more. This would typically need a laptop to use large amounts, and the alternative would have been to use your broadband connection directly.
Have I got my numbers wrong? Too simplistic? Correct or clarify these by commenting below (you can remain anonymous if you want).