Tuesday, May 6, 2014

The cost of sharing a docomo data plan between a phone and a tablet

UPDATE: I misunderstood the cost of the docomo data-only plans. I neglected to account for the ¥1,700 "data plan" price, which is a newly added base fee onto data-only devices. This over doubles the cost of data for a tablet and drastically changes the outcome of the below simulation. See here for a revised estimate.

In the previous post, I described the effect of variable monthly usage on the total, longterm cost of a data plan. To keep things simple, I considered only one person (me) using one device (my phone). The outcome was that both the 5 GB and 2 GB plans (coming soon from Docomo) would be less expensive for me than just continuing to use my current 7 GB plan. There was a good chance that the 2 GB would offer the most savings, but only by a very modest amount of a few thousand yen over 2 years if I don’t modify my future behavior to minimize the number of times I would exceeding the 2 GB quota.

This result is really not surprising and could be obtained without all the extra work of a Monte Carlo simulation, but it was a good way to introduce the framework. Now, I’ll expand it to consider more sources of variance (i.e., more devices). For now I’ll continue to keep it simple and consider just two devices, so the result will again be a bit obvious. (For the next post on this topic, I’ll consider multiple people sharing a large family data plan, the results of which will be much more difficult to predict without simulation.)

Current data plans

Like the previous example, this post will also consider my real usage. I currently have two active SIM cards, one voice+data and one data-only:
The FAIR data plan is the most expensive offered by b-mobile, but it is the only one that has the same average speed and latency as a standard docomo plan. The 1 GB is good for 4 months, so it is ideally suited for devices that consume only 250 MB per month. Of course there are much cheaper ways to get this small amount of monthly data, but speed and responsiveness are hugely sacrificed. (See here for a series of posts on the b-mobile FAIR SIM)
  1. 7 GB Docomo Xi Flat rate (¥5,700/month)
  2. 1 GB b-mobile FAIR (¥7,953/4 months)
Both the 2 GB and 5 GB plans are for individuals but will allow sharing of data between and phone and a data-only device, so I could potentially cancel my current b-mobile plan and get a second docomo SIM for my data-only device. (Swapping out the FAIR SIM for a less expensive but much slower sub-1,000 yen SIM card is not an option for this particular application and would cause me massive headaches if I did, so I find it worth the premium price.)

The above probability density functions (PDFs) show how I expect my phone (right) and tablet (left) to consume data in the future. The b-mobile FAIR SIM uses around 250 MB per month, often a bit less, so you can probably already imagine, based on the result outlined in my previous post, that canceling it would save money.

Data plan options

To keep the high speed and low latency of the FAIR SIM, I have three options:
  1. 7 GB Xi and b-mobile FAIR: ¥5,700+ approx. ¥2,000/month
  2. 2 GB Xi and additional data-only SIM: ¥3,500+¥800/month ¥2,400
  3. 5 GB Xi and additional data-only SIM: ¥5,000+¥800/month ¥2,400
This would work out to monthly costs of at least ¥7,700 for the current set up of docomo + b-mobile, a minimum of ¥4,300 ¥5,900 for the 2 GB share plan, and ¥6,300 ¥7,400 for the 5 GB share plan. (Each additional shared line costs ¥1,700 for the "data plan," ¥500 and the ISP fee, which is ¥200 for the cheapest Mopera U plan.)

The outcome

See this new post for new results taking into account the data plan cost.

Below are the results of performing 100,000 simulations using the PDFs shown above. In 62% of the simulations, the 2 GB plan (red) works out to be less expensive than the 5 GB plan (green), but on average, only by a very very tiny amount (¥40/month!). The cost of having two separate plans from b-mobile and docomo jumps every four months, reflecting the recharge interval of the FAIR SIM.

Outlined below (from the least expensive over 2 years) are the three ways to proceed:
  • ¥140,000 (Both the 2 GB and 5 GB plan split across two devices)
  • ¥170,000 (General cost for the 2 GB or the 5 GB and the FAIR SIM (not shown)
  • ¥185,000 (Cost of 7 GB plan and FAIR SIM — status quo)
Not surprisingly, continuing to pay for two separate data plans has zero chance of being less expensive, and the total potential savings of combining the two devices onto one data plan is over ¥40,000 — enough for a new Nexus phone/tablet after 2 years. Though the 2 GB has a slightly higher chance of costing less in total, the mean cost of the 2 GB and 5 GB plans is virtually identical.

The primary thing that I learn from this exercise is that my usage really fits neither the 2 GB plan nor the 5 GB plan; something in the middle (that doesn't exist, probably for this exact reason) would be the most effective. I am unlikely to exceed the 2 GB quota often enough to make the 5 GB plan less expensive. This indicates that If I made a conscious effort to reduce usage, I could probably save an additional ¥10,000 over 2 years (based on the standard deviation of the 2 GB result). On the other hand, I could continue using my devices as I do now, without really caring at all about how much data I consume. I could even increase my usage and still save money.


  1. Thanks for a very interesting post.

    Out of interest, what software did you use to generate the probability density functions? And perform the Monte Carlo simulations?

  2. Thanks. I used MATLAB, though the same code will run on octave.