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Roaming experience in Japan


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I'm currently typing this from a bullet train headed from Tokyo to Osaka. Using a roaming eSIM rather than T-Mobile as it's a lot cheaper, but I'll start with T-Mobile's roaming experience.

Since I have a business line, I can't add data packs online, so I'm just using the 256 kbps baseline service you get by default. That service runs on Softank 4G. SoftBank has a well-built-out LTE network though, with plenty of B41, falling back to B1/3/8 as needed. 5G roaming from T-Mobile doesn't appear to exist though. I've seen 20+10 MHz B41 when I've looked, generally speaking.

WiFi calling works well, and voice calls over LTE work fine too (I forgot to turn WiFi back on after doing some testing, so I expect my bill to be a dollar more next month). I want to say I even got HD voice over the cell network for the VoLTE call I did.

I have a bunch of eSIMs and a couple of physical SIMs to try out. I've gotten the eSIMs up and running, but last I checked the physical SIM wasn't working even after activation so I'll run through eSIMs for the moment and update this thread with pSIM info and details on not-Tokyo in the coming days.

First off, there's US Mobile's complimentary East Asia eSIM (5GB) that I grabbed before my unlimited plan Stateside expired. That SIM uses SIM Club, routing through Singapore, running on SoftBank LTE and 5G. I've seen 40 MHz n77, as well as 10x10 n28, and have seen download speeds in excess of 200 Mbps with uploads of more than 50 Mbps, though typical speeds are slower. Routing is via Equinix/Packet.net. 5G coverage is rather spotty, but LTE is plenty fast enough; either my phone doesn't want to use the 5G band combos that have more coverage or 5G coverage is just spottier here than in the US (at least on T-Mibile). Latency is as low as 95ms to sites in Singapore (usually closer to 120ms), which is pretty great considering the 3300 mi between Tokyo and Singapore.

Next there's Ubigi. It also routes through Singapore via Transatel (despite being owned by NTT), and sites on top of NTT docomo's network. I didn't see NTT 5G in Tokyo when I tested it, but since then I've seen 10x10 n28, and have seen B1/B3/B19 on the LTE side. So far it's not the fastest thing out there, but I'm guessing coverage will be a little better...or maybe not. This was $17 for 10GB. Latency is a bit higher to Singapore, but still under 150ms it seems.

Then there's Airalo, which was the cheapest when I bought it at $9 for 10GB. It also routes through Singapore (on Singtel), but on my S24 I have my pick of KDDI (au) or SoftBank. KDDI has extensive B41 coverage and I've seen 20+20 with UL CA. While waiting for the train at HND Terminal 3 (Keikyu line) I hit 250+ Mbps down and 10+ Mbps up...over LTE...with pretty respectable latency numbers (not much above 100ms). This is in adition to supporting SoftBank, also on LTE (my S24 defaulted to KDDI, while my wife's Pixel 8 defaulted to SoftBank and didn't seem to want to connect to KDDI). Of the various carriers mentioned, I'd say this was the best pick, though prices have bumped back up to $18 for the 10GB plan...but it's probably still what I'd pick if I had to pick just one carrier.

Then there's Saily, which uses Truphone out of Hong Kong. I haven't used this as much, as I only grabbed 3GB for $7. It runs on NTT but doesn't seem to have 5G access and doesn't seem to have as good speeds. Yes, Hong Kong is way closer to Japan, but latency didn't seem to be any better, at ~150ms.

In all cases, I've had reception even in train tunnels and even at high speed on the bullet train, on all three carriers I've tried (I don't think I'll be able to play with a Rakuten SIM, which is rather disappointing). There have been cases where service has degraded, but it looks like you'd have reasonable cell service no matter which of the big three carriers you picked...and since T-Mobile roams on one of them, that's good enough if you're content to buy day passes.

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I've now seen 100 MHz n77 from SoftBank and 100 MHz n78 from NTT. NTT seems to be a bit better south of Osaka, though in some cases it drops down to B19 LTE as some areas around here are pretty rural. SoftBank has n77 around, but it's flakey enough that I switched eSIMs earlier this morning.

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I've now seen how things work in Kobe, Hiroshima, and Osaka, as well as some areas south of Osaka (e.g. Wakayama, Kinokawa), and tried three more SIMs. The two physical SIMs (different branding for each) both use IIJ, which provides a Japanese IP address/routing on NTT, aleit LTE-only, so latency is ~45ms to Tokyo.

The catch with NTT is that it uses two frequency bands (B42/3500 MHz LTE, n79/4900 MHz NR) that you're not going to get on an Android sold in the US, and I'm guessing that B42 would be helpful speed-wise on that network, as it doesn't have B41.

I also found one place that doesn't have cell service: a vending machine in the back of the Osaka Castle tower. Or, rather, the B8/18/19 signal is weak enough there to be unusable.

Going back to 5G for a moment, I saw a fair amount of Softbank n257 in Hiroshima, as well as in some train stations between Osaka and Kobe. 4x100 MHz bandwidth, anchored by B1/3/8, with speeds sometimes exceeding 400 Mbps on the US Mobile roaming eSIM. Not quite the speeds I've seen on mmW in the States, but I've probably been on mmW for more time over the past few days than I have in the US over the past year, so I'll take it.

My fastest speed test was actually on SoftBank n77 though, with 100 MHz of that plus 10x10 B8 hitting ~700 Mbps down and ~80 Mbps up with ~100ms latency...on the roaming eSIM...on the 4th floor of the hotel near Shin-Kobe station. Guessing B8 was a DAS or small cell based on signal levels, and the n77 might have been (or was just a less-used sector of the site serving the train station).

I'm now 99% sure that all three providers are running DSS on band 28, and I've seen 10x10 on similar frequencies from both NTT and SoftBank IIRC, on both LTE and 5G.

I also picked up one more eSIM: my1010, which is different from 1010/csl used by US Mobile's eSIM unfortunately, as it's LTE-only. On the bright side, it's cheap (10GB/7 days is like $11, and 20GB for the same period would be around $15), and can use both KDDI and SoftBank LTE. It also egresses from Taiwan (Chunghwa Telecom), though latency isn't really any better than the Singapore based eSIMs.

Tomorrow will include the most rural part of our journey, so we'll see how networks hold up there, and from tomorrow night on we'll be in Tokyo, so any further reports after that will be Tokyo-centric.

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So, the rural part of the journey still has cell service for nearly all the way, usually on B18/19/8 (depending on whether we're talking about KDDI/NTT/SoftBank). I think I saw a bit of B28 and even n28 early on in the trip, though that faded out after a bit.

Once we got to where we were going though, KDDI had enough B41 to pull 150+ Mbps, while NTT and SoftBank had B1/B3 IIRC. Cell service was likewise generally fine from Kawaguchiko Station to Tokyo on the express bus to Shinjuku Station, though there were some cases where only low-band LTE was available and capacity seemed to struggle.

I also figured out what I was seeing with SoftBank on 40 MHz vs. 100 MHz n77: the 40 MHz blocks are actually inside the n78 band class, but SoftBank advertises them as n77, probably to facilitate NR CA. My phone likely preferred the 40 MHz slices as they're *much* lower-frequency, ~3.4 GHz rather than ~3.9, though of course I did see the 100 MHz slice being used rather often. By contrast, when I got NR on NTT it was either n28 10x10 or, more often, 100 MHz n78.

As usual, EMEA bands on my S24 don't CA, so any data speeds I saw were the result of either one LTE carrier or one LTE carrier plus one NR carrier...except for B41 LTE. KDDI seems to have more B41 bandwidth live at this point, so my1010 or Airalo works well for this, and honestly while SoftBank and NTT 5G (in descending order of availability) have 5G that's readily available it may be diminishing returns, particularly given that I still don't know how to, as someone not from Hong Kong, get an eSIM that runs on SoftBank 5G that isn't the USM "comes for free with the unlimited premium package" roaming eSIM (NTT is easy enough thanks to Ubigi).

In other news, I was able to borrow someone's Rakuten eSIM and...got LTE with it. 40 Mbps down, 20 Mbps up, 40ms latency to Tokyo while in Tokyo...which isn't any worse than the Japan-based physical SIMs I had used earlier. But not getting n77 or n257 was disappointing, though I had to test the eSIM from one spot rather than bouncing around the city to find somewhere with better reception. It's currently impossible to get a SIM as a foreigner that runs on Rakuten, so that was the best I could do.

Also, I know my phone doesn't have all the LTE and 5G bands needed to take full advantage of Japanese networks. My S24 is missing:

  • B21 (1500 MHz) - NTT
  • B11 (1500 MHz) - KDDI, SoftBank
  • B42 (3500 MHz) - NTT, KDDI, SoftBank
  • n79 (4900 MHz) - NTT

Of the above, B42/n79 are available on the latest iPhones, though you lose n257, and I'm guessing you're not going to find B11/B21 on a phone sold outside Japan.

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So, in summary, here are the options I tested:

  • T-Mobile intl roaming - LTE on SoftBank, routes back to the US (~220ms to 4.2.2.4)
  • IIJ physical SIM - LTE on NTT, local routing
  • Airalo - LTE on SoftBank and KDDI (seems to prefer SoftBank), routed through Singapore (SingTel)
  • Ubigi - 5G on NTT, routed through Singapore (Transatel)
  • US Mobile East Asia roaming - 5G on SoftBank, routed through Singapore (Club SIM)
  • Saily - 5G on NTT, routed through Hong Kong (Truphone)...seems to be poorer routing
  • my1010 - LTE on SoftBank and KDDI (seems to prefer KDDI), routed through Taiwan (Chunghwa Telecom)

I wouldn't buy up on the T-Mobile international roaming, but it's a solid fallback. If you have the US Mobile roaming eSIM that's a great option. Otherwise Ubigi, Airalo, or my1010 are all solid options, so get whatever's cheapest.

I wouldn't bother trying to find a physical SIM from IIJ...the Japanese IP is nice but there's enough WiFi that you can get a Japanese IP enough for whatever you need, and eSIM flexibility is great (IIJ as eSIM but seems a bit more involved to get it to work).

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