A new report from the analysts at Signals Research Group (SRG) indicates that Rakuten is capping high-speed access to its 5G network at 5GB per day and 20GB per month.

"The strict policy was a bit surprising and not consistent with what we've observed in the past, including on other 5G networks in Japan, but in hindsight it should be expected given the operator's modest amount of LTE spectrum and its 5G network coverage," the analysts wrote in their report.

Mike Thelander, founder of SRG, said the Rakuten data speeds dropped into the low single-digit Mbit/s range after hitting the cap, and appeared to do so regardless of whether the network was congested or not.

The findings are noteworthy considering Rakuten bills its service as unlimited. However, if the throttling is indeed widespread across Rakuten's network, the operator isn't necessarily the only one in the world applying such caps. In the US market, operators including AT&T and Verizon have long applied such high-speed usage caps. After customers consume the allotted data, their speeds are slowed dramatically.

Figure 1: (Source: PriceMinister on Flickr CC2.0)

SRG said it obtained its findings during a lengthy series of tests across Tokyo using five phones with unlimited data usage SIMs. Two of the phones also offered the ability to log chipset data. SRG used professional network testing equipment from Accuver Americas and Spirent Communications. The company has conducted similar tests all over the world.

"Although the data usage may have been unlimited, there was data throttling with relatively minor data usage – we believe these commercial SIMs had a 5 GB daily cap and a 20 GB monthly cap, after which point the throughput was severely reduced. It was crystal clear when the throttling occurred," they wrote.

In response to the report, Rakuten officials suggested the issue may have to do with its roaming parameters. "The testing devices might have latched to a roaming network despite Rakuten Mobile coverage availability and testing might have occurred in roaming partner coverage areas. We’re continually working to increase the overall network performance through regular network firmware upgrades," the company said in a statement to Light Reading.

But SRG's Thelander said the company's testing was conducted on Rakuten's network.

Continued Rakuten in its written statement: "Users can enjoy unlimited data use when connected to a Rakuten Mobile base station. As mentioned on our website (Japanese only), we have a fair use policy in place where exceeding a daily data use cap, network speeds are throttled to moderate speeds until midnight when normal speeds are resumed. This ensures a fair service to our customers. When roaming on a domestic network, connection speeds are throttled to 1Mbps once the 5GB monthly cap is reached. When roaming on an international network, connection speeds are throttled to 128kbps when reaching 2GB a month. Customers can purchase data top-up to resume normal speeds."

Analysts at SRG also reported a number of other anomalies in Rakuten's network.

"Although the network supported most features associated with a best-in-class 5G network, there are opportunities for meaningful improvements to many of these features," they wrote. "The most obvious opportunity pertains to the simultaneous scheduling of data packets over 5G and LTE with the HTTP protocol. It was very clear in our results that network performance was severely impacted with HTTP data transfers, degrading performance over both LTE and especially 5G."

Another surprise was coverage: "Given the population density of Tokyo and our familiarity with midband 5G coverage in the US, we expected to encounter midband 5G much of the time in our testing. Expectations did not meet reality. Although Rakuten's 5G coverage was relatively modest, it wasn't that much different from KDDI's and NTT DoCoMo's 5G coverage," they wrote.

The findings are noteworthy considering Rakuten has gained worldwide attention in its efforts to build a cloud-native 5G network with open RAN specifications – one of the first in the world to do so. The company is also in the midst of leveraging its networking learnings in Japan into a product it calls Symphony. It's now selling that platform worldwide to mobile network operators.

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— Mike Dano, Editorial Director, 5G & Mobile Strategies, Light Reading | @mikeddano