AT&T Announces 5G Roaming in Japan when 5G roam-capable devices are available

AT&T 5G customers will be the first U.S. mobile customers to have the ability to access 5G in Japan through an international roaming agreement with NTT DOCOMO, Inc. and by using a 5G roam-capable device – the first of which will become available on the AT&T network this week.

The agreement delivers on AT&T’s commitment to keeping its customers connected and providing them with access to 5G – at home and around the globe – as capable devices become available.  However, this agreement is only for 5G NSA which uses an LTE anchor/core for all non-RAN related functions.

“As a longtime industry leader providing global connectivity, we’re evolving the breadth and depth of our global coverage with 5G,” said Susan A. Johnson, executive vice president – Global Connections & Supply Chain, AT&T. “This marks a significant milestone for AT&T and our customers.”

AT&T customers on a wireless plan that includes access to 5G in the U.S. will be able to access 5G in Japan through NTT DOCOMO, Inc., when they use a 5G roam-capable device.

But what are those 5G roam-capable devices and when will they be available?  They will surely be based on 3GPP Release 15 5GNR for the data plane, and 4G LTE for everything else, e.g. signaling, network management, security, 4G core/EPC, etc.  Hence, this arrangement might be called “5G NSA roaming with a LTE core network/EPC.”  5G SA roaming will be much more complicated as there is no standard for 5G core network and all the “cloud native” functions (like network slicing and virtualization) it will support.

“We’re excited to launch 5G roaming in Japan on NTT DOCOMO, Inc.’s network because of what it means for our customers,” said JR Wilson, vice president – Tower Strategy & Roaming, AT&T. “Our teams never stop innovating, developing and deploying the latest technology to provide our customers with access to a next-generation network – at home and abroad.”

AT&T claims they have historically been a leader in global roaming. So, while the company continues innovating to enable customers with compatible devices to access 5G internationally, those with LTE devices can still expect the same great roaming coverage they’re used to while traveling.

NTT DOCOMO is the first mobile operator in Japan to offer an inbound 5G roaming service.  Going forward, DOCOMO plans to expand its inbound 5G roaming service with mobile operators around the globe.  Again, that will surely be for 5G NSA service.


Addendum:  5G SA roaming will be much more difficult

Roaming with a 5G SA endpoint device (smartphone, tablet, notebook PC, robot/industrial equipment, etc. won’t be achieved anytime soon. Why? There is no standard for 5G core network which have to interoperate with one another for roaming.

The 3GPP core network specs are architecture documents that do NOT specify implementation details.  Therefore, every 5G network provider will have its own 5G core spec which is implemented in the core network routers/compute servers and endpoints.  So each 5G endpoint (e.g. smartphone or tablet/notebook PC) will have to download a 5G SA software update from the 5G network provider.  That means that unless there are bilateral 5G carrier roaming agreements, a 5G end point can only work on the 5G core network it’s subscribed to, e.g. no portability or interoperability!

For example, there are many software choices for implementing a “cloud native” 5G Core: containers, virtualized network functions, kubernetes, micro-services. Each Network Function (NF) offers one or more services to other NFs via Application Programming Interfaces (APIs).  And there is no standard for the APIs associated with a given Network Function!

3GPP Rel 16 5G Core/Architecture specs: 

  • TS 23.501 5G Systems Architecture, with annexes which describe 5G core deployment scenarios:
  • TS 23.502 [3] contains the stage 2 procedures and flows for 5G System
  • TS 23.503 [45] contains the stage 2 Policy Control and Charging architecture for 5G System