T-Mobile US’ 5G standalone core is novel insofar as it’s an industry first, but the achievement was also delegated to the same mix of vendors that provision and supply its 4G LTE network core.

That relatively direct transition and unified vendor pool from 4G to 5G allowed the operator to move quickly and beat its competitors in deploying the world’s first 5G standalone (SA) network, according to Kishen Mangat, VP and GM of mobility and automation at Cisco, one of three primary vendors involved in that effort.

Cisco has a long history with T-Mobile. The vendor provided its packet gateway for T-Mobile’s 4G mobile core, later added its evolved packet core (EPC), and eventually virtualized the operator’s entire packet core in 2017, making it the “largest fully virtualized packet core of any operator” at that time, he said. 

“We have an established track record of innovating and pioneering new mobile core technology with T-Mobile. They invest pretty significantly around new technology introduction and testing, so it’s actually a really good place for us to take our new innovations and really put them to the test at large scale with a customer that really likes to move fast,” Mangat said. 

T-Mobile’s 5G Standalone Rides on Cisco’s Service Core

T-Mobile was also the first major operator to introduce Cisco’s 4G control and user plane separation (CUPS) in the EPC at production scale in 2018, and that quickly led to the introduction of a 5G SA core, according to Mangat.

Within T-Mobile’s 5G core, Cisco provides three primary network functions that Mangat refers to as the service core. This includes the user plane, session management, and policy control functions. Those network functions run on Cisco servers, switching, and its virtualization orchestration stack, Mangat explained. 

“T-Mobile has taken a domain approach to the way they’ve approached the disaggregation architecture,” he said. While it follows a common approach to the network core vendor landscape, the operator does move quickly and excels at bringing multiple vendors together for projects of this scale, he added.

“Nothing changed too much from 4G to 5G in terms of the vendor landscape,” Mangat said.

Nokia and Ericsson Extend 4G to 5G Functions

T-Mobile uses Nokia’s mobility management entity (MME) for 4G and its access and mobility management function (AMF) for 5G.  The Finnish vendor extended other 4G to 5G core functions, including policy and charging rules function (PCRF) to policy control function (PCF), and home subscriber server front end to unified data management (UDM), a Nokia spokesperson explained.

Nokia also, according to a spokesperson, added new 5G SA core functions to T-Mobile's network, including network repository function (NRF), network slicing, authentication server function (AUSF), user data repository (UDR), and network exposure function (NEF).

Likewise, Ericsson extended many of the 4G functions it already provided to T-Mobile to a new 5G architecture, according to Mangat.

“They’re discreet subdomains within a domain,” he said. “Vendors develop more expertise in different areas,” and Cisco’s experience with service core technologies in its IP core stands out in that regard. “The core is where the radio network meets the IP network” and the service core provided by Cisco is “like a service brain” for mobile network operators, he said. 

The packet core or mobile core network is typically, but not always, unified by a single vendor by function, Mangat explained. “That split between the user plane, the session management, the policy, and then the signaling gateways, such as MME or AMF, is not uncommon. But it would be somewhat unusual to have multiple vendors within a given function” because of the amount of complexity at the service layer and call flows, he said. 

As such, the current vendor mix on T-Mobile’s 5G core doesn’t differ from the pool of vendors it uses for its 4G network core.

Evolution of the 5G Standalone Core

The development of Cisco’s 5G SA core was “an entirely ground-up effort” that cast aside legacy technology because when mobile networks transition to a highly distributed architecture, management becomes impossible without complete automation, Mangat explained.

“Right now we have 5G NSA (non-standalone) and 5G SA running in parallel,” he said. “The next big milestone is to deploy the converged core next year.”

That effort to add network-wide automation and the ability to “validate multiple Gs into a common G” on the network core has some huge benefits, but it’s not without its challenges, Mangat said. 

“There’s some difficult parts and then there’s also some big payoffs in terms of getting to converged core. Supporting multiple interfaces with performance in a single software instance is going to be non-trivial. We’re already well into the (research and development) work on it,” he said. 

“The key for getting the 4G core onto the 5G cloud-native platform is to take those 4G interfaces and support them in microservices,” he explained. That “critical hurdle,” which Cisco intends to overcome in 2021, involves porting legacy 4G interfaces into microservices technology.

The converged 5G core will be fully built out with T-Mobile next year, according to Mangat.

Editor's note: This story has been updated to clarify the specific network functions that Nokia provides on T-Mobile's 5G SA core.