Not all 5G is equal
Interoperability between devices in private 5G networks today
Article by Paul Cooper, Director of Engineering, Zeetta Networks
5G standalone (SA) private cellular networks (PCN) are evolving at speed. As this new technology emerges interoperability challenges need to be addressed. This article looks at some of the challenges seen connecting devices to an Open RAN (O-RAN) 5G SA PCN and current workarounds required to test interoperability .
In the 5G ENCODE project we are supporting there are multiple 5G SA PCN environments for both use case testing and interoperability testing. There are two types of environments:
- Test laboratories (testlabs)
- An O-RAN trial network containing multiple cells deployed across multiple sites
Whilst the trial network is O-RAN the interoperability issues discussed in this article may also occur in other implementations of 5G SA RAN.
Within each test and trial environment consideration needs to be given to the configuration of the following parameters:
- Public Land Mobile Network (PLMN)
- International Mobile Subscriber Identity (IMSI) and Subscriber Identity Module (SIM)
- Access Point Name (APN)
Note: a PLMN is composed of two parts; the mobile country code (MCC) and mobile network code (MNC)
Figure – Simplified 5G and 4G RAN data networks 
In the testlabs, 5G SA PCN solutions from two different vendors exist. The testlabs for 5G ENCODE are, currently, single radio environments intended to create end to end-user connections to the internet.
End to end connectivity: Ensures that the end-user devices used and 5G carrier network pass data traffic. Often, speedtest.net and youtube.com are used to check end to end connectivity, however, many other sites and methods may be used.
SIMs and PLMN: testlabs use MCC 001, MNC 01, thus, IMSI’s are prefixed with 00101
APN: this is a text string. Typically, the text string contains some reference to the testlab name or 5G technology.
Figure: 5G SA transportable solution
The trial 5G SA PCN network has been built inside two industrial buildings and is designed for mobility, high bandwidth and low latency use case proving. The network in each building is, in fact, a separate network for easier integration and to remove reliance on the interbuilding IT connection outside of project control.
End to end connectivity: Ensures end-user devices and 5G carrier network pass data traffic end to end. Download speeds need to reach 300-400Mbps and upload speeds need to reach or exceed 50-70Mbps. This differs to the public 5G access speeds where uncontrollable variables are introduced, for example, other users on the network.
Note: for completeness of information 4G traffic download speeds can reach 300Mbps and upload speeds of 150Mbps in good radio conditions using LTE-Advanced. Theoretical maximum speeds in 5G exceed in downlink 10Gbps and on uplink 1Gbps .
SIMs and PLMN: the trial network uses a test MCC: 999, and a two-digit MNC: 42. The MCC and MNC form the PLMN that is used in both 4G and 5G RANs, thus, IMSI’s are prefixed with PLMN 99942. Each IMSI can attach to either RAN technology and is independent of the device in which it is used. This introduces a complexity that the device attachment conditions need to be managed. The APN configured for data services is used to manage attachments.
APN: To devices in the trials network attach and use the desired data service for the use case in the test, the 4G APN and 5G APN are named differently. This means a device will not pass data on a RAN technology that it is not supposed to when testing.
UE and CPE connection
The maturity and availability of fully standalone 5G enabled end-user devices, both UE and CPE, is limited at present.
Note: many communication service providers (CSP) have launched networks with a 5G RAN known as 5G non-standalone (NSA). In a 5G NSA network UE’s and CPE’s attach using the 4G LTE service. Once attached UE and CPE connections are then transferred to the 5G NSA service as needed. User transfer to 5G NSA may be initiated when a high bandwidth or low latency data service is requested, for example, live streaming video.
IMPORTANT: The UE and CPE devices used in 5G NSA differ from 5G SA devices. This must be considered when making UE and CPE device purchases. For example, a device packaged as 5G ready may be ready for 5G NSA only. 5G NSA devices cannot make a connection to a 5G SA network.
With the 5G SA capable UE and CPE devices acquired there are further limitations on many devices to consider. These are as follows:
- IMSI on SIM must use test PLMN, thus the IMSI must begin with the characters 00101
- UE’s and CPE’s need roaming enabled where network differs from the test PLMN 00101
- Devices may need manual configuration to force scanning of 5G SA RAN
The radio spectrum available for use in mobile telecommunications is defined as bands of frequencies. 5G RAN, in the UK, trials and shared access licenses are issued for radio spectrum in bands n77 and n78 .
Note: Trials licenses may be purchased from the licensing authority (in the UK this is OFCOM) for short periods of testing. Shared access licenses may be purchased for extended trials or longer-term use of radio capacity. Different countries have different processes and regulations governing radio spectrum and licensing.
It is recommended that CPE (and UE when possible) user devices are configured to only scan the bands used. In the testbed and trial networks bands n77 and n78 are used. This recommendation can only be applied on certain devices (mostly CPE’s). When the device is constrained to scanning certain bands, scan and attach times are improved.
IMSI limitation and roaming
The Network PLMN does not need changing to accommodate IMSI’s with PLMN 00101 (test network), however, the IMSI with PLMN 00101 needs to be registered in the HSS (part of the core) in the RAN. As the network is using a different PLMN the UE and CPE need to have roaming enabled to attach.
Manual configuration for 5G SA
To force a user device to scan 5G radio availability its configuration must be changed in developer or engineering settings (this setting is normally hidden to prevent device misconfiguration by inexperienced users). Each device has a different method to enable 5G radio scanning. Device suppliers will provide engineering mode/developer mode access method when requested, however, configuration methods and code for commonly available devices can be found on the internet.
With the user device in engineering or developer mode, locate the setting that requests a 5G SA / NSA scan by the device. Your 5G SA RAN will now be listed as available to be attached to when a manual network scan is triggered on the UE. CPE devices will likely auto scan and show the 5G network will appear.
Summary and findings
Whilst 5G SA and 5G SA PCN remain as emerging technologies, the high bandwidth low latency features offered by local 5G solutions will be attractive for industrial and commercial use. Large costs can be avoided by not needing to install network cabling each time a new device is needed or a device needs repositioning. Mobility to move on a public CSP when leaving a PCN enables uninterrupted tracking of mobile assets.
Many parts of the 5G SA and PCN solutions need additional work to complete features and interoperability, finalise RAN selection processes and preferences and create the seamless mobility needed.
 O-RAN alliance: O-RAN ALLIANCE
 4G LTE overview – Full LTE architecture and components (yatebts.com)
 4G and 5G speeds: https://www.4g.co.uk/how-fast-is-4g/
 5G new radio (NR) bands: 5G NR frequency bands – Wikipedia
This article could not exist without the invaluable input from the following contributors:
Marc Ranford – NCC new technologies smart industry
Sachin Sree Raman – NCC project manager smart factories
Luis Amaral – Zeetta delivery engineer
David Hepton – Zeetta delivery engineer
Alan Holt – Zeetta network specialist
It is also acknowledged that there are many other contributors to the information included in this article.