Page 15 - EETimes Europe June 2021

EE|Times EUROPE 15

Testing Times for 5G Network Synchronization

radio-interface–based techniques that synchronize distributed radio TIME-SENSITIVE NETWORKING
units in the evolved RAN architecture. As already noted, to address fronthaul requirements, the ITU-T
In a typical setup, the upper and lower parts of the 5G New Radio has defined a standard (G.8273.2) that mandates the addition of
(NR) RAN are separated in the different logical units: the centralized enhanced boundary clocks to meet the stringent synchronization
unit (CU), the distributed unit (DU), and the radio unit (RU). In this requirements of disaggregated 5G networks. These specialist timing
way, the baseband function in a base station is separated into two devices allow accurate distribution of timing in the network — for
logical units; the CU would host the higher-layer protocols, while a DU example, by using time-sensitive networking (TSN) Ethernet bridges
would handle the lower layers to the user equipment. that incorporate such telecom boundary clocks.
Moreover, network-based timing offers improved visibility of
SATELLITE SIGNALS flaws and, in combination with PTP, makes it possible to monitor the
While synchronization for backhaul in 5G will be similar to that for flow from the grandmaster clock to the PTP client. That offers the
LTE, there are significant challenges for fronthaul in the absence of a network provider a complete view of all synchronized PTP clients,
synchronous interface, said Hussain. enhancing network visibility and therefore making the network
Deploying satellite receivers at each RU and using GPS or GNSS, as is much easier to control.
common now, will not be cost-effective, notably for small cells, C-band TSN, of course, has been widely used in the fixed network,
radios, and millimeter-wave (mmWave) radios. Hussain said there are where very low latency is required. To promote TSN’s use in the
likely to be satellite connections at the Centralized RAN (C-RAN) hub wireless sector, silicon providers and networking gear vendors
location with tight timing controls out to the radios. have established an interoperability and testing program under the
In effect, this means timing and synchronization distribution is auspices of the Avnu Alliance. The initiative includes companies
collapsed to work over Ethernet, and in many cases, PTP will be used such as Intel, Broadcom, NXP, Microchip, Cisco, Extreme Networks,
to distribute time of day (ToD) and Synchronous Ethernet (SyncE) to and Keysight.
distribute frequency so that RUs will be synchronized over Ethernet. Importantly, the latest Release 16 for 5G under the auspices of the
Ricardo Querios, head of RAN Security OAM and Transport at 3GPP includes TSN support.
Ericsson, concurred. “Both GNSS and PTP can be sufficient, and the But the TSN standards were specified by the IEEE with Ethernet
choice will depend on timing and costs,” he said in an e-mail exchange in mind and thus target the link layer of a network, which is not the
with EE Times Europe. “Many operators prefer to have redundancy, case with 3GPP 5G standards or the 802.11 Wi-Fi communications
though … and will likely use several sources and combinations of GNSS links. In both the latter, these specifications are embedded in the
and PTP to synchronize the RAN nodes. communications layer.
“Complementary technologies may be relevant for cases that require The emerging Wi-Fi 6 and 6E networks, based on the 802.11ax
GNSS redundancy” — for example, to protect against jamming or spoof- standard, deploy a different scheduling mechanism that can more
ing events, he added. “And there will be cases where GNSS is just not accurately and efficiently schedule simultaneous transmissions from
feasible, due to satellites’ visibility issues.” multiple devices. That makes it possible to provide bounded latency
Hussain noted that “reliability is arguably the biggest challenge, and high reliability.
and the problem with in-building access to satellite signal operation is TSN can be made to work with Wi-Fi and 5G by integrating it over
not far behind. And if you think about scaling that up, that is just not the top so that it will have minimal impact on the RAN. The 3GPP
economically feasible.” approach outlined in Release 16 calls for the TSN time-domain
information to be distributed between the
TSN translator functions in the network and
the device.
The next iteration of the 3GPP specifi-
cations, Release 17, is expected to simplify
matters so that the TSN capability will
reside within the 5G device.
There are other issues to be resolved,
most importantly those related to mobility,
because as devices move from one cell site
to another, connectivity can be disrupted,
impacting latency and reliability.
But since the applications in which wireless
TSNs are most likely to figure — such as indus-
trial IoT and robotics — have relatively low
mobility requirements, those working on the
dilemma believe the challenge can be solved.
It is highly likely that as operators con-
tinue to roll out 5G, they will increasingly
look to network-based timing as a backup
source of synchronization. The advantages
include a way to circumvent the security
issues around satellite-based systems,
clearer visibility of synchronization flows,
and the potential to reduce cell site costs. ■
John Walko is a technology writer and an
Recommended synchronization solutions for 5G (Source: Ericsson) EE Times contributing editor.

www.eetimes.eu | JUNE 2021