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August 4, 2020
Time-sensitive networking (TSN) may not be a familiar acronym to most in the wireless industry, but it is to players in the Ethernet world. TSN is the ability to ensure data traffic is delivered in a timely manner over a network, and it has typically been used by wired networks because it requires very low latency. However, the latest 5G and 802.11 Wi-Fi standards, which support ultra-reliable low latency communications (URLLC), make TSN over wireless networks a possibility.
To push TSN into the wireless realm, a group of silicon suppliers and networking vendors working under the auspices of the Avnu Alliance is creating an interoperability testing and device certification program for TSN in wireless. Members of the Avnu Alliance include some familiar names like Intel, Keysight Technologies, General Electric and Extreme Networks.
The TSN standard was initially developed because standard IT networks didn't have any concept of time and couldn't synchronize or provide precision timing. This made the delivery of data unreliable, particularly if there is network congestion. Congestion, throttling and buffering cause delays, which are unacceptable for many applications.
According to Dave Cavalcanti, principal engineer at Intel and Wireless TSN workgroup chair at Avnu Alliance, the TSN family of standards was developed by the IEEE and originally crafted with Ethernet in mind. "It was designed to make Ethernet more deterministic," Cavalcanti said, alluding to the fact that a deterministic system removes the randomness of any output.
The TSN standards, according to Cavalcanti, are involved at the link layer of the network, which is different from the 3GPP 5G standards or the 802.11 Wi-Fi communications standards, which reside in the communications layer of the network.
Originally, TSN was developed for applications such as professional video and audio. However, Cavalcanti said that it has now expanded to include other applications such as industrial manufacturing and Internet of Things (IoT).
"Those applications, such as control systems in factories or robots, are the things that are driving most of the development today," he noted.
Wireless is hard
One of the main criteria for making wireless TSN a reality is a network that can provide "bounded" latency. In other words, a time-sensitive application needs to know what the worst case is, rather than the average latency. That means that the network must guarantee a bounded latency number, which is really the worst-case scenario.
For example, a TSN-enabled network must ensure that every packet is delivered within a given latency bound with no packet losses and delays due to congestion.
But in a wireless network this can be a challenge because delays are often introduced in the network. However, according to an Avnu white paper, Wi-Fi 6 networks, which are based on the latest 802.11ax standard, have a different scheduling mechanism than prior Wi-Fi network standards. This new scheduling mechanism can more efficiently schedule simultaneous transmissions from multiple devices. This can eliminate delays, and can make it possible to provide bounded latency and high reliability.
Likewise, the 5G standard, 3GPP Release 16, introduces TSN support over 5G. But unlike Wi-Fi 6, 5G isn't a local area network technology so it can't be directly integrated with Ethernet TSN standards. Instead, the 3GPP approach is to integrate TSN over the top, which means TSN will have little impact on the radio access network. In 3GPP Release 16, the TSN time domain information is distributed between the TSN translator functions in the network and the device using the 802.1AS standard protocol. More work is expected to occur in 3GPP Release 17 and it is anticipated that in this release the TSN will reside in the 5G device.
Mobility is a different story
Making TSN work with Wi-Fi or 5G network is one thing, but making it work when a device is moving is a whole other challenge because as devices move from cell site to cell site that can disrupt connectivity and impact latency and reliability. "How do you keep the TSN performance guarantees when you are moving?" asked Cavalcanti. "That is one of the challenges we are exploring now."
Cavalcanti added that the mobility factor is a big issue because there can be no disruptions with wireless TSN. However, he added that many of the current applications for wireless TSN, such as industrial IoT and robotics, tend to have very low mobility requirements. "Robots move at certain speeds but it is not extremely fast," he said.
For wireless operators, however, Cavalcanti said that ensuring 5G networks support wireless TSN is important because it will make it more likely that certain industrial applications will be able to use 5G. And ultimately that will make their networks more valuable.
— Sue Marek, special to Light Reading. Follow her @suemarek.
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