The rise of single-pair Ethernet in IIoT (part 2: standards)
21 April 2022
In 2016, the IEEE 802.3 Ethernet Working Group initiated a drive to create a unified network as an alternative to the fragmented Fieldbus landscape, using single-pair cabling.
The automotive industry was one of the first sectors to recognise the advantages of using Ethernet to meet the data rates needed in modern cars. Like industrial applications, automotive applications rely on a form of Fieldbus technology, namely the CAN bus, for communications between the Electronic Control Unit (ECU) and sensors and actuators. However, the slow data rate of CAN (1Mbit/s) or even the faster CAN-FD (5Mbit/s) was running out of steam to support the multiple cameras, radars and LiDAR range sensors used in Advanced Driver-Assistance Systems (ADAS).
Ethernet can deliver data rates up to 400Gbit/s. But compared to a single-pair CAN bus, traditional Ethernet 2- or 4-pair cabling would add to the overall cost and weight of the vehicle. Together, automotive OEMs began developing a set of Ethernet standards that could handle data rates in the 1Gbit/s range for short-distance communication links using single pair cabling. The result was the beginning of xBASE-T1 SPE.
Convergence from the cloud to the field level
IIoT requires industrial system integration to become vendor-independent and support end-to-end interoperability from the sensor to cloud. Here, standardised communication supports the digital transformation across all industries, including factory automation and process automation. Seamless access to production data and process conditions facilitate the availability and optimisation of production processes.
Communication standards, such as the Open Platform Communications Unified Architecture (OPC UA), standardise device models for the uniform configuration and diagnostics of devices from different manufacturers in the network. Ethernet technologies of particular note are the Advanced Physical Layer (APL) and Time-Sensitive Networking (TSN). APL enables seamless Ethernet connectivity down to the field level, providing power and communication over two wires with long cable lengths and intrinsic safety. TSN makes Ethernet deterministic by default, allowing IT and OT protocols to coexist in a common network infrastructure.
The original 10BASE-T implementation uses two wires for transmitting the data and the other pair for receiving. At 10Mbit/s, this standard is much faster than the original coax cable for IT protocols and uses CAT 5 or higher cable with RJ-45 connectors. However, it uses a physical star topology using a logical bus, rather than the bus topology of the coax solution. This star topology requires a centralised hub, or Ethernet switch, to handle data movement between devices connected to its ports.
Leveraging these existing IEEE 802.3u standard rules, 100Base-T, otherwise known as Fast Ethernet, transmits data at 100Mbit/s and requires CAT 5 UTP cable. The fastest form of Ethernet is Gigabit Ethernet (1000BASE-T), which is an order of magnitude higher with data rates to 1Gbit/s. It requires the use of CAT 5 or higher grade cable and uses all four pairs.
Most of today’s industrial automation profiles are based on the 4-pair Ethernet specification, making cable dimensions and the associated connector sizable. The size was never an issue for commercial applications, but it did limit its use in industrial applications. For instance, the thicker CAT 5e/CAT 6 Ethernet cables meant fewer could run side-by-side in a conduit. They also took longer to pull and attach, which extends deployment and commissioning times. In addition to the cable’s physical size, its weight and maximum bending radius were also limiting factors.
The success of xBASE-T1 SPE in automotive applications brings potential advantages of developing the IEEE 808.3 standard further to bring IIoT to field-level devices. The IEEE 802.3bw (100Base-T1) and IEEE 802.3bp (1000Base-T1) SPE standards for use in cars deliver end-to-end communication using unshielded twisted pair (UTP) cables for distances up to 15m.
Compared to CAT 6 Ethernet IEEE 802.3ab, which operates at 125MHz and uses all four pairs, the IEEE 802.3bp standard needs to operate at 600MHz to achieve the same 1Gbits/s (1000Base-T1) data transfer rate, hence the requirement for shielding for transmission lengths longer than 15m up to a maximum of 40m.
For industrial network applications, the IEEE 802.3cg variant is of particular interest. This standard operates at 20MHz to enable transmission rates of up to 10Mbit/s over a maximum distance of 1,000m on shielded twisted pair (STP) cable, which can replace almost all Fieldbuses. The newer IEEE 802.3ch standard enables data rates of up to 10Gbit/s (4GHz) over a distance of 15m of STP cable, fulfilling the needs for high-resolution sensors and video transmissions.
In addition to changes in the data rates, the technique known as Power over Ethernet (PoE), defined by the IEEE 802.at standard, enables power delivery on the same Ethernet cable as the data. Supporting the delivery of up to 25.5W at 48Vdc, PoE is used mainly in building automation and communications infrastructure, powering remote devices such as surveillance cameras and wireless access points. The simultaneous transmission of data and power via ‘classic’ industrial Ethernet requires two pairs of copper wires for fast Ethernet (100MB) and four pairs for Gigabit Ethernet.
Like PoE, the capability to deliver power to the device was developed specifically for the xBASE-T1 SPE market. The IEEE 802.3bu standard employs the Power over Data Lines (PoDL) technique. PoDL allows for both data and power to be delivered via a single wire pair.
This specification outlines ten classes (0 – 9) of power delivery capability, from 0.5W (Class 0) up to a maximum of 50W at the connected device. Each class provides a range of working voltages and maximum current to deliver the maximum power – up to 10W at 24Vdc and up to 50W at 48Vdc. The more recent IEEE 802.3cg standard adds six more classes of PoDL-powered devices (10 – 15).
For data transmission speeds of 10Mbit/s up to 100Gbit/s, a point-to-point (PtoP) connection is typically used. All these protocols can be combined with PoDL for remote powering applications.
IEEE 802.3cg also defines the 10BASE-T1S as a PHY that can be employed in two possible ways - a PtoP system with a reach of at least 15m and a point to multipoint (PtoMP) communication, also called a MultiDrop (MD) segment. This PtoMP topology is more or less a ‘classical’ bus system with at least a 25m reach and up to 8 edge nodes. Unfortunately, 10BASE-T1S does not yet support PoDL in PtoMP.
The next aspect to consider is how to deploy SPE into the sometimes hazardous and environmentally challenging industrial domain. Here, close attention, particularly from the interconnect perspective, is required.
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