Draft:10BASE-T1S

The 10BASE-T1S is a short-reach, 10 Mb/s IEEE 802.3 Ethernet PHY, standardized in 2019 by the IEEE 802.3cg group. It belongs to the family of Single-Pair-Ethernet (SPE) physical layers, among other notable standards like 10BASE-T1L, 100BASE-T1 and 1000BASE-T1. It is designed to operate over a single balanced pair of conductors (e.g., twisted-pair cables) in full-duplex point-to-point, half-duplex point-to-point or half-duplex multi-drop mode (linear bus topology). Nominally, the 10BASE-T1S reaches at least 15 meters in point-to-point mode and at least 25 meters in multi-drop mode.

When operating in multi-drop mode, the 10BASE-T1S may optionally support the Physical Layer Collision Avoidance (PLCA) Reconciliation Sublayer (RS) for improving media access performance. The PLCA RS, once properly configured, allows for maximizing the network throughput especially under high bus load conditions, as well as minimizing latencies and improving medium-access fairness. As the latencies are bounded (i.e., the worst case is predictable), PLCA makes the 10BASE-T1S suitable for many applications with deterministic and real-time requirements such as industrial process control, automotive, etc.

The 10BASE-T1S was designed to be significantly lower in cost compared to traditional Ethernet technologies, allowing competition also with existing non-Ethernet low-speed network solutions (e.g., CAN, FlexRay, RS-485, LIN, Fieldbus, etc.). At the same time, the 10BASE-T1S architecture addresses the strict EMC requirements that are typical of industrial and automotive applications.

Basic features

 * 10 Mb/s data rate
 * Reaches (at least) up to 15 meters in full-duplex point-to-point mode
 * Reaches (at least) up to 25 meters in half-duplex multi-drop mode
 * Line topology with up to 10 cm stubs
 * Daisy-Chain (single PHY per node) or T-shaped connectors
 * Supports up to (at least) 8 nodes on the same mixing-segment
 * Compatible with existing Ethernet MACs exposing an MII including the CRS and COL signals (required for half-duplex operation)
 * Supports the PLCA Reconciliation Sublayer
 * Allows meeting automotive and industrial EMC requirements using unshielded cables
 * Supports engineered (i.e., non plug & play) Power over Data Lines (PoDL)

NOTE: while the above requirements reflect the minimum ones defined by the IEEE 802.3cg-2019 standard, several existing 10BASE-T1S implementations are known to exceed those limits. In fact, some 10BASE-T1S PHY implementations have shown reliable operations with 40 nodes on a 25 meters link in half-duplex multi-drop mode. Additionally, the Automotive Ethernet Congress in 2022 showed a live demo of a 50 meters multi-drop network with 16 nodes belonging to different vendors.

Advanced Features

 * Wake/Sleep function: allows to selectively disable nodes on a shared bus, putting them into a deep-sleep state where the PHY draws less than 35 μA . Sleeping nodes are not affected by the normal communication of the active nodes on the shared media. Any active node can globally wake-up the sleeping nodes using a dedicated physical layer signaling. As of 2022-09-15, the Wake/Sleep specification for 10BASE-T1S is undergoing editorial review in the OPEN Alliance TC14 committee.
 * Topology Discovery: allows measuring the distance between nodes on the shared-medium with a 10 cm accuracy (TODO: link to IEEE E&IP presentation, create dedicated page to topdisc). This feature can be useful to locate and enumerate nodes plugged to the mixing-segment and assign identifiers to them (e.g., IP addresses, MAC addresses, PLCA IDs, etc.). The ability to dynamically assign roles and IDs to potentially identical nodes drastically simplifies network installation and maintenance because it eliminates the necessity of pre-configuring the network devices. As of 2022-09-15, the Topology Discovery specification for 10BASE-T1S is undergoing editorial review in the OPEN Alliance TC14 committee.
 * SQI (Signal Quality Indicator): allows monitoring the quality of the PHY signal when transmitting and receiving data. This feature is useful for a variety of purposes, for example to assess the quality of a new installation, or to get early notification of the BER degradation over time due to aging. The SQI function is part of the OPEN Alliance TC14 Advanced Diagnostic Feature specification.

Application Fields
Although the 10BASE-T1S was defined to support both point-to-point and multi-drop modes, its ability to operate on a mixing-segment (shared bus in IEEE terminology) is what makes this technology attractive for a number of low-cost and (relatively) low-speed applications. The main application fields for 10BASE-T1S include automotive, industrial automation, building automation and intra-system communication (e.g., backplanes). Since the 10BASE-T1S is part of the Ethernet ecosystem, it is expected to be pervasive and potentially reach the consumer market in the near future.

The common benefits that make 10BASE-T1S attractive for a wide variety of applications include:


 * The ability to use lower cost unshielded single-pair cables
 * The ability to support multiple nodes on a single segment, reducing the number of required PHYs, connectors, cables, and other PCB components
 * Simplification of network maintenance by reducing the number of standards that must be supported (i.e., use an all-Ethernet solution down to the network edge)
 * Lower software maintenance costs by using fewer and consolidated protocols in the system (also maximizes software re-use)
 * Can be used in noisy industrial applications that require deterministic and real-time performance
 * Can replace legacy multi-drop communications with lower complexity and lower cost Ethernet installations
 * Takes Ethernet all the way to the edge (e.g. sensors, actuators) enabling a wider set of applications to use Ethernet
 * Lower network maintenance by eliminating the need for large switches, gateways, protocol translators, and the additional wiring and power they require


 * add known example of applications for industrial and automotive.

History
In the 1980's, multi-drop Ethernet networks were dominated by the 10BASE-2 and 10BASE-5 standards (now deprecated ). From 1990 onwards, these technologies were progressively abandoned in favor of switched point-to-point networks that allowed pushing the Ethernet technology towards higher bit rates (e.g., 100BASE-TX and 1000BASE-T ). High-speed switched networks determined the success of Ethernet for all modern telecommunication infrastructures, including the World Wide Web.

In the automotive industry, Ethernet was first introduced with support for 100 Mb/s point-to-point. The first use case employed 100BASE-TX for a high-speed communication link between the head unit and the rear seat entertainment of a BMW 7 series in 2008. However, a shielded two-pair (four wires) cable was not cost and weight efficient enough for that application. Therefore, the first SPE technology, namely 100BASE-T1/IEEE 802.3bw (still called BroadR-Reac h at that time), was introduced in 2013 for connecting cameras to a surround-view control unit in a BMW X5 using unshielded single-pair (two wires) cables. The development for 1 Gb/s SPE immediately followed (completed as IEEE 802.3bp/1000BASE-T1 in 2016). Also, data rates beyond 1 Gb/s were addressed (completed, e.g., as IEEE 802.3ch/MGBASE-T1 in 2020). However, the vast majority of communication in cars required the support of data rates below 10 Mb/s. In order to support this market efficiently with an Ethernet capable technology, the development of 10BASE-T1S (the "S" standing for "short" reach) was initiated in 2016, along with 10BASE-T1L (the "L" standing for "long" reach).

TODO: describe how modern industry 4.0, autonomous vehicles, and IoT drove the need for higher bit rates and Ethernet network integration. Explain that high-speed p2p links are too expensive and (especially) too power-hungry for these applications. Explain that pushing existing technology (CAN, FlexRay, RS422/485, Field Bus, etc.) to higher speeds did not allow network integration and did not allow to get rid of gateways for accessing low-end devices (sensors, actuators, e.g., microphones, low-res cameras, radars, PLCs, ...)

The 10BASE-T1S standard was eventually approved in 2019 and published in 2020 as an amendment to the IEEE Std. 802.3-2018.

Architecture
The 10BASE-T1S PHY uses 4B/5B and Differential Manchester Encoding (DME) with a baud rate of 25 MHz. Within the PCS, 4 bits of data at 2.5 MHz (i.e., 10 Mb/s) coming from the MII, are scrambled and converted into 5-bit codes at the same frequency, producing an effective line bit rate of 12.5 MHz. The PMA serializes the 5B data from LSB to MSB and encodes each bit using the (inverted) DME scheme, where clock transitions occur every 80 ns and "ones" add a single data transition in between two clock transitions.

A self-synchronizing (multiplicative) scrambler and de-scrambler provide sufficient randomization to significantly reduce electromagnetic emissions caused by potentially repetitive data patterns within the packets payload.

Unlike many other Point-to-Point Ethernet standards, 10BASE-T1S does not transmit any signal while idle, yielding a significant power saving depending on the actual bus utilization. In 100BASE-T1 and 1000BASE-T1, for example, the transmitter is always active to drive a special "idle" symbol onto the line to allow the link partner to keep its clock & data recovery (CDR) function trained at all times. A 10BASE-T1S PHY, instead, quickly re-trains the CDR each time a new packet or physical layer signaling is received. This characteristic is essential for supporting multi-drop busses where each transmitter is fed by its own clock.

Link segment characteristics
The 10BASE-T1S PHY supports two modes of operation: Point-To-Point (Full-Duplex or Half-Duplex) and Multi-Drop (Half-Duplex). Any PHY implementation may support only one or both operating modes but shall support Half-Duplex as a minimum common ground for interoperability.

Each mode defines different characteristics for the link segment.

Point-To-Point Link Segment
Although the IEEE 802.3cg-2019 standard defines both Point-To-Point and Multi-Drop modes of operation, the latter provides the highest benefits in terms of complexity and cost reduction. In fact, at the time of writing this article (2022-09-17), there are no known PHY implementations available on the market supporting Full-Duplex Point-To-Point mode.

Multi-Drop Mixing Segment
A typical 10BASE-T1S multi-drop network consists of a number of nodes connected together by a single unshielded twisted-pair cable (i.e., exactly two copper wires) in a line topology. As opposed to a star topology, where nodes are typically connected to a central controller, a line topology consists of a (potentially) long trunk with short stubs running across all nodes in a snake-like fashion. Depending on the physical layout of the network, a line topology may be overall shorter or longer (in terms of cable length) than a star topology; however, it provides a better signal integrity (due to reduced reflections and impedance changes) and simplifies the termination scheme. The below picture shows an example of a 10BASE-T1S multi-drop network with typical limits.



The physical connection of the nodes to the trunk can be achieved in several ways. The two most common options are the "T" connection and the "Daisy-Chain" methods. The "T" connection requires a special connector that joins three segments of twisted pair cable, similar to the Coaxial Tee connectors. The "T" connection may go through a short stub of twisted-pair cable (nominally, up to 10 cm), or it may plug into the node directly (not shown in the above picture). On the other hand, a "Daisy-Chain" approach consists of multiple trunk segments joined by the nodes themselves. In this case, each node typically has two connectors wired together on the PCB and routed to the line pins of the same PHY chip.

The advantage of the "T" connection is the preservation of the trunk integrity, allowing nodes to be added or removed dynamically without disrupting the link. While in a "Daisy-Chain" approach, the addition or removal of a node implies breaking the trunk and generating a temporary loss of traffic. On the other hand, a "Daisy-Chain" approach is typically easier to implement for engineered networks and may provide better signal integrity due to the reduced number of connectors and the lack of stubs.

Other approaches exists, such as vampire-like connections.

Terminations
A 10BASE-T1S mixing-segment requires a 100Ω differential termination at each end of the trunk to guarantee signal integrity. In its simplest form, the terminator is a 100Ω resistor connecting the two wires of the cable. When using PoDL, however, each terminator shall include a series capacitor (typically 100 nF) to block the DC component of the voltage and avoid wasting power on the resistors.

Optionally, the differential termination can be replaced with a "split-termination" to ground, consisting of two 50Ω resistors (i.e., 100Ω differential) connected in the middle point to ground through a 4.7 nF capacitor. That provides a 50Ω common-mode termination in the frequency range from 1 to 20 MHz, where the 10BASE-T1S modulation lies. Terminating the common mode of the line may help achieve better EMI and EME performance, depending on the grounding characteristics of the system.



Cabling and Harness
TODO: need contribution

MDI characteristics
TODO

Host Interface Flavors
TODO: SPI, MII, PMD

10BASE-T1S PHY implementations
P2MP: Point-To-Multipoint (synonym of multi-drop); P2P: Point-To-Point

Software support
TODO: 10BASE-T1S is supported by the Linux kernel since version 6.3

Temporary - sources to be cited
Thesis on T1S, PLCA and automotive requirements

Automotive Ethernet Book Third Edition

802.3cg tutorial