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While much of the focus in recent Ethernet development has centered on extremely high data rates, not every application requires speeds up to 400 Gbps. For some applications -- including IoT, industrial and automotive -- 10 Mbps is sufficient. Factors like cost, weight, distance and the space required for cable are much more important for these use cases.
Various network technologies have supported these factors in the past, but increasingly complex applications require higher data rates. For example, Controller Area Network (CAN), designed primarily for automotive use cases, is a multidrop technology that carries messages along a shared wire to operate turn signals, door locks and similar functions. CAN supports a maximum data rate of approximately 1 Mbps, which is adequate for unlocking doors and controlling turn signals but not for backup cameras and the radar units now installed in many cars.
Conventional four-pair Ethernet cables support these rate requirements but add weight and take up space. Similarly, IoT devices have become more complicated and exchange more data. Further, the current 100-meter range limit of shared Ethernet is not adequate for factory installations.
IEEE 802.3cg standard to extend Ethernet range
Recognizing these evolving requirements, IEEE began work in early 2017 to define IEEE 802.3cg, a standard for single-pair Ethernet that would support rates up to 10 Mbps. The goals of IEEE 802.3cg were to define the following:
- a point-to-point and a multidrop short-distance standard with a maximum length of 25 meters; and
- a long-distance point-to-point standard that supports distances up to 1,000 meters.
Other objectives for 802.3cg included maintaining compatibility with existing Ethernet standards. The standard maintains the same packet format, size, temperature and emission standards, and it supports autonegotiation between endpoints on point-to-point links. Also, 802.3cg supports Power over Ethernet.
By supporting Ethernet throughout the enterprise, 802.3cg simplifies connections between industrial networks and corporate networks. The advantage of single twisted pair over four-pair cable is a single pair is one-fourth the weight, takes one-fourth the space and is cheaper than four pairs.
10Base-T1S and 10Base-T1L
IEEE 802.3cg includes two link-layer standards, both of which support rates up to 10 Mbps over a single twisted pair:
- 10Base-T1S for networks up to 25 meters
- 10Base-T1L for networks up to 1,000 meters
The 10Base-T1S short-range standard is primarily targeted at automobile and truck applications. It includes options for both point-to-point and multidrop networks. Stations on a multidrop network can share a cable using the standard Ethernet carrier-sense multiple access with collision detection (CSMA/CD) access method or operate using PHY-Level Collision Avoidance (PLCA).
PLCA defines a way to operate a shared network at a high level of utilization without the collisions that reduce throughput when a CSMA/CD link nears maximum capacity. PLCA uses a method somewhat similar in concept to Token Ring or time-division multiple access (TDMA) but differs from each.
With PLCA, each node is configured with a node ID, and the node assigned ID 0 is designated the PLCA coordinator. The coordinator initiates a cycle by sending out a beacon message, which the other nodes use to coordinate their clocks.
Each node determines when it is permitted to send data by listening on the cable for when the node with an ID one lower than itself stops transmitting. Then, a 20-bit time quiet period called the transmit opportunity period ensues. When that period completes, the next node can transmit. Each node is permitted to transmit all the data it has.
After the last node receives an opportunity to transmit data, the PLCA coordinator initiates the next cycle with another beacon. PLCA achieves higher throughput than TDMA or Token Ring because nodes don't have to split messages to fit into multiple time slots and the 20-bit transmit opportunity time consumes less bandwidth than a token packet.
The long-range option for 802.3cg -- 10Base-T1L -- is designed for IoT and industrial control applications. The 1,000-meter range is sufficient for use in large factories or warehouses, and 10 Mbps is sufficient for gathering data from sensors and to monitor and control many types of industrial machinery. It shares the advantages of the short network variant: compatibility with four-pair Ethernet and lower cost, weight and required space.
The IEEE 802.3cg standard was released in November 2019. Currently, few types of equipment have been modified with single-pair connectors, but due to the cost, weight and space advantages, compatible equipment will likely appear soon.