Wednesday, November 27, 2013

History of Real-Time IEEE Ethernet

The industry control applications with Ethernet started back in mid-1980s with IEEE802.3 Ethernet bus using CSMA/CD medium access with token passing. Today, switched full-duplex Ethernet dominates the industry Ethernet market, and the networks based on CSMA/CD are phased out. Ethernet-based fieldbuses - the modified and not fully compatible variants of switched full-duplex Ethernet (modified 802.3&802.1) are used in line or ring topologies in high speed control market niches and in low-cost applications which do not require Ethernet switches.

Early Days of Ethernet Bus: 802.3 & CSMA/CD
Ethernet is a set of frame-based network technologies for LAN communications developed since 1980s, but Ethernet has changed significantly over the last 30 years. The advantage of Ethernet standards was always the simplicity and low implementation costs, and broad industry support. 
IEEE802 standardization environment has facilitated the process - a "melting pot" environment which enabled selection of promising ideas and disqualification of dead ends.
Ethernet was only one of technologies capable of supporting TCP/IP comunication in LAN networks. There were two "de jure" standards in IEEE802 focusing on LANs:
  • Token Ring IEEE802.5
  • Ethernet IEEE802.3
Ethernet, would become the "de facto" standard by mid-1990, and mid-2000, the only IEEE802 standard for LAN networks. IEEE802.4 and IEEE802.5 working groups have been simply disbanded in 2004 and 2008 respectively due to the lack of industry interest.
Ethernet technology has developed gradually from a simple databus technology with a notoriously inneficent half-duplex media access (CSMA/CD), and prevailed over potent (and more deterministic) competitors such as IBM's Token Ring IEEE802.3 by the end of 1990s.

Token Bus Story: 1st Real-Time Industrial Ethernet
Ethernet databus was considered completely unreliable for real-time applications within defined temporal boundaries in 1980s and 1990s.
In order to improve the determinism and reduce congestions, Token Bus IEEE802.4 protocol has been established - it has been supported by General Motors for their industry process control applications. This protocol could imitate some of the positive properties of Token Ring, and minimize shortcomings of Ethernet bus media access CSMA/CD defined in IEEE802.3.
Virtually all real-time Ethernet networks used in process automation from 1990s are based on Token Bus which could operate with Ethernet physical layer and CSMA/CD.

Switched Ethernet: Fast Ethernet and Gigabit Ethernet for Industrial Applications 
Rapid change in the networking market can swiftly change the competitive environments in industrial Ethernet. However, significant changes cannot be expected as the IEEE802 Ethernet technology built on IEEE802.3 and IEEE802.1 activities. Ethernet has settled around the following success-drivers and properties which can satisfy the majority of industrial and enterprise networking requirements:
  • full-duplex Ethernet
  • star and line topology
  • 100MBit/s
While the star topology minimizes the number of message hops, the store-and-forward buffering in line topology can introduce significant latency for more then 10 hops (approx. 124µs per hop for a full frame). Therefore the industrial automation vendors designed Ethernet-based fieldbuses which can exchange Ethernet frames and use existing physical layer componts.

Ethernet-based Fieldbusses
Ethernet-based fieldbuses improve the real-time performance (communication cycles of min. 12µs) and support the design of switchless automation networks.

Those capabilities are ideal for design of low-cost automation systems with fast real-time performance and without separate switches. A two- or three-port cut-through "switch" is sufficient for every node and can be designed using a simple FPGA or chips provided by TI, Innovasic, Hilscher, IXXAT and others. This portion of industrial market will continue its expansion in industrial automation and motion / drives control, primarily supported by automation companies and PLC OEMs. With growing number of products, this class of modified "Ethernet" networks can speed-up the replacement of traditional fieldbus application domains and creates a "de-facto" standard in selected niches.

Any additional QoS capabilities within IEEE802 can further advance use of Ethenret in hard real-time and integrated applications, however IEEE802 will most probably not be able to make concessions in 802.1 to support summation frames and other changes in MAC for efficent line topologies, as they are not required in high-volume mainstream markets, and do not create business for networking OEMs and semiconductor companies such as Broadcom, Marvell, Cisco, HP, Dell, Huawei and others. This will keep the Ethernet fieldbuses incompatible to switched Ethernet for a foreseable future.