Proposed Network Hierarchy for Open Control : ETHERNET
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ETHERNET ARCNET CAN Proposed Network Hierarchy for Open Control By George Thomas Contemporary Controls Embedded Networking INTRODUCTION from the cable. Communication proposed by some in our industry. messages are usually short, fast and The arguments for its use include When we discuss control strategy, frequent. Delivery must be low cost, good connectivity and the issue of networks is always dependable. simple migration to higher speed raised. Since network requirements networks. The cry to use "standard" vary depending upon the Ethernet ignores the attributes of complexity of control, different Control Level other open standards such as network technologies are usually At this level controllers ARCNET and CAN that are better specified for the various levels of communicate to other controllers. suited at the lower levels of the control hierarchy. It is common to Message length increases as well as network hierarchy. identify three different networks data speed. Message delivery must when describing a control system. be dependable and predictable to The lowest level is the device ensure real-time coordination of the What is standard Ethernet? network that is used to link sensors control strategy. I am not sure what standard and actuators to controllers. Above Ethernet is but it certainly is not the that is the control level that links 2.94 Mbps version that came out of the various controllers together. Information Level Xerox’s Palo Alto Research Center The highest level is the information At this level data from the control (PARC) in the early 70s. In 1980, level used to link the control system is made available to the Digital Equipment Corporation system to the enterprise-wide enterprise-wide network. Usually (DEC), Intel and Xerox published information system. With the the timeliness of delivery of the the DIX V1.0 standard which movement toward open control data is unimportant. What is boosted the speed of Ethernet to 10 systems, it is only logical to pick important is that connectivity to Mbps while maintaining Ethernet’s three open networking standards to worldwide standards such as the thick trunk cabling scheme. In 1982 complete the control system Internet is achieved. Message length the DIX V2.0 standard was released networking hierarchy. What is can be long. which is now commonly referred to recommended here is Ethernet for as Ethernet II. Xerox then the information network, ARCNET relinquished its trademark. for the control network and CAN ETHERNET EVERYWHERE? for the device network. At the time of the first DIX There has been much discussion standard, the Institute of Electrical recently regarding the applicability and Electronic Engineers (IEEE) NETWORK HIERARCHY of using Ethernet at all levels of the were attempting to develop open control hierarchy. Since Ethernet is network standards through the 802 Device Level so prevalent in the office and committee. In 1985 the IEEE 802.3 Devices such as sensors and frequently used as the enterprise committee published "IEEE 802.3 actuators share a common network for high-end controllers, it Carrier Sense Multiple Access with communications bus with an I/O would seem to be a natural to use Collision Detection (CSMA/CD) scanner located in a controller. Ethernet at the control level or Access Method and Physical Layer Power for the devices might come even at the device level as Specifications." This technology is Figure 1 depicts the generalized network model consisting of three separate networks. Although actual systems may not incorporate three separate networks, it is helpful to discuss the services expected at each of the levels. called 802.3 CSMA/CD and not network before actual data arrives. good record keeping, will assign Ethernet; however, it is frequently Ethernet uses Manchester encoding. sequential numbers to each adapter referred to as Ethernet even though card he makes thereby creating a the frame definition differs from The IEEE redefined the preamble to worldwide unique address. With 24- DIX V2.0. Although 802.3 and DIX be seven bytes of preamble, the bits to work with, a lot of adapters frames can coexist on the same same as the DIX preamble, followed can be produced from a single cable, interoperability is not by a one-byte start of frame manufacturer. A list of OUI assured. Therefore, when discussing delimiter (SFD) which looks like the assignments can be found on "Ethernet," it is necessary to clarify last byte of the DIX preamble. There the Internet. 802.3 frames or DIX V2.0 frames. is no change in operation between the DIX preamble and the IEEE To further confuse issues, standard preamble and SFD byte. Both Type and Length Field Ethernet sometimes means an preambles are not considered part The original intention of Ethernet attached protocol — mainly TCP/IP. of the frame when calculating the was never to use its data link layer Ethernet only defines the data link size of the overall frame. as the means for providing and physical layers of the Open guaranteed delivery of data. It was Systems Interconnect (OSI) always the intent that a higher layer Reference Model whereas TCP/IP Destination Address protocol would do that service. defines the transport and network In the DIX standard the first bit of Therefore it was only necessary to layers respectively of the same the 48-bit destination address identify by number which higher model. Therefore, when the indicates if the address is a layer protocol was being used suggestion is made to use standard multicast address or a physical through the two-byte field in the Ethernet at the information, control address. A "0" indicates a unicast DIX frame. Originally, Xerox and device levels, does this mean transmission to the indicated maintained the assignments and TCP/IP connectivity as well? destination while a "1" indicates a now IEEE provides the multicast or group address. administration. ETHERNET FRAMES The IEEE standard further defines The 802.3 standard does not the second bit of the 48-bit include the type field but instead The two types of Ethernet frames destination to indicate if the defines it as a length field. Per the used in industry are similar. The DIX address is locally administered or 802.3 standard, a value in this field V2.0 frame,frequently referred to as globally administered. This bit is a of 1518 or less indicates the length the Ethernet II frame, consists of an "0" when the address is globally of the data field, while values above eight-byte preamble, six-byte source administered; that is, assigned by this may be ignored, discarded or and destination addresses, a two-byte the Ethernet interface manufacturer. used in a private manner. These out type field used to identify higher of bound values could then be used layer protocols, a variable data byte A 48-bit address of all "1s" is a to identify higher layer protocols field followed by a four-byte frame broadcast address in both DIX and just like DIX frames. check sequence (FCS) field. The IEEE IEEE formats indicating that the 802.3 frame divides the preamble transmission is directed to all What is important here is that since into a seven-byte preamble followed devices on the network. DIX and IEEE frames are identical in by a single byte start of frame terms of the number of bits and delimiter (SFD).The two-byte type length of fields, both frames can field now becomes a two-byte length Source Address coexist on the same network but field. The data field now includes an The 48-bit source address is may not be able to communicate to 802.2 logical link control (LLC) field appended to the transmission as an one another. Much of the existing that precedes the actual data. The aid to the higher layer protocols. It TCP/IP software that binds to FCS remains the same. is not used for medium access Ethernet uses DIX frames and not control. To avoid duplicate node IDs 802.3 frames, so care must be for global addresses, the Ethernet exercised when selecting or Preamble adapter manufacturer obtains an developing software or claiming The DIX preamble consists of 64 Organizationally Unique Identifier interoperability. bits of alternating "1s" and "0s" but (OUI) from the IEEE (for an ending with two "1s" to indicate administration fee).The OUI is 24- that a valid frame is to begin. This bits long and is used as the most Data Field creates a 10 MHz signal that significant portion of the 48-bit A raw Ethernet frame (no synchronizes the receivers on the address. The manufacturer, using encapsulated protocol or LLC) can be up to 1500 bytes long but no transceiver called a medium to introduce twisted-pair cabling less than 46 bytes. This is the attachment unit (MAU) clamps at and star topology to Ethernet DIX frame. particular points on the cable installations. The10BASE-T Ethernet marked by stripes every 2.5 meters. adapters have internal transceivers Although the total available length From the transceiver, an attachment and RJ-45 connectors. Usually two- of the IEEE data field is the same as unit interface (AUI) cable connects pair unshielded cabling is attached the DIX frame, the LLC header to an AUI port on the actual to a hub in a point-to-point fashion. reduces the amount of field Ethernet adapter that fits into the Bus connections are not allowed. available for actual data or payload computer. The AUI port is a DB-15 The connection between an as it is sometimes referred to. If the connector. A coaxial segment can adapter and hub cannot exceed 100 LLC header and actual payload are be up to 500 meters long and AUI meters in length.