Volume 1 • Issue 3 Fall 1999 This article appears in The Industrial Ethernet Book, issue 1, published by Fieldbus.pub. For more information on The Industrial Ethernet Book, please the contact Adrian Chesney, [email protected] or +44 (0) 1329 846166. EXTENSION A Technical Supplement to control NETWORK INTRODUCTION TO ETHERNET ETHERNET FRAMES The two types of Ethernet frames Ethernet for Control—Understanding the Basics used in industry are similar. The DIX V2.0 frame, frequently referred to as the Ethernet II frame, consists of an eight-byte preamble, six-byte INTRODUCTION and Electronic Engineers (IEEE) was attempting to develop open source and destination addresses, a There has been much discussion network standards through the 802 two-byte type field used to identify recently regarding the applicability committee. In 1985 the IEEE 802.3 higher layer protocols, a variable of using Ethernet at various levels committee published “IEEE 802.3 data byte field followed by a four- of the control hierarchy. Since Carrier Sense Multiple Access with byte frame check sequence (FCS) Ethernet is so prevalent in the Collision Detection (CSMA/CD) field. The IEEE 802.3 frame divides office and frequently used as the Access Method and Physical Layer the preamble into a seven-byte enterprise network for high-end Specifications.” This technology is preamble followed by a single byte controllers, it would seem to be called 802.3 CSMA/CD and not start of frame delimiter (SFD). The a natural to use Ethernet at the Ethernet; however, it is frequently two-byte type field now becomes a control level or even at the device referred to as Ethernet even two-byte length field. The data level as proposed by some in our though the frame definition differs field now includes an 802.2 logical industry. The arguments for its use from DIX V2.0. Although 802.3 and link control (LLC) field that include low cost, good connectivity DIX frames can coexist on the precedes the actual data. The FCS and simple migration to higher same cable, interoperability is not remains the same. speed networks. The cry to use assured. Therefore, when “standard” Ethernet for control discussing “Ethernet,” it is Preamble applications requires an necessary to clarify 802.3 frames or understanding of the basics of DIX V2.0 frames. The DIX preamble consists of 64 Ethernet. bits of alternating “1s” and “0s” but To further confuse issues, standard ending with two “1s” to indicate Ethernet sometimes means an that a valid frame is to begin. This What is standard attached protocol—mainly TCP/IP. creates a 10 Mhz signal that Ethernet? Ethernet only defines the data link synchronizes the receivers on the We are not sure what standard and physical layers of the Open network before actual data arrives. Ethernet is but it certainly is not Systems Interconnect (OSI) Ethernet uses Manchester the 2.94 Mbps version that came Reference Model whereas TCP/IP encoding. out of Xerox’s Palo Alto Research defines the transport and network Center (PARC) in the early 70s. In layers respectively of the same 1980, Digital Equipment model. Therefore, when the The IEEE redefined the preamble Corporation (DEC), Intel and Xerox suggestion is made to use standard to be seven bytes of preamble, the published the DIX V1.0 standard Ethernet for control does this mean same as the DIX preamble, which boosted the speed of TCP/IP connectivity as well? followed by a one-byte start of frame delimiter (SFD) which looks Ethernet to 10 Mbps while APPLICATION like the last byte of the DIX maintaining Ethernet’s thick trunk PRESENTATION cabling scheme. In 1982 the DIX preamble. There is no change in SESSION V2.0 standard was released which operation between the DIX TRANSPORT is now commonly referred to as preamble and the IEEE preamble Ethernet II. Xerox then NETWORK and SFD byte. Both preambles are relinquished its trademark. DATA LINK not considered part of the frame PHYSICAL when calculating the size of the At the time of the first DIX overall frame. standard, the Institute of Electrical Figure 1—Ethernet defines the lower two layers of the OSI Reference Model. 1 Ethernet II DIX Frame What is important here is that since 64 bits 48 bits 48 bits 16 bits 368 to 12000 bits 32 bits DIX and IEEE frames are identical (46 to 1500 bytes) Preamble Individual/ Destination Source Type Data Frame in terms of the number of bits and Group Address Address Check length of fields, both frames can Address Bit Sequence coexist on the same network but may not be able to communicate to IEEE 802.3 Frame one another. Much of the existing 56 bits 8 bits 48 bits 48 bits 16 bits 368 to 12000 bits 32 bits TCP/IP software that binds to (46 to 1500 bytes) Preamble SFD Individual/ Globally/ Destination Source Length LLC/Data Frame Ethernet uses DIX frames and not Group Locally Address Address Check 802.3 frames, so care must be Address Administered Sequence exercised when selecting or Bit Address Bit developing software or claiming Figure 2—Two types of Ethernet frames are used in industry. interoperability. Destination Address good recordkeeping, will assign Data Field In the DIX standard the first bit of sequential numbers to each adapter A raw Ethernet frame (no the 48-bit destination address card he makes thereby creating a encapsulated protocol or LLC) can be indicates if the address is a worldwide unique address. With 24- up to 1500 bytes long but no less multicast address or a physical bits to work with, a lot of adapters than 46 bytes. This is the DIX frame. address. A “0” indicates a unicast can be produced from a single transmission to the indicated manufacturer. A list of OUI destination while a “1” indicates a assignments can be found on the Although the total available length multicast or group address. Internet. of the IEEE data field is the same as the DIX frame, the LLC header reduces the amount of field The IEEE standard further defines Type and Length Field available for actual data or payload the second bit of the 48-bit as it is sometimes referred to. If the destination to indicate if the address The original intention of Ethernet LLC header and actual payload are is locally administered or globally was never to use its data link layer less than 46 bytes, the data field administered. This bit is a “0” when as the means for providing must be padded to 46 bytes to the address is globally administered; guaranteed delivery of data. It was ensure that the transmission is not that is, assigned by the Ethernet always the intent that a higher layer interpreted as a runt packet or interface manufacturer. protocol would do that service. Therefore it was only necessary to packet fragment. identify by number which higher A 48-bit address of all “1s” is a layer protocol was being used Frame Check Sequence broadcast address in both DIX and through the two-byte field in the IEEE formats indicating that the DIX frame. Originally, Xerox Both the DIX and IEEE standard transmission is directed to all maintained the assignments and use four bytes to hold the CRC-32 devices on the network. now IEEE provides the check on the complete frame from administration. destination address all the way to the end of the data field. The Source Address receiving station calculates its own The 48-bit source address is The 802.3 standard does not include CRC-32, checks on the received appended to the transmission as an the type field but instead defines it data and compares the results with aid to the higher layer protocols. It as a length field. Per the 802.3 the transmitted CRC-32 value for a is not used for medium access standard, a value in this field of match indicating a successful control. To avoid duplicate node 1518 or less indicates the length of reception. Note that there is no IDs for global addresses, the the data field, while values above inherent mechanism in the Ethernet Ethernet adapter manufacturer this may be ignored, discarded or data link layer protocol to inform obtains an Organizationally Unique used in a private manner. These out the source node that a reception Identifier (OUI) from the IEEE (for of bound values could then be used was accepted or rejected due to a an administration fee). The OUI is to identify higher layer protocols failed CRC-32 check. That task is 24-bits long and is used as the most just like DIX frames. left to the higher layer protocol. significant portion of the 48-bit address. The manufacturer, using 2 ETHERNET PHYSICAL standardized in 1985 as 10BASE2. higher-speed migration, noise LAYERS Thinnet again was a bus topology immunity and electrical isolation. but this time with internal There are three media standards: Although Ethernet was originally transceivers. A thin RG-58/u coaxial designed as a coaxial bus system, cable interconnects up to 30 alternate physical layers have stations to a maximum length of 10BASE-FL—This fiber link evolved since the early 80s. The 185 meters. Segments can be standard replaces older FOIRL IEEE 802 committee has defined repeated up to 740 meters. BNC standard. several physical layers and that is style connectors, terminators and 10BASE-FB—This backbone why it is important to specify the taps are used to cable the system. standard is not very popular. correct option when selecting Although easier to install than Ethernet. 10BASE-FP—This passive hub 10BASE5, the focus on new technology is also not popular.