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INTRODUCTION to ETHERNET ETHERNET FRAMES the Two Types of Ethernet Frames Ethernet for Control—Understanding the Basics Used in Industry Are Similar

INTRODUCTION to ETHERNET ETHERNET FRAMES the Two Types of Ethernet Frames Ethernet for Control—Understanding the Basics Used in Industry Are Similar

Volume 1 • Issue 3 Fall 1999

This article appears in The Industrial Book, issue 1, published by .pub. For more information on The 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 , 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 (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 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 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 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 ’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), 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 A raw (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 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. . 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 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 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 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 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 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 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 topology immunity and electrical isolation. but this time with internal There are three media standards: Although Ethernet was originally . 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. installations is towards twisted-pair 10BASE5 cabling. This cable is likewise not supported by . The 10BASE-FL standard requires a The original Ethernet was duplex 62.5/125µm fiber optic configured as a bus system with a cable for each link. Transmission thick as the medium. 10BASE-T distances of up to 2km are possible That is what was specified in the In 1990 the IEEE published as well as full-duplex operation. 1980 DIX standard. An external 10BASE-T after pioneering work called a medium was done to introduce twisted-pair attachment unit (MAU) clamps at cabling and star topology to particular points on the cable Ethernet installations. The10BASE-T What follows is a discussion of the marked by stripes every 2.5 meters. Ethernet adapters have internal medium access control protocol for From the transceiver, an attachment transceivers and RJ-45 connectors. a 10 Mbps half-duplex Ethernet unit interface (AUI) cable connects Usually two-pair unshielded cabling network operating with several to an AUI port on the actual is attached to a hub in a point-to- nodes. Ethernet adapter that fits into the point fashion. Bus connections are . The AUI port is a DB-15 not allowed. The connection connector. A coaxial segment can between an adapter and hub When a station wants to transmit, it be up to 500 meters long and AUI cannot exceed 100 meters in first waits for an absence of a cables are each restricted to 50 length. Hub-to-hub connection carrier, which would indicate that meters in length. A total of 100 length can vary depending upon some other station is transmitting. transceivers can occupy one trunk the medium used. If another As soon as silence is detected, the segment. Individual trunk segments twisted-pair connection is used, the station waiting to transmit can be cascaded using repeaters up maximum length is again 100 continues to defer until the to 2000 meters. In 1985 the IEEE meters. With Thinnet it is 185 Interframe Gap (IFG) time has standardized this configuration as meters and with thick coaxial cable expired which is a minimum of 96- 10BASE5 to signify 10 Mbps 500 meters. bit times (9.6µs). If a carrier still signaling up to 500 appears to be absent, the station meters in length. begins to transmit while observing The star topology is much easier to its collision sense circuitry. If no troubleshoot than a bus system; collision is detected, the Thick coaxial cable is indeed bulky however, the reliability of the hub transmitting station assumes the and its topology is not always now must be considered in the transmission was sent successfully. convenient to wire in a plant. overall reliability of the system. If the detects an early Troubleshooting a 100-station Another reason for the focus on collision, one which occurred segment could be a nightmare, so twisted-pair is that development of during the preamble, the station you do not see new 10BASE5 Fast Ethernet is based on twisted- continues to send the preamble installations. There is no support pair and not coaxial cable plus 32 bits of data called a jam for this cable with Fast Ethernet providing no migration path for signal. This ensures that other technology. installed coaxial cable. stations will note the collision as well. After the collision, the 10BASE2 transmitting station will backoff 10BASE-F from retransmitting based upon a The answer to the bulkiness of The 10BASE-F standard is actually a backoff algorithm. If no collisions 10BASE5 along with its expense series of fiber optic standards. Fiber are detected after 512-bit times (not was Thinnet or Cheapernet optics provides long distance, counting the preamble), the station

3 is assumed to have acquired the Increased network before attempting a new channel and no late collisions aggravates the collision problem. transmission sequence. The integer should occur on a properly Silence on the line does not is determined by the equation: working network. The collision necessarily mean a distant counter is cleared. This 512-bit transmitter has not already sent a k time (51.2µs) is called the slot time packet down the cable, which will 0 < r < 2 where k = min (n, 10) and is critical in the way Ethernet eventually result in a collision. arbitrates access to the cable. The variable k is actually the number of collisions capped at a maximum of 10. Therefore, r can range from 0 to 1023 when k = 10. This slot time defines the The actual value for r is determined upper bound limit of the total by a random process within each propagation delay of a transmitted Ethernet node. As the number of symbol from one end of the Figure 3—For proper operation, a consecutive collisions increases, the network to the farthest end and collision domain must be within the range of possible backoff times back. This includes the time it maximum network diameter. increases exponentially. The number takes the symbol to travel through of possible retries is also capped but cables, repeaters and MAUs and at 16. varies with devices used. However, Collision Detection regardless of the path, the resulting propagation delay must be less A collision is defined as two For example, assume two stations than the slot time. Therefore the stations attempting to transmit at A and B on the network wanting slot time defines Ethernet’s the same time. On coaxial cable to transmit. They both wait for an maximum network diameter which transceivers, there is circuitry to absence of carrier and then wait limits its collision domain. A detect the DC level of the signal for the IFG time to expire before collision domain that exceeds the on the cable. This is the indicator initiating a transmission. It does maximum network diameter of a collision. On fiber optic and not matter if they are 10 meters or violates Ethernet’s medium access twisted-pair interfaces with 2500 meters apart. They could both control mechanism resulting in separate receive and transmit be sensing silence and unreliable operation. circuitry, a collision is detected by simultaneously begin to transmit the simultaneous receiving and causing a collision at some point. transmitting of data. Remember They each sense the collision and Collisions can generate runt that we are discussing half-duplex back off for either 0 or 1 slot time. packets that are less than 512 bits Ethernet that allows either The odds are 50-50 they will pick in length. These can be detected transmitting or receiving but not at the same value and collide again. by the receiving nodes and the same time. Only If they do, they will now back off discarded accordingly. That is why look for collisions and it is their for either 0, 1, 2 or 3 slot times. it is important that a minimum responsibility to reinforce a The probability of collision is now valid Ethernet frame always be collision with a jam signal. 25%. Eventually, one will win in sent to distinguish valid packets Receivers only look for valid which case its collision timer is from packet fragments. A minimum packets and automatically discard cleared to zero while the other of 46 bytes in the data field runt packets that are caused by collision timer continues to ensures that a valid Ethernet frame collisions. Once a collision is increment until a successful is 512-bits long. Control messages detected by simultaneous transmission. are typically short so it should be transmitters, these transmitters will remembered that the shortest follow a backoff algorithm Ethernet frame is 64 bytes in A high number of retires indicates length. a busy network with more stations Backoff Algorithm wanting to transmit than originally When a collision occurs on the assumed. That is why the backoff If the network diameter is small, network, the colliding transmitters time range is increased collision detection is faster and the will backoff from retransmitting for exponentially to provide more resulting collision fragments are a time determined by a backoff possible slot times for the smaller. As the network diameter algorithm. This algorithm requires additional stations. At ten retries, it increases more time is lost each transmitter to wait an integral is assumed that 1024 simultaneous detecting collisions and the number of slot times (51.2µs) transmitters exist. This becomes the collision fragments get larger.

4 upper bound limit of stations that experienced the first collision and transparent to the application can coexist on one Ethernet will probably select a much lower program requiring no development. network. Actually this is the logical timeout allowing it to transmit and limit. Physically it may be assemble another packet and could impossible to have that many beat station B again in the backoff Others suggest simply increasing stations on one collision domain contest. This phenomenon of the data rate to 100 Mbps by using without violating cabling rules. channel capture is real and Fast Ethernet technology. By simply demonstrates that access to the using raw horsepower messages BACKOFF RANGE AS A FUNCTION OF COLLISIONS will get through with or without Collision Estimate of Range Range network is neither fair nor on Attempt Number of of Random of Backoff collisions. The collision domain Number Other Stations Numbers Times (µs) predictable. The time around decreases by a factor of 10 when 1 1 0……..1 0……..51.2 station B could get the upper hand 2 3 0……..3 0……153.6 and limit A’s access. If another migrating to 100 Mbps Ethernet 3 7 0……..7 0……358.4 station C decides to transmit as resulting in a maximum network 4 15 0……15 0……768.0 well, it could beat out station A diameter of only 205 meters, which 5 31 0……31 0…..1587.2 is a small size network. Of course 6 63 0……63 0…..3225.6 due to the state of A’s collision all nodes would need to be 7 127 0…..127 0…..6502.4 counter. In actuality a station that 8 255 0…..255 0…13056.0 was last to arrive could transmit first. capable of communicating at 100 9 511 0…..511 0…26163.2 Mbps which could be a burden for 10 1023 0…1023 0…52377.6 under-powered . 11 1023 0…1023 0…52377.6 Improving Ethernet’s 12 1023 0…1023 0…52377.6 Determinism 13 1032 0…1023 0…52377.6 One approach is to avoid collisions 14 1023 0…1023 0…52377.6 There has been much discussion in altogether by using full-duplex 15 1023 0…1023 0…52377.6 the literature about implementing 16 Too High N/A Discard Frame technology and switched hubs. In methods to improve the this scheme each node is paired Table 1—Backoff range increases determinism of Ethernet. One with a port on the hub. Each exponentially with the number of approach is to incorporate a node/port arrangement creates it collisions. master/slave protocol such as own collision domain separate or on top of from all others. There are no Ethernet. In this situation, the Channel Capture collisions with a full-duplex link. slaves only respond to the master’s The switching hub directs messages As shown above, the Ethernet commands thereby controlling the to other links by observing the backoff algorithm provides a means traffic on the cable and thus destination address within the for peer stations to each gain avoiding collisions. The downside frame. Switching hubs are more access to the network. Access is of this approach is that you forfeit expensive than non-switched hubs provided to all but in an the inherent multimaster capability and they introduce more latency by unpredictable fashion. The question of Ethernet. their "" nature. is if access is fair. The switching hub now becomes Another suggestion is to develop a an integral component of the Assume the same two stations token-passing protocol that would control strategy. A and B as before. This time, be implemented in Ethernet’s data however, they both have high field. This would have to be There is an IEEE 802.1p task group amounts of data to send and they developed and its acceptance studying schemes that would attempt to send at the same time would have to be sought. The provide higher priorities to the and collide on the first attempt. software burden would increase transmission of time-critical data. They both back off but this time and technologies such as ARCNET This activity is mainly addressing A was successful. A’s collision already can do this with the way multicast frames are sent. counter is cleared but B’s does not built-in firmware clear. If station A has more data to send and it is quick to assemble another packet to send, it might collide with B again. This time B could be selecting higher and higher backoff times as its collision counter continues to increment. However, station A feels it has only Figure 4—A switching hub, bridge or is required to interconnect two or more collision domains.

5 Expanding an Ethernet 5-4-3 Rule ETHERNET MAXIMUM MEDIA SEGMENT LENGTH Network The 5-4-3 rule states that a Media type Maximum number Maximum Expanding an Ethernet network is system can have up to five of MAUs per segment segment length (m) possible by the use of repeaters segments in series, with up Mixing segment while maintaining one collision to four repeaters and no 10BASE5 100 500 (trunk) domain. If expansion is required more than three mixing 50 (AUI) beyond a collision domain, this segments. The remaining 10BASE2 30 185 can only be accomplished by the two segments must be link use of bridges, switches or routers. segments. A mixing Link segment To maintain one collision domain, segment is defined as a FOIRL 2 1000 a symbol sent from the extreme segment that may be 10BASE-T 2 100 end of the network must be able connected to more than 10BASE-FL 2 2000 to make a complete round trip two transceivers. In other within the slot time of 512-bits words, a bus segment. Table 2—Expansion rules require that (51.2µs at 10 Mbps). Calculating Only coaxial cable can be used for segments be identified as being either the complete propagation delay a bus segment (we are ignoring mixing or link. through adapters, AUI cables, 10BASE-FP) while fiber optic and transceivers, trunk cables and twisted-pair cable can be used as repeaters is possible but is also a link segments. A link segment can challenge. Table 2 provides only have two transceivers and it information on the maximum must support full-duplex operation case, all segments can be mixing number of MAUs per segment (separate transmit and receive providing a network diameter of and the maximum segment length. channels) to speed up collision 2000 meters for 10BASE5 and 740 The maximum allowable segment detection. This simplified rule does meters for 10BASE2. For other length, as well as the repeaters not address all the possible configurations you need to refer to themselves, has been assigned combinations but it does yield approach 2. delay values by the 802.3 some gross network diameters. For specification. example, all five segments cannot SUMMARY be 10BASE5 or 10BASE2. If all five were 10BASE-T then the diameter What has been discussed is the The 802.3 specification discusses would be 500 meters. With fiber operation of Ethernet’s physical ways to interconnect cable optics it is different. You cannot and data link layers. This alone segments with repeater sets use the maximum segment length does not implement an industrial without exceeding the collision for all five segments. In the case of communication network. What is domain. A repeater set is defined 10BASE-F the maximum diameter needed is for as repeater electronics and two or is 2500 meters. You need to read reliable transfers of messages and more attached MAUs—one for the standard to understand this an which each segment to be connected. restriction. provides the actual control The system designer can use either commands and responses. We will transmission system model 1 or discuss these topics in the next transmission system model 2. The 5-4-3 rule does not address issue of the EXTENSION. Approach 2 is the detailed the three repeater configuration approach where exact delay which yields four segments. In this calculations and Interframe Gap shrinkage calculations are made. Approach 1 is the simplified approach, which is not as exacting REFERENCES as approach 2. Approach 1 has Practical Networking With Ethernet, Charles E. Spurgeon, 1997, been further simplified by creating International Thomson Computer Press the 5-4-3 rule. Switched and Fast Ethernet, Second Edition, Robert Breyer and Sean Riley, 1996, Macmillan Computer Publishing USA

International Standard ISO/IEC 8802-3 ANSI/IEEE Std 802.3, 1996, The Institute of Electrical and Electronic Engineers, Inc.

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