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Full-duplex is one optional setting of procedure allowingsimultaneous communication between a pair ofstations. The link in between thestations have to use a point-to-point media segment, together astwisted-pair or fiber optic media, to provide independent transmitand get data paths. Through full-duplex mode enabled, both stationscan simultaneously transmit and also receive, which doubles the aggregatecapacity the the link. Because that example, a half-duplex fast Ethernettwisted-pair segment provides a preferably of 100 Mbps the bandwidth.When activate in full-duplex, the same 100BASE-TX twisted-pairsegment can administer a total bandwidth the 200 Mbps.

Another significant advantage the full-duplex operation is that the maximumsegment size is no longer restricted by the timing requirements ofshared channel half-duplex Ethernet. In full-duplex mode, the onlylimits are those collection by the signal-carrying ability of the mediasegment. This is especially helpful for fiber optic segments.

The optional full-duplex setting is specified in the 802.3x supplementto the standard, which formally describes the techniques used forfull-duplex operation. This complement was authorized for fostering intothe IEEE 802.3 typical in in march 1997. The 802.3x complement alsodescribes an optional collection of mechanisms used for flow control overfull-duplex links. The mechanisms used to establish flow regulate arecalled MAC Control and also PAUSE. First we’ll describe howfull-duplex mode works, and also then we’ll display how the MAC Controland stop mechanisms can be used to administer flow control over afull-duplex link.

The media system must have actually independent transmit and receive datapaths that have the right to operate simultaneously. Together data routes are typicallyfound top top twisted-pair and fiber optic links.

The original mode of Ethernet procedure is half-duplex, based onCSMA/CD. The procedure ofCSMA/CD is explained in information in chapter 3. Inthe CSMA/CD-based half-duplex setting of operation, only one station cantransmit at any kind of given time; other stations should defer until thattransmission is complete.


Ethernetrepeaters as characterized inthe IEEE 802.3 standard are not stations and cannot be operated infull-duplex mode. Ethernet repeaters just pass Ethernet signalsbetween fastened segments, and also a repeater deserve to only run inhalf-duplex mode.

Beware of confuse terms that you may discover in declaring or networktrade journals, such as “full-duplex repeater.” Vendorshave produced devices which lock call"buffered repeaters,”“buffered distributors,” and “full-duplexrepeaters,” however these are not true repeaters. The operation andconfiguration of these gadgets is not explained in the Ethernetstandard and system construction guidelines. Instead, this devicestypically operate as switching hubs. Switching hubs are described inChapter 18.


figure 4.1 reflects two stations simultaneouslysending and receiving over a full-duplex connect segment. The segmentprovides elevation data paths so that both stations deserve to be activewithout interfering v one another’s transmissions. Toprovide full-duplex operation, both the station interface(controller ) and also the transceiver at eachstation should support and be configured for full-duplex operation.

When sending out a framework in full-duplex mode, the terminal ignorescarrier sense anddoes not defer to website traffic being obtained on the channel. However, thestation quiet waits because that an interframe gap period betweenframe transmissions as Ethernet interfaces room designed to suppose aninterframe gap between successive frames. Providing the interframegap ensures the the interfaces at each end of the attach can save upwith the full structure rate the the link. In full-duplex mode, thestations at each end of the attach ignore anycollision detect signalsthat come native the transceiver. If the transceiver has actually a collisiondetect light, climate placing the transceiver in full-duplex mode meansthat the significance of the collision detect irradiate is undefined, andthe light have to be ignored.

The CSMA/CD algorithm provided on shared half-duplex Ethernet channels isnot provided on a connect operating in full-duplex mode. A station on afull-duplex connect sends whenever that likes, skip carriersense (CS). There is no multipleaccess (MA) because there is just one terminal ateach end of the link and also the Ethernet channel in between them is not thesubject of accessibility contention by multiple stations. Since there is noaccess contention, there will certainly be no collisions either, so the stationat each end of the link is free to overlook collisiondetect (CD).

While full-duplex operation has actually the potential todouble the bandwidth of anEthernet attach segment, it normally won’t result in a largeincrease in power on a attach that connects to a user’scomputer. That’s since most network protocols are designed tosend data and also then wait because that an acknowledgment. This leads to asymmetric data patterns, in whichmost of the data is sent out in one direction, and also smaller quantities ofdata (in the form of acknowledgments) return in the other direction.

On the various other hand, full-duplex web links betweenswitching hubs in a network backbone system will generally carrymultiple conversations between many computers. Therefore, theaggregated website traffic on backbone networks will be an ext symmetric, withboth the transmit and also receive channels seeing about the exact same amountof traffic. For the reason, the largest advantage of a full-duplexbandwidth rise is typically seen in backbone links.

Nonetheless, configuring a connect to a user’s computer system forfull-duplex operation deserve to still it is in useful. Because that example, if it’sa modern-day computer capable of place a hefty load on one Ethernetchannel, it could encounter the Ethernet record effect explained inChapter 3. Configuring full-duplex setting shuts offthe CSMA/CD algorithm and also avoids any type of problems with record effect.

It is important that both end of alink operation infull-duplex mode are configured correctly, or the link will haveserious data errors. To ensure correct configuration, the standardrecommends that Ethernet Auto-Negotiation (see thing 5) be used whenever possible to automaticallyconfigure full-duplex mode.

However, making use of Auto-Negotiation to configure full-duplex procedure ona attach may no be as basic as that sounds. For one thing, assistance forAuto-Negotiation is optional for most Ethernet media systems, inwhich instance the seller is not compelled to administer Auto-Negotiationcapability. Furthermore, Auto-Negotiation was originally developedfor twisted-pairEthernet gadgets only, and also thus the is not sustained on all Ethernetmedia types. The 10 Mbps and 100 Mbps fiber optic media systems execute notsupport the Auto-Negotiation standard, while Gigabit Ethernet fiberoptic systems have actually their very own auto-configuration scheme. Therefore,you may discover that you have to manually configure full-duplex supporton the terminal at each finish of the link.

On a manually configured link, the is essential that both ends of thelink be properly configured for full-duplex operation. If only oneend the the link is in full-duplex mode, and the various other is inhalf-duplex mode, then the half-duplex end of the attach will loseframes as result of errors, such together late collisions. Data will certainly still flowacross the link, yet as the full-duplex finish will be sending datawhenever that pleases, it will not be obeying the exact same CSMA/CD rules asthe half-duplex end. Due to the fact that the misconfigured attach will stillsupport the flow of data (despite the errors), it is possible thatthis trouble may no be detected appropriate away. Therefore, you require tobe aware that this condition can occur, and make absolutely certain thatboth end of a manually configured link are set for the same setting ofoperation.

At each finish of the link, you must likewise ensure thatboth the Ethernet interface and also the transceiverare configured because that full-duplex operation. This can be complicated insome cases, due to the fact that the 10 MbpsAttachment Unit interface (AUI) used between an Ethernet interfaceand one externaltransceiver was designedbefore full-duplex mode was developed and does not support automaticfull-duplex configuration.<1>

For this reason, the typical recommends that just 10 Mbps Ethernetinterfaces with integrated transceivers be provided for full-duplexsegments. That’s since the 15-pin AUI connector used toconnect to external transceivers does not administer any way for theEthernet interface in the terminal to uncover the ability of theexternal transceiver and also to correctly set the setting of thetransceiver. Together a result, as soon as using an outside transceiver with a15-pin connection, you could end up with a transceiver in half-duplexmode linked to an user interface in full-duplex mode. ~ above the otherhand, the newer 40-pin medium Independent Interface(MII), which support both 10 and 100 Mbps Ethernet systems, renders itpossible because that the interface to finding and set the operational mode ofan outside transceiver.

If an outside transceiver is used, climate it is crucial that themode of procedure of the Ethernet interface and the transceiver at agiven station match. There have the right to be difficulties if one Ethernet user interface in full-duplexmode is associated to a transceiver in half-duplex mode, or if thestation interfaces are not in the same mode. The typical notes thatthis sort ofconfusion

... Can lead to major network power degradation, increasedcollisions, so late collisions, CRC errors, and undetected datacorruption.<2>

Table 4.1 gives a complete list the Ethernet media systems, and also showswhich ones deserve to support the full-duplex mode of operation.

Media System

Cable Type

Full-Duplex Support?


50 ohm thick coaxial cable



50 ohm slim coaxial cable



2-pair group 3/4/5 twisted-pair



2 multimode optical fibers (MMF)



2 multimode optical fibers



2 multimode optical fibers



75 ohm coaxial cable



2-pair category 5 twisted-pair



2 multimode optical fibers



4-pair group 3 twisted-pair



2-pair group 3/4/5 twisted-pair



2 multimode optical fibers



2 MMF or single-mode optical fibers



2-pair shielded twisted-pair



4-pair group 5 twisted-pair


<3> although the media system supports full-duplexmode, an external transceiver with a 15-pin AUI interface must bespecifically configured for full-duplex operation. Much more modernexternal transceivers with 40-pin MII interfaces room automaticallyconfigured through the terminal interface.

<4> The 10BASE-FB, 10BROAD36 and 100BASE-T4equipment to be sold just by a couple of vendors, and these media systems arenot commonly used.

<5> The 10BASE-FP and also 100BASE-T2 media systemswere never emerged by any kind of vendor, and are no deployed.

When asegment is operation infull-duplex mode, the CSMA/CD-based MAC is closeup of the door off. As a result,cable length boundaries imposed by the round-trip timing constraints ofthe CSMA/CD algorithm no much longer exist. Since there is no round-triptiming limit, the just limit on the length of the cabling is the oneimposed through the signal transmission features of the cable. Forthat reason, some full-duplex segments have the right to be much longer than thesame segments operated in half-duplex mode.

Twisted-pair segment arelimited in distance due to the signal carrying characteristics of thecable, and cannot be expanded in size when operated in full-duplexmode. A best cabling distance reference of 100 meter (328feet) because that unshielded twisted-pair cable is common to the 10BASE-T,100BASE-TX and also 1000BASE-T media systems. Because of the minimal signalcarrying capability of twisted-pair cable, the maximum limit for atwisted-pair cable segment is the same whether the segment isoperated in full-duplex or half-duplex mode.

Fiber optic segments, onthe various other hand, have actually very an excellent signal carrying features andare mostly minimal in size by the timing constraints the half-duplexoperation. For that reason, a full-duplex mode fiber optic segmentcan be significantly longer than the exact same segment form operating inhalf-duplex mode. Together an example, a 100BASE-FX fiber optic segmentusing a typical multimode fiber optic cable islimited come segment lengths of 412 meter (1351.6 feet) in half-duplexmode. However, the very same media system can reach as far as 2 kilometres (6561.6feet) as soon as operated in full-duplex mode.

Single-mode fiber optic mediacan bring signals end longer ranges than multi-mode fiber.Therefore, a full-duplex fiber attach can be stretched considerablyfurther if single mode fiber media is used. In the instance of a100BASE-FX link, single-mode fiber can administer link ranges of 20km or more. For full-duplex links, you must consult the equipmentvendor for specifications top top the maximum size of the segment.

The rate of website traffic onnetworkbackbones is constantly growing, and also as a an outcome backbone switchesconnected along with full-duplex links deserve to be greatly loaded withtraffic. A convert hub frequently has a fixed collection ofresources, in the form of internal switching bandwidth and packetbuffers, which it apportions to its switching ports. Resources likepacket buffer memory space expensive, and also many low-cost switches limitthese resources. To keep these limited resources from beingoverwhelmed, a variety of non-standard flow regulate mechanisms weredeveloped by convert hub vendors for usage onhalf-duplex segments. These includethe use of a quick burst ofcarrier signalsent by the switching hub to cause stations on a half-duplex segmentto protect against sending data when the buffers on a convert port space full.These mechanisms are defined in more detail in thing 18.

This type of device is based upon the half-duplex setting ofoperation—it will certainly not occupational on a full-duplex segment that is notusing the CSMA/CD algorithm and also ignores things favor carrier. Together aresult, a convert hub associated to full-duplex segments needs a newmechanism to send a flow regulate message. To that end, an explicitflow regulate message is provided by the optional MAC control andPAUSE specifications in the 802.3xFull-Duplex supplement.

The optional MAC Control portion of the 802.3xsupplement gives a mechanism for real-time control andmanipulation the theframe transmission and also receptionprocess in one Ethernet station. In typical Ethernet operation, themedia access control (MAC) protocol specifies how to walk abouttransmitting and also receiving frames. In the optional Ethernet flowcontrol system, the MAC manage protocol offers mechanisms tocontrol as soon as Ethernet frames room sent.

When implemented, the MAC manage system gives a way for thestation to obtain a MAC manage frame and also act top top it. The operationof the MAC regulate system is transparent come the regular media accesscontrol features in a station. MAC regulate is not used for anon-real-time function like configuring interfaces, which is handledby network management mechanisms. Instead, MAC control is draft toallow train station to connect in real time to manage the circulation oftraffic. New functions beyond flow regulate may be added in thefuture.

MAC manage frames are established with a kind value that 0x8808 (hex). Astation equipped v optional MAC regulate receives all frames usingthe normal Ethernet MAC functions, and also then overcome the frames come theMAC manage software because that interpretation. If the frame has thehex worth 0x8808 in the kind field, climate the MAC manage functionreads the frame, in search of MAC control operation codes brought inthe data field. If the frame does not contain the 0x8808 value in thetype field, climate MAC control takes no action, and the frame is passedalong to the normal framework reception software on the station.

MAC regulate frames save work codes(opcodes) in the data field of the frame. Theframe dimension is solved at the minimum framework size enabled in thestandard, i beg your pardon is 46 bytes. The opcode is included in the an initial twobytes of the data field. There is no dependable transport mechanism, soMAC manage must be able to deal with the reality that MAC Controlframes might be lost, discarded, damaged, or delayed.

The PAUSE device of flow regulate onfull-duplex link segments is defined in 802.3x and uses MAC Controlframes to bring the stop commands.<6> TheMAC manage opcode because that a pause commandis 0x0001 (hex). A terminal that receive a MAC control frame withthis opcode in the very first two bytes the the data ar knows that thecontrol frame is being supplied to perform the stop operation, for thepurpose of offering flow regulate on a full-duplex connect segment. Onlystations configured because that full-duplex operation might send pause frames.

Another advantage of making use of this multicast attend to arises native the useof flow regulate on full-duplex segments in between switching hubs. The particularmulticast deal with used is selected native a variety of addresses whichhave been reserved by the IEEE 802.1D typical (whichspecifies the procedure of convert hubs). Normally, a frame with amulticast destination attend to that is sent out to a switch will certainly beforwarded the end all other ports of the switching hub. However, thisrange the multicast addresses is special and also will no be forwarded byan 802.1D-compliant switch. Instead, frames sent out to this attend to areunderstood by the move to it is in frames expected to be acted ~ above withinthe switch.

A station sending out a PAUSE frame to the distinct multicast addressincludes the pause opcode, and additionally includes the period of stop timebeing requested, in the kind of a two byte integer. This numbercontains the length of time because that which the receiving station isrequested to avoid transmitting data. The stop time is measure inunits of stop “quanta,” wherein eachunit is equal to 512 little times. The range of feasible pause timerequests is fromthrough 65,535 units.

number 4.2 mirrors what aPAUSE framework looks like. The stop fieldsare lugged in the data ar of the MAC control frame. The MAC regulate opcode of0x0001 shows that this is a pause frame. ThePAUSE framework carries a solitary parameter, defined as thepause_time in the standard. In this example, thecontents of pause_time is 2, indicating arequest that the machine at the other finish of the link stoptransmitting because that a period of two slot times.


By making use of MAC manage frames to send pause requests, a station at oneend that a full-duplex link can inquiry the station at the other end ofthe connect to avoid transmitting data because that a duration of time. Thisprovides real-time circulation control in between switching hubs, or evenbetween a switching hub and also a server which space equipped through the optional MAC control software and also connected by a full-duplex link.

<6> “PAUSE” is not an acronym. Instead, pause iswritten in uppercase letters to indicate that words is a formallydefined function in the full-duplex Ethernet standard. This is commonpractice because that formally identified words and also phrases in thestandard.

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