If The Vlan Tag Is Present In An Ethernet Frame, What Is The Maximum Frame Size?

Chapter 4. The Ethernet Frame and Full-Duplex Mode

The tutorial in Chapter 3 introduced the Ethernet system and provided a brief look at how information technology works. In this chapter, nosotros take a more detailed look at the Ethernet frame and the total-duplex mode of performance. You don't demand to know all the details of the frame and Ethernet system operation in order to build and use Ethernets. Nevertheless, an understanding of these elements can certainly assist when designing networks or troubleshooting bug.

The original half-duplex mode Media Admission Control (MAC) protocol was designed to allow a set of stations to compete for access to a shared Ethernet aqueduct, based on coaxial cable segments linked with indicate repeaters. The half-duplex media access command protocol is based on carrier sense with multiple admission and collision detection, which gives rise to the CSMA/CD acronym.

The evolution of total-duplex media systems made it possible for Ethernet links to operate in total-duplex mode, providing a higher-performance way of operation than the i supported over shared channels using CSMA/CD. The Auto-Negotiation protocol described in Affiliate 5 automatically selects the highest-functioning mode of operation over a link, typically resulting in full-duplex mode for Ethernet connections. Today, the vast majority of Ethernet links operate in full-duplex mode, which we volition describe in this chapter.

Even so, half-duplex mode is even so supported for Ethernet interfaces operating at 10 or 100 Mb/s over twisted-pair cables, and you may find a station connected to a switch port over a link that is in half-duplex mode. The performance of the original one-half-duplex fashion is described in detail in Appendix B.

Tip

A twisted-pair link segment is capable of supporting full-duplex operation by virtue of having two pairs of wires that support information being sent in both directions. A station operating in half-duplex mode while information technology is connected to a twisted-pair media system may bespeak a misconfigured link, or an upshot with the Auto-Negotiation arrangement. See Chapter v for details.

To simplify the description of these elements, this affiliate is in two parts. The first ii sections look at the structure of the frame and the full-duplex media access command system. The following two sections examine flow control and draw how the high-level network software on a computer uses Ethernet frames to send data.

The Ethernet Frame

The organization of the Ethernet frame is central to the operation of the system. The Ethernet standard determines both the structure of a frame and when a station is allowed to transport a frame. The frame was offset defined in the original Ethernet December-Intel-Xerox (DIX) standard, and was afterwards redefined and modified in the IEEE 802.3 standard. The changes between the two standards were mostly cosmetic, except for the type or length field.

The DIX standard defined a type field in the frame. The first 802.iii standard (published in 1985) specified this field equally a length field, with a mechanism that allowed both versions of frames to coexist on the same Ethernet system. Well-nigh networking software kept using the type field version of the frame. A later version of the IEEE 802.3 standard was inverse to define this field of the frame every bit being either length or type, depending on usage.

Figure 4-1 shows the DIX and IEEE versions of the Ethernet frame. In that location are three sizes of frame currently defined in the standard, and a given Ethernet interface must support at least one of them. The standard recommends that new implementations support the almost recent frame definition, called an envelope frame , which has a maximum size of 2,000 bytes. The two other sizes are basic frames , with a maximum size of one,518 bytes, and Q-tagged frames with a maximum of ane,522 bytes.

DIX Ethernet and IEEE 802.3 frames

Figure 4-1. DIX Ethernet and IEEE 802.iii frames

Because the DIX and IEEE basic frames both have a maximum size of one,518 bytes and are identical in terms of the number and length of fields, Ethernet interfaces can ship either DIX or IEEE basic frames. The only difference in these frames is in the contents of the fields and the subsequent interpretation of those contents by the network interface software.

Next, we'll have a detailed bout of the frame fields.

Preamble

The frame begins with the 64-fleck preamble field, which was originally incorporated to allow ten Mb/s Ethernet interfaces to synchronize with the incoming data stream before the fields relevant to carrying the content arrived.

The preamble was initially provided to allow for the loss of a few $.25 due to indicate showtime-upwardly delays as the signal propagates through a cabling system. Like the heat shield of a spacecraft, which protects the spacecraft from called-for up during reentry, the preamble was originally developed as a shield to protect the $.25 in the rest of the frame when operating at 10 Mb/s.

Tip

The original 10 Mb/s cabling systems could include long stretches of coaxial cables, joined by signal repeaters. The preamble ensures that the entire path has enough time to get-go up, so that signals are received reliably for the rest of the frame.

The higher-speed Ethernet systems use more circuitous mechanisms for encoding the signals that avoid whatsoever point beginning-upward losses, and these systems don't need a preamble to protect the frame signals. All the same, information technology is maintained for backward compatibility with the original Ethernet frame and to provide some actress timing for interframe housekeeping, as demonstrated, for example, in the twoscore Gb/s system.

While at that place are differences in how the ii standards formally defined the preamble bits, there is no practical divergence between the DIX and IEEE preambles. The design of bits existence sent is identical:

DIX standard: In the DIX standard, the preamble consists of eight "octets," or viii-bit bytes. The start seven comprise a sequence of alternate ones and zeros. The eighth byte of the preamble contains 6 bits of alternating ones and zeros, only ends with the special design of "1, 1." These two bits signal to the receiving interface that the end of the preamble has been reached, and that the bits that follow are the actual fields of the frame.
IEEE standard: In the 802.3 specification, the preamble field is formally divided into two parts consisting of 7 bytes of preamble and one byte called the outset frame delimiter (SFD). The last two bits of the SFD are 1, 1, as with the DIX standard.

Destination Address

The destination accost field follows the preamble. Each Ethernet interface is assigned a unique 48-bit address, chosen the interface's physical or hardware accost . The destination address field contains either the 48-bit Ethernet address that corresponds to the address of the interface in the station that is the destination of the frame, a 48-bit multicast address, or the broadcast address.

Ethernet interfaces read in every frame upward through at least the destination address field. If the destination address does not match the interface'south own Ethernet accost, or one of the multicast or circulate addresses that the interface is programmed to receive, then the interface is gratis to ignore the rest of the frame. Hither is how the two standards implement destination addresses:

DIX standard: The first bit of the destination address, as sent onto the network medium, is used to distinguish physical addresses from multicast addresses. If the first bit is zero, and so the address is the physical address of an interface, which is also known as a unicast address , because a frame sent to this address but goes to one destination. If the kickoff bit of the address is a 1, so the frame is beingness sent to a multicast address . If all 48 bits are ones, this indicates the circulate , or all-stations, address.
IEEE standard: The IEEE 802.iii version of the frame adds significance to the 2nd scrap of the destination accost, which is used to distinguish betwixt locally and globally administered addresses. A globally administered address is a physical address assigned to the interface by the manufacturer, which is indicated by setting the second chip to zero. (DIX Ethernet addresses are always globally administered.) If the address of the Ethernet interface is administered locally for some reason, and so the second fleck is supposed to be set to a value of one. In the example of a circulate address, the second bit and all other bits are ones in both the DIX and IEEE standards.

Tip

Locally administered addresses are rarely used on Ethernet systems, because each Ethernet interfaces is assigned its ain unique 48-fleck address at the factory. Locally administered addresses, nonetheless, were used on some other local surface area network systems.

Understanding physical addresses

In Ethernet, the 48-bit concrete accost is written every bit 12 hexadecimal digits with the digits paired in groups of two, representing an octet (8 bits) of data. The octet lodge of manual on the Ethernet is from the leftmost octet (as written or displayed) to the rightmost octet. The bodily transmission social club of bits within the octet, all the same, goes from the least meaning bit of the octet through to the about pregnant scrap.

This means that an Ethernet accost that is written as the hexadecimal string F0-2E-xv-6C-77-9B is equivalent to the post-obit sequence of bits, sent over the Ethernet channel from left to correct: 0000 1111 0111 0100 1010 1000 0011 0110 1110 1110 1101 1001.

Therefore, the 48-bit destination address that begins with the hexadecimal value 0xF0 is a unicast accost, because the first flake sent on the channel is a aught.

Source Address

The next field in the frame is the source address. This is the concrete accost of the device that sent the frame. The source accost is not interpreted in whatever way by the Ethernet MAC protocol, although it must always be the unicast address of the device sending the frame. It is provided for the apply of high-level network protocols, and as an aid in troubleshooting. It is as well used by switches to build a table associating source addresses with switch ports. An Ethernet station uses its physical accost as the source address in any frame it transmits.

The DIX standard notes that a station can change the Ethernet source address, while the IEEE standard does not specifically state that an interface may have the power to override the 48-fleck physical address assigned by the manufacturer. All the same, all Ethernet interfaces in use these days appear to allow the physical address to be changed, which makes it possible for the network administrator or the loftier-level network software to alter the Ethernet interface accost if necessary.

To provide the physical address used in the source address field, a vendor of Ethernet equipment acquires an organizationally unique identifier (OUI), which is a unique 24-bit identifier assigned past the IEEE. The OUI forms the starting time one-half of the physical accost of any Ethernet interface that the vendor articles. As each interface is manufactured, the vendor also assigns a unique address to the interface using the second 24 bits of the 48-bit address space, and that, combined with the OUI, creates the 48-chip address. The OUI may brand it possible to place the vendor of the interface fleck, which can sometimes be helpful when troubleshooting network problems.

Q-Tag

The Q-tag is so called because it carries an 802.1Q tag, besides known as a VLAN or priority tag. The 802.1Q standard defines a virtual LAN (VLAN) equally one or more switch ports that function as a separate and contained Ethernet system on a switch. Ethernet traffic inside a given VLAN (e.thousand., VLAN 100) will exist sent and received only on those ports of the switch that are defined to be members of that particular VLAN (in this case, VLAN 100). A four-byte-long Q-tag is inserted in an Ethernet frame between the source address and the length/type field to identify the VLAN to which the frame belongs. When a Q-Tag is nowadays, the minimum data field size is reduced to 42 bytes, maintaining a minimum frame size of 64 bytes.

Switches can be connected together with an Ethernet segment that functions as a body connectedness that carries Ethernet frames with VLAN tags in them. That, in turn, makes it possible for Ethernet frames belonging to VLAN 100, for example, to be carried between multiple switches and sent or received on switch ports that are assigned to VLAN 100.

VLAN tagging, a vendor innovation, was originally accomplished using a multifariousness of proprietary approaches. Development of the IEEE 802.1Q standard for virtual bridged LANs produced the VLAN tag as a vendor-neutral machinery for identifying which VLAN a frame belongs to.

The addition of the 4-byte VLAN tag causes the maximum size of an Ethernet frame to exist extended from the original maximum of 1,518 bytes (non including the preamble) to a new maximum of 1,522 bytes. Because VLAN tags are but added to Ethernet frames past switches and other devices that have been programmed to send and receive VLAN-tagged frames, this does not bear on traditional, or "archetype," Ethernet operation.

The first ii bytes of the Q-tag contain an Ethernet type identifier of 0x8100. If an Ethernet station that is not programmed to ship or receive a VLAN tagged frame happens to receive a tagged frame, information technology will see what looks like a blazon identifier for an unknown protocol type and merely discard the frame. VLANs and the contents and system of VLAN tags are described in Affiliate 19.

Envelope Prefix and Suffix

Every bit networks grew in complication and features, the IEEE received requests for more tags to accomplish new goals. The VLAN tag provided space for a VLAN ID and Class of Service (CoS) $.25, but vendors and standards groups wanted to add extra tags to support new bridging features and other schemes.

To accommodate these requests, the 802.iii standards engineers defined an "envelope frame," which adds an actress 482 bytes to the maximum frame size. The envelope frame was specified in the 802.3as supplement to the standard, adopted in 2006. In another change, the tag data was added to the data field to produce a MAC Client Data field. Because the MAC client information field includes the tagging fields, information technology may seem like the frame size definition has changed, but in fact this is merely a way of referring to the combination of tag data and the data field for the purpose of defining the envelope frame.

The 802.3as supplement modified the standard to state that an Ethernet implementation should support at least one of iii maximum MAC client information field sizes. The data field size continues to be defined as 46 to ane,500 bytes, but to that is added the tagging data to create the MAC client data field, resulting in the following MAC client information field sizes:

1,500-byte "basic frames" (no tagging information)
1,504-byte "Q-tagged frames" (1,500-byte data field plus 4-byte tag)
one,982-byte "envelope frames" (one,500-byte data field plus 482 bytes for all tags)

The standard notes that:

The envelope frame is intended to allow inclusion of additional prefixes and suffixes required by college layer encapsulation protocols … such as those divers past the IEEE 802.1 working grouping (such as Provider Bridges and MAC Security), ITU-T or IETF (such as MPLS). The original MAC Client Data field maximum remains 1500 octets while the encapsulation protocols may add upwards to an boosted 482 octets.^[xiii]

The contents of the tag space are not defined in the Ethernet standard, allowing maximum flexibility for the other standards to provide tags in Ethernet frames. Either or both prefix and suffix tags can be used in a given frame, occupying a maximum tag space of 482 bytes if either or both are present. This tin can issue in a maximum frame size of 2,000 bytes.

The latest standard merely includes the Q-tag as one of the tags that can be carried in an envelope prefix. The standard notes, "All Q-tagged frames are envelope frames, but not all envelope frames are Q-tagged frames." In other words, you can use the envelope space for any kind of tagging, and if y'all use a Q-tag, then information technology is carried in the envelope prefix as defined in the latest standard. An envelope frame carrying a Q-tag will have a minimum data size of 42 bytes, preserving the minimum frame size of 64 bytes.

Tagged frames are typically sent between switch ports that have been configured to add and remove tags as necessary to attain their goals. Those goals can include VLAN operations and tagging a frame equally a member of a given VLAN, or more circuitous tagging schemes to provide information for use by higher-level switching and routing protocols. Normal stations typically send basic Ethernet frames without tags, and will drop tagged frames that they are non configured to accept.

Blazon or Length Field

The old DIX standard and the IEEE standard implement the type and/or length fields differently:

DIX standard: In the DIX Ethernet standard, this 16-bit field is called a type field , and it always contains an identifier that refers to the type of loftier-level protocol data being carried in the information field of the Ethernet frame. For example, the hexadecimal value 0x0800 has been assigned as the identifier for the Internet Protocol (IP). A DIX frame existence used to carry an IP bundle is sent with the value of 0x0800 in the type field of the frame. All IP packets are carried in frames with this value in the type field.
IEEE standard: When the IEEE 802.3 standard was showtime published in 1985, the blazon field was not included, and instead the IEEE specifications called this field a length field . Type fields were added to the IEEE 802.3 standard in 1997, and then the use of a type field in the frame is officially recognized in 802.3. This modify simply fabricated the common exercise of using the type field an official part of the standard. The identifiers used in the type field were originally assigned and maintained past Xerox, but with the type field now part of the IEEE standard, the responsibleness for assigning type numbers was transferred to the IEEE.

In the IEEE 802.iii standard, this field is called a length/type field, and the hexadecimal value in the field indicates the way in which the field is existence used. The starting time octet of the field is considered the most pregnant octet in terms of numeric value.

If the value in this field is numerically less than or equal to 1,500 (decimal), and so the field is existence used as a length field. In that case, the value in the field indicates the number of logical link control (LLC) data octets that follow in the data field of the frame. If the number of LLC octets is less than the minimum required for the data field of the frame, and then octets of padding data will automatically be added to make the data field large plenty. The content of the padding data is unspecified by the standard. Upon reception of the frame, the length field is used to determine the length of valid data in the data field, and the padding data is discarded.

If the value in this field of the frame is numerically greater than or equal to 1,536 decimal (0x600 hex), so the field is being used every bit a type field.

Tip

The range of 1,501 to 1,535 was intentionally left undefined in the standard.

In that instance, the hexadecimal identifier in the field is used to indicate the blazon of protocol information beingness carried in the data field of the frame. The network software on the station is responsible for providing whatsoever padding data required to ensure that the information field is 46 bytes in length. With this method, there is no disharmonize or ambiguity well-nigh whether the field indicates length or type.

Information Field

Next comes the information field of the frame, which is besides treated differently in the two standards:

DIX standard: In a DIX frame, this field must contain a minimum of 46 bytes of data, and may range up to a maximum of 1,500 bytes of data. The network protocol software is expected to provide at least 46 bytes of data.
IEEE standard: The total size of the data field in an IEEE 802.iii frame is the same as in a DIX frame: a minimum of 46 bytes and a maximum of ane,500. Still, a logical link command protocol defined in the IEEE 802.2 LLC standard may ride in the data field of the 802.three frame to provide control data. The LLC protocol is also used equally a manner to identify the type of protocol data being carried by the frame if the type/length field is used for length data. The LLC protocol data unit (PDU) is carried in the kickoff set of bytes in the data field of the IEEE frame. The structure of the LLC PDU is defined in the IEEE 802.two LLC standard.

The process of figuring out which protocol software stack gets the data in an incoming frame is known every bit demultiplexing . An Ethernet frame may use the blazon field to place the high-level protocol data beingness carried by the frame. In the LLC specification, the receiving station demultiplexes the frame by deciphering the contents of the logical link control protocol data unit of measurement. These issues are described in more than detail subsequently in this chapter.

FCS Field

The last field in both the DIX and IEEE frames is the frame check sequence (FCS) field, likewise called the cyclic redundancy check (CRC). This 32-flake field contains a value that is used to check the integrity of the diverse bits in the frame fields (not including the preamble/SFD). This value is computed using the CRC, a polynomial that is calculated using the contents of the destination, source, type (or length), and data fields. As the frame is generated past the transmitting station, the CRC value is simultaneously being calculated. The 32 bits of the CRC value that are the result of this calculation are placed in the FCS field every bit the frame is sent. The x³¹ coefficient of the CRC polynomial is sent equally the first bit of the field, and the ten⁰ coefficient as the last.

The CRC is calculated again by the interface in the receiving station as the frame is read in. The result of this 2nd calculation is compared with the value sent in the FCS field by the originating station. If the ii values are identical, then the receiving station is provided with a high level of assurance that no errors have occurred during transmission over the Ethernet channel. If the values are not identical, then the interface can discard the frame and increment the frame mistake counter.

Finish of Frame Detection

The presence of a betoken on the Ethernet channel is known as carrier . The transmitting interface stops sending data after the last bit of a frame is transmitted, which causes the Ethernet channel to go idle. In the original 10 Mb/s system, the loss of carrier when the channel goes idle signals to the receiving interface that the frame has concluded. When the interface detects loss of carrier, it knows that the frame transmission has come to an cease. The higher-speed Ethernet systems use more complex signal encoding schemes, which have special symbols available for signaling to the interface the start and stop of a frame.

A bones frame carrying a maximum data field of 1,500 bytes is actually one,518 bytes in length (not including the preamble) when the 18 bytes needed for the addresses, length/blazon field, and the frame check sequence are included. The addition of a further 482 bytes for envelope frames makes the maximum frame size become 2,000 bytes. This was chosen every bit a useful maximum frame size that could exist handled by a typical Ethernet implementation in an interface or switch port, while providing plenty room for current and future prefixes and suffixes.

The full-duplex mode of operation was added to the standard in 1997, to let simultaneous advice between a pair of stations over a link. The link between the stations must be composed of a signal-to-point media segment, such every bit twisted-pair or fiber optic media, that provides independent transmit and receive data paths. In full-duplex fashion, both stations tin can simultaneously transmit and receive, which doubles the amass capacity of the link. For example, a half-duplex Fast Ethernet twisted-pair segment provides a maximum of 100 Mb/s of bandwidth. When operated in total-duplex mode, the aforementioned 100BASE-TX twisted-pair segment tin provide a total aggregate bandwidth of 200 Mb/s.

Some other major advantage of full-duplex operation is that the maximum segment length is no longer express by the timing requirements of the original shared-channel half-duplex Ethernet organisation. In total-duplex mode, the only limits are those set by the bespeak-conveying capabilities of the media segment. This is especially useful for fiber optic segments, allowing those segments to span long distances.

The full-duplex mode was specified in the 802.3x supplement to the standard. This supplement was approved for adoption into the IEEE 802.iii standard in March 1997. The 802.3x supplement also describes an optional gear up of mechanisms used for flow control over total-duplex links. The mechanisms used to establish catamenia control are chosen MAC control and Suspension . Offset we'll describe how total-duplex way works, and then we'll bear witness how the MAC control and Interruption mechanisms tin can be used to provide flow control over a full-duplex link.

Full-Duplex Operation

The following requirements must be met for full-duplex operation:

The media system must have independent transmit and receive data paths that tin operate simultaneously.
Exactly two stations can be continued past any total-duplex point-to-point link. At that place is no contention for use of a shared medium, and so the multiple admission algorithm (i.e., CSMA/CD) is unnecessary and is not used.
Both stations on the network link must be capable of, and accept been configured to utilise, the total-duplex mode of operation. This means that both Ethernet interfaces must have the capability to simultaneously transmit and receive frames.

Figure 4-2 shows two stations simultaneously sending and receiving over a full-duplex link segment. The segment provides contained data paths so that both stations can be active without interfering with one another'southward transmissions.

Full-duplex operation

Figure four-two. Full-duplex operation

When sending a frame in full-duplex mode, the station does non defer to traffic existence received on the channel. Yet, the station still waits for an interframe gap period between frame transmissions, as Ethernet interfaces are designed to await a gap between successive frames. Providing the interframe gap ensures that the interfaces at each end of the link tin can keep upwardly with the full frame rate of the link.

A station on a full-duplex link transmits whenever it wishes to, without respect to carrier sense (CS), which indicates frames being received from the other station on the receive side of the link segment. There is no multiple access (MA), as there is but one station at each end of the link and the Ethernet aqueduct between them is non the bailiwick of admission contention past multiple stations. Because there is no admission contention, in that location will be no collisions either, and so the stations at each end of the link also ignore standoff detection (CD), which indicates frame reception while transmitting.

Furnishings of Full-Duplex Performance

While total-duplex functioning has the potential to double the bandwidth of an Ethernet link segment, it usually won't result in a large increase in performance on a link that connects to a user's calculator. That's because few applications send and receive the same corporeality of data simultaneously. Instead, many applications send some data (due east.chiliad., the data resulting from a web click) and and so wait for a response. This leads to asymmetric data patterns, in which data that is making requests is sent in one direction, and so larger amounts of data return with the response, oft including text, images, or video streams.

On the other hand, full-duplex links betwixt switches in a network courage system volition typically carry multiple conversations between many computers. Therefore, the aggregated traffic on courage channels will be more symmetric, with both transmit and receive channels seeing roughly the aforementioned amount of traffic. For that reason, the largest benefits of a total-duplex bandwidth increase are usually seen in courage links.

Configuring Total-Duplex Operation

To ensure correct configuration of the Ethernet interfaces at each end of a link, the standard recommends that Ethernet Auto-Negotiation (run into Affiliate five) be used whenever possible to automatically configure total-duplex mode. The vast majority of twisted-pair Ethernet interfaces and switch ports back up Auto-Negotiation, which volition automatically support the highest-operation manner of operation between two stations on a link segment.

Information technology is essential that both ends of a link operating in full-duplex mode are configured correctly, or the link volition have data errors. However, using Auto-Negotiation to configure total-duplex performance on a link may non be every bit elementary every bit it sounds. For one thing, back up for Auto-Negotiation is optional for some Ethernet media systems, in which example the vendor is non required to provide Auto-Negotiation capability.

Motorcar-Negotiation was originally developed for twisted-pair Ethernet devices only, and after the original development of 10BASE-T; thus, it is not supported on all Ethernet media types or older 10BASE-T systems. The older 10 Mb/s and 100 Mb/south fiber optic media systems also do non support the Auto-Negotiation standard, while Gigabit Ethernet fiber optic systems have their own auto-configuration scheme. Therefore, you may notice that you have to manually configure full-duplex support on the stations at each cease of the link.

On a manually configured link, if just ane end of the link is in full-duplex mode and the other is in one-half-duplex manner, then the one-half-duplex end of the link will lose frames due to errors, such every bit late collisions. Data will nevertheless menses across the link, but equally the full-duplex terminate volition be sending data whenever it pleases, it will not exist obeying the same CSMA/CD rules as the half-duplex end. Because the misconfigured link will still back up the menstruation of data (despite the errors), it is possible that this trouble may not be detected right away. Therefore, you lot need to be enlightened that this status tin can occur, and brand sure that both ends of a manually configured link are set for the same mode of functioning.

Total-Duplex Media Back up

Table iv-ane provides a list of copper Ethernet media systems, and indicates which ones tin support the total-duplex mode of operation.

Table iv-one. Full-duplex media support

Media organization	Cable type	Full-duplex support?
10BASE5	50 ohm thick coaxial cable	No
10BASE2	50 ohm sparse coaxial cable	No
10BASE-T	2-pair twisted-pair	Yeah
10BROAD36	75 ohm coaxial cable	No
100BASE-TX	two-pair twisted-pair	Aye
100BASE-T4	4-pair twisted-pair	No
100BASE-T2	2-pair twisted-pair	Yes
1000BASE-SX	2 multimode optical fibers	Yeah
1000BASE-LX	two multimode or single-mode optical fibers	Yes
1000BASE-CX	ii-pair shielded twisted-pair	Yep
1000BASE-T	four-pair twisted-pair	Yep
10GBASE-T	4-pair twisted-pair	Yes
10GBASE-CR4	Short-range twinaxial cables	Aye
40GBASE-CR4	Curt-range twinaxial cables	Yes

Full-Duplex Media Segment Distances

When a segment is operating in full-duplex mode, CSMA/CD-based MAC operation is disabled. As a result, the cablevision length limits imposed by the round-trip timing constraints of the CSMA/CD algorithm no longer be. In the absence of a round-trip timing limit imposed by the CSMA/CD MAC algorithm, the simply constraint on cable length is the one imposed past the bespeak manual characteristics of the cable. For that reason, some full-duplex segments can exist much longer than the same segments operating in one-half-duplex mode.

For twisted-pair cabling, information technology is the signal-carrying characteristics of the wires that limit segment length. The 10/100/1000BASE-T and 10GBASE-T media systems have a maximum cabling distance recommendation of 100 meters (328 feet) for twisted-pair cable. This limit is the aforementioned whether the segment is operated in full-duplex or one-half-duplex mode.

Cobweb optic segments, with their excellent signal-carrying characteristics, are mostly express in length by the timing constraints of half-duplex operation. For that reason, a full-duplex mode fiber optic segment can be considerably longer than the same segment type operating in half-duplex style. As an example, a 100BASE-FX fiber optic segment using a typical multimode cobweb optic cable is express to segment lengths of 412 meters (1351.half-dozen anxiety) in one-half-duplex style. However, the same media organization can reach every bit far every bit two kilometers (6561.half-dozen feet) when operated in total-duplex mode.

Single-way fiber optic media tin can behave signals over longer distances than multimode cobweb. Therefore, a full-duplex fiber link can work over considerably longer distances if single-mode fiber is used. In the example of a 100BASE-FX link, single-way fiber can provide link distances of twenty kilometers (12.42 miles) or more than. For full-duplex links, you need to consult the equipment vendor for specifications on the maximum length of the segment.

Ethernet Menses Control

Ethernet flow control is a mechanism that allows an interface or switch port to send a indicate requesting a curt pause in frame transmission. At the time that this characteristic was developed, vendors were implementing diverse approaches to controlling Ethernet frame transmission, in an attempt to manage limited switch and interface resources on busy networks. To provide a vendor-neutral way to signal a request for a cursory pause in frame transmission, an explicit flow command message is provided by the optional MAC control and Pause specifications in the 802.3x total-duplex supplement.

Today, switch and interface resources are no longer every bit limited every bit they once were, and while Ethernet menses control is implemented by vendors, information technology is not widely used for its original purpose. Instead, you will find Break-based flow command used in data heart switch implementations, for example, to provide quality of service for file storage information flows.

The optional MAC control portion of the 802.3x supplement provides a machinery for real-time command and manipulation of the frame manual and reception process in an Ethernet station. In normal Ethernet functioning, the Media Access Control (MAC) protocol defines how to go most transmitting and receiving frames. In the Ethernet flow control system, the MAC control protocol provides mechanisms to control when Ethernet frames are sent.

The MAC control system provides a way for the station to receive a MAC control frame and act upon it. The operation of the MAC control organisation is transparent to the normal media access control functions in a station. MAC control is not used for not-real-fourth dimension functions, such as configuring interfaces, that are handled past network management mechanisms. Instead, MAC control is designed to allow stations to interact in real time to control the menstruation of traffic. The specification allows for new functions beyond flow command to exist added in the futurity.

MAC command frames are identified with a type value of 0x8808 (hex). A station equipped with optional MAC control receives all frames using the normal Ethernet MAC functions, and so passes the frames to the MAC command software for interpretation. If the frame contains the hex value 0x8808 in the blazon field, and then the MAC control function reads the frame, looking for MAC command operation codes carried in the information field. If the frame does not comprise the 0x8808 value in the type field, then MAC control takes no action, and the frame is passed along to the normal frame reception software on the station.

MAC command frames contain performance codes ( opcodes ) in the data field of the frame. The frame size is fixed at the minimum frame size allowed in the standard, with 46 bytes in the data field. The opcode is contained in the get-go ii bytes of the data field. There is no reliable transport machinery, and then MAC control must be able to bargain with the fact that MAC control frames may exist lost, discarded, damaged, or delayed.

PAUSE Operation

The PAUSE organization of catamenia command on full-duplex link segments, originally divers in 802.3x, uses MAC command frames to carry the PAUSE commands. The MAC command opcode for a Interruption command is 0x0001 (hex). A station that receives a MAC control frame with this opcode in the first 2 bytes of the data field knows that the control frame is beingness used to implement the PAUSE performance, for the purpose of providing menstruum control on a full-duplex link segment. Only stations configured for full-duplex functioning may transport PAUSE frames.

Tip

"Break" is not an acronym. Instead, PAUSE is written in uppercase messages to indicate that the word is a formally defined function in the MAC command standard. This is common practice for formally defined words and phrases in the standard.

When a station equipped with MAC control wishes to transport a PAUSE command, it sends a PAUSE frame to the 48-bit destination multicast accost of 01-eighty-C2-00-00-01. This detail multicast address has been reserved for utilize in Pause frames. Having a well-known multicast accost simplifies the flow control process past making it unnecessary for a station at one end of the link to discover and store the address of the station at the other end of the link.

Some other advantage of using this multicast address arises from the use of menstruum control on full-duplex segments between switches. The particular multicast accost used was selected from a range of addresses reserved by the IEEE 802.1D standard, which specifies basic Ethernet switch (bridge) performance. Normally, a frame with a multicast destination address that is sent to a switch will exist forwarded out all other ports of the switch. However, this range of multicast addresses is special—they will not be forwarded by an 802.1D-compliant switch. Instead, frames sent to these addresses are understood by the switch to be frames meant to be acted upon within the switch.

A station sending a PAUSE frame to the special multicast accost includes not but the Suspension opcode, but also the flow of pause time being requested, in the form of a ii-byte integer. This number contains the length of time for which the receiving station is requested to cease transmitting data. The break fourth dimension is measured in units of break "quanta," where each unit is equal to 512 bit times. The range of possible pause time requests is from 0 through 65,535 units.

Figure 4-3 shows what a PAUSE frame looks like. The PAUSE frame is carried in the data field of the MAC control frame. The MAC control opcode of 0x0001 indicates that this is a Intermission frame. The PAUSE frame carries a single parameter, defined as the pause_time in the standard. In this instance, the content of pause_time is 2, indicating a asking that the device at the other stop of the link stop transmitting for a flow of ii pause quantas (i,024 fleck times total).

PAUSE frame

Figure iv-three. PAUSE frame

By using MAC control frames to send PAUSE requests, a station at ane stop of a full-duplex link can request the station at the other end of the link to stop transmitting frames for a period of time. This provides real-time flow control between switches, or between a switch and a server that are equipped with the optional MAC control software and continued past a total-duplex link.

High-Level Protocols and the Ethernet Frame

The procedure of identifying which loftier-level network protocol data is being carried in the data field of an Ethernet frame is called multiplexing . In multiplexing, multiple sources of data tin be carried over a single system. In this case, multiple high-level protocols can be sent over the aforementioned Ethernet system in split up Ethernet frames.

Multiplexing Data in Frames

The original arrangement of multiplexing for Ethernet is based on using the type field in the Ethernet frame. For example, the high-level protocol software on a computer can create a packet of IP data, and so manus the packet to software that understands how to create Ethernet frames with type fields. The software inserts a hexadecimal value into the type field of the frame; this value corresponds to the type of high-level protocol being carried past the frame. It then hands the data to the interface driver software for transmission over the Ethernet.

The Ethernet interface driver software deals with the details of interacting with the Ethernet interface to send the frame over the Ethernet aqueduct. When carrying IP packets, the type field will be assigned the hexadecimal value 0x0800. The receiving station so uses the value in the type field to identify the protocol information beingness carried, and thus demultiplex the received frame.

Each layer of the network system is substantially independent from the other layers. Encapsulating the data being passed betwixt layers helps maintain independence between the layers, making it possible for a complex arrangement of network software to be broken downwards into more manageable chunks. By providing standardized operating system interfaces to the network programmers, the complication of each network layer is effectively hidden from view.

The programmer is free to write software that hands the completed high-level protocol bundle to the advisable computer system software interface. The details of placing the protocol packet into the data field of an Ethernet frame are automatically dealt with. In this mode, an IP-based application, and the IP software itself, tin function without major changes regardless of which physical network system the computer happens to be fastened to.

Things are made somewhat more complex because of the presence of two methods of identifying data in a frame: one using a blazon field to place data, and one using the IEEE 802.two logical link control (LLC) standard. However, many network drivers are capable of identifying and dealing with multiple frame formats.

IEEE Logical Link Control

As nosotros've seen, the value of the identifier in the length/type field determines which way the field is existence used. When used equally a length field, the task of identifying the type of high-level protocol beingness carried in the frame is moved to the 802.2 LLC fields carried in the first few bytes of the data field. Let's look at the LLC fields in a little more detail.

Figure 4-iv shows an IEEE 802.2 LLC protocol data unit, or PDU. The LLC PDU contains a destination service access point (DSAP), which identifies the high-level protocol that the data in the frame is intended for, much like the blazon field does. After a source service admission point (SSAP) and some command data, the bodily user data (the data that makes up the high-level protocol parcel) follows the LLC fields.

LLC PDU carried in an Ethernet frame

Figure 4-4. LLC PDU carried in an Ethernet frame

Tip

Given that TCP/IP uses the type field in Ethernet frames, there is very lilliputian use of the IEEE LLC encapsulation on Ethernet systems.

When network protocol software uses the 802.2 LLC fields, multiplexing and demultiplexing work in the same fashion that they do for a frame with a type field. The divergence is that the identification of the type of loftier-level protocol data is shifted to the DSAP, which is located in the LLC PDU. The whole LLC PDU fits inside the first few bytes of the data field of the Ethernet frame. In frames carrying LLC fields, the actual corporeality of high-level protocol data that can be carried is a few bytes less than in frames that use a type field.

Yous may be wondering why the IEEE went to all the trouble of defining the 802.2 LLC protocol to provide multiplexing when the blazon field seems to exist able to do the job just every bit well. The reason is that the IEEE 802 committee was created to standardize a set of LAN technologies, and not just the 802.3 Ethernet system. To do that, they needed something that would work no thing which LAN engineering was in apply.

Because at that place was no guarantee that all LAN frames would accept a type field, the IEEE 802 committee provided the LLC protocol every bit a method of identifying the blazon of data beingness carried by the frame. All LAN systems take a information field, then it is easy enough to write network protocol software that can wait at the first few bytes of information in the information field, and so interpret that data in terms of the LLC specifications.

The LLC Sub-Network Access Protocol

Only to make things more than interesting, the 802.2 LLC protocol tin likewise be used to carry the original Ethernet blazon identifiers. In other words, when you lot send a frame on a non-Ethernet LAN technology that does not provide a type field in its frame, at that place'southward a fashion to employ the LLC fields to provide a blazon identifier. The rationale for this approach comes from the fact that the LLC fields are non big. Given that limitation, the IEEE didn't want to use up the limited number of bits in the LLC fields to provide identifiers for the older high-level protocol types. Instead, a method was created to preserve the existing prepare of loftier-level protocol type identifiers, and to reuse them in the IEEE LLC system.

This approach, known as LLC Sub-Network Access Protocol (SNAP) encapsulation, provides yet another set of bytes in the information field of the frame. The contents of the LLC fields of the frame are used to identify another set of bits in the data field, organized according to the SNAP specification, and the SNAP fields are used to carry the older protocol blazon identifiers. The standard for the apply of SNAP encapsulation via IP is documented in RFC 1042. (RFCs tin can be found at http://tools.ietf.org; for more information, see Appendix A.)

If you're writing network protocol software, then SNAP encapsulation is a handy way to proceed using the aforementioned high-level protocol type identifiers when sending frames over other LAN systems. In the Ethernet arrangement itself, of grade, TCP/IP protocol software just uses the type field, and you don't demand to business organisation yourself with any of this. Yet, yous will probably meet SNAP encapsulation if you deal with multiple LAN systems at this level of detail.

As a network user, you don't need to lose sleep over which frame format your computers may be using. The option of frame format is built into your networking software, and there'due south null you need to do well-nigh it.