TechFest Ethernet Technical Summary

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4.0 Ethernet Physical Layer Specifications

4.1 Physical Layer Summary

The following table provides a summary of the various physical layer specifications defined for Ethernet. The maximum cable segment lengths listed for half-duplex operation are valid for networks with only one repeater or repeating hub. Further analysis is required to determine the maximum cable segment lengths for multi-segment networks with more than one repeater. The maximum segment lengths listed for full-duplex operation are valid regardless of the size of the network.

4.2 10 Mb/s Physical Layers

4.2.1 10Base5

10Base5 is the original Ethernet system that supports a 10 Mb/s transmission rate over "thick" (10mm) coaxial cable. The "10Base5" identifier is shorthand for 10 Mb/s transmission rate, the baseband form of transmission, and the 500 meter maximum supported segment length.

Thick Ethernet coaxial cabling includes a "mark" every 2.5 meters to indicate proper placement of the 10Base5 transceivers (or MAUs) used to connect stations to the network. Transceivers may be placed at any multiple of 2.5 meter intervals. This minimizes signal reflections that may degrade the transmission quality of the cable segment. The outer jacket of Thick Ethernet cables is typically a bright color (often yellow) with black bands at 2.5 meter intervals to mark valid transceiver placement points.

10Base5 transceivers are attached through a clamp that makes physical and electrical contact with the cable. They are also called "transceiver taps" because they are connected through a process known as "tapping" that drills a hole in the cable to allow electrical contact to be made. The transceivers are called "non-intrusive" taps because the connection can be made on an active network without disrupting traffic flow.

Stations attach to the transceiver through a transceiver cable that is also called an "attachment unit interface", or AUI. Typically computer stations that attach to 10Base5 include an Ethernet "network interface card" (NIC) or "adapter card" with a 15-pin AUI connector. The transceiver cable connects between the 15-pin AUI connector on the NIC to another 15-pin connector on the transceiver. The standard allows the transceiver cable to be up to 50 meters in length. This permits stations to be located up to 50 meters from the coaxial cable segment.

The standard allows a 10Base5 coaxial cable segment to be up to 500 meters in length. Up to 100 transceivers may be connected to a single segment at any multiple of 2.5 meters apart. A 10Base5 segment may consist of a single continuous section of cable, or be assembled from multiple cable sections that are attached end to end. If multiple cable sections are used, it can result in "impedance mismatches" that are caused by slight differences in the impedance of each cable section. When excessive, these mismatches can cause signal reflections that result in bit errors and discarded frames. Segments with multiple sections are often built with cable that comes from a single spool. This ensures each section of the cable segment will have consistent impedance since it was built by one manufacturer, at one time, using the same equipment. Cable segments can be joined at any point along their length and are not restricted to 2.5 meter intervals like transceivers.

Multiple 10Base5 segments may be connected through "repeaters" to form a larger network that still consists of a single collision domain. A repeater regenerates the signal by receiving it from one cable segment and then re-transmitting it at its original strength over all the other cable segments. The 10Base5 standard permits up to five cable segments, and thus up to four repeaters, in the path between two stations. Up to three of the segments may be coaxial cable segments (up to 1500 meters), and the other two segments must be point to point inter-repeater links (up to 1000 meters). Each end of the three coaxial cable segments may have a 50 meter AUI cable (up to 300 meters). This results in a maximum total network span of 2800 meters.

10Base5 coaxial cable segments are built using "N-type" connectors. Each end of a segment must have a N-type coaxial connector with N-type 50-ohm terminators installed. Two sections of a segment are interconnected using two N-type coaxial connectors that are mated together through a N-type barrel connector. Long 10Base5 segments typically have one or more barrel connectors to allow the segment to be split for purposes of problem isolation. For safety reasons, the standard specifies that a cable segment should be connected to earth ground at one and only one point. This may be done at the terminator at the end of the cable, or at a barrel connector where two segments are joined.

10Base5 Advantages: 1) very reliable when properly installed, and 2) new stations easily added by tapping into existing cable segment.
10Base5 Disadvantages: 1) thick, heavy, and inflexible cable makes installation a challenge, 2) bus topology makes problem isolation difficult, and 3) the coaxial medium does not support higher speed Ethernet standards.

4.2.2 10Base2

10Base2 supports a 10 Mb/s transmission rate over "thin" (5mm) coaxial cable. It is also known as "thin Ethernet", or "cheapernet". It was the first new variety of physical medium to be adopted after the original thick Ethernet standard.

The 10Base2 standard is similar in many ways to 10Base5. Both standards implement a bus topology using a 50-ohm coaxial cable. They both implement the same signal encoding, signal transmission, and collision detection mechanisms. The thinner cable used by 10Base2 has the advantages of being cheaper, lighter, more flexible, and easier to install than the thick cable used by 10Base5. However the thin cable has the disadvantage that its transmission characteristics are not as good. It supports only a 185 meter maximum segment length (vs. 500 meters for 10Base5) and a maximum of 30 stations per cable segment (vs. 100 for 10Base5).

The spacing between stations is not critical with the 10Base2 specification, but it does require that a minimum of 0.5 meters of cable separate any two stations. This spacing acts to minimize signal reflections caused by the cable connections.

Transceivers are connected to the cable segment through a "BNC Tee" connector, and not through "tapping" as with 10Base5. As the name implies, the BNC Tee connector is shaped like the letter "T". The horizontal part of the "T" includes female connectors that mate with the male BNC coaxial connectors on each end of the attaching cable sections. The vertical part of the "T" includes a male BNC connector that either plugs directly into the Ethernet network interface card (NIC) in the computer station, or to an external thin Ethernet transceiver that is then attached to the NIC through a standard AUI cable. If stations are removed from the network, the "T" connector is removed and replaced with a "BNC Barrel" connector that provides a straight through connection.

Each end of a 10Base2 coaxial segment must be terminated with a BNC 50-ohm terminator. For safety reasons, a ground wire should connect the segment to earth ground at one point, typically at the terminator on the end of the segment.

Unlike 10Base5, the 10Base2 standard optionally allows the transceiver (or MAU) function to be integrated directly into the network interface card. This further reduces cost by eliminating the need for the external transceiver component and AUI cable. It does require however that the cable segment be routed in direct proximity of the computer station as the "T" connector must be plugged directly into the female BNC connector on network interface card. No cable "stub" is allowed between the NIC and the "T" connector. 10 Mb/s Ethernet NICs commonly include both a BNC connector and AUI connector to allow them to be attached directly to a 10Base2 "T" connector, or to an external transceiver through an AUI cable.

Two wiring topologies are supported by 10Base2. The BNC Tee connectors easily support a "daisy chain" topology where the cable segment is routed directly from one computer to the next. Terminators are installed on the unused "T" connector at each end of the segment. The other topology is "point-to-point" where the cable segment attaches only a single station to a 10Base-2 repeater. The "point-to-point" topology is useful in office environments where the structure of the building makes it impractical to daisy chain multiple computers together. The cable segment is terminated at the computer station on one end, and routed to a repeater in a wiring closet on the other end.

10Base2 Advantages: 1) thinner cable makes installation easier than 10Base-5, and 2) cost is reduced by eliminating the external transceivers required by 10Base5.
10Base2 Disadvantages: 1) daisy chained segments are unreliable and difficult to troubleshoot, and 2) daisy chained segments can be difficult to route in an office environment, and 3) the coaxial medium does not support higher speed Ethernet standards.

4.2.3 10Base-T

10Base-T supports a 10 Mb/s transmission rate over two pairs of Category 3 or better telephone twisted pair cabling, also known as "voice grade" twisted pair. The widespread use of twisted pair wiring has made 10Base-T the most popular version of Ethernet.

10Base-T uses one pair of wires for transmitting data, and the other pair for receiving data. The two pairs of wires are bundled into a single cable that may often include two additional pairs of wires which are unused for 10Base-T. Each end of the cable is terminated with an 8 position RJ-45 connector, or "jack".

All 10Base-T connections are point-to-point. This implies that a 10Base-T cable can have a maximum of two Ethernet transceivers (or MAUs), with one at each end of the cable. One end of the cable is typically attached to a 10Base-T "repeating hub". The other end is attached directly to a computer station's network interface card (NIC), or an external 10Base-T transceiver. The transceiver function is integrated into most 10Base-T NICs allowing the cable to be plugged directly into an RJ-45 connector on the NIC without any external components or termination. The AUI interface on older NICs may be used attached to a 10Base-T network through an external transceiver.

Two 10Base-T NICs may be directly attached to each other without a 10Base-T repeating hub. In this case a special "crossover cable" is required that attaches the transmit pair of one station to the receive pair of the other station, and vice versa. When attaching a NIC to a repeating hub, a normal "straight through" cable is used and the cross over function is performed inside the repeating hub.

The target segment length for 10Base-T with Category 3 wiring is 100 meters. Longer segments can be accommodated as long as signal quality specifications are met. Higher quality cabling such as Category 5 wiring may be able to achieve longer segment lengths in the order of 150 meters while still maintaining the signal quality required by the standard.

The point-to-point cable connections of 10Base-T result in a "star" topology for the network. A star topology consists of a central hub with point-to-point links that appear to radiate out from the center like light from a star. The star topology simplifies maintenance, allows for faster troubleshooting, and isolates cable problems to a single link. 10Base-T is well suited for use in structured cabling systems. Structured cabling systems locate hubs in central wiring closets. Cables fanout to wall outlets in each office. In the office, a "patch" cable connects the computer station to the wall outlet.

10Base-T transceivers continually monitor the receive data path for activity to check that the link is working correctly. During periods when the network is idle, transceivers send a "link pulse" to each other as a means of verifying the integrity of the twisted pair connections. 10Base-T transceivers may optionally provide a "link light" that remains lit as long as the transceiver receives frames or link pulses from the other end of the segment. If the link lights are "on" at both ends of the segment, then you have an indication that the segment is wired correctly. This function is known as a "link integrity test".

The independent transmit and receive paths of the 10Base-T media allow the full-duplex mode of operation to be optionally supported. To support full-duplex mode, both the NIC and the hub must be capable of, and be configured for, full-duplex operation.

10Base-T Advantages: 1) the star wiring topology supports easier maintenance and troubleshooting, 2) twisted pair wiring is inexpensive and widely used, and 3) optionally supports full-duplex operation.
10Base-T Disadvantages: 1) 10Base5 and 10Base2 support longer segment lengths.

4.2.4 10Broad36

10Broad36 supports a 10 Mb/s transmission rate over a "broadband" cable system. The "36" in the name refers to the 3600 meter total span supported between any two stations.

The broadband cable used with 10Broad36 is the same inexpensive coaxial cable used in cable TV (CATV) transmission systems. Broadband cable systems support transmission of multiple services over a single cable by dividing the bandwidth into separate frequencies, with each frequency assigned to a different service. This technique is used in cable TV transmission systems to transmit multiple channels over a single cable. Each channel uses a different frequency range. This capability can allow 10Broad36 share a single cable with other services such as video.

Broadband cable also has the advantage of being able to support transmission of signals over longer distances than the baseband coaxial cable used with 10Base5 and 10Base2. Single 10Broad36 segments can be as long as 1800 meters. All 10Broad36 networks are terminated by a "head end" device. The head end can be at the end of a single segment, or at the root of multiple segments. With multiple segments the broadband cable media can span a total distance of up to 3600 meters. 10Broad36 supports attachment of stations through transceivers that are physically and and electrically attached to the broadband cable. Computers attach to the transceivers through an AUI cable that can be up to 50 meters in length. This technically allows the maximum total distance spanned between two stations to be up to 3700 meters.

Broadband transmission differs from baseband transmission in the direction of signal flow. With baseband transmission (10Base5 and 10Base2), signal flow is bidirectional. The signal flows in both directions away from the transmitting station. With broadband transmission, signal flow is unidirectional. The signal moves in only one direction along the cable. In order for signals to reach all the devices in the network, there must be two paths for data flow. This may be accomplished through either a "single cable" or "dual cable" configuration.

In a single cable configuration, the cable carries transmissions over two channels, each using a different frequency range. One channel is used to transmit signals, and the other is used to receive. When a signal is transmitted, it travels to the end of the cable where the head end device is located. The head end includes a frequency converter that changes the frequency of the signal and re-transmits it in the opposite direction along the same cable. The signal is then received by all devices on the cable.

In a dual cable configuration, each station is attached to two cables. One cable is used to send, and the other is used to receive. When a signal is transmitted, it reaches the head end where it is passed via a a connector to the other cable without any change in frequency. The signal can then be received as it passes along the second cable.

When introduced, 10Broad36 offered the advantage of supporting much longer segment lengths than 10Base5 and 10Base2. But this advantage was diminished with introduction of the fiber based FOIRL and 10Base-F standards. 10Broad36 is not capable of supporting the full-duplex mode of operation.

4.2.5 FOIRL

Fiber Optic Inter-Repeater Link (FOIRL) supports a 10 Mb/s transmission rate over two fiber optic cables. It was designed to provide a relatively long distance point-to-point connection between two repeaters. The FOIRL standard was originally released in 1987. In 1993, the 10Base-FL ("fiber link") standard was released which updated and expanded the FOIRL concept.

FOIRL supports point-to-point links up to 1000 meters in length allowing longer distances to be spanned than with a coaxial or twisted pair link. As defined in the standard, FOIRL is restricted to links between two repeaters. But vendors have adapted the technology to also support long distance links between an computer and a repeater. The standard also defined the use of "SMA" fiber connector technology, but the more popular "ST" fiber connector is often used with FOIRL.

The newer 10Base-FL standard supports interoperability with the older FOIRL technology. A 10Base-FL transceiver may be used at one end of a fiber link while an FOIRL transceiver is used at the other end. However, the maximum segment length is limited to the 1000 meters length specified by FOIRL, and not the 2000 meter length supported by 10Base-FL.

4.2.6 10Base-F

The 10Base-F standard defines 10 Mb/s operation over fiber optic media. It was released to enhance the prior Fiber Optic Inter-Repeater Link (FOIRL) standard. The identifier "10Base-F" refers collectively to three fiber optic segment types described in the following sections: 10Base-FL, 10Base-FB, and 10Base-FP. The three 10Base-F implementations are not compatible with each other at the fiber interface.

4.2.7 10Base-FL

10Base-FL ("fiber link") supports a 10 Mb/s transmission rate over two fiber optic cables. The 10Base-FL standard updates and expands the prior Fiber Optic Inter-Repeater Link (FOIRL) standard. 10Base-FL supports a maximum segment length of 2000 meters compared with 1000 meters supported by FOIRL.

10Base-FL may be used to connect two computers, two repeaters, or or a computer and a repeater port. All 10Base-FL segments are point-to-point with one transceiver on each end of the segment. A computer typically attaches through an external 10Base-FL transceiver. The network interface card (NIC) in the computer attaches to the external transceiver through an AUI cable. The transceiver attaches to the two fiber optic cables through connectors that are commonly known as "ST" connectors, but officially called "BFOC/2.5" connectors in the standard. One fiber optic cable is used to transmit data, and the other is used to receive data.

The fiber optic cable typically used with 10Base-FL is multi-mode fiber (MMF) known as "62.5/125". This designation indicates the fiber optic core of the cable is 62.5 microns in diameter, and the outer cladding is 125 microns in diameter. Other types of multi-mode fiber optic cable such as 50/125, 85/125, and 100/140 may be used in 10Base-FL links, but they may not achieve the same distance as 62.5/125. The wavelength of light used with 10Base-FL is 850 nanometers.

The independent transmit and receive paths of the 10Base-FL media allow the full-duplex mode of operation to be optionally supported. When operating in full-duplex mode, 10Base-FL can support segment lengths longer than 2000 meters. In full-duplex mode, segment lengths are no longer restricted by the round trip timing requirements of a CSMA/CD collision domain. High quality multi-mode fiber and transceivers can support segment lengths of 5 km. Even longer distances can be supported with the more expensive single mode fiber (SMF).

10Base-FL is ideal for connecting between buildings. In addition to supporting longer segment lengths, fiber optic cables are immune to electrical hazards such as lightning strikes and ground currents that can occur when connecting separate buildings. Fiber is also immune to electrical noise that can be generated by motors or other electrical equipment.

4.2.8 10Base-FB

10Base-FB ("fiber backbone") supports a 10 Mb/s transmission rate over a special synchronous signaling link that is optimized for interconnecting repeaters. The synchronous signaling protocol allows the number of repeaters that can be used in a 10 Mb/s Ethernet system to be extended. Individual 10Base-FB segments may be up to 2000 meters in length.

There are two factors that limit the number of repeaters in the path between two stations: 1) repeaters add signal delay that can cause the maximum time for collisions to propagate throughout the network to exceed the required 512 bit time limit, and 2) repeaters introduces a somewhat random bit loss in the preamble that can result in "shrinkage" of the interframe gap below the required 10 Mb/s limit of 9.6 microseconds. 10Base-FB increases the number of repeaters allowed in the network by reducing the amount of interframe gap shrinkage.

All frames are initially transmitted with a 56-bit preamble and 8-bit start of frame delimiter. When the frame is received by a repeater, there is a variability in the amount of time it takes the repeater's electronic circuitry to recognize that a new frame is being transmitted. To the repeater it appears that bits of the preamble have been lost. Successive frames may experience different levels of bit loss. Repeaters are required to regenerate the lost preamble bits as each frame is re-transmitted. If one frame experiences more bit loss than the next frame, the interframe gap between the two frames will shrink.

Some interframe gap shrinkage is expected and allowed by the standard. However if successive frames pass through too many repeaters, the interframe gap may shrink to the point that the receiving end looses one or both frames. Hence the number of repeaters allowed in the path between any two stations is restricted.

10Base-FB reduces the amount of interframe gap shrinkage by synchronizing the transmission between two repeaters. The interframe gap at the output of a normal repeater may shrink by as many as 8 bits. With 10Base-FB repeaters the variability of the shrinkage is reduced by a factor of four to only 2 bits. This simplifies network design as the time required for collisions to propagate throughout the network becomes the only practical limit on the size of the network.

10Base-FB repeaters are synchronized through a 2.5 MHz "Idle" signal that is transmitted over the link when packets are not present. This allows a receiver to be continuously locked onto the transmit signal so no bits are lost at the beginning of a packet. Since no bits are lost, there is no need to regenerate the preamble, and interframe gap shrinkage is minimized.

10Base-FB is restricted to use as a point-to-point link between repeaters. The repeaters on both ends of the link must be specifically designed to support 10Base-FB. 10Base-FB cannot be used to connect a computer directly to a repeater. 10Base-FB supports the same cable and connector types as 10Base-FL. However a 10Base-FB port on one repeater cannot be directly connected to a 10Base-FL port on another repeater as the signaling protocols are not compatible. 10Base-FB is also not capable of supporting the full-duplex mode of operation.

4.2.9 10Base-FP

10Base-FP ("fiber passive") supports a 10 Mb/s transmission rate over a "fiber optic passive star" system. 10Base-FP segments may be up to 500 meters in length, and a single 10Base-FP "Star" may link up to 33 computers.

The 10Base-FP Star is passive device which implies it requires no power. It is ideal for use in locations where no active power supply is available. The Star acts as a passive hub that receives optical signals from special 10Base-FP transceivers (or MAUs) and passively distributes the signal uniformly to all the other 10Base-FP transceivers connected to the Star, including the one from which the transmission originated. A 10Base-FP Star is comprised of a passive-star coupler and fiber optic connectors packaged in a mechanical enclosure.

10Base-FP is not capable of supporting the full-duplex mode of operation. Use of 10Base-FP has not been widely adopted at this time.

4.3 100 Mb/s Physical Layers

4.3.1 100Base-T

The identifier "100Base-T" refers collectively to the entire set of specifications and media standards for 100 Mb/s Ethernet, or "Fast Ethernet". Four 100 Mb/s media standards have been defined: 100Base-TX, 100Base-FX, 100Base-T4, and 100Base-T2. Each of these standards are described in the following sections.

All 100Base-T standards share a common "Media Access Control" (MAC) specification, but each has its own unique "Physical Layer" (PHY), or transceiver, specification. 100 Mb/s transceivers may be integrated directly into a network device such as a repeater or network interface card (NIC), or located external to the device. If located externally, the transceiver is attached to the repeater or NIC through a 40-pin "Media Independent Interface" (MII) connector. The transceiver may be plugged directly into the MII connector, or be attached through a MII cable that is analogous to the AUI cable defined as part of the 10 Mb/s standard. If present, the MII cable may be a maximum of 0.5 meters in length.

The MII supports Ethernet operation at either 10 Mb/s or 100 Mb/s. Many Fast Ethernet transceivers (PHYs) include the necessary electronics that allow them to support operation at either speed. A device configures itself for operation at the proper speed through a protocol known as "Auto-Negotiation".

4.3.2 100Base-X

The identifier "100Base-X" refers collectively to the 100Base-TX and 100Base-FX standards described in the following sections. Both 100Base-TX and 100Base-FX share a common signaling specification, called "4B/5B", that originated with the ANSI X3T9.5 standard for Fiber Distributed Data Interface (FDDI). An existing signaling specification was adopted to help speed 100Base-X products to market.

With 4B/5B signaling, every 4-bits of user data are converted into a 5-bit code prior to transmission over the media. The overhead associated with the extra bit requires that a signal transmission rate of 125 megabaud be used to transfer a net 100 Mb/s of user data. However, the extra bit allows 5-bit "symbols" to be defined that transfer control information in addition to user data. The 5-bit symbols are also defined in a manner that ensures there will be periodic transitions in the signal to allow the receiver to maintain synchronization with the incoming data stream.

4.3.3 100Base-TX

100Base-TX supports a 100 Mb/s transmission rate over two pairs twisted pair cabling. It uses one pair of wires for transmitting data, and the other pair for receiving data. The two pairs of wires are bundled into a single cable that may often include two additional pairs of wires. If present, the two additional pairs of wires must remain unused since 100Base-TX is not designed to tolerate the "cross talk" that can occur when the cable is shared with other signals. Each end of the cable is terminated with an 8 position RJ-45 connector, or "jack".

100Base-TX supports transmission over up to 100 meters of 100 ohm Category 5 unshielded twisted pair (UTP) cabling. Category 5 cabling is a "higher grade" wiring than the Category 3 cabling used with 10Base-T. It is rated for transmission at frequencies up to 100 MHz. Category 3 cabling supports transmission only up to 16 MHz. 100Base-TX transmits data using the "4B/5B" signal encoding scheme that originated with the ANSI X3T9.5 standard for Fiber Distributed Data Interface (FDDI). Note that use of the 4B/5B encoding scheme requires a signal transmission rate of 125 megabaud to transfer a net 100 Mb/s of data. But 125 megabaud equates to a maximum frequency of 62.5 MHz, which is within the maximum 100 MHz frequency supported by Category 5 cabling.

All 100Base-TX segments are point-to-point with one transceiver (PHY) at each end of the cable. Most 100Base-TX connections link a computer station to a repeating hub. 100Base-TX repeating hubs typically have the transceiver (PHY) function integrated internally so the Category 5 cable plugs directly into an RJ-45 connector on the hub. Computer stations attach through a network interface card (NIC). The transceiver function may be integrated into the NIC allowing the Category 5 twisted pair cable to be plugged directly into an RJ-45 connector on the NIC. Or the cable may plug into an external 100Base-TX transceiver that attaches to the NIC through a 40-pin Media Independent Interface (MII) connector.

Two 100Base-TX NICs may also be directly attached to each other without a 100Base-TX repeating hub. In this case a special "crossover cable" is required that attaches the transmit pair of one station to the receive pair of the other station, and vice versa. When attaching a NIC to a repeating hub, a normal "straight through" cable is used and the cross over function is performed inside the repeating hub.

The 100Base-TX standard supports the option of using 150 ohm shielded twisted pair (STP) cabling. STP cabling is found in buildings wired to support the 802.5 Token Ring standard. When using STP cabling a 9-pin "D-shell" connector replaces the RJ-45 connector used with UTP cabling.

The independent transmit and receive paths of the 100Base-TX media allow the full-duplex mode of operation to be optionally supported. To support full-duplex mode, both the NIC and the hub must be capable of, and be configured for, full-duplex operation.

4.3.4 100Base-FX

100Base-FX supports a 100 Mb/s transmission rate over two fiber optic cables. It allows maximum segment lengths of 412 meters for half-duplex links, and 2000 meters or more for full-duplex links. 100Base-FX is essentially a "fiber" version of the 100Base-TX standard. The twisted pair cabling and connectors used in 100Base-TX components are replaced with fiber optic cabling and connectors in 100Base-FX components. Both standards use the same 4B/5B signal encoding scheme.

The fiber optic cable typically used with 100Base-FX is multi-mode fiber (MMF) known as "62.5/125". This designation indicates the fiber optic core of the cable is 62.5 microns in diameter, and the outer cladding is 125 microns in diameter. Other types of multi-mode fiber optic cable such as 50/125, 85/125, and 100/140 may be used in 100Base-FX links, but they may not achieve the same distance as 62.5/125. The wavelength of light used with 100Base-FX is 1300 nanometers.

The 100Base-FX standard allows several types of fiber optic connectors to be used. Duplex "SC" connectors are recommended, but "ST" and FDDI "MIC" connectors are also permitted. Two fiber optic cables are used in each 100Base-FX segment. One cable is used for transmit data, and the other is used for receive data.

All 100Base-FX segments are point-to-point with one transceiver (PHY) at each end of the link. Use of 100Base-FX requires unique transceiver (PHY) hardware. 100Base-FX repeating hubs typically have built-in fiber optic connectors and transceivers. Network Interface Cards (NICs) may have integrated 100Base-FX connectors and transceiver, or a 100Base-FX transceiver may be attached externally through a 40-pin Media Independent Interface (MII) connector.

The independent transmit and receive paths of the 100Base-FX media allow the full-duplex mode of operation to be optionally supported. When operating in full-duplex mode, 100Base-FL segment lengths can be increased from 412 meters to 2000 meters. Even longer distances can be supported with the more expensive single mode fiber (SMF). In full-duplex mode, segment lengths are no longer restricted by the round trip timing requirements of a CSMA/CD collision domain.

4.3.5 100Base-T4

100Base-T4 supports a 100 Mb/s transmission rate over four pairs of Category 3 or better twisted pair cabling. It allows 100 Mb/s Ethernet to be carried over inexpensive Category 3 cabling as opposed to the Category 5 cabling required by 100Base-TX.

Of the four pairs of wire used by 100Base-T4, one pair is dedicated to transmit data, one pair is dedicated to receive data, and two bidirectional pairs are used to either transmit or receive data. This scheme ensures that one dedicated pair is always available to allow collisions to be detected on the link, while the three remaining pairs are available to carry the data transfer.

Data is transmitted over 100Base-T4 using an "8B6T" signal encoding scheme in which 8 bits of binary data are converted into 6 "ternary" signals for transmission over the twisted pair wires. A ternary signal can have one of three values: -1, 0, and +1. This is opposed to the binary signaling method used in other physical layers such as 100Base-TX that can have only two values: 0 and 1. This scheme effectively splits the 100 Mb/s data rate over three twisted pairs that each carry 33.333... Mb/s. Since the ternary signaling requires only six bauds to transfer eight bits of information, the maximum signal transmission rate on each of the twisted pairs is 25 megabaud. This equates to a maximum frequency of 12.5 MHz, which is within the 16 MHz limit supported by Category 3 cabling.

All 100Base-T4 segments are point-to-point with one transceiver (PHY) at each end of the cable. Each end of the cable is terminated with an 8 position RJ-45 connector, or "jack". Use of 100Base-T4 requires unique transceiver (PHY) hardware. 100Base-T4 repeating hubs typically have built-in transceivers. Network Interface Cards (NICs) may have an integrated 100Base-T4 transceiver, or a 100Base-T4 transceiver may be attached externally through a 40-pin Media Independent Interface (MII) connector.

100Base-T4 does not support the full-duplex mode of operation since it cannot support simultaneous transmit and receive at 100 Mb/s.

4.3.6 100Base-T2

100Base-T2 is the only Ethernet standard that supports a 100 Mb/s transmission rate over two pairs of Category 3 twisted pair cabling. If the cable has more than two twisted pairs, it also permits the additional pairs to carry other services such as digital phone, 10Base-T, or more 100Base-T2 connections.

100Base-T2 employs a "dual duplex baseband transmission" scheme to transmit data over each wire pair in each direction simultaneously. It uses a complex signal encoding scheme called "Five-level Pulse Amplitude Modulation", or PAM5x5, that transmits data using a "quinary" (five level) signal that can have the following values: -2, -1, 0, +1, or +2. It allows four bits of information to be transmitted per signal transition on each wire pair. With a 25 megabaud transition rate, and two pairs of wire, it can support transmission of 100 Mb/s of data in each direction simultaneously (full-duplex).

The 100Base-T2 standard was approved in March 1997, and is not widely used at this time.

4.4 1000 Mb/s Physical Layers

4.4.1 1000Base-X

The identifier "1000Base-X" refers collectively to the 1000Base-LX, 1000Base-SX, and 1000Base-CX "Gigabit Ethernet" standards described in the following sections. Each of these standards are based on physical layer specifications adopted from the ANSI X3.230-1994 standard for Fibre Channel. In particular, 1000Base-X uses the same "8B/10B" coding scheme as Fibre Channel, and similar optical and electrical specifications. The adoption of the existing Fibre Channel physical layer specifications helped speed 1000Base-X products to market. Note that 1000Base-T, which is an additional Gigabit Ethernet standard currently being defined, will not use the Fibre Channel physical layer specifications and is not part of the 1000Base-X family of standards.

With 8B/10B signaling, every 8-bits of user data are converted into a 10-bit symbol prior to transmission over the media. The overhead associated with the extra bits requires that a signal transmission rate of 1.25 gigabaud be used to transfer a net 1 Gb/s of user data. However, the extra bits permit a unique symbol to be assigned for each valid 8-bit data combination, while allowing additional symbols to be defined to transfer control and other information. Control symbols are used to signal conditions such as start of packet, end of packet, and idle. Many symbols are invalid, and if received are an indication that a transmission error has occurred. All valid symbols are defined to include five '1's and five '0's to allow the signal transmission to be "DC balanced". This also permits the receiver to easily perform symbol alignment, and ensures the incoming bit stream has frequent transitions for performing clock recovery.

The 1000Base-X standard defines a Gigabit Media Independent Interface (GMII) that attaches the Media Access Control (MAC) and Physical Layer (PHY) functions of a Gigabit Ethernet device. GMII is analogous to the Attachment Unit Interface (AUI) in 10 Mb/s Ethernet, and the Media Independent Interface (MII) in 100 Mb/s Ethernet. However, unlike AUI and MII, no connector is defined for GMII to allow a transceiver (PHY) to be attached externally via a cable. The transceiver function is built into all Gigabit Ethernet devices, and the GMII exists only as an internal component interface. It is not practical to expose the GMII as an external interface due to the high frequency (125 MHz) of its parallel 8-bit data transfers.

4.4.2 1000Base-LX

The "L" in 1000Base-LX stands for "long" as it uses long wavelength lasers to transmit data over fiber optic cable. The long wavelength lasers specified by the standard operate in the wavelength range of 1270 to 1355 nanometers. Both single mode and multi-mode optical fibers are supported. Long wavelength lasers are more expensive than short wavelength, but have the advantage of being able to drive longer distances.

4.4.3 1000Base-SX

The "S" in 1000Base-SX stands for "short" as it uses short wavelength lasers to transmit data over fiber optic cable. The short wavelength lasers specified by the standard operate in the wavelength range of 770 to 860 nanometers. Only multi-mode optical fiber is supported. Short wavelength lasers have the advantage of being less expensive than long wavelength lasers.

4.4.4 1000Base-CX

The "C" in 1000Base-CX stands for "copper" as it uses specially shielded balanced copper jumper cables also called "twinax" or "short haul copper". Segment lengths are limited to only 25 meters which restricts 1000Base-CX to connecting equipment in small areas like wiring closets.

4.4.5 1000Base-T

The 1000Base-T standard is was defined by the IEEE 802.3ab working group and formally released in June 1999. The standard supports Gigabit Ethernet over 100 meters of Category 5 balanced copper cabling. It provides the following definition for 1000Base-T:

The 1000BASE-T PHY employs full-duplex baseband transmission over four pairs of Category 5 cabling. The aggregate data rate of 1000 Mb/s is achieved by transmission at a data rate of 250 Mb/s over each wire pair. The use of hybrids and cancellers enables full-duplex transmission by allowing symbols to be transmitted and received on the same wire pairs at the same time. Baseband signaling with a modulation rate of 125 Mbaud is used on each of the wire pairs. The transmitted symbols are selected from a four dimensional 5 level symbol constellation. Each four dimensional symbol can be viewed as a 4-tuple (A n , B n , C n , D n ) of one dimensional quinary symbols taken from the set {-2, -1, 0, +1, +2}. Idle mode is a subset of code groups in that each symbol is restricted to the set {2, 0, -2} to improve synchronization. Five level Pulse Amplitude Modulation (PAM5) is employed for transmission over each wire pair. The modulation rate of 125 MBaud matches the GMII clock rate of 125 MHz and results in a symbol period of 8 ns. This specification permits the use of category 5 or better balanced cabling, installed according to ANSI/TIA/EIA-568-A.

The 1000BASE-T standard makes use of two signaling methods already used in earlier IEEE standards: 100BASE-TX (125 Mbaud three level baseband signaling) and 100BASE-T2 (25 Mbaud PAM5 baseband signaling.) These methods were chosen to make the 1000BASE-T PHY more 100BASE-T friendly for 100/1000 dual speed Ethernet PHY implementations, and to make the standards development less time consuming.

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