Gigabit Ethernet
Encyclopedia
Gigabit Ethernet is a term describing various technologies for transmitting Ethernet frame
s at a rate of a gigabit per second (1,000,000,000 bits per second), as defined by the IEEE 802.3-2008 standard. It came into use beginning in 1999, gradually supplanting Fast Ethernet
in wired local networks where it performed considerably faster. The cables and equipment are very similar to previous standards, and as of 2011 are very common and economical.
Half-duplex gigabit links connected through hub
s are allowed by the specification but in the marketplace full-duplex with switches
is normal.
in the early 1970s, Ethernet
evolved into a widely implemented physical
and link
layer
protocol. Fast Ethernet
increased speed from 10 to 100 megabits per second (Mbit/s). Gigabit Ethernet was the next iteration, increasing the speed to 1000 Mbit/s. The initial standard for gigabit Ethernet was produced by the IEEE in June 1998 as IEEE 802.3z, and required optical fiber
. 802.3z is commonly referred to as 1000BASE-X, where -X refers to either -CX, -SX, -LX, or (non-standard) -ZX.
IEEE 802.3ab, ratified in 1999, defines gigabit Ethernet transmission over unshielded
twisted pair
(UTP) category 5, 5e
, or 6
cabling and became known as 1000BASE-T. With the ratification of 802.3ab, gigabit Ethernet became a desktop technology as organizations could use their existing copper cabling infrastructure.
IEEE 802.3ah, ratified in 2004 added two more gigabit fiber standards, 1000BASE-LX10 (which was already widely implemented as vendor specific extension) and 1000BASE-BX10. This was part of a larger group of protocols known as Ethernet in the First Mile
.
Initially, gigabit Ethernet was deployed in high-capacity backbone network
links (for instance, on a high-capacity campus network). In 2000, Apple's
Power Mac G4
and PowerBook G4
were the first mass produced personal computers featuring the 1000BASE-T connection. It quickly became a built-in feature in many other computers. Gigabit NICs (1000BASE-T) are included in almost all desktop, laptop and server computer systems.
Higher bandwidth 10 Gigabit Ethernet
standards have since become available as the IEEE ratified a fiber-based standard in 2002, and a twisted pair standard in 2006. 10Gb Ethernet is replacing 1Gb as the backbone network and has begun to migrate down to high-end server systems.
standards for gigabit Ethernet using optical fiber
(1000BASE-X), twisted pair cable (1000BASE-T), or balanced copper cable (1000BASE-CX).
The IEEE 802.3z standard includes 1000BASE-SX for transmission over multi-mode fiber, 1000BASE-LX for transmission over single-mode fiber, and the nearly obsolete 1000BASE-CX for transmission over balanced copper cabling. These standards use 8b/10b encoding
, which inflates the line rate by 25%, from 1000 Mbit/s to 1250 Mbit/s, to ensure a DC balanced signal. The symbols are then sent using NRZ
.
IEEE 802.3ab, which defines the widely used 1000BASE-T interface type, uses a different encoding scheme in order to keep the symbol rate as low as possible, allowing transmission over twisted pair.
Ethernet in the First Mile
later added 1000BASE-LX10 and -BX10.
(NIR) light
wavelength
.
The standard specifies a distance capability between 220 metres (62.5/125 µm fiber with low modal bandwidth
) and 550 metres (50/125 µm fiber with high modal bandwidth). In practice, with good quality fiber, optics, and terminations, 1000BASE-SX will usually work over significantly longer distances.
This standard is highly popular for intra-building links in large office buildings, co-location facilities and carrier neutral internet exchanges.
Optical power specifications of SX interface: Minimum output power = −9.5 dBm
. Minimum receive sensitivity = −17 dBm.
1000BASE-LX is specified to work over a distance of up to 5 km over 10 µm single-mode fiber.
1000BASE-LX can also run over all common types of multi-mode fiber with a maximum segment length of 550 m. For link distances greater than 300 m, the use of a special launch conditioning patch cord may be required. This launches the laser at a precise offset from the center of the fiber which causes it to spread across the diameter of the fiber core, reducing the effect known as differential mode delay which occurs when the laser couples onto only a small number of available modes in multi-mode fiber.
task group. It is very similar to 1000BASE-LX, but achieves longer distances up to 10 km over a pair of single-mode fiber due to higher quality optics. Before it was standardized 1000BASE-LX10 was essentially already in widespread use by many vendors as a proprietary extension called either 1000BASE-LX/LH or 1000BASE-LH.
1000BASE-T (also known as IEEE 802.3ab) is a standard for gigabit Ethernet over copper
wiring.
Each 1000BASE-T network segment can be a maximum length of 100 meters (328 feet), and must use Category 5 cable
or better. Category 5e cable or Category 6 cable
may also be used.
Autonegotiation
is a requirement for using 1000BASE-T according to Section 28D.5 Extensions required for Clause40 (1000BASE-T). At least the clock source has to be negotiated, as one has to be master and the other slave.
In a departure from both 10BASE-T
and 100BASE-TX, 1000BASE-T uses all four cable pairs for simultaneous transmission in both directions through the use of adaptive equalization and a 5-level pulse amplitude modulation (PAM-5) technique. The symbol rate is identical to that of 100BASE-TX (125 Mbaud
) and the noise immunity of the 5-level signaling is also identical to that of the 3-level signaling in 100BASE-TX, since 1000BASE-T uses 4-dimensional trellis coded modulation
(TCM) to achieve a 6 dB
coding gain across the 4 pairs.
Since negotiation takes place on only two pairs, if two gigabit devices are connected through a cable with only two pairs, the devices will successfully choose 'gigabit' as the highest common denominator (HCD), but the link will never come up. Most gigabit physical devices have a specific register to diagnose this behaviour. Some drivers offer an "Ethernet@Wirespeed" option where this situation leads to a slower yet functional connection.
The data is transmitted over four copper pairs, eight bit
s at a time. First, eight bits of data are expanded into four 3-bit symbols through a non-trivial scrambling procedure based on a linear feedback shift register
; this is similar to what is done in 100BASE-T2, but uses different parameters. The 3-bit symbols are then mapped to voltage levels which vary continuously during transmission. An example mapping is as follows:
Automatic MDI/MDI-X Configuration is specified as an optional feature in the 1000BASE-T standard, meaning that straight-through cables will often work between gigabit-capable interfaces. This feature eliminates the need for crossover cables
, making obsolete the uplink/normal ports and manual selector switches found on many older hubs and switches and greatly reducing installation errors.
(TIA) created and promoted a standard similar to 1000BASE-T that was simpler to implement, calling it 1000BASE-TX (TIA/EIA-854). The simplified design would, in theory, have reduced the cost of the required electronics by only using one pair of wires in each direction. However, this solution required Category 6 cable and has been a commercial failure, likely due to the cabling requirement as well as the rapidly falling cost of 1000BASE-T products. Many 1000BASE-T products are advertised as 1000BASE-TX due to lack of knowledge that 1000BASE-TX is actually a different standard. The confusion between 1000BASE-T and 1000BASE-TX probably stems from the fact that most popular form of Fast Ethernet (100 Mbit/s) is known as 100BASE-TX, and the fact that many products support multiple speeds of 10/100/1000Mb/sec and are often promoted as 10/100/1000BASE-TX.An example of a product being specifying 10/100/1000BASE-TX ports can be found at http://www.cisco.com/en/US/products/ps10018/index.html.
Ethernet frame
A data packet on an Ethernet link is called an Ethernet frame. A frame begins with Preamble and Start Frame Delimiter. Following which, each Ethernet frame continues with an Ethernet header featuring destination and source MAC addresses. The middle section of the frame is payload data including any...
s at a rate of a gigabit per second (1,000,000,000 bits per second), as defined by the IEEE 802.3-2008 standard. It came into use beginning in 1999, gradually supplanting Fast Ethernet
Fast Ethernet
In computer networking, Fast Ethernet is a collective term for a number of Ethernet standards that carry traffic at the nominal rate of 100 Mbit/s, against the original Ethernet speed of 10 Mbit/s. Of the fast Ethernet standards 100BASE-TX is by far the most common and is supported by the...
in wired local networks where it performed considerably faster. The cables and equipment are very similar to previous standards, and as of 2011 are very common and economical.
Half-duplex gigabit links connected through hub
Ethernet hub
An Ethernet hub, active hub, network hub, repeater hub or hub is a device for connecting multiple Ethernet devices together and making them act as a single network segment. A hub works at the physical layer of the OSI model. The device is a form of multiport repeater...
s are allowed by the specification but in the marketplace full-duplex with switches
Network switch
A network switch or switching hub is a computer networking device that connects network segments.The term commonly refers to a multi-port network bridge that processes and routes data at the data link layer of the OSI model...
is normal.
History
The result of research done at Xerox PARCXerox PARC
PARC , formerly Xerox PARC, is a research and co-development company in Palo Alto, California, with a distinguished reputation for its contributions to information technology and hardware systems....
in the early 1970s, Ethernet
Ethernet
Ethernet is a family of computer networking technologies for local area networks commercially introduced in 1980. Standardized in IEEE 802.3, Ethernet has largely replaced competing wired LAN technologies....
evolved into a widely implemented physical
Physical layer
The physical layer or layer 1 is the first and lowest layer in the seven-layer OSI model of computer networking. The implementation of this layer is often termed PHY....
and link
Data link layer
The data link layer is layer 2 of the seven-layer OSI model of computer networking. It corresponds to, or is part of the link layer of the TCP/IP reference model....
layer
OSI model
The Open Systems Interconnection model is a product of the Open Systems Interconnection effort at the International Organization for Standardization. It is a prescription of characterizing and standardizing the functions of a communications system in terms of abstraction layers. Similar...
protocol. Fast Ethernet
Fast Ethernet
In computer networking, Fast Ethernet is a collective term for a number of Ethernet standards that carry traffic at the nominal rate of 100 Mbit/s, against the original Ethernet speed of 10 Mbit/s. Of the fast Ethernet standards 100BASE-TX is by far the most common and is supported by the...
increased speed from 10 to 100 megabits per second (Mbit/s). Gigabit Ethernet was the next iteration, increasing the speed to 1000 Mbit/s. The initial standard for gigabit Ethernet was produced by the IEEE in June 1998 as IEEE 802.3z, and required optical fiber
Optical fiber
An optical fiber is a flexible, transparent fiber made of a pure glass not much wider than a human hair. It functions as a waveguide, or "light pipe", to transmit light between the two ends of the fiber. The field of applied science and engineering concerned with the design and application of...
. 802.3z is commonly referred to as 1000BASE-X, where -X refers to either -CX, -SX, -LX, or (non-standard) -ZX.
IEEE 802.3ab, ratified in 1999, defines gigabit Ethernet transmission over unshielded
Shielded cable
A shielded or screened cable is an electrical cable of one or more insulated conductors enclosed by a common conductive layer. The shield may be composed of braided strands of copper , a non-braided spiral winding of copper tape, or a layer of conducting polymer. Usually, this shield is covered...
twisted pair
Twisted pair
Twisted pair cabling is a type of wiring in which two conductors are twisted together for the purposes of canceling out electromagnetic interference from external sources; for instance, electromagnetic radiation from unshielded twisted pair cables, and crosstalk between neighboring pairs...
(UTP) category 5, 5e
Category 5 cable
Category 5 cable is a twisted pair cable for carrying signals. This type of cable is used in structured cabling for computer networks such as Ethernet. It is also used to carry other signals such as telephony and video. The cable is commonly connected using punch down blocks and modular connectors...
, or 6
Category 6 cable
Category 6 cable, commonly referred to as Cat 6, is a cable standard for Gigabit Ethernet and other network physical layers that is backward compatible with the Category 5/5e and Category 3 cable standards. Compared with Cat 5 and Cat 5e, Cat 6 features more stringent specifications for crosstalk...
cabling and became known as 1000BASE-T. With the ratification of 802.3ab, gigabit Ethernet became a desktop technology as organizations could use their existing copper cabling infrastructure.
IEEE 802.3ah, ratified in 2004 added two more gigabit fiber standards, 1000BASE-LX10 (which was already widely implemented as vendor specific extension) and 1000BASE-BX10. This was part of a larger group of protocols known as Ethernet in the First Mile
Ethernet in the First Mile
Ethernet in the first mile refers to using one of the Ethernet family of computer network protocols between a telecommunications company and a customer's premise. From the customer's point of view it is their "first" mile, although from the access networks' point of view it is known as the "last...
.
Initially, gigabit Ethernet was deployed in high-capacity backbone network
Backbone network
A backbone network or network backbone is a part of computer network infrastructure that interconnects various pieces of network, providing a path for the exchange of information between different LANs or subnetworks. A backbone can tie together diverse networks in the same building, in different...
links (for instance, on a high-capacity campus network). In 2000, Apple's
Apple Computer
Apple Inc. is an American multinational corporation that designs and markets consumer electronics, computer software, and personal computers. The company's best-known hardware products include the Macintosh line of computers, the iPod, the iPhone and the iPad...
Power Mac G4
Power Mac G4
The Power Mac G4 was a series of personal computers that was designed, manufactured, and sold by Apple between 1999 and 2004. They used the PowerPC G4 series of microprocessors. They were heralded by Apple to be the first personal supercomputers, reaching speeds of 4 to 20 Gigaflops...
and PowerBook G4
PowerBook G4
The PowerBook G4 are a series of notebook computers that were manufactured, marketed, and sold by Apple, Inc. between 2001 and 2006 as part of its PowerBook line. It uses the PowerPC G4 processor, initially produced by Motorola and later by Freescale, after Motorola spun off its semiconductor...
were the first mass produced personal computers featuring the 1000BASE-T connection. It quickly became a built-in feature in many other computers. Gigabit NICs (1000BASE-T) are included in almost all desktop, laptop and server computer systems.
Higher bandwidth 10 Gigabit Ethernet
10 Gigabit Ethernet
The 10 gigabit Ethernet computer networking standard was first published in 2002. It defines a version of Ethernet with a nominal data rate of 10 Gbit/s , ten times faster than gigabit Ethernet.10 gigabit Ethernet defines only full duplex point to point links which are generally connected by...
standards have since become available as the IEEE ratified a fiber-based standard in 2002, and a twisted pair standard in 2006. 10Gb Ethernet is replacing 1Gb as the backbone network and has begun to migrate down to high-end server systems.
Varieties
There are five physical layerPhysical layer
The physical layer or layer 1 is the first and lowest layer in the seven-layer OSI model of computer networking. The implementation of this layer is often termed PHY....
standards for gigabit Ethernet using optical fiber
Optical fiber
An optical fiber is a flexible, transparent fiber made of a pure glass not much wider than a human hair. It functions as a waveguide, or "light pipe", to transmit light between the two ends of the fiber. The field of applied science and engineering concerned with the design and application of...
(1000BASE-X), twisted pair cable (1000BASE-T), or balanced copper cable (1000BASE-CX).
The IEEE 802.3z standard includes 1000BASE-SX for transmission over multi-mode fiber, 1000BASE-LX for transmission over single-mode fiber, and the nearly obsolete 1000BASE-CX for transmission over balanced copper cabling. These standards use 8b/10b encoding
8B/10B encoding
In telecommunications, 8b/10b is a line code that maps 8-bit symbols to 10-bit symbols to achieve DC-balance and bounded disparity, and yet provide enough state changes to allow reasonable clock recovery. This means that the difference between the count of 1s and 0s in a string of at least 20 bits...
, which inflates the line rate by 25%, from 1000 Mbit/s to 1250 Mbit/s, to ensure a DC balanced signal. The symbols are then sent using NRZ
Non-return-to-zero
In telecommunication, a non-return-to-zero line code is a binary code in which 1's are represented by one significant condition and 0's are represented by some other significant condition , with no other neutral or rest condition. The pulses have more energy than a RZ code...
.
IEEE 802.3ab, which defines the widely used 1000BASE-T interface type, uses a different encoding scheme in order to keep the symbol rate as low as possible, allowing transmission over twisted pair.
Ethernet in the First Mile
Ethernet in the First Mile
Ethernet in the first mile refers to using one of the Ethernet family of computer network protocols between a telecommunications company and a customer's premise. From the customer's point of view it is their "first" mile, although from the access networks' point of view it is known as the "last...
later added 1000BASE-LX10 and -BX10.
| Name | Medium | Specified distance |
|---|---|---|
| 1000BASE‑CX | Twinaxial cabling Twinaxial cabling Twinaxial cabling, or "Twinax", is a type of cable similar to coax, but with two inner conductors instead of one. Due to cost efficiency it is becoming common in modern very-short-range high-speed differential signaling applications.-IBM:... |
25 meters |
| 1000BASE‑SX | Multi-mode fiber | 220 to 550 meters dependent on fiber diameter and bandwidth |
| 1000BASE‑LX | Multi-mode fiber | 550 meters |
| 1000BASE‑LX | Single-mode fiber | 5 km |
| 1000BASE‑LX10 | Single-mode fiber using 1,310 nm wavelength | 10 km |
| 1000BASE‑ZX | Single-mode fiber at 1,550 nm wavelength | ~ 70 km |
| 1000BASE‑BX10 | Single-mode fiber, over single-strand fiber: 1,490 nm downstream 1,310 nm upstream | 10 km |
| 1000BASE‑T | Twisted-pair cabling (Cat‑5, Cat‑5e, Cat‑6, or Cat‑7) | 100 meters |
| 1000BASE‑TX | Twisted-pair cabling (Cat‑6, Cat‑7) | 100 meters |
1000BASE-X
1000BASE-X is used in industry to refer to gigabit Ethernet transmission over fiber, where options include 1000BASE-CX, 1000BASE-LX, and 1000BASE-SX, 1000BASE-LX10, 1000BASE-BX10 or the non-standard -ZX implementations.1000BASE-CX
1000BASE-CX is an initial standard for gigabit Ethernet connections over twinaxial cabling with maximum distances of 25 meters using balanced shielded twisted pair and either DE-9 or 8P8C connector. The short segment length is due to very high signal transmission rate. Although, it is still used for specific applications where cabling is done by IT professionals, for instance the IBM BladeCenter uses 1000BASE-CX for the Ethernet connections between the blade servers and the switch modules, 1000BASE-T has succeeded it for general copper wiring use.1000BASE-SX
1000BASE-SX is a fiber optic gigabit Ethernet standard for operation over multi-mode fiber using a 770 to 860 nanometer, near infraredInfrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...
(NIR) light
Light
Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...
wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
.
The standard specifies a distance capability between 220 metres (62.5/125 µm fiber with low modal bandwidth
Modal bandwidth
Modal Bandwidth, in the discipline of telecommunications, refers to the signalling rate per distance unit. The signalling rate can typically be measured in MHz, and the modal bandwidth is expressed as MHz·km ....
) and 550 metres (50/125 µm fiber with high modal bandwidth). In practice, with good quality fiber, optics, and terminations, 1000BASE-SX will usually work over significantly longer distances.
This standard is highly popular for intra-building links in large office buildings, co-location facilities and carrier neutral internet exchanges.
Optical power specifications of SX interface: Minimum output power = −9.5 dBm
DBm
dBm is an abbreviation for the power ratio in decibels of the measured power referenced to one milliwatt . It is used in radio, microwave and fiber optic networks as a convenient measure of absolute power because of its capability to express both very large and very small values in a short form...
. Minimum receive sensitivity = −17 dBm.
1000BASE-LX
1000BASE-LX is a fiber optic gigabit Ethernet standard specified in IEEE 802.3 Clause 38 which uses a long wavelength laser (1,270–1,355 nm), and a maximum RMS spectral width of 4 nm.1000BASE-LX is specified to work over a distance of up to 5 km over 10 µm single-mode fiber.
1000BASE-LX can also run over all common types of multi-mode fiber with a maximum segment length of 550 m. For link distances greater than 300 m, the use of a special launch conditioning patch cord may be required. This launches the laser at a precise offset from the center of the fiber which causes it to spread across the diameter of the fiber core, reducing the effect known as differential mode delay which occurs when the laser couples onto only a small number of available modes in multi-mode fiber.
1000BASE-LX10
1000BASE-LX10 was standardized six years after the initial gigabit fiber versions as part of the Ethernet in the First MileEthernet in the First Mile
Ethernet in the first mile refers to using one of the Ethernet family of computer network protocols between a telecommunications company and a customer's premise. From the customer's point of view it is their "first" mile, although from the access networks' point of view it is known as the "last...
task group. It is very similar to 1000BASE-LX, but achieves longer distances up to 10 km over a pair of single-mode fiber due to higher quality optics. Before it was standardized 1000BASE-LX10 was essentially already in widespread use by many vendors as a proprietary extension called either 1000BASE-LX/LH or 1000BASE-LH.
1000BASE-BX10
1000BASE-BX10 is capable of up to 10 km over a single strand of single-mode fiber, with a different wavelength going in each direction. The terminals on each side of the fibre are not equal, as the one transmitting downstream (from the center of the network to the outside) uses the 1,490 nm wavelength, and the one transmitting upstream uses the 1,310 nm wavelength.1000BASE-ZX
1000BASE-ZX is a non-standard but industry accepted term to refer to gigabit Ethernet transmission using 1,550 nm wavelength to achieve distances of at least 70 km over single-mode fiber.1000BASE-T
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
wiring.
Each 1000BASE-T network segment can be a maximum length of 100 meters (328 feet), and must use Category 5 cable
Category 5 cable
Category 5 cable is a twisted pair cable for carrying signals. This type of cable is used in structured cabling for computer networks such as Ethernet. It is also used to carry other signals such as telephony and video. The cable is commonly connected using punch down blocks and modular connectors...
or better. Category 5e cable or Category 6 cable
Category 6 cable
Category 6 cable, commonly referred to as Cat 6, is a cable standard for Gigabit Ethernet and other network physical layers that is backward compatible with the Category 5/5e and Category 3 cable standards. Compared with Cat 5 and Cat 5e, Cat 6 features more stringent specifications for crosstalk...
may also be used.
Autonegotiation
Autonegotiation
Autonegotiation is an Ethernet procedure by which two connected devices choose common transmission parameters, such as speed, duplex mode, and flow control. In this process, the connected devices first share their capabilities regarding these parameters and then choose the highest performance...
is a requirement for using 1000BASE-T according to Section 28D.5 Extensions required for Clause40 (1000BASE-T). At least the clock source has to be negotiated, as one has to be master and the other slave.
In a departure from both 10BASE-T
10BASE-T
Ethernet over twisted pair technologies use twisted-pair cables for the physical layer of an Ethernet computer network. Other Ethernet cable standards employ coaxial cable or optical fiber. Early versions developed in the 1980s included StarLAN followed by 10BASE-T. By the 1990s, fast, inexpensive...
and 100BASE-TX, 1000BASE-T uses all four cable pairs for simultaneous transmission in both directions through the use of adaptive equalization and a 5-level pulse amplitude modulation (PAM-5) technique. The symbol rate is identical to that of 100BASE-TX (125 Mbaud
Baud
In telecommunications and electronics, baud is synonymous to symbols per second or pulses per second. It is the unit of symbol rate, also known as baud rate or modulation rate; the number of distinct symbol changes made to the transmission medium per second in a digitally modulated signal or a...
) and the noise immunity of the 5-level signaling is also identical to that of the 3-level signaling in 100BASE-TX, since 1000BASE-T uses 4-dimensional trellis coded modulation
Trellis modulation
In telecommunication, trellis modulation is a modulation scheme which allows highly efficient transmission of information over band-limited channels such as telephone lines...
(TCM) to achieve a 6 dB
Decibel
The decibel is a logarithmic unit that indicates the ratio of a physical quantity relative to a specified or implied reference level. A ratio in decibels is ten times the logarithm to base 10 of the ratio of two power quantities...
coding gain across the 4 pairs.
Since negotiation takes place on only two pairs, if two gigabit devices are connected through a cable with only two pairs, the devices will successfully choose 'gigabit' as the highest common denominator (HCD), but the link will never come up. Most gigabit physical devices have a specific register to diagnose this behaviour. Some drivers offer an "Ethernet@Wirespeed" option where this situation leads to a slower yet functional connection.
The data is transmitted over four copper pairs, eight bit
Bit
A bit is the basic unit of information in computing and telecommunications; it is the amount of information stored by a digital device or other physical system that exists in one of two possible distinct states...
s at a time. First, eight bits of data are expanded into four 3-bit symbols through a non-trivial scrambling procedure based on a linear feedback shift register
Linear feedback shift register
A linear feedback shift register is a shift register whose input bit is a linear function of its previous state.The most commonly used linear function of single bits is XOR...
; this is similar to what is done in 100BASE-T2, but uses different parameters. The 3-bit symbols are then mapped to voltage levels which vary continuously during transmission. An example mapping is as follows:
| Symbol | 000 | 001 | 010 | 011 | 100 | 101 | 110 | 111 |
|---|---|---|---|---|---|---|---|---|
| Line signal level | 0 | +1 | +2 | −1 | 0 | +1 | −2 | −1 |
Automatic MDI/MDI-X Configuration is specified as an optional feature in the 1000BASE-T standard, meaning that straight-through cables will often work between gigabit-capable interfaces. This feature eliminates the need for crossover cables
Ethernet crossover cable
An Ethernet crossover cable is a type of Ethernet cable used to connect computing devices together directly. Normal straight through or patch cables were used to connect from a host network interface controller to a network switch, hub or router.A cable with connections that "cross over" was used...
, making obsolete the uplink/normal ports and manual selector switches found on many older hubs and switches and greatly reducing installation errors.
1000BASE-TX
The Telecommunications Industry AssociationTelecommunications Industry Association
The Telecommunications Industry Association is accredited by the American National Standards Institute to develop voluntary, consensus-based industry standards for a wide variety of ICT products, and currently represents nearly 400 companies...
(TIA) created and promoted a standard similar to 1000BASE-T that was simpler to implement, calling it 1000BASE-TX (TIA/EIA-854). The simplified design would, in theory, have reduced the cost of the required electronics by only using one pair of wires in each direction. However, this solution required Category 6 cable and has been a commercial failure, likely due to the cabling requirement as well as the rapidly falling cost of 1000BASE-T products. Many 1000BASE-T products are advertised as 1000BASE-TX due to lack of knowledge that 1000BASE-TX is actually a different standard. The confusion between 1000BASE-T and 1000BASE-TX probably stems from the fact that most popular form of Fast Ethernet (100 Mbit/s) is known as 100BASE-TX, and the fact that many products support multiple speeds of 10/100/1000Mb/sec and are often promoted as 10/100/1000BASE-TX.An example of a product being specifying 10/100/1000BASE-TX ports can be found at http://www.cisco.com/en/US/products/ps10018/index.html.
Further reading
- Norris, MarkMark Norris (technology writer)Mark Norris is a British consultant in the field of software engineering and telecommunications, particularly noted as a prolific writer on technology subjects....
, Gigabit Ethernet Technology and Applications, Artech House, 2002. ISBN 1-58053-505-4
External links
- Get IEEE 802.3
- IEEE 802.3
- IEEE and Gigabit Ethernet Alliance Announce Formal Ratification of gigabit Ethernet Over Copper Standard - Announcement from IEEE 28 June 1999
- IEEE P802.3ab 1000BASE-T Task Force (historical information)
- IEEE 802.3 CSMA/CD (ETHERNET)
- 1000BASE-T Whitepaper from 10GEA
- Gigabit Ethernet Auto-Negotiation