Gigabit Ethernet
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Gigabit Ethernet (GbE or 1 GigE) is a term describing various technologies for transmitting Ethernet frames at a rate of a gigabit per second, as defined by the IEEE 802.3-2008 standard. Half-duplex gigabit links connected through hubs are allowed by the specification but in the marketplace full-duplex with switches are normal.
History
The result of research done at Xerox Corporation in the early 1970s, Ethernet has evolved into the most widely implemented physical and link layer protocol today. 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.[1] It quickly became a built-in feature in many other computers. As of 2009[update] Gigabit NICs (1000BASE-T) are included in almost all desktop and server computer systems.
Faster 10 gigabit Ethernet standards have become available as the IEEE ratified a fiber-based standard in 2002, and a twisted pair standard in 2006. As of 2009[update] 10Gb Ethernet is replacing 1Gb as the backbone network and has just begun to migrate down to high-end server systems.
[edit] Summary
There are four different physical layer 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.
name medium specified
distance
1000BASE-CX Balanced copper cabling 25 meters
1000BASE-SX Multi-mode fiber 220 to 550 meters dependent on fiber diameter and bandwidth[2]
1000BASE-LX Multi-mode fiber 550 meters[3]
1000BASE-LX Single-mode fiber 5 km[3]
1000BASE-LX10 Single-mode fiber using 1310 nm wavelength 10 km
1000BASE-ZX Single-mode fiber at 1550 nm wavelength ~ 70 km
1000BASE-BX10 Single-mode fiber, over single-strand fiber: 1490 nm downstream 1310 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 copper cabling with maximum distances of 25 meters using balanced shielded twisted pair. It is still used for specific applications where cabling is not done by general users, for instance the IBM BladeCenter uses 1000BASE-CX for the Ethernet connections between the blade servers and the switch modules. 1000BASE-T 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 infrared (NIR) light wavelength.
The standard specifies a distance capability between 220 meters (62.5/125 µm fiber with low modal bandwidth) and 550 meters (50/125 µm fiber with high modal bandwidth). In practice, with good quality fibre and terminations, 1000BASE-SX will usually work over significantly longer distances.[citation needed]
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
1000BASE-LX is a fiber optic gigabit Ethernet standard specified in IEEE 802.3 Clause 38 which uses a long wavelength laser (1270 to 1355 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.[4] 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 Mile 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.[5]
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 1490 nm wavelength, and the one transmitting "upstream" uses the 1310 nm wavelength.
Non IEEE versions
1000BASE-ZX
1000BASE-ZX is a non-standard but industry accepted[citation needed] term to refer to gigabit Ethernet transmission using 1550 nm wavelength to achieve distances of at least 70 km over single-mode fiber.
1000BASE-T
1000BASE-T capable network interface card made by Intel, which connects to the computer via PCI-X
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 at a minimum. Category 5e cable or Category 6 cable may also be used.
Autonegotiation is a requirement for using 1000BASE-T[6] according to Section 28D.5 Extensions required for Clause40 (1000BASE-T).[7] At least the clock source has to be negotiated, as one has to be master and the other slave.
1000BASE-T requires all four pairs to be present. If two gigabit devices are connected through a non-compliant Cat5 cable with two pairs only, negotiation takes place on two pairs only, so the devices successfully choose 'gigabit' as the highest common denominator (HCD), but the link never comes 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[8].
1000BASE-T details
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 echo cancellation 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.
The data is transmitted over four copper pairs, eight bits 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. One example mapping is as follows:
Symbol Line signal level
000 0
001 +1
010 +2
011 -1
100 0
101 +1
110 -2
111 -1
Automatic crossover
Automatic MDI/MDI-X Configuration is specified as an optional feature in the 1000BASE-T standard[9], 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.
1000BASE-TX
The Telecommunications Industry Association (TIA) created and promoted a version of 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 most popular form of Fast Ethernet (100 Mbit/s) is known as 100BASE-TX.
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