Gigabit Ethernet

In computer networking, Gigabit Ethernet is the term applied to transmitting Ethernet frames at a rate of a gigabit per second. The most popular variant, 1000BASE-T, is defined by the IEEE 802.3ab standard. It came into use in 1999, and has replaced Fast Ethernet in wired local networks due to its considerable speed improvement over Fast Ethernet, as well as its use of cables and equipment that are widely available, economical, and similar to previous standards. The first standard for faster 10 Gigabit Ethernet was approved in 2002.

History

was the result of research conducted at Xerox PARC in the early 1970s, and later evolved into a widely implemented physical and link layer protocol. Fast Ethernet increased the speed from 10 to 100 megabits per second. Gigabit Ethernet was the next iteration, increasing the speed to.
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 -ZX. IEEE 802.3ab, ratified in 1999, defines Gigabit Ethernet transmission over unshielded twisted pair 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 GbE fiber standards: 1000BASE-LX10 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. In 2000 and 2001, Apple's Power Mac G4 and PowerBook G4 respectively were the first mass-produced personal computers to feature the 1000BASE-T connection. It quickly became a built-in feature in many other computers.
Half-duplex GbE links connected through repeater hubs were part of the IEEE specification, but the specification has not been maintained and full-duplex operation with switches is, in practice, used exclusively.

Varieties

There are five physical layer standards for Gigabit Ethernet using optical fiber, twisted pair cable, or shielded balanced copper cable.
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 shielded balanced copper cabling. These standards use 8b/10b encoding, which adds a 25% overhead, to ensure a DC balanced signal and allow for clock recovery. The symbols are then sent using NRZ line code.
Optical fiber transceivers are most often implemented as user-swappable modules in SFP form or GBIC on older devices.
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.
IEEE 802.3ap defines Ethernet operation over electrical backplanes at different speeds.
Ethernet in the first mile later added 1000BASE-LX10 and -BX10 extended-distance fiber-optic variants.

Copper

1000BASE-T

IEEE 802.3ab is the original standard for Gigabit Ethernet over twisted-pair wiring, known as 1000BASE-T.
Each 1000BASE-T network segment is recommended to be a maximum length of, and must use Category 5 cable or better.
Autonegotiation is a requirement for 1000BASE-T implementations as minimally the clock source for the link has to be negotiated, as one endpoint must be master and the other endpoint must be slave.
In a departure from both 10BASE-T and 100BASE-TX, which use a cable pair in each direction, 1000BASE-T uses four cable pairs for simultaneous transmission in both directions through the use of echo cancellation with adaptive equalization, the resulting circuits being called hybrid circuits as used in telephone hybrids. Line coding is five-level pulse-amplitude modulation. The symbol rate is identical to that of 100BASE-TX and the noise immunity of the five-level signaling is also identical to that of the three-level signaling in 100BASE-TX, since 1000BASE-T uses four-dimensional trellis coded modulation to achieve a 6 dB coding gain across the four pairs.
Since autonegotiation takes place on only two pairs, if two 1000BASE-T interfaces are connected through a cable with only two pairs, the interfaces will complete negotiation and choose gigabit as the best common operating mode, but the link will never come up because, in most cases, all four pairs are required for data communications. Most 1000BASE-T implementations have a specific register to diagnose this behavior.
The data is transmitted over four pairs, eight bits at a time. First, eight bits of data are expanded into four three-bit symbols through a scrambling procedure based on a linear-feedback shift register; this is similar to what is done in 100BASE-T2, but uses different parameters. The three-bit symbols are then mapped to voltage levels that 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, and is commonly implemented meaning that straight-through cables will often work between two GbE-capable network node interfaces and between two switch or hub interfaces. This feature eliminates the need for Ethernet crossover cables, making obsolete the uplink vs. normal port choices and manual selector switches found on many older hubs and switches and reducing wiring errors.
In order to extend and maximize the use of existing Cat-5e and Cat-6 cabling, the newer standards 2.5GBASE-T and 5GBASE-T operate at 2.5 and, respectively, on existing copper infrastructure designed for use with 1000BASE-T. They are based on 10GBASE-T but use lower signaling frequencies.

1000BASE-T1

IEEE 802.3 standardized 1000BASE-T1 in IEEE Std 802.3bp-2016. It defines Gigabit Ethernet over a single twisted pair for automotive and industrial applications. It includes cable specifications for 15 meters or 40 meters reach. The transmission is done using PAM-3 at 750 MBd.

1000BASE-TX

In 2001, the Telecommunications Industry Association created and promoted a standard similar to 1000BASE-T that was simpler to implement, calling it 1000BASE-TX. The simplified design would have, in theory, reduced the cost of the required electronics by only using four unidirectional pairs instead of four bidirectional pairs.

1000BASE-CX

802.3z-1998 CL39 standardized 1000BASE-CX is an initial standard for Gigabit Ethernet connections 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 a 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-KX

802.3ap-2007 CL70 standardized 1000BASE-KX as part of the IEEE 802.3ap standard for Ethernet Operation over Electrical Backplanes. This standard defines one to four lanes of backplane links, one RX and one TX differential pair per lane, at link bandwidth ranging from 100 Mbit to 10 Gbit per second. The 1000BASE-KX variant uses 1.25 GBd electrical signaling speed.

Fiber optics

1000BASE-X is used in industry to refer to Gigabit Ethernet transmission over fiber, where options include 1000BASE-SX, 1000BASE-LX, 1000BASE-LX10, 1000BASE-BX10 or the non-standard -EX and -ZX implementations. Included are copper variants using the same 8b/10b line code. 1000BASE-X is based on the physical-layer standards developed for Fibre Channel.

1000BASE-SX

1000BASE-SX is an optical fiber Gigabit Ethernet standard for operation over multi-mode fiber using a 770 to 860 nanometer, near infrared light wavelength.
The standard specifies a maximum length of 220 meters for 62.5 μm/160 MHz×km multi-mode fiber, 275 m for 62.5 μm/200 MHz×km, 500 m for 50 μm/400 MHz×km, and 550 m for 50 μm/500 MHz×km multi-mode fiber. Fiber optic cable manufacturers have extended the reach of 1000BASE-SX to at least 1km when used with more modern fiber optic grades such as OM3 and OM4.
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-LSX

1000BASE-LSX is a non-standard but industry accepted
term to refer to Gigabit Ethernet transmission. It is very similar to 1000BASE-SX but achieves longer distances up to 2 km over a pair of multi-mode fibers due to higher quality optics than a SX, running on 1310 nm wavelength lasers. It is easily confused with 1000BASE-SX or 1000BASE-LX because the use of -LX, -LX10 and -SX is ambiguous between vendors.
The range is achieved with use of Fabry Perot laser transmitter.

1000BASE-LX

1000BASE-LX is an optical fiber Gigabit Ethernet standard specified in IEEE 802.3 Clause 38 which uses a long wavelength laser, 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.