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Gigabit Ethernet (GbE) is a term describing various technologies for transmitting Ethernet packets at a rate of a gigabit per second, as defined by the IEEE 802.3-2002 standard.
History
As a result of research done at Xerox Corporation in the early 1970s, Ethernet has evolved into the most widely implemented networking 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 standardized by the IEEE in June 1998 as IEEE 802.3z. 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 utilize their existing copper cabling infrastructure.
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 has since become a built-in feature in many Macintoshes and PCs.
Fiber Gigabit Ethernet has recently been overtaken by 10 Gigabit Ethernet which was ratified by the IEEE in 2002 and provided data rates 10 times greater than that of Gigabit Ethernet. Work on copper 10 Gigabit Ethernet over twisted pair has just been completed, but as of June 2006, the only currently available adapters for 10 gigabit Ethernet over copper requires special cabling and is limited to 15 m.
Summary
The Gigabit Ethernet data link layer encapulates packets with a frame header (which includes MAC addresses and other header information) and a 32-bit Frame Check Sequence (FCS) before encoding to send them across the physical (fiber or copper) medium.
There are four different physical layer standards for Gigabit Ethernet using optical fiber, twisted pair cable, or balanced copper cable.
The IEEE 802.3z standard includes 1000BASE-SX transmission over multi-mode fiber, 1000BASE-LX transmission over single-mode fiber, and the now obsolete 1000BASE-CX for transmission over balanced copper cabling. These standards use 8B/10B encoding, inflating the line rate by 25%, from 1000 Mbit/s to 1250 Mbit/s. (Note that 8B/10B is an encoding scheme, not a specific code - there exist multiple incompatible implementations; for a specific one, see the standard.) The symbols are then sent using NRZ, as 8B/10B mapping already handles the DC balancing properties of the signal (compare to 1000BASE-T).
IEEE 802.3ab defines 1000BASE-T.
1000BASE-SX
1000BASE-SX is a fiber optic Gigabit Ethernet standard. It operates over multi-mode fiber using a 850 nanometer, near infrared (NIR) light wavelength. The specification allows for a maximum distance between endpoints of 220 m over 62.5/125 µm fibre although in practice, with good quality fibre and terminations, 1000BASE-SX will usually work over significantly longer distances. Modern 50/125 µm fibres can reliably extend the signal to 500 m or more. This standard is highly popular for intra-building links in large office buildings, colocation facilities and carrier neutral internet exchanges.
1000BASE-LX
1000BASE-LX is a fiber optic Gigabit Ethernet standard, using a long wavelength laser (See IEEE 802.3 Clause 38). Signaling speed 1.25±100 ppm GBd, wavelength 1270 to 1355 nm, RMS spectral width (max) 4 nm. Typically, GbE lasers will be specified as having a 1300 or 1310 nm wavelength.
1000BASE-LX is specified to work over a distance of up to 2 km over 9 µm single-mode fiber. In practice it will often operate correctly over a much greater distance. Many manufacturers will guarantee operation up to 10 or 20 km, provided that their equipment is used at both ends of the link. 1000BASE-LX can also run over multi-mode fiber with a maximum segment length of 550 m. For any link distance greater than 300 m, the use of a special launch conditioning patchcord 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. This reduces 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-CX
1000BASE-CX is an obsolete standard for Gigabit Ethernet connections over short distances (maximum of 25 meters per segment) using copper cable (balanced shielded twisted pair). It was succeeded by 1000BASE-T.
1000BASE-T
1000BASE-T (also known as IEEE 802.3ab) is a standard for Gigabit Ethernet over copper wiring. It requires, at a minimum, Category 5 cable (the same as 100BASE-TX), but Category 5e ("Category 5 enhanced") and Category 6 cable may also be used and is often recommended. 1000BASE-T requires all four pairs to be present and is far less tolerant of poorly installed wiring than 100BASE-TX.
Each network segment can have a maximum distance of 100 meters, although several chip manufacturers claim 150 meters. Autonegotiation is a requirement for using 1000BASE-T*.
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 MBd) 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, 8 bits per symbol. First, an 8-bit symbol is 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. Example symbol mapping to PAM-5 during test pattern transmission goes like this (generally, mapping is non-constant and fast varying):
| Symbol | Line signal level |
|---|---|
| 000 | 0 |
| 001 | +1 |
| 010 | +2 |
| 011 | -1 |
| 100 | 0 |
| 101 | +1 |
| 110 | -2 |
| 111 | -1 |
Non-trivial DSP algorithms and processing power were involved with the introduction of PAM-5, hence its delayed introduction after 802.3z.
1000BASE-TX
The Telecommunications Industry Association (TIA) created and promoted a version of 1000BASE-T that was simpler to implement, calling it 1000BASE-TX. The simplified design would, in theory, have reduced the cost of the required electronics by only using two pair in each direction. However, the two-pair solution required Category 6 cable and has been a commercial failure, likely due to the rapidly falling cost of 1000BASE-T products combined with the Category 6 cable requirement. Many 1000BASE-T products are advertised as 1000BASE-TX due to lack of knowledge that 1000BASE-TX is actually a different standard.
See also
External links
- Making the Transition to Gigabit Ethernet - Advice on how to change your network to Gigabit Ethernet
- IEEE and Gigabit Ethernet Alliance Announce Formal Ratification of Gigabit Ethernet Over Copper Standard - Announcement from IEEE Jun 28, 1999
- Gigabit Ethernet Alliance - Page from Lancaster University about the former Gigabit Ethernet Alliance, w/ cabling details
- IEEE P802.3ab 1000BASE-T Task Force (historical information)
- IEEE 802.3 CSMA/CD (ETHERNET)
Gigabit Ethernet | Gigabit Ethernet | Gigabit Ethernet | Gigabit ethernet | 1000BASE-TX
"Gigabit Ethernet"