Gigabit Ethernet

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 802.3-2005 standard. Half-duplex gigabit links connected through hubs are allowed by the specification but in the marketplace full-duplex with switches is the norm.

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 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. [cite web |url=http://www.apple-history.com/frames/body.php?page=gallery&model=g4giga |title=Power Macintosh G4 (Gigabit Ethernet) |publisher=apple-history.com |accessdate=2007-11-05] It quickly became a built-in feature in many other computers.

Since that time, 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.

ummary

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.

1000BASE-X

1000BASE-X is used in industry to refer to gigabit Ethernet transmission over fiber, where options include 1000BASE-SX, -LX, -BX10, or the non-standard -LH/-ZX implementations.

1000BASE-SX

1000BASE-SX is a fiber optic gigabit Ethernet standard for operation over multi-mode fiber using a 850 nanometer, near infrared (NIR) light wavelength. The standard specifies a distance capability 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 550 m or more. This standard is highly popular for intra-building links in large office buildings, co-location facilities and carrier neutral internet exchanges.Typical optical power parameters of SX interface: maximum mean output power = -5 dBm; stressed receiver sensitivity = -14 dBm.

1000BASE-LX

1000BASE-LX is a fiber optic gigabit Ethernet standard which uses a long wavelength laser (See IEEE 802.3 Clause 38), with a wavelength of 1270 to 1355 nm, and a maximum RMS spectral width of 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 5 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 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-LH

1000BASE-LH is a non-standard but industry accepted term to refer to gigabit Ethernet transmission using 1300 or 1310nm wavelength. It is very similar to 1000BASE-LX, but achieves longer distances up to 10km over single-mode fiber due to higher quality optics. 1000BASE-LH is backwards compatible with 1000BASE-LX. [ [http://www.iol.unh.edu/services/testing/ethernet/training/The%20Ethernets%20new%20handout.pdf Wrap-Up ] ]

1000BASE-ZX

1000BASE-ZX is a non-standard but industry accepted term to refer to gigabit Ethernet transmission using 1550 nm wavelength to achieve distances of at least 70 km over single-mode fiber.

1000BASE-BX10

This latest addition to the standard also includes the 1000-BASE-BX10 transmission over a single strand of fiber (which is itself single-mode fiber), with one 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.

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-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" cabling, while Category 5e cable or 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. If two Gigabit devices are connected through a non-compliant Cat5 cable with four pairs, many FCS errors and retransmissions may occur. If two Gigabit devices are connected through a non-compliant Cat5 cable with two pairs only, negotiation takes place on two pairs only, so it ends up successfully choosing gigabit as the Highest Common Denominator (HCD), but the link never goes up. Most gigabit physical devices have a specific register to diagnose this behaviour.

Each network segment can have a maximum distance of 100 meters. Autonegotiation is a requirement for using 1000BASE-T [cite web |url=http://standards.ieee.org/reading/ieee/interp/IEEE802.3af-2003interp-6.pdf |title=Auto-Negotiation; 802.3-2002 |work=IEEE Standards Interpretations |publisher=IEEE |format=PDF |accessdate=2007-11-05] according to the standard. At least clock source has to be negotiated, as one has to be Master and the other Slave. Several physical layer devices and drivers will allow you to force 1000 Mbit/s full duplex to eliminate autonegotiation issues. In this out of standard case, net admin had to care that one peer is clock Master, and the other is clock Slave.

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:

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 (TIA/EIA-854). The simplified design would, in theory, have reduced the cost of the required electronics by only using two pairs 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.

ee also

* 10 Gigabit Ethernet
* 100 Gigabit Ethernet
* Ethernet Alliance
* GBIC
* SFP transceiver
* Jumbo Frames

Footnotes

Bibliography

*Norris, Mark, "Gigabit Ethernet Technology and Applications", Artech House, 2002. ISBN 1-580-53505-4

External links

* [http://standards.ieee.org/getieee802/802.3.html Get IEEE 802.3]
* [http://www.ieee802.org/3/ IEEE 802.3]
* [http://standards.ieee.org/announcements/802.3ab.html IEEE and Gigabit Ethernet Alliance Announce Formal Ratification of gigabit Ethernet Over Copper Standard] - Announcement from IEEE 28 June 1999
* [http://grouper.ieee.org/groups/802/3/ab/ IEEE P802.3ab 1000BASE-T Task Force] (historical information)
* [http://grouper.ieee.org/groups/802/3/ IEEE 802.3 CSMA/CD (ETHERNET)]
* [http://www.10gea.org/1000base-t.htm 1000BASE-T Whitepaper from 10GEA]