Gigabit Ethernet

Posted on May 18, 2009 in Knowledge Base

In Computer Networkinggigabit Ethernet (GbE or 1 GigE) is a term describing various technologies for transmitting Ethernet frames 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 were very common and economical by 2010.

Half-duplex gigabit links connected through hubs are allowed by the specification, but full-duplex usage withswitches is much more common.



There are five physical layer standards for gigabit Ethernet using Optical Fiber (1000BASE-X), twisted pair cable (1000BASE-T), or shielded 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 shielded 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.

IEEE 802.3ap defines Ethernet Operation over Electrical Backplanes at different speeds.

Ethernet in the First Mile later added 1000BASE-LX10 and -BX10.

Name Medium Specified distance
1000BASE‑CX Shielded balanced copper cable 25 meters
1000BASE‑KX Copper backplane 1 meter
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‑EX Single-mode fiber at 1,310 nm wavelength ~ 40 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 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 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 (with a pinout different from 1000BASE-T). 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-KX is part of the IEEE 802.3ap standard for Ethernet Operation over Electrical Backplanes. This standard defines 1-4 lanes of backplane links, one RX and one TX differential pair per lane, at link bandwidth ranging from 100Mbit to 10Gbit per second (from 100BASE-KX to 10GBASE-KX4). The 1000BASE-KX variant uses 1.25 GBd electrical signalling speed.


1000BASE-SX is a Fiber Optic gigabit Ethernet standard for operation over multi-mode fiber using a 770 to 860nanometer, near infrared (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 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 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.


1000BASE-EX is a non-standard but industry accepted term to refer to gigabit Ethernet transmission. It is very similar to 1000BASE-LX10 but achieves longer distances up to 40 km over a pair of Single-Mode Fibers due to higher quality optics than a LX10, running on 1310 nm wavelength lasers. It is sometimes referred to as LH (Long Haul). Easily confused with a 1000BASE-LX10 or 1000BASE-ZX because some vendors use the LH term.


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 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 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 overcopper wiring.

Each 1000BASE-T network segment can be a maximum length of 100 meters (328 feet), and must use Category 5 cable or better (including Cat 5e and Cat 6).

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 endpoint must be master and the other endpoint must be 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 ofadaptive 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-dimensionaltrellis 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 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. 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, making obsolete the uplink/normal ports and manual selector switches found on many older hubs and switches and greatly reduces installation errors.


The Telecommunications Industry Association (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 have, in theory, reduced the cost of the required electronics by only using one pair of wires in each direction. However, this solution has been a commercial failure, likely due to the required Category 6 cabling and the rapidly falling cost of 1000BASE-T products.

1000BASE-T products are sometimes marketed as 1000BASE-TX despite the difference in standards. The confusion possibly stems from the most popular form of Fast Ethernet (100 Mbit/s) is known as 100BASE-TX, leading to many products supporting multiple speeds of 10/100/1000Mbit/s marketed as “10/100/1000BASE-TX”.