Gigabit Ethernet was first specified by two standards: IEEE 802.3z-1998 and 802.3ab-1999. The IEEE 802.3z standard specifies the operation of Gigabit Ethernet over fiber and coaxial cable and introduced the Gigabit Media-Independent Interface (GMII). These standards have been superseded by the latest revision of all the 802.3 standards included in IEEE 802.3-2002.
The IEEE 802.3ab standard specified the operation of Gigabit Ethernet over CAT 5 UTP. Gigabit Ethernet still retains the frame formats and frame sizes, and it still uses CSMA/CD. As with Ethernet and Fast Ethernet, full-duplex operation is possible. Differences appear in the encoding; Gigabit Ethernet uses 8B10B coding with simple nonreturn to zero (NRZ). Because of the 20% overhead, pulses run at 1250 MHz to achieve 1000 Mbps throughput.
Table 6-3 provides an overview of Gigabit Ethernet scalability constraints.
Table 6-3 Gigabit Ethernet Scalability Constraints
| Type | Speed | Maximum Segment Length | Encoding | Media |
| 1000BASE-T | 1000 Mbps | 100 m | Five-level | CAT 5 UTP |
| 1000BASE-LX (long wavelength) | 1000 Mbps | 550 m | 8B10B | Single-mode/multimode fiber |
| 1000BASE-SX (short wavelength) | 1000 Mbps | 62.5 micrometers: 220 m 50 micrometers: 500 m | 8B10B | Multimode fiber |
| 1000BASE-CX | 1000 Mbps | 25 m | 8B10B | Shielded balanced copper |
The following are the physical specifications for Gigabit Ethernet, which are described in the following sections:
- 1000BASE-LX
- 1000BASE-SX
- 1000BASE-CX
- 1000BASE-T
1000BASE-LX Long-Wavelength Gigabit Ethernet
IEEE 1000BASE-LX uses long-wavelength optics over a pair of fiber strands. The specifications are as follows:
- It uses long wavelengths (1300 nm [nanometers]).
- It can be used on multimode or single-mode fiber.
- Maximum lengths for multimode fiber are as follows:
- 62.5-micrometer fiber: 440 meters
- 50-micrometer fiber: 550 meters
- The maximum length for single-mode fiber (9 micrometers) is 5 km.
- It uses 8B10B encoding with simple NRZ.
1000BASE-SX Short-Wavelength Gigabit Ethernet
IEEE 1000BASE-SX uses short-wavelength optics over a pair of multimode fiber strands. The specifications are as follows:
- It uses short wavelengths (850 nm).
- It can be used on multimode fiber.
- Maximum lengths are as follows:
- 62.5-micrometer fiber: 260 m
- 50-micrometer fiber: 550 m
- It uses 8B10B encoding with simple NRZ.
1000BASE-CX Gigabit Ethernet over Coaxial Cable
The IEEE 1000BASE-CX standard is for short copper runs between servers. The specifications are as follows:
- It is used on short-run copper.
- It runs over a pair of 150-ohm balanced coaxial cables (twinax).
- The maximum length is 25 meters.
- It is mainly for server connections.
- It uses 8B10B encoding with simple NRZ.
1000BASE-T Gigabit Ethernet over UTP
The IEEE standard for 1000 Mbps Ethernet over CAT 5 UTP was IEEE 802.3ab; it was approved in June 1999. It is now included in IEEE 802.3-2002. This standard uses the four pairs in the cable. (100BASE-TX and 10BASE-T Ethernet use only two pairs.) The specifications are as follows:
- It uses CAT 5 four-pair UTP.
- The maximum length is 100 meters.
- The encoding defined is a five-level coding scheme.
- One byte is sent over the four pairs at 1250 MHz.