The 1000-Mbps Ethernet or Gigabit Ethernet standards represent transmission using both fiber and copper media.  The 1000BASE-X standard, IEEE 802.3z, specifies 1 Gbps full duplex over optical fiber. The 1000BASE-X standard, IEEE 802.3z, specifies 1 Gbps full duplex over optical fiber.1000BASE-TX, 1000BASE-SX, and 1000BASE-LX use the same timing parameters, as shown in Figure The differences between standard Ethernet, Fast Ethernet and Gigabit Ethernet occur at the physical layer. Due to the increased speeds of these newer standards, the shorter duration bit times require special considerations. Since the bits are introduced on the medium for a shorter duration and more often, timing is critical. This high-speed transmission requires frequencies closer to copper medium bandwidth limitations. This causes the bits to be more susceptible to noise on copper media. These issues require Gigabit Ethernet to use two separate encoding steps. Data transmission is made more efficient by using codes to represent the binary bit stream. The encoded data provides synchronization, efficient usage of bandwidth, and improved Signal-to-Noise Ratio characteristics. At the physical layer, the bit patterns from the MAC layer are converted into symbols. The symbols may also be control information such as start frame, end frame, medium idle conditions. The frame is coded into control symbols and data symbols to increase in network throughput. Fiber-based Gigabit Ethernet (1000BASE-X) uses 8B/10B encoding which is similar to the 4B/5B concept. This is followed by the simple Non-Return to Zero (NRZ) line encoding of light on optical fiber. This simpler encoding process is possible because the fiber medium can carry higher bandwidth signals. |
| As Fast Ethernet was installed to increase bandwidth to workstations, this began to create bottlenecks upstream in the network. 1000BASE-T (IEEE 802.3ab) was developed to provide additional bandwidth to help alleviate these bottlenecks. It provided more "speed" for applications such as intra-building backbones, inter-switch links, server farms, and other wiring closet applications as well as connections for high-end workstations. Fast Ethernet was designed to function over existing Cat 5 copper cable and this necessitated that cable would pass the Cat 5e test. Most installed Cat 5 cable can pass 5e certification if properly terminated. One of the most important attributes of the 1000BASE-T standard is that it be interoperable with 10BASE-T and 100BASE-TX. Because Cat 5e cable can reliably carry up to 125 Mbps of traffic, getting 1000 Mbps (Gigabit) of bandwidth was a design challenge. The first step to accomplish 1000BASE-T is to use all four pairs of wires instead of the traditional two pairs of wires used by 10BASE-T and 100BASE-TX. This is done using complex circuitry to allow full duplex transmissions on the same wire pair. This provides 250 Mbps per pair. With all four-wire pairs, this provides the desired 1000 Mbps. Since the information travels simultaneously across the four paths, the circuitry has to divide frames at the transmitter and reassemble them at the receiver. The 1000BASE-T encoding with 4D-PAM5 line encoding is used on Cat 5e or better UTP. Achieving the 1 Gbps rate requires use of all four pairs in full duplex simultaneously. That is the transmission and reception of data happens in both directions on the same wire at the same time. As might be expected, this results in a permanent collision on the wire pairs. These collisions result in complex voltage patterns. With the complex integrated circuits using techniques such as echo cancellation, Layer 1 Forward Error Correction (FEC), and prudent selection of voltage levels, the system achieves the 1Gigabit throughput. In idle periods there are nine voltage levels found on the cable, and during data transmission periods there are 17 voltage levels found on the cable. The data from the sending station is carefully divided into four parallel streams, encoded, transmitted and detected in parallel, and then reassembled into one received bit stream. Figure |
The IEEE 802.3 standard recommends that Gigabit Ethernet over fiber be the preferred backbone technology.
The timing, frame format, and transmission are common to all versions of 1000 Mbps. Two signal-encoding schemes are defined at the physical layer. 
The 8B/ 10B scheme is used for optical fiber and shielded copper media, and the pulse amplitude modulation 5 (PAM5) is used for UTP.
1000BASE-X uses 8B/10B encoding converted to non-return to zero (NRZ) line encoding. NRZ encoding relies on the signal level found in the timing window to determine the binary value for that bit period. Unlike most of the other encoding schemes described, this encoding system is level driven instead of edge driven. That is the determination of whether a bit is a zero or a one is made by the level of the signal rather than when the signal changes levels.
The NRZ signals are then pulsed into the fiber using either short-wavelength or long-wavelength light sources. The short-wavelength uses an 850 nm laser or LED source in multimode optical fiber (1000BASE-SX). It is the lower-cost of the options but has shorter distances. The long-wavelength 1310 nm laser source uses either single-mode or multimode optical fiber (1000BASE-LX). Laser sources used with single-mode fiber can achieve distances of up to 5000 meters. Because of the length of time to completely turn the LED or laser on and off each time, the light is pulsed using low and high power. A logic zero is represented by low power, and a logic one by high power.
The Media Access Control method treats the link as point-to-point. Since separate fibers are used for transmitting (Tx) and receiving (Rx) the connection is inherently full duplex. Gigabit Ethernet permits only a single repeater between two stations. Figure 
is a 1000BASE Ethernet media comparison chart.
Gigabit Ethernet architecture
| The distance limitations of full-duplex links are only limited by the medium, and not the round-trip delay. Since most Gigabit Ethernet is switched, the values in Figures and are the practical limits between devices. Daisy-chaining, star, and extended star topologies are all allowed. The issue then becomes one of logical topology and data flow, not timing or distance limitations. A 1000BASE-T UTP cable is the same as 10BASE-T and 100BASE-TX cable, except that link performance must meet the higher quality Category 5e or ISO Class D (2000) requirements. Modification of the architecture rules is strongly discouraged for 1000BASE-T. At 100 meters, 1000BASE-T is operating close to the edge of the ability of the hardware to recover the transmitted signal. Any cabling problems or environmental noise could render an otherwise compliant cable inoperable even at distances that are within the specification. It is recommended that all links between a station and a hub or switch be configured for Auto-Negotiation to permit the highest common performance. This will avoid accidental misconfiguration of the other required parameters for proper Gigabit Ethernet operation. |
10-Gigabit Ethernet
| IEEE 802.3ae was adapted to include 10 Gbps full-duplex transmission over fiber optic cable. The basic similarities between 802.3ae and 802.3, the original Ethernet are remarkable. This 10-Gigabit Ethernet (10GbE) is evolving for not only LANs, but also MANs, and WANs. With the frame format and other Ethernet Layer 2 specifications compatible with previous standards, 10GbE can provide increased bandwidth needs that are interoperable with existing network infrastructure. A major conceptual change for Ethernet is emerging with 10GbE. Ethernet is traditionally thought of as a LAN technology, but 10GbE physical layer standards allow both an extension in distance to 40 km over single-mode fiber and compatibility with synchronous optical network (SONET) and synchronous digital hierarchy (SDH) networks. Operation at 40 km distance makes 10GbE a viable MAN technology. Compatibility with SONET/SDH networks operating up to OC-192 speeds (9.584640 Gbps) make 10GbE a viable WAN technology. 10GbE may also compete with ATM for certain applications. To summarize, how does 10GbE compare to other varieties of Ethernet?
The basic standard governing CSMA/CD is IEEE 802.3. An IEEE 802.3 supplement, entitled 802.3ae, governs the 10GbE family. As is typical for new technologies, a variety of implementations are being considered, including:
The IEEE 802.3ae Task force and the 10-Gigabit Ethernet Alliance (10 GEA) are working to standardize these emerging technologies. 10-Gbps Ethernet (IEEE 802.3ae) was standardized in June 2002. It is a full-duplex protocol that uses only optic fiber as a transmission medium. The maximum transmission distances depend on the type of fiber being used. When using single-mode fiber as the transmission medium, the maximum transmission distance is 40 kilometers (25 miles). Some discussions between IEEE members have begun that suggest the possibility of standards for 40, 80, and even 100-Gbps Ethernet. |
| As with the development of Gigabit Ethernet, the increase in speed comes with extra requirements. The shorter bit time duration because of increased speed requires special considerations. For 10 GbE transmissions, each data bit duration is 0.1 nanosecond. This means there would be 1,000 GbE data bits in the same bit time as one data bit in a 10-Mbps Ethernet data stream. Because of the short duration of the 10 GbE data bit, it is often difficult to separate a data bit from noise. 10 GbE data transmissions rely on exact bit timing to separate the data from the effects of noise on the physical layer. This is the purpose of synchronization. In response to these issues of synchronization, bandwidth, and Signal-to-Noise Ratio, 10-Gigabit Ethernet uses two separate encoding steps. By using codes to represent the user data, transmission is made more efficient. The encoded data provides synchronization, efficient usage of bandwidth, and improved Signal-to-Noise Ratio characteristics. Complex serial bit streams are used for all versions of 10GbE except for 10GBASE-LX4, which uses Wide Wavelength Division Multiplex (WWDM) to multiplex four bit simultaneous bit streams as four wavelengths of light launched into the fiber at one time. Figure Currently, most 10GbE products are in the form of modules, or line cards, for addition to high-end switches and routers. As the 10GbE technologies evolve, an increasing diversity of signaling components can be expected. As optical technologies evolve, improved transmitters and receivers will be incorporated into these products, taking further advantage of modularity. All 10GbE varieties use optical fiber media. Fiber types include 10µ single-mode Fiber, and 50µ and 62.5µ multimode fibers. A range of fiber attenuation and dispersion characteristics is supported, but they limit operating distances. Even though support is limited to fiber optic media, some of the maximum cable lengths are surprisingly short. As with 10 Mbps, 100 Mbps and 1000 Mbps versions, it is possible to modify some of the architecture rules slightly. Possible architecture adjustments are related to signal loss and distortion along the medium. Due to dispersion of the signal and other issues the light pulse becomes undecipherable beyond certain distances. |
Ethernet has gone through an evolution from Legacy → Fast → Gigabit → MultiGigabit technologies. While other LAN technologies are still in place (legacy installations), Ethernet dominates new LAN installations. So much so that some have referred to Ethernet as the LAN “dial tone”. Ethernet is now the standard for horizontal, vertical, and inter-building connections. Recently developing versions of Ethernet are blurring the distinction between LANs, MANs, and WANs.
While 1-Gigabit Ethernet is now widely available and 10-Gigabit products becoming more available, the IEEE and the 10-Gigabit Ethernet Alliance are working on 40, 100, or even 160 Gbps standards. The technologies that are adopted will depend on a number of factors, including the rate of maturation of the technologies and standards, the rate of adoption in the market, and cost.
Proposals for Ethernet arbitration schemes other than CSMA/CD have been made. The problem of collisions with physical bus topologies of 10BASE5 and 10BASE2 and 10BASE-T and 100BASE-TX hubs is no longer common. Using UTP and optical fiber with separate Tx and Rx paths, and the decreasing costs of switches make single shared media, half-duplex media connections much less important.
The future of networking media is three-fold:
- Copper (up to 1000 Mbps, perhaps more)
- Wireless (approaching 100 Mbps, perhaps more)
- Optical fiber (currently at 10,000 Mbps and soon to be more)
Copper and wireless media have certain physical and practical limitations on the highest frequency signals that can be transmitted. This is not a limiting factor for optical fiber in the foreseeable future. The bandwidth limitations on optical fiber are extremely large and are not yet being threatened. In fiber systems, it is the electronics technology (such as emitters and detectors) and fiber manufacturing processes that most limit the speed. Upcoming developments in Ethernet are likely to be heavily weighted towards Laser light sources and single-mode optical fiber.
When Ethernet was slower, half-duplex, subject to collisions and a “democratic” process for prioritization, was not considered to have the Quality of Service (QoS) capabilities required to handle certain types of traffic. This included such things as IP telephony and video multicast.
The full-duplex high-speed Ethernet technologies that now dominate the market are proving to be sufficient at supporting even QoS-intensive applications. This makes the potential applications of Ethernet even wider. Ironically end-to-end QoS capability helped drive a push for ATM to the desktop and to the WAN in the mid-1990s, but now it is Ethernet, not ATM that is approaching this goal.
Cisco Systems, Inc.
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