10 and 100 Mbps Ethernet
The principal 10 Mbps implementations of Ethernet include:
10BASE5 using Thicknet coaxial cable
10BASE2 using Thinnet coaxial cable
10BASE-T using Cat3/Cat5 unshielded twisted-pair cable
The early implementations of Ethernet, 10BASE5, and 10BASE2 used coaxial cable in a physical bus. These implementations are no longer used and are not supported by the newer 802.3 standards.
10 Mbps Ethernet - 10BASE-T
10BASE-T uses Manchester-encoding over two unshielded twisted-pair cables. The early implementations of 10BASE-T used Cat3 cabling. However, Cat5 or later cabling is typically used today.
10 Mbps Ethernet is considered to be classic Ethernet and uses a physical star topology. Ethernet 10BASE-T links could be up to 100 meters in length before requiring a hub or repeater.
10BASE-T uses two pairs of a four-pair cable and is terminated at each end with an 8-pin RJ-45 connector. The pair connected to pins 1 and 2 are used for transmitting and the pair connected to pins 3 and 6 are used for receiving. The figure shows the RJ45 pinout used with 10BASE-T Ethernet.
10BASE-T is generally not chosen for new LAN installations. However, there are still many 10BASE-T Ethernet networks in existence today. The replacement of hubs with switches in 10BASE-T networks has greatly increased the throughput available to these networks and has given Legacy Ethernet greater longevity. The 10BASE-T links connected to a switch can support either half-duplex or full-duplex operation.
100 Mbps - Fast Ethernet
In the mid to late 1990s, several new 802.3 standards were established to describe methods for transmitting data over Ethernet media at 100 Mbps. These standards used different encoding requirements for achieving these higher data rates.
100 Mbps Ethernet, also known as Fast Ethernet, can be implemented using twisted-pair copper wire or fiber media. The most popular implementations of 100 Mbps Ethernet are:
100BASE-TX using Cat5 or later UTP
100BASE-FX using fiber-optic cable
Because the higher frequency signals used in Fast Ethernet are more susceptible to noise, two separate encoding steps are used by 100-Mbps Ethernet to enhance signal integrity.
100BASE-TX was designed to support transmission over either two pairs of Category 5 UTP copper wire or two strands of optical fiber. The 100BASE-TX implementation uses the same two pairs and pinouts of UTP as 10BASE-T. However, 100BASE-TX requires Category 5 or later UTP. The 4B/5B encoding is used for 100BASE-T Ethernet.
As with 10BASE-TX, 100Base-TX is connected as a physical star. The figure shows an example of a physical star topology. However, unlike 10BASE-T, 100BASE-TX networks typically use a switch at the center of the star instead of a hub. At about the same time that 100BASE-TX technologies became mainstream, LAN switches were also being widely deployed. These concurrent developments led to their natural combination in the design of 100BASE-TX networks.
The 100BASE-FX standard uses the same signaling procedure as 100BASE-TX, but over optical fiber media rather than UTP copper. Although the encoding, decoding, and clock recovery procedures are the same for both media, the signal transmission is different - electrical pulses in copper and light pulses in optical fiber. 100BASE-FX uses Low Cost Fiber Interface Connectors (commonly called the duplex SC connector).
Fiber implementations are point-to-point connections, that is, they are used to interconnect two devices. These connections may be between two computers, between a computer and a switch, or between two switches.
1000 Mbps - Gigabit Ethernet
The development of Gigabit Ethernet standards resulted in specifications for UTP copper, single-mode fiber, and multimode fiber. On Gigabit Ethernet networks, bits occur in a fraction of the time that they take on 100 Mbps networks and 10 Mbps networks. With signals occurring in less time, the bits become more susceptible to noise, and therefore timing is critical. The question of performance is based on how fast the network adapter or interface can change voltage levels and how well that voltage change can be detected reliably 100 meters away, at the receiving NIC or interface.
At these higher speeds, encoding and decoding data is more complex. Gigabit Ethernet uses two separate encoding steps. Data transmission is more efficient when codes are used to represent the binary bit stream. Encoding the data enables synchronization, efficient usage of bandwidth, and improved signal-to-noise ratio characteristics.
1000BASE-T Ethernet provides full-duplex transmission using all four pairs in Category 5 or later UTP cable. Gigabit Ethernet over copper wire enables an increase from 100 Mbps per wire pair to 125 Mbps per wire pair, or 500 Mbps for the four pairs. Each wire pair signals in full duplex, doubling the 500 Mbps to 1000 Mbps.
1000BASE-T uses 4D-PAM5 line encoding to obtain 1 Gbps data throughput. This encoding scheme enables the transmission signals over four wire pairs simultaneously. It translates an 8-bit byte of data into a simultaneous transmission of four code symbols (4D), which are sent over the media, one on each pair, as 5-level Pulse Amplitude Modulated (PAM5) signals. This means that every symbol corresponds to two bits of data. Because the information travels simultaneously across the four paths, the circuitry has to divide frames at the transmitter and reassemble them at the receiver. The figure shows a representation of the circuitry used by 1000BASE-T Ethernet.
1000BASE-SX and 1000BASE-LX Ethernet Using Fiber-Optics
The fiber versions of Gigabit Ethernet - 1000BASE-SX and 1000BASE-LX - offer the following advantages over UTP: noise immunity, small physical size, and increased unrepeated distances and bandwidth.
All 1000BASE-SX and 1000BASE-LX versions support full-duplex binary transmission at 1250 Mbps over two strands of optical fiber. The transmission coding is based on the 8B/10B encoding scheme. Because of the overhead of this encoding, the data transfer rate is still 1000 Mbps.
Each data frame is encapsulated at the Physical layer before transmission, and link synchronization is maintained by sending a continuous stream of IDLE code groups during the interframe spacing.