Summary of Module 7

Summary
This page summarizes the topics discussed in this module.


Ethernet is a technology that has increased in speed one thousand times, from 10 Mbps to 10,000 Mbps, in less than a decade. All forms of Ethernet share a similar frame structure and this leads to excellent interoperability. Most Ethernet copper connections are now switched full duplex, and the fastest copper-based Ethernet is 1000BASE-T, or Gigabit Ethernet. 10 Gigabit Ethernet and faster are exclusively optical fiber-based technologies.

10BASE5, 10BASE2, and 10BASE-T Ethernet are considered Legacy Ethernet. The four common features of Legacy Ethernet are timing parameters, frame format, transmission process, and a basic design rule.

Legacy Ethernet encodes data on an electrical signal. The form of encoding used in 10 Mbps systems is called Manchester encoding. Manchester encoding uses a change in voltage to represent the binary numbers zero and one. An increase or decrease in voltage during a timed period, called the bit period, determines the binary value of the bit.

In addition to a standard bit period, Ethernet standards set limits for slot time and interframe spacing. Different types of media can affect transmission timing and timing standards ensure interoperability. 10 Mbps Ethernet operates within the timing limits offered by a series of no more than five segments separated by no more than four repeaters.

A single thick coaxial cable was the first medium used for Ethernet. 10BASE2, using a thinner coax cable, was introduced in 1985. 10BASE-T, using twisted-pair copper wire, was introduced in 1990. Because it used multiple wires 10BASE-T offered the option of full-duplex signaling. 10BASE-T carries 10 Mbps of traffic in half-duplex mode and 20 Mbps in full-duplex mode.

10BASE-T links can have unrepeated distances up to 100 m. Beyond that network devices such as repeaters, hub, bridges and switches are used to extend the scope of the LAN. With the advent of switches, the 4-repeater rule is not so relevant. You can extend the LAN indefinitely by daisy-chaining switches. Each switch-to-switch connection, with maximum length of 100m, is essentially a point-to-point connection without the media contention or timing issues of using repeaters and hubs.

100-Mbps Ethernet, also known as Fast Ethernet, can be implemented using twisted-pair copper wire, as in 100BASE-TX, or fiber media, as in 100BASE-FX. 100 Mbps forms of Ethernet can transmit 200 Mbps in full duplex.

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.

Gigabit Ethernet over copper wire is accomplished by the following:

• Category 5e UTP cable and careful improvements in electronics are used to boost 100 Mbps per wire pair to 125 Mbps per wire pair.
• All four wire pairs instead of just two. This allows 125 Mbps per wire pair, or 500 Mbps for the four wire pairs.
• Sophisticated electronics allow permanent collisions on each wire pair and run signals in full duplex, doubling the 500 Mbps to 1000 Mbps.

On Gigabit Ethernet networks bit signals occur in one tenth of the time of 100 Mbps networks and 1/100 of the time of 10 Mbps networks. With signals occurring in less time the bits become more susceptible to noise. The issue becomes how fast the network adapter or interface can change voltage levels to signal bits and still be detected reliably one hundred meters away at the receiving NIC or interface. At this speed encoding and decoding data becomes even more complex.

The fiber versions of Gigabit Ethernet, 1000BASE-SX and 1000BASE-LX offer the following advantages: noise immunity, small size, and increased unrepeated distances and bandwidth. The IEEE 802.3 standard recommends that Gigabit Ethernet over fiber be the preferred backbone technology.