Handout 3 - Communications Media

LAN Transmission Media

The communications medium is the medium across which data communications occur between two computing devices.

There are two categories of communications medium: wire medium and wireless medium. Wire medium uses a physical medium to transmit the data over, whereas a wireless medium uses electromagnetic waves to transmit the data, and do so through the air.

Wire Media

There are three basic types of wire media:

• » Twisted Pair Wires - This communications medium consists of wire strands twisted in sets of two and bound into a cable. This is either shielded (shielded twisted pair - STP) or unshielded (unshielded twisted pair - UTP).
• » Coaxial Cable - This communications medium consists of a centre wire inside a grounded, cylindrical shield. These are referred to as Ethernet cables and are either thick or thin Ethernet.
• » Fibre-Optic Cable - This medium is composed of a hair-thin, glass fibre along which lasers carry data as light waves.

Wireless Media

There are two widely used media for wireless communications:

• » Microwave - These are high frequency electromagnetic waves.
• » Infrared - This technology sends data as light waves through the air.

Data Transmission Media

When data is transmitted across wired communications media, it is transmitted as voltages of electricity at discrete values (digital signals) or pulses of light (fiber optic).

In high speed LANs and WANs digital signals are used to transmit data from one device to another. These signals are discrete – they take one of two discrete values, and are used to represent the binary form of the date being transmitted.

Factors Affecting Wired Data Transmission

As data is transmitted over wired transmission media (twisted pair, coaxial, UTP) there are several factors that degrade the quality of the transmission:

• » Noise – This is caused by stray electromagnetic waves that interfere with the transmitted signals.
• » Attenuation – When an electrical signal is transmitted over an electrical cable, it becomes weaker as the length of the cable increased. This weakening of the strength of the signal is called attenuation.
• » Delay Distortion – This occurs because the velocity of electromagnetic signals (voltages in this case) varies with frequency. Because of this, the data from one bit may spill over into the other bit.

LAN Transmission Media

The transmission media used to transmit data over LANs are of four basic types: STP, UTP, Coaxial and Fiber Optic. The IEEE 802.3 specifications define the characteristics of these media, which are used for Ethernet LANs:

• » 10Base-2 – 10 Mbps baseband Ethernet over coaxial cable with a maximum distance of 185 meters. Also referred to as Thin Ethernet or Thinet or Thinwire..
• » 10Base-5 – 10 Mbps baseband Ethernet over coaxial cable with a maximum distance of 500 meters. Also referred to as Thick Ethernet, Thicknet or Thickwire.
• » 10Base-T – 10 Mbps baseband Ethernet over twisted pair cables with a maximum length of 100 meters.
• » 10Base-F – 10 Mbps baseband Ethernet over optical fiber.
• » 100Base-T – 100 Mbps baseband Ethernet over twisted pair cable.
• » 100Base-TX – 100 Mbps baseband Ethernet over two pairs of shielded twisted pair cable or category 4.
• » 100Base-FX – 100 Mbps baseband Ethernet over two multimode optical fibers with a maximum distance of 412 meters.
• » 1000Base-T – 1000 Mbps baseband Ethernet over four pairs of category 5 unshielded twisted pair cable.

Further Reading

Chapter 3, pp. 71 – 91 of the course text