A transmission medium refers to any material that can propagate waves or energy. In a communication context, it can be defined as the physical means that enables the transfer of information.
Transmission media can be classified as guided or unguided (Kularatna and Dias, 2004).
- Guided transmission media incorporate a physical path along which the waves travel. These include twisted pair cable, coaxial cable and optical fiber. In short, these broadly include copper cables (twisted pair and coaxial) and fiber optics.
- Unguided media, also referred to as wireless transmission media, make use of an antenna for transmitting electromagnetic waves through air, water or vacuum (Kularatna and Dias, 2004). These include microwave transmission and satellites. Unguided media make use of the electromagnetic spectrum and include radio waves of different frequencies propagating in different ways.
This report reviews the different types of transmission media that may be used in communication systems. It presents a brief outline of each reviewed medium followed by a detailed description of its uses in contemporary systems.
It consists of two insulated copper wires arranged in a regular spiral pattern with many pairs bundled together into a cable protected by a tough sheath (Kularatna and Dias, 2004). Twisting is employed to decrease the crosstalk. The wires typically range in thickness from 0.4mm to 0.9mm. There are two types of twisted pair: unshielded twisted pair (UTP) and shielded twisted pair (STP). UTP is the ordinary telephone wire and is susceptible to external influence and noise. STP consists of a metallic shield around the wire pairs that minimizes the effect of external interference. However, it is more expensive and difficult to work with. Hence, UTP finds more application in current communication systems.
UTP is divided into various categories based on the maximum data are possible. Category 1 is designed for voice telephony and has one twist per foot. Due to advances in modulation techniques, it also finds applications in long range Ethernet and DSL (Digital Subscriber Line) with data rates of 10Mbps or more. The other commonly used categories include Category 3 and 5 (also Category 5e -extended) for deployment in Local Area Networks (LANs). The Cat 5 cable has one to three twists per inch. Cat 3 is used in 10Mbps Ethernet and 4Mbps token ring LANs, while Cat 5 finds use in 10Mbps or 100Mbps Ethernets. Cat 6 and 7 are also coming into use, with data rates of up to 1Gbps. Cat 6 is the standard for Gigabit Ethernet while Cat 7, which has four twisted copper pairs, is designed for Ultra Fast Ethernet.
Applications of twisted pair
Twisted pair finds its main applications in telephony, private branch exchanges (PBXs) between telephone sets and switching cabinets, LANs and local loops, including both analog telephone lines and broadband DSL.
Traditional telephony makes use of the 4 kHz range of the twisted pair to provide analog subscriber lines. The local loop, connecting the local telephone exchange to the subscriber premises, is however, capable of carrying frequencies beyond the 3.4 kHz voice channel. This unused bandwidth is utilised by DSL to provide digital data by avoiding the voice frequency band.
The most commonly used standards of the DSL family are Asymmetrical DSL (ADSL) and Symmetrical (or Single-Line) DSL (SDSL). ADSL is an asymmetrical channel with the majority of the bandwidth devoted to the downlink channel and a smaller return path. It currently supports up to 7Mbps downstream and up to 800Kbps upstream and is thus finds applications in providing Internet connections to residences. SDSL is a symmetrical service deployed in multiples of 64 kbps, with a maximum of 2 Mbps in each direction. It finds application in businesses, small and home offices, and for remote access into corporate facilities (Jarrett and Goleniewski, 2006)
A coaxial cable consists of a single inner conductor surrounded by a hollow outer cylindrical conductor. The outer conductor is typically braided-copper shielding or aluminium foil-type covering. The center conductor is thicker than in twisted pair and is surrounded by plastic insulation, which helps filter out external interference. The outer conductor is covered by a jacket or shield, with the number and type of jackets depending upon the cable’s intended place of deployment (to be strung in air or underground). Coax offers slightly better performance than twisted-pair because it is less susceptible to outside interferences due to the metallic shielding that protects the center conductor. It also has a much larger usable bandwidth.
Applications of coaxial cable
Coaxial cable has typically been used to provide services by cable TV operators. It is currently used to provide local-loop services and cable TV, with the cable connecting to set-top boxes or cable-ready TV sets. This is usually deployed in an HFC (hybrid fiber coax) architecture, with fiber terminated on a neighbourhood node and coaxial cable connecting individual homes to the node. This architecture can support 750MHz or 1,000MHz systems, from which individual channels can be carved out, making coaxial cable a broadband facility. Cable modems provide high-speed Internet access, with each channel being 8 MHz wide in the Phase Alternate Line (PAL) standard being used in Europe. However, each channel is shared by everyone getting the service from one coaxial cable running from the neighbourhood node, which can range from 200 to 2,000 homes (Jarrett and Goleniewski, 2006).
Traditionally, coaxial cable has been used in telephony networks as interoffice trunks as it is more convenient to lay down a smaller coaxial cable to replace copper cable bundles with 1,500 or 3,000 pairs of copper wires in them.
Early LAN architectures also employed coaxial cable as the transmission media from the host to the terminal.
An optical fibre is a thin, flexible strand of glass or plastic, which guides light from one end to the other. It works in the visible and infrared spectrum, with typical wavelengths being 850 nm, 1300 nm and 1550 nm. An optical fiber cable has three concentric sections: the core, the cladding and the jacket. The core is surrounded by the cladding, with diameters ranging from 8 to 100 µm and with different optical properties than the core. This difference in optical properties gives rise to the phenomenon of total internal reflection, upon which principle fiber works. This means that light shone in one end of the fibre remains within it. The light just travels inside the fibre until it comes to the far end (Open University, 2006). The jacket limits the stress that can be placed on the cable, protecting it from being bent to such an angle that the inner core breaks. The jacket is reinforced with Kevlar reinforcing material to prevent other intrusions and is in turn covered by a number of other outer jackets. The two components of an optical fiber system are the cable and the light source used. There are two types of light sources that are used with fiber: light-emitting diodes (LEDs) and laser diodes. LEDs are capable of coupling only about 3% of the incident light into the fiber, thus providing low data rates at 500 Mbps. They are relatively inexpensive, have a long life and tolerant of extreme temperatures. Laser diodes can provide a coherent light source with little distortion and can thus support much higher transmission rates. However, they are more expensive and are thus used for long-haul and high speed transmission.
Applications of Optical Fiber
The key application of optical fiber is in providing the majority of public and private network backbones. It is deployed in the backbone infrastructures of telephony networks, Cable TV systems, Internet providers as well as electric power utilities. Fiber is also used as undersea submarine cable to carry telecommunications traffic, including telephone as well as Internet and private data traffic, between countries. “As of 2005, such cables link all the continents except Antarctica.” (Wikipedia 2007)
Fiber also finds use in the local loop, being deployed in various forms, including HFC, fiber to the curb (fiber to a neighbourhood node with twisted pairs on to homes) and passive optical networking.
Another application of fiber is in LANs implementing Gigabit Ethernet and 10 Gigabit Ethernet.
Another emerging use of fiber is in applications with critical requirements of high resolution imagery or video, for instance, telemedicine. An example scenario involves an optical fiber connection between an imaging centre and a doctor’s office. Other environments that can make use of the imaging application include universities, health care environments, and entertainment applications (Jarrett and Goleniewski, 2006).
“The term microwave identifies a particular range of frequencies used for radio communications”. (Open University, 2006) Though the range of frequencies used is different in different contexts, it is approximately in the 1 GHz to 100 GHz band. Microwave requires line of sight operation and its operation is also dependent upon the physical environment. Thus, the distance that a beam can travel is also restricted by the curvature of the Earth, which interrupts the line of sight at about 90 miles (144 km). (Jarrett and Goleniewski, 2006) Environmental factors such as precipitation, fog and presence of metals also impair the signal through reflection and absorption. In addition to these factors, the distance at which repeaters are needed also depends upon the frequency of operation, with lower frequencies travelling farther than the higher ones before attenuation.
Applications of Microwave
The initial application of microwave was to act as a replacement for leased lines in private networks. Due to the relative ease of setting up a microwave system and the cost factor, this setup is commonly used by multi-location customers to connect their multiple facilities with microwave links, e.g. a university with multiple campuses or a bank with multiple branches. It is also used to act as a bridge to connect two different LANs.
In places where the terrain makes it difficult to lay cables, microwave can be deployed for the local loop infrastructure.
The relatively quick deployment of microwave systems also lends them to a disaster-recovery role where wired systems or structures have been destroyed.
Microwave is increasingly finding more use with wireless applications being deployed in about every network domain. In the wide-area domain, microwave supports the 2G cellular services (GSM – Global System for Mobile Communication), 2.5G enhanced data services (GPRS – General Packet Switched Radio) and the 3G high-speed data and multimedia services (WCDMA – Wideband Code Division Multiple Access, UMTS – Universal Mobile Telecommunication System). These systems provide mobile radio access over a range of frequencies.
Wireless LANs are increasing in popularity and include the IEEE 802.11 family of protocols that operate in the unlicensed microwave bands of 2.4GHz and 5GHz. The ease of setting up these LANs, as they do not require licensing from the spectrum management agency, have seen them being deployed in university campuses, business organizations, airports and other commercial places.
Wireless personal area networks (WPANs) also operate in the 2.4 GHz frequency range and are used to connect a small number of networked devices. The popular ones include the IEEE 802.15.1 Bluetooth standard, IEEE 802.15.4 ZigBee and some applications of RFID (Radio Frequency Identification).
Satellites act as communications broadcast tools, being placed at orbits of varying elevations above the Earth’s atmosphere. The key communications component in a satellite is the transponder which accepts the incoming signal, shifts it to another frequency and then amplifies and transmits it back on the downlink channel. Thus, the frequency allocations always specify two different bands: one for the uplink channel and the other for the downlink. A satellite network has three segments: the space segment that concerns the actual design of the satellite and the orbit at which it is placed; the control segment which defines the operating frequency spectrum and the signalling techniques used; and the ground segment that consists of the Earth station, the antenna design and the access techniques to multiplex various channels to the satellite. Orbits also affect the application of satellites. The three major orbits include the geosynchronous orbit (GEO) at 36000 km, middle earth orbit (MEO) at elevations of 10000 to 15000 km and low earth orbit (LEO) at 640 to 1600 km.
Applications of Satellites
GEO satellites are primarily used for one-way broadcasts and point-to-multipoint links. The major application in this case is international television. These satellites also support VSAT (very-small-aperture terminal) networks to enable private networking by setting up point-to-point links between two locations or for point-to-multipoint business video. VSATs are also employed in vehicle-tracking systems to communicate with drivers. An emerging application is broadband Internet access with download speeds of up to 1 Mbps.
“The main applications for MEOs are in regional networks, to support mobile voice and low-speed data, in the range of 9.6Kbps to 38Kbps”. (Jarrett and Goleniewski, 2006) LEOs find application in messaging, paging and vehicle location services.
LEOs operating at the 2 GHz range offer mobile voice services and those operating at 20 GHz to 30 GHz provide high-speed data and multimedia services at up to 155 Mbps.
Traditionally, satellites have been used for serving remote areas where ground-based facilities are not available, to provide disaster-recovery support, remote monitoring and control, maritime and air navigation, to distribute TV, video and multimedia, to gather weather information and for defence communications.
Satellites find increasing use in automotive navigation systems to provide navigational assistance and also support emergency services. Another recent application is “digital audio radio, such as the SIRIUS system, with which one can receive more than 100 radio stations throughout the footprint of the satellite”. (Jarrett and Goleniewski, 2006)
Other emerging applications include provisioning of Internet backbones where terrestrial facilities cannot handle the traffic load, telemedicine, distance learning and remote imaging.
As presented in this report, different transmission media find application in different areas, with some overlaps among them. The choice of which media to use for deployment of any planned system thus depends on the intended application as well as the financial resources available.