CONTENTS Communications channel Physical transmission media – Twisted-pair cables – Coaxial cables – Fibre-optic cable Wireless transmission media – Broadcast radio – Cellular radio – Microwaves – Communications satellite – Infra-red Questions
The communications path between two devices. The width of the communications channel is referred to as bandwidth. The higher this is, the faster transmission of data. Lower bandwidth is OK for transmission of text documents, but higher bandwidth is needed for most other data transmission. If the bandwidth is too low, there is a considerable slow down in system performance. Consists of one or more transmission media.
When sending data from your computer to another device, the signal carrying the data usually travels over a variety of transmission media. Despite this, it still usually only takes a moment for the communications process to occur. Baseband media transmits only one signal at a time, broadband transmits multiple signals. Broadband is much faster. Transmission media are physical or wireless. Physical include wires and cables, wireless send communication signals through air/space.
Twisted-pair cables Commonly used for network cabling and telephone systems. Each twisted-pair wires consists of two separate insulated copper wires that are twisted together. The twisting reduces noise. Noise is an electrical disturbance that can degrade communications.
Coaxial cables Often referred to as coax, consist of a single copper wire surrounded by at least three layers: insulating material; woven or braided metal; and plastic outer coating. Often used for cable TV as it can be laid over longer distances. Not often used for computer networks today as fibre-optic has a faster transmission rate.
Fibre-optic cable The core consists of dozens or hundreds of thin strands of glass or plastic that use light to transmit signals. These strands (optical fibres) are as thin as a human hair. Inside the cable, each optical fibre is surrounded by an insulating glass cladding and a protective coating. Several advantages over cables which use wires: carry significantly more signals, faster data transmission, less susceptibility to noise from other devices, better security for signals due to reduced noise, and smaller in size (thinner and lighter).
Disadvantages include: costs much more than twister-pair or coaxial cables, and it can be difficult to install and modify. Despite these, many telephone and cable TV companies are replacing existing coaxial cables with fibre-optic cables. Many businesses are also using fibre-optic cabling.
Used when it is inconvenient, impractical or impossible to install cables. Transmission mediumTransfer rates (maximum) Broadcast radio Bluetooth HomeRF a b g n 1-2 Mbps Mbps 54 Mbps 11 Mbps 54 Mbps Mbps Cellular radio 2G 3G Mbps 144 Kbps-2 Mbps Microwave radio150 Mbps Communications satellite1 Gbps Infra-red115 Kbps-4 Mbps
Broadcast radio Need a transmitter to send the broadcast radio signal and a receiver to accept it. The receiver has an antenna located in the range of the signal. Some networks use a transceiver, which both sends and receives signals from wireless devices. Slower and more susceptible to noise than physical transmission media, however it provides flexibility and portability. Short-range broadcast radio communications such as Bluetooth are becoming popular for home and small-business users.
Bluetooth uses short-range radio waves to transmit data among Bluetooth-enabled devices. To communicate with one another they must be within a specified range. Wi-Fi communications technology also uses broadcast radio.
Cellular radio Widely used for mobile communications, specifically wireless modems and mobile phones. Several categories of cellular transmission exist: – 1G (first generation) – transmitted analogue data only – 2G (second generation) – transmitted digital data at speeds of Kbps – 3G (third generation) – transmitted digital data at speeds from 144 Kbps to 2.4 Mbps.
Microwaves Radio waves that provide a high-speed signal transmission. Microwave transmission involves sending signals from one microwave station to another. Can transmit data up to 4500 times faster than a dial-up modem. Uses line-of-sight transmission which means that microwaves must transmit in a straight line with no obstructions between microwave antennas. Often sit on top of buildings, towers or mountains to avoid possible obstructions.
Used in environments where installing physical transmission media is difficult or impossible, and where line-of-sight transmission is available (e.g. Used in wide open areas such as deserts or lakes, or between buildings). Often used at universities, hospitals, cable V providers and phone companies.
Communications satellite Space station that receives microwave signals from an Earth-based station, strengthens the signals and broadcasts the signals back over a wide area to any number of Earth-based stations (which are often microwave stations). Devices such as hand-held computers and GPS receives can also function as Earth-based stations. Transmission from an Earth-based station to a satellite is an uplink, and transmission from a satellite to an Earth-based station is a downlink.
Applications such as air navigation, television and radio broadcasts, videoconferencing, paging and GPS use communications satellites. With the proper satellite dish and modem card, users can access the Internet using satellite technology. With this, the uplink transmissions are slower than downlink transmissions.
Infra-red Infra-red Data Association (IrDA) is a wireless transmission standard that sends signals using infra-red light waves. An IrDA port is included in many mobile computers and devices. Infra-red transmission requires a direct line of sight between the sending and receiving devices. Any obstructions to the path will block the transmission of data.
Questions Whats the difference between broadband and baseband transmission? What is the advantage of using coaxial cable over twisted-pair wire? When would you use twisted-pair? What advantage does fibre-optics have over wire cables? If you were designing a network, in what circumstances would you consider using wireless transmission media? Briefly describe the broadcast radio and cellular radio wireless transmission media. What is the main limitation of microwave transmission? Why might Bluetooth take over from infra-red transmission in some networks?