There two types of data transmission modes (see figure 3.6) parallel transmission and serial transmission.
- Parallel Transmission: Parallel data transmission involves the concurrent flow of bits of data through separate communication lines (see figure 3.7). This pattern resembles the flow of automobile traffic on a multi-lane highway. Internal transfer of binary data in a computer uses a parallel mode. If the computer uses a 32-bit internal structure, all the 32-bits of data are transferred simultaneously on 32 lane connections. Parallel data transmission is commonly used for interactions between a computer and its printing unit. The printer usually located close to the computer, because parallel cables need many wires and may not work stably in long distance.
- Serial Data Transmission: Most data transmitted over telephone lines use a serial pattern. That is , each individual bit of information travels along its own communications channel. This pattern is analogous to the flow of traffic down a one-lane residential street (see fig 3.8). Serial transmission is typically slower than parallel transmission, because data are sent sequentially in a bit-by – bit fashion. Another way of classifying data communications flow is synchronous or asynchronous.
- Asynchronous Transmission: A type of communication that sends data using flow control rather than a clock to synchronize data between the source and destination.When asynchronous transmission (also called start/stop transmission) is used, a special start signal is transmitted at the beginning of each group message bits. When a character is about to be transmitted a start bit is sent. A start bit has a value of 0, (also called a space state) . Thus, when the line switches from a value of 1 to a value of 0, the receiver is alerted that a data character is about to come down the line. An asynchronous line that is idle is identified with a value of 1, (also called a mark state). By using this value to indicate that no data is currently being sent, the devices are able to disconnected line. Conversely , asynchronous transmission state and a disconnected line. Conversely, asynchronous transmission involves the sending and receiving of one byte of data at a time. This type of transmission is most often used by computers and other systems characterized by slow speeds.
- Synchronous Transmission: A type of transmission that uses a clock to control the timing of bits being sent. Large volumes of information can be transmitted at a single time with synchronous transmission . This type of transmission involves the simultaneous flow of several bytes of data . Because a large block of data being sent synchronously cannot be interrupted, a synchronized clock is necessary to carefully schedule the transmission of information. This special communications equipment is expensive; but this cost can be made up in part by faster , less expensive transmission of information.
Bandwidth
Each type of communications media has different transmission speed . The bandwidth is a measure of the transmission rate of communications channels.
- Base-band: Digital signals are commonly called base-band signals. Base-band is a communication technique in which digital signals are placed onto the transmission line without change in modulation.It transmits up to a couple of miles, and does not require the complex modems. Typical Token Ring and Ethernet use base-band signals.
- Broadband: Broadband is a technique for transmitting large amounts of data, voice and video onto a different frequency. Using the FDM (Frequency division multiplexing) technique, several streams of data can be transmitted simultaneously.
Broadband is the bandwidth used for direct communication between very high-speed computers(e.g., large mainframe computers). This bandwidth includes microwave, satellite , coaxial cable , and fiber-optic media.
Communication Media
For data to be transmitted from one location to another , some of pathway or medium must be used . These pathway are called communication channels. The communication channels can be divided into two types of media .
- Guided Media : it refers to channels that allow the transmission of data through a physical media such as a twisted pair wire , coaxial cable , or fiber optic cable. These are also called bounded media.
- Twisted Pair: The telephone lines used to carry most of the voice and data communications consist of a pair of thin-diameter insulated copper wires (called twisted pairs) see in fig 3.9. The wires are twisted around each other to minimize interference from other twisted pairs in the cable . Twisted pairs have fewer bandwidth than coaxial cable or optical fiber. Twisted pairs have fewer bandwidth than coaxial cable or optical fiber. They have been the standard communication channels for voice , data and information, but are now diminishing because of more reliable media such as coaxial cable, Optical fiber, microwave, or satellite.
- Coaxial Cable: Coaxial cable can be used for telephone lines for transmission at a high frequency. Coaxial cable consists of a single core of solid copper (see fig 3.10). A coaxial cable can handle 80 times as many telephone transmission as twisted pair media . Many computers in local area networks are linked by coaxial cables. Because of its sturdiness, coaxial cable is often used for telephone lines that must be carried under water. Because coaxial cables have very little distortion and are less prone to interference, they have low error rates. Coaxial cable contains from four to twenty –two coaxial units called tubes, Each coaxial tube consists of a 0.100-inch copper outer conductor by polyethylene insulating disks space about 1 inch apart and is held closed by interlocking serrated edges along its longitudinal seam. Two steel tapes are wound around the outer conductor for added strength.
In addition to coaxial tubes , coaxial cable contains a small number of twisted wire pairs and single wires that are used for maintenance and alarm important advantage coaxial cable has over paired cable is its capability to operate at very high frequencies,. Coaxial , the unique operating environment dictates design,
Operational , and reliability requirements different from those for cable used on land.
- Fiber- Optic Cable: A fiber-optic cable consists of tubes of glass through which data are transmitted as pulses of light . optical fiber consists of thin glass fibers that can carry information at frequencies in the visible light spectrum and beyond. The typical fiber consists of a very narrow and beyond.The typical optical fiber consists of a very narrow stand of glass called the core. Around the core is a concentric layer of glass called the cladding. A typical core diameter is 62.5 microns (1 micron=10 meters). Typically Cladding has a diameter of 125 microns. Coating the cladding is a protective coating consisting of plastic, it is called the Jacket.
An Important characteristic of fiber optics is refraction . Refraction is the characteristic of a material to either pass or reflect light. When light passes through a medium , it bends as it passes from one medium to the other. An example of this is when we look into a pond of water , Although a fiber- optic media is its high level of security. These communications channels are not susceptible to electronic interference .
Therefore, they are a more reliable form of data transmission . Fiber-optic disadvantage of fiber-optic channels is that they cannot carry information over great distances.
- Unguided Media : It refers to those channels that transmit data and information in the form in the form of wave . Unguided transmission media consists of a means for the data signals are not bound to a cabling media and are example of unguided media such as microwave , or satellite.
- Microwave data transmission differs from the previously mentioned communications channels in that data is transmitted through the air instead of through cables or wires (see in fig3.13) Microwaves are high-frequency radio waves that can only be directed in straight lines. Consequently , microwave transmission is usually limited to communications occurring within the limits of a particular city or community . For microwave transmissions to be able to occur over larger distance, data messages must be relayed from one location to another using antennas placed at high altitudes usually twenty to thirty miles apart.
- Satellites: Instead of antennas , satellites can also be used to transfer microwave messages from one location to another . Satellites rotate-approximately 23,300 miles above the earth in precise locations.
Satellite transmission stations that can both send and receive messages are known as earth stations. A major advantage of satellite transmission is that large volumes of data can be communicated at once. A particular drawback is bad weather can severely affect the quality of satellite transmissions. Another one is that it has a serious security problem, because it is easy to intercept the transmission as it travels through the air.
Satellite communications is the generic term for communication via satellite. Transmitting a signal from each station up-to a satellite and then receiving that signal back at earth. The satellite does not have to be a huge expensive array of electronics and solar panels. In fact the moon was used, in the early years of satellite communications research, to bounce the signals off. The time it took for our signals to get to the moon and back was about 2 seconds. This time is called satellite delay and is still present today in all our satellite communications.This wide range broadcasting ability has given rise to a multitude of different satellites carrying all kinds of information such as telephone calls, television channels, Internet traffic military communications,weather data , and even radio stations.
- Mobile Communication: It is radio-based networks that transmit data to and from mobile computer. Computer can be connected to the network through wired ports or through wireless connections.