Wk 3: Short Answer Paper
Network and Telecommunications Concept/NTC-360
June 19, 2011
Here are the short answer responses for the following terms:
• Synchronous and asynchronous
Synchronous systems negotiate the communication parameters at the data link layer before communication begins. Basic synchronous systems will synchronize both clocks before transmission begins, and reset their numeric counters for errors etc. More advanced systems may negotiate things like error correction and compression.
Asynchronous communication utilizes a transmitter, a receiver and a wire without coordination about the timing of individual bits. There is no coordination between the two end points on just how long the transmiter leaves the signal at a certain level to represent a single digital bit. Each device uses a clock to measure out the 'length' of a bit. The transmitting device simply transmits. The receiving device has to look at the incoming signal and figure out what it is receiving and coordinate and retime its clock to match the incoming signal. • Analog and digital
A string tied to a doorknob is a practical example of an 'analog' system. The string, when shaken, creates waves that travel down the length of the string until they hit the doorknob. Shake the string harder and you get bigger waves that travel faster down the string and hit the doorknob harder. Shake the string more gently and you get smaller waves. Shake the string up and down and the waves are oriented vertically, shake the string sideways and the waves are oriented horizontally. One thing is constant--the shape created by the string is wave-like.
A digital system would be more like flipping a light switch on and off. Speaking in general terms, there is no 'in between' values, unlike our doorknob and string example above. If the switch you are using is not a dimmer switch, then the light is either on, or off. Our light-switch example uses the light bulb as our transmitter, the transmission media is the space between the lightbulb and your eyes, and the receiver of the signal is your eye. This would be a digital system. A graph showing the properties of a digital transmission would look something like this:
• XON and XOFF
Xon/Xoff (X-on/X-off or XON/XOFF)
Xon/Xoff (sometimes written "X-on/X-off" or "XON/XOFF" and pronounced eks-AWN eks-AWF ) is a protocol for controlling the flow of data between computers and other device s on an asynchronous serial connection. For example, a computer typically sends data to a printer faster than the printer can print. The printer contains a buffer where data is stored until the printer catches up with the computer. If the buffer becomes full before the printer catches up, a small microprocessor in the printer sends back an X/off signal to stop sending data. When enough data is printed and buffer storage becomes free, the printer sends an X/on signal telling the computer to resume sending data. The "X" stands for "transmitter" so the X/on and X/off are signals to turn a transmitter on or off. The actual signal for X/on is the same bit configuration as the ASCII Ctrl-Q keyboard combination. The X/off signal is the Ctrl-S character. When you define your modem to your computer's operating system, you may need to specify the use of flow control with X/on/Xoff or with CTS/RTS (Clear to Send/Ready to Send). When sending binary data, Xon/Xoff may not be recognized because it is character-encoded. o http://whatis.techtarget.com/definition/0,,sid9_gci213406,00.html • Simplex and duplex
Simplex communication is permanent unidirectional communication. Some of the very first serial connections between computers were simplex connections. For example, mainframes sent data to a printer and never checked to see if the...
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