Presentation on theme: "ITEC 352 Lecture 34 Communications. Review USB –How did you like the material (1-10) from bad to good –What did you learn? Differences between bus types."— Presentation transcript:
Review USB –How did you like the material (1-10) from bad to good –What did you learn? Differences between bus types Electricity level Packet level Code level
Communications P3 Write a paper on a topic relevant to this class –Goal: Investigate new technology, make a case on what to adopt Have at least 3 different sources Compare and contrast with another product or approach (i.e. PPC versus P4 CPU) Good – There are certain situations where each CPU trumps the other, however due to the long pipeline of the P4 the PPC CPU is a better fit for a codebase that relies heavily on branching. Bad –P4 is better because it has more MHZ. Length 2-3 pages double spaced Due date: Next Wednesday at 10:00PM
Communications Rubric 78 points (HW + Project) Rubric (A gets 100%, B gets 89%, etc…) –A – Meets all requirements and is well written –B – Meets most of the requirements but may contain some grammatical errors and/or may not be as well written –C – Grammatical errors common and several requirements are not met in some way –D/F – Barely any effort invested (i.e. project images take up ½ a page) Well written = Clear, concise, easy to understand
Communications Plan Communications + Misc. topics this week Next week M – Ethics of computer organization W – Review of semester F – Question / Answer session
Communications Communicatio n We will look into: –Types of signals that are used to exchange messages. –Type of messages sent using the signals (how are the bits transmitted from one component to another). –What happens if the message we send ends up with an error: error correcting codes.
Communications Types of signals The ability for one computing component to talk to another component. –This requires that the components send each other “signals” There are many of sending signals. We hear these terms often: –AM, FM, PM, PCM etc… We know what AM and FM are….
Communications Amplitude Modulation What is the amplitude of a signal? –Diagram on board. –Amplitude is the height of the wave from 0 position. What is modulation? AM: Consider a brand new and the coolest radio station in town: 352CoolAM: your one stop radio station for all the news on computer architecture. Suppose we want to transmit music from this station. –The station first plays the music from say a CD player A wave corresponding to the music comes out. This is called the “signal wave” –The station then has to transmit the music across the air. It pays for a frequency that it can reserve for itself. This frequency is in the form of another wave called carrier wave. –You then “add” the signal wave to the carrier wave, in order to get the AM signal. In other words, you are changing (or dampening) the amplitude of the carrier wave to obtain the signal.
Communications Frequency modulation In the wave on the board, what is the frequency? –The thickness of the wave. Just like AM, FM changes the wave – –But unlike AM it changes the frequency Advantage: Frequency is not effected (as much as amplitude) by noise or by obstructions (e.g., a bridge or an electricity pole).
Communications Modem Communication –Communication over a telephone line with modems:
Communications Modulation Schemes for digital signal waves –Consider a digital signal wave – this has only 1’s and 0’s. The AM, FM and PM waves after modulation look like the figures shown below. These represent the 0’s in the digital signal. The amplitude is different between waves for 0 and 1 Notice that the frequencies are same for both 0’s and 1’s These represent the 0’s in the digital signal. Notice that the amplitude is same for both 0’s and 1’s Frequency Amplitude
Communications Pulse Code Modulation (PCM) –Conversion of an analog signal into a PCM binary sequence: analog signal wave is sampled multiple times (in this case twice) Each of these is a sampling of the signal wave The amplitudes of each sample form the PCM sequence 011
Communications Error correction Signal waves may have errors in them –Errors can be due to noise introduced during transmission (e.g., a bit 1 in a PCM wave may change to bit 0). –Errors can also be due to problems with transmission media or due to collisions with other messages etc.. It is important for a receiver of a signal to be able to –At least detect errors, and, –Preferably correct them.
Communications Options Consider a PCM signal wave sent by a sender –Sender sends: 011 110 011 001 100 –Receiver receives: 111 110 011 001 100 –Can you design a protocol between the sender and receiver, such that the receiver can detect the error? – What about a protocol that will allow the receiver to correct the error?
Communications Parity: detecting odd number of bit errors –Even parity bits are assigned to a few ASCII characters. A parity “P” of “1” is assigned when the number of 1’s in the code are odd. By appending the parity bit to ASCII codes, we can increase the hamming distance to 2. Hamming code is a great way to detect errors. It is based on calculating the hamming distance: the number of bits by which two advanced code’s differ
Communications Error Correction Consider the ascii code for the character “a”: 1100001 Consider the possibility of a 1 bit error in this code. –What are the possible erroneous a’s due to a 1 bit error? (Hint: there are 7 possible erroneous a’s). Hence, for every ascii code of 7 bits, there are 7 possible erroneous codes. Hence, to correct a 1 bit error in a 7 bit code, we can represent each ASCII character using any of the 8 bit patterns (1 correct pattern + 7 erroneous patterns). –E.g., “a” can be represented in ASCII as: 1100001, 1000001, 1110001, etc… But this will reduce the number of bits we can use to represent an ASCII code. Why?
Communications Summary Wave transmissions in communications Next time –Error detection / correction
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