Computer Networks. Week 03 Founda(on Communica(on Concepts. College of Information Science and Engineering Ritsumeikan University

Transcription

1 Computer Networks Week 03 Founda(on Communica(on Concepts College of Information Science and Engineering Ritsumeikan University

2 Agenda l Basic topics of electromagnetic signals: frequency, amplitude, degradation l Multiple devices sharing a single, limited medium: collision handling and multiplexing l Concepts of error detection and correction 2

3 Amplitude l Amplitude is the height of a wave, which can be voltage, current, or field intensity l In telecommunications, amplitude is usually measured as the average wave height l Amplitude modulation (AM) in radio signals is used, for example, in AM broadcast radio the amplitude of a fixed-frequency carrier wave is modulated (changed) by the signal being sent l Optical signals are usually sent using amplitude modulation

4 Frequency l Frequency is the number of waves per given time interval l Alternatively, the inverse is used the distance between waves, or wavelength l Frequency modulation (FM) in radio signals is used, for example, in FM broadcast radio The frequency of a fixed-amplitude carrier is modulated by the signal being sent

5 Signal Degradation l Degradation means a signal s quality is reduced as it moves through a medium l Over distance, the signal strength is reduced, which is called attenuation when a signal is attenuated too much relative to noise, it is no longer reliable: it has degraded l Occlusion (or obstruction) means a physical object is blocking a signal l Interference happens when nearby signals become mixed together, corrupting the signal

6 Multiplexing l Multiplexing means sending two or more signals through a single channel l Signals are encoded by analog or digital means before being transmitted l At the receiver, the signals must be decoded l One kind of multiplexing sends signals using different frequencies l Another kind breaks a signal into small parts, and sends different parts at different times parts from different signals are interleaved

7 Collisions l A collision happens when two or more devices attempt to use a single channel at the same time l When a collision happens, both devices are usually unable to transmit l Media Access Control (MAC), in the Data Link Layer, performs the task of handling collisions and allowing multiple devices to share a limited medium

8 CSMA/CD l Carrier Sense Multiple Access with Collision Detection (CSMA/CD) is one solution for handling collisions, used on Ethernet l When a device attempts to send a frame, it listens for a collision l If there is a collision, the device waits for a pseudo-random time before attempting to resend the frame l After a certain number of failed attempts, the device gives up, and the transmission fails

9 CSMA/CD l Very abbreviated flowchart for CSMA/CD Start Backoff routine: Wait a pseudorandom period of microseconds true Transmit if idle Collision detected true Attempts <16 false false The method used to detect depends on the physical and data-link layer implementation Transmission success Too many collisions

10 CSMA/CA l Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) can be used to solve the collision problem l Before transmitting, a device listens and waits if the medium is busy l The sender may request a period of time to send, with a Request to Send (RTS) packet l A central controller device acknowledges the request with a Clear to Send (CTS) packet, giving the sender permission to transmit

11 CSMA/CA with RTS/CTS l Very abbreviated flowchart for IEEE Start Backoff routine: Wait a pseudorandom period of microseconds false Transmit RTS if idle CTS received? Both CSMA/CD and CSMA/CA enter a backoff or wait routine if another device is already transmitting true Transmit frame Transmission success

12 Error Detection and Correction l Because digital data is discrete, it can be checked and even corrected l The correct value is determined by the transmitting party l The simplest form of correction is simple redundancy but that is not very efficient l Error detection and correction is handled in the Data Link layer at the bit level, and at the Transport layer for whole packets

13 Parity Bits l One simple way of checking data in a signal is to count the number of bits l The sender must have some way of telling the correct number of 1 bits and 0 bits l One parity bit per byte (or word) can check for errors, but not correct them l Multiple parity bits can be used redundantly to allow error correction

14 Block Parity Example (Message, Part 1)

15 Block Parity Example (Message, Part 2)

16 Checksums l A checksum performs arithmetic on the values of digits or bits transmitted and adds the result to the data sent l Examples of checksums are all around you l UPC checksums are based on multiplying every other digit by 3 and adding the result (UPC bar codes also have parity) l ISBN 10 codes have checksum values of 0 to 10 (where 10 is represented by the digit X) Note: ISBN 13 has a different checksum formula

17 ISBN 10 example l ISBN l 4*1 + 5*2 + 6*3 + 3*4 + 0*5 + 1*6 + 3*7 + 9*8 + 9*9 = l = l 224 l 224 % 11 = 4

18 Transport Layer Checksums l Internet Checksum simply uses addition on each set of 16 bits (as an unsigned integer) and the final checksum is the one s complement l Cyclic Redundancy Check (CRC) is one type of checksum used in the Transport Layer l CRC uses simple XOR, multiplication, and division operations on each set of bits to encode and decode the data while checking for errors l A parity bit is an example of a single-bit CRC l CRC encoding does not make the data encrypted or secret; it is only for detecting errors

19 Cyclic Redundancy Check l Transmitter performs polynomial division to calculate an N-bit remainder (the CRC), which is appended to the data l When the same division is performed by the receiver (with CRC attached) the remainder will be zero if the data contains no errors l Division performed by exclusive-or between input and N+1-bit divisor, with MSbit set l Divisor shifted right so dividend and divisor MSbits are aligned l When dividend is zero, remainder is CRC

20 Cyclic Redundancy Check transmit C R C divisor divisor divisor divisor divisor remainder receive divisor divisor divisor divisor divisor remainder OK 0 0 0

21 Hamming Distance l Hamming Distance is a form of error correction using special encoding to send values that are very different from each other l If one byte or word is wrong, the distance to the nearest correct byte or word in the set is the most probable value l The difference between data sent and expected data is known as the Hamming distance

22 Hamming Code l Is a way to mix parity and data bits to correct single-bit errors l Bits are numbered from 1 and each powerof-2 bit position becomes a parity bit for all bits whose binary number has that bit set l The sum of the bit numbers of any incorrect parity bit(s) identifies an incorrect bit Bit number Coded data p1 p2 d1 p4 d2 d3 d4 p8 d5 d6 d7 d8 d9 d10 d11 p16 d12 d13 d14 d15 bits p1 x x x x x x x x x x Parity Bit coverage p2 x x x x x x x x x x p3 x x x x x x x x x p4 x x x x x x x x p5 x x x x x

23 Conclusions l Signals are sent using basic principles of electromagnetic waves: amplitude and frequency l Media Access Control (MAC) allows several devices to share a medium that is limited to one signal at a time l Multiplexing allows many signals to be sent over a limited number of lines or channels l Errors must be detected and corrected using parity, checksums, or encoding methods such as Hamming Distance