جلسه ششم شبکه های کامپیوتری به نــــــــــــام خدا

Telephone system 2 Public Switched Telephone Network (PSTN) Originally designed to transmit human voice Telephone system organization: hierarchical network with three important components: Local loops Switching centers Trunks: connecting switching offices using various transmission technologies

Telephone Modem Modem: A device that receives a stream of digital bits and sends/receives it properly from the transmission medium (such as twisted pair) Transmission problems: Attenuation Noise (Thermal Noise, Impulse Noise,…) Delay Distortion Attenuation and Distortion which are frequency dependent 3

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Baseband Transmission 5 UniPolar Polar BiPolar Line Coding

Unipolar Encoding 6 In unipolar encoding, the polarity is assigned to ‘1’ bit, while the ‘0’ bit is represented by zero voltage.

Polar Encoding 7 Polar encoding uses two voltage levels (positive and negative). NRZ RZ Manchester Differential Manchester Polar Encoding

8 In NRZ-Level (NRZ-L), a positive voltage means ‘0’ bit, while a negative voltage means ‘1’ bit. In NRZ-Invert (NRZ-I), it is the transition between positive and negative voltage, not the voltage itself, that represents a ‘1’ bit. A ‘0’ bit is represented by no change. Return to Zero (RZ) uses three values: positive, negative and zero. A 1 bit is represented by positive-to-zero and a 0 bit by negative-to-zero. Manchester Encoding Differential Manchester Encoding

Bipolar AMI Encoding 9 in bipolar encoding is used to represent binary 0. The 1s are represented by alternating positive and negative voltages

Passband Transmission 10

Amplitude-Shift Keying (ASK) 11

Frequency-shift keying (FSK) 12

Phase-shift keying (PSK) No change 0 ° Change 180° 13

14 64-QAM QPSK-QAM16-QAM64

Local Loop, Broadband Access -ADSL Objective: Offering data services without impacting the voice service on twisted pair Central Office (CO) side: Use splitter to feed low pass part to the voice switch while the higher frequencies to the feed the DSLAM Customer Premise End (CPE) Use splitter or in-line filters to feed the low pass part of the spectrum to the telephone and the high pass part to the CPE ADSL modem 15

Multiplexing In telecommunications and computer networks multiplexing is a method by which multiple analog message signals or digital data streams are combined into one signal over a shared medium. The aim is to share an expensive resource. 16

Frequency Division Multiplexing Information from multiple sources can be multiplexed in frequency domain by assigning different carriers to different sources. Usually there is a guard band between different frequency bands 17

Wavelength Division Multiplexing 18 Several independent streams of data each carried in a different wavelength can be multiplexed in optical domain and transmitted over a single fiber. This technology has drastically increased the capacity and lowered to cost of transmission networks

Time Division Multiplexing Different sources of data can be transmitted by dividing time into different slots TDM is used widely in telephone and cellular networks 19