Yerel Eşzamansız (Asenkron) İletim (RS-232) 1Veri İletimi

Eşzamansız İletim Veri, belirli bir zamanda bir karakter olarak iletilir. –5-8 bit arası Zamanlama, her karakter arasındaki düzenlemeyi içerir Her karakter yeniden eşzamanlı (senkronize) edilir. 2Veri İletimi

Eşzamanlı İletimin Şekli 3Veri İletimi

Arayüz Özellikleri Mekanik –Başlantı soketleri Elektriksel –Voltaj, zamanlama, kodlama Fonksiyonel –Veri, kontrol, zamanlama, topraklama Yöntemsel –İşlemlerin ardışıklığı 4Veri İletimi

International Telecommunications Union (ITU) Electronic Industries Assosiation (EIA) (RS-232) Institute for Electrical and Electronic Engineering (IEEE) İletim Standartları 5Veri İletimi

V.24/EIA-232-F ITU-T v.24 Yalnız fonksiyonel ve yöntemsel tanımlamaları içerir –Elektrik ve Mekanik için diğer standartlara atıfta bulunur. EIA-232-F (USA) –Mekanik: ISO 2110 –Elektriksel: v.28 –Fonksiyonel: v.24 –Yöntemsel: v.24 6Veri İletimi

Mekanik Tanımlamalar 7Veri İletimi

Elektriksel Tanımlamalar Sayısal sinyaller Değerler, devrenin özelliğine bağlı olarak veri veya kontrol sinyali olarak değerlendirilir. Voltaj değerine göre değer belirlenir. –-3v’un üzeri = ikili sistemde 1, –+3v ’un üzeri = ikili sistemde 0 (NRZ-L) Sinyal oranı < 20kbps Mesafe <15m Kontrol için : > -3v = “off"; > +3v = “on” 8Veri İletimi

İşlevsel Tanımlamalar Devreler, kategoriler altında gruplandırılır: –Veri –Kontorl –Zamanlama –Toprak Her yöne br devre verilir. –Çift yönlü (Full duplex) İki adet ikincil veri devresi bulunur: –Yarı çift yönlü (Half duplex) işlemde durdurmak veya akış kontrolünü sağlamak 9Veri İletimi

Çift Yönlü ve Yarı Çift Yönlü Eşzamansız İletim Bağlantı 50 feet (~15 m) den daha kısa olmalıdır Seri, eşzamansız iletim tanımlanmaktadır. 10Veri İletimi

Yöntemsel Tanımlamalar Örneğin: Eşzamansız özel hatlı modem Çalışıp hazır konuma geldiğinde modem (DCE) “DCE hazır” mesajını gönderir “Veri Uçbirim Ekipmanı (Data Terminal Equipment: DTE)” veri göndermek için hazır olduğunda, “Gönderme Talebi (Request to Send: RTS)” mesajını gönderir. –Yarı eşzamansızda alma modunu da belirtir. Modem, hazır olduğunda, “Gönderme için Açık (Clear to Send: CTS)” mesajı ile cevap veriyor. VUE veriyi gönderiyor Veri eriştiğinde, yerel modem “Hat Sinyal Algılayıcıyı Al (Receive Line Signal Detector: RLSD)” mesajını gönderiyor ve veriyi iletiyor. 11Veri İletimi

Gerçek Donanımların Sınırları Sinyal enerji kaybeder Voltaj değişimleri ideal değildir. İletim ortamında “gürültü” olabilir. 12Veri İletimi

Transmission Impairments Signal received may differ from signal transmitted Analog - degradation of signal quality Digital - bit errors Caused by –Attenuation and attenuation distortion –Delay distortion –Noise 13Veri İletimi

Attenuation Signal strength falls off with distance Depends on medium Received signal strength: –must be enough to be detected –must be sufficiently higher than noise to be received without error Attenuation is an increasing function of frequency 14Veri İletimi

Delay Distortion Only in guided media Propagation velocity varies with frequency 15Veri İletimi

Noise (1) Additional signals inserted between transmitter and receiver Thermal –Due to thermal agitation of electrons –Uniformly distributed –White noise Intermodulation –Signals that are the sum and difference of original frequencies sharing a medium 16Veri İletimi

Noise (2) Crosstalk –A signal from one line is picked up by another Impulse –Irregular pulses or spikes –e.g. External electromagnetic interference –Short duration –High amplitude 17Veri İletimi

Channel Capacity Data rate –In bits per second –Rate at which data can be communicated Bandwidth –In cycles per second of Hertz –Constrained by transmitter and medium 18Veri İletimi

Nyquist Bandwidth If rate of signal transmission is 2B then signal with frequencies no greater than B is sufficient to carry signal rate Given bandwidth B, highest signal rate is 2B Given binary signal, data rate supported by B Hz is 2B bps Can be increased by using M signal levels C= 2B log 2 M 19Veri İletimi

Shannon Capacity Formula Consider data rate,noise and error rate Faster data rate shortens each bit so burst of noise affects more bits –At given noise level, high data rate means higher error rate Signal to noise ratio (in decibels) SNR db = 10 log 10 (signal/noise) Capacity C=B log 2 (1+SNR) This is error free capacity 20Veri İletimi

SIGNAL ENCODING and MODULATION TECHNIQUES 21Veri İletimi

Modulation Modulation converts an digital computer signal into a form that can travel down an ordinary analog telephone line PSTN Client A Server A Telephone 33.6 kbps Modem Binary Data Analog Modulated Signal Modem 22Veri İletimi

Digital Data, Analog Signal Public telephone system –300Hz to 3400Hz –Use modem (modulator-demodulator) Amplitude shift keying (ASK) Frequency shift keying (FSK) Phase shift keying (PK) 23Veri İletimi

Modulation Techniques 24Veri İletimi

Amplitude Modulation (AM) Low Amplitude (0) High Amplitude (1) Amplitude Modulation (1011) Amplitude (low) Amplitude (high) 25Veri İletimi

Amplitude Shift Keying Values represented by different amplitudes of carrier Usually, one amplitude is zero –i.e. presence and absence of carrier is used Susceptible to sudden gain changes Inefficient 26Veri İletimi

Binary Frequency Shift Keying Most common form is binary FSK (BFSK) Two binary values represented by two different frequencies (near carrier) Less susceptible to error than ASK High frequency radio Even higher frequency on LANs using co-ax 27Veri İletimi

Figure B-1: Frequency Modulation (FM) Low Frequency (0) High Frequency (1) Frequency Modulation (1011) Wavelength Veri İletimi

FSK on Voice Grade Line 29Veri İletimi

Phase Shift Keying Phase of carrier signal is shifted to represent data Binary PSK –Two phases represent two binary digits Differential PSK –Phase shifted relative to previous transmission rather than some reference signal 30Veri İletimi

Phase Two signals can have the same frequency and amplitude but have different phases--be at different points in their cycles at a given moment Basic Signal 180 degrees out of phase 31Veri İletimi

Phase Modulation (PM) In Phase (0) 180 degrees out of phase (1) Frequency Modulation (1011) 32Veri İletimi

Differential PSK 33Veri İletimi

Analog Data, Digital Signal Digitization –Conversion of analog data into digital data –Digital data can then be transmitted (for example using NRZ-L) –Digital data can then be converted to analog signal –Pulse code modulation –Delta modulation 34Veri İletimi

Digitizing Analog Data 35Veri İletimi

Pulse Code Modulation(PCM) (1) If a signal is sampled at regular intervals at a rate higher than twice the highest signal frequency, the samples contain all the information of the original signal Voice data limited to below 4000Hz Require 8000 sample per second Each sample assigned digital value 36Veri İletimi

Pulse Code Modulation(PCM) (2) 4 bit system gives 16 levels Quantized –Quantizing error or noise –Approximations mean it is impossible to recover original exactly 8 bit sample gives 256 levels 8000 samples per second of 8 bits each gives 64kbps 37Veri İletimi

PCM Example 38Veri İletimi

PCM Block Diagram 39Veri İletimi

Delta Modulation Analog input is approximated by a staircase function Move up or down one level (  ) at each sample interval Binary behavior –Function moves up or down at each sample interval 40Veri İletimi

Delta Modulation - example 41Veri İletimi

Delta Modulation - Performance Good voice reproduction –PCM levels (7 bit) –Voice bandwidth 4khz –Should be 8000 x 7 = 56kbps for PCM Data compression can improve on this 42Veri İletimi

Analog Data, Analog Signals Why modulate analog signals? For example permits frequency division multiplexing Types of modulation –Amplitude –Frequency –Phase 43Veri İletimi

Analog Modulation 44Veri İletimi

Modem Hardware Used For Modulation and Demodulation Optical, Radio Frequency and Dialup Modems 45Veri İletimi

Multiplexing 46Veri İletimi

Frequency Division Multiplexing FDM Each signal is modulated to a different carrier frequency Carrier frequencies separated so signals do not overlap (guard bands) e.g. broadcast radio Channel allocated even if no data 47Veri İletimi

Frequency Division Multiplexing 48Veri İletimi

FDM System 49Veri İletimi

Wavelength Division Multiplexing Multiple beams of light at different frequency Carried by optical fiber A form of FDM Each color of light (wavelength) carries separate data channel 1997 Bell Labs –100 beams –Each at 10 Gbps –Giving 1 terabit per second (Tbps) 50Veri İletimi

WDM Operation Same general architecture as other FDM Number of sources generating laser beams at different frequencies Multiplexer consolidates sources for transmission over single fiber Demux separates channels at the destination Mostly 1550nm wavelength range Was 200MHz per channel Now 50GHz 51Veri İletimi

Dense Wavelength Division Multiplexing DWDM No official or standard definition Implies more channels more closely spaced that WDM 200GHz or less 52Veri İletimi

Synchronous Time Division Multiplexing, TDM May be at bit level of blocks Time slots preassigned to sources and fixed Time slots allocated even if no data Guarantees on fairness 53Veri İletimi

Statistical Time Division Multiplexing Similar to sync. TDM except, if a source does not have data to send then multiplexor skips that source 54Veri İletimi

Time Division Multiplexing 55Veri İletimi

TDM System 56Veri İletimi

References Comer, D. E.: Computer Networks and Internets 4/e, Prentice Hall, ISBN 0 ‑ 13 ‑ ‑ X, New Jersey, Tannenbaum, Andrew S., Computer Networks 3/e, Prentice Hall, ISBN , New Jersey, 1996 Stallings, William, Data and Computer Communications 7/e, Pearson, ISBN , New Jersey, 2004 Panko, Raymond, Business Data Networks and Telecommunications, 4/e, Prentice Hall, Veri İletimi