Chapter 02 基本通訊概念
2024/06/18 Edited by Ting
三種資料傳輸方式
- 單工 Simplex
- 半雙工 Half-duplex
- 可以雙向傳輸,但一次只能傳一方向,不可同時進行傳輸。
- 例如:無線電對講機。
- 全雙工 Full-duplex
傳輸訊號
傳輸訊號可以分成2種:
- 類比訊號(Analog Signal):存在於自然界的一種連續訊號。
- 數位訊號(Digital Signal):一般數位式電腦中的高低訊號。
訊號的特性
3個重要的因子:
- 振幅 Amplitude,即大小 Magnitude,m(t)。
- 頻率 Frequency,f(t)。
- 相位 Phase,θ(t)。
訊號
傳輸距離
無線電以光速 ( ) 進行傳輸:
- 愈大,訊號含括範圍愈大,但相對低,傳送速度慢(網速慢)
- 一般信號傳輸是用微波(:1mm 至 1m,: 300GHz 至 300MHz)或紅外線(:760nm 至 1mm,:400THz 至 300GHz)。
調變(Modulation) & 解調(Demodulation)
- 調變:將一個或多個週期性的載波混入想傳送之訊號。
- 解調:調變的逆過程叫做「解調」,用以解出原始的訊號。
- 載波
- 常見調變方式:
-
AM(Amplitude Modulation; 振幅調變):
- 載波的頻率、相位不變,僅在"振幅大小"變化。
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-
FM(Frequency Modulation; 頻率調變):
- 載波的振幅、相位不變,僅在"頻率"上變化。
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-
PM(Phase Modulation; 相位調變):改相位
數位調變處理
從類比訊號到數位傳輸,要經過3個流程(取樣)->(量化)->(編碼)。
- ASK(Amplitude Shift Keying; 振幅位移鍵)
- 透過載波的振幅變化來表示數位訊號的一種振幅調變方式。
- 當訊號為1,振幅變大。
- FSK(Frequrncy Shift Keying; 頻率位移鍵)
- 利用頻率差異的訊號來傳送資料的調變方式。
- 當訊號為1,頻率變大。
- PSK(Phase Shift Keying; 相位位移鍵)
- 相位差異的訊號來傳送資料的調變方式。
- 當訊號為變化,相位保持不變。
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取樣 Sample
- 在某個時間間隔內,對類比訊號進行取樣,即將連續時間性的訊號在時間軸上進行分割。
- 取樣次數愈密,還原訊號與原始訊號愈接近。但處理時間愈長,且記憶體容量較大。
注意: 需 ,否則取樣訊號將重疊而無法還原。
- 此時,最小的取樣頻率 稱為奈奎斯特頻率(Nyquist frequency)。其中:基頻頻率 、取樣頻率(即一個切換開關以 的速率週期性關閉,並停留秒)。( = 脈波寬度)。
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編碼(Encodeing)處理
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- NRZ-L
- (1 = v, 0 = -v)。
- 有2V的差距,抗干擾性佳。
- 不歸零(non-return-to-zero level)。
- NRZ-I
- (1 = inverse, 0 = keep)。
- 此模式要設初始值(-v, picture)。
- 反不歸零(non-return-to-zero, Invert on ones)。
- Manchester
- (1 = 左下右上,0 = 右下左上)。
- 曼徹斯特編碼。
- Differential Manchester
- (1 = inverse,0 = keep)。
- 微分曼徹斯特編碼。
- MLT-3
- (0 -> -v -> +v -> 0 ->..)。
多工處理
- 分頻多工(FDM)
- 訊號間不會互相干擾,因為彼此的頻率範圍不相同,可以在單一通道達到多工。
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- 分時多工(TDM)
- 時間軸上切成很多時間間格[時槽(time slot)]給多人使用。
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- 分碼多工(CDM)
- 具備很好的抗雜訊特性,每個使用者的信號轉成的長串數碼具有互不干擾性,在同個頻帶中,就像是把其他使用者訊號視為背景雜音,不必考慮碰撞(Collision)問題。
- 分相多工(PDM)
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通訊交換方式
- 電路交換 (Circuit Switching)
- 環境中有許多交換機,藉由交換機內的交換電路,在發送端與接收端之間串接起一條專屬路徑,通訊過程中,其他節點無法對收發雙方進行通訊,此電路在通訊結束後切斷。
- 例如:電話。
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- 分封交換 (Packet Switching)
- 傳送資料前,先將資料分割成一連串的小封包(Packet),並在封包加上部分的控制訊息,以便封包依照控制訊息,將資料送達目的端,目的端所有資料都到齊後,會將所有小封包重組成原始資料。
- 例如:網路傳送。
- 優點:資源效率高。僅在需要時使用資源,避免了連接保持帶來的浪費。
- 缺點:延遲較高。封包可能經過不同路徑,因此傳輸延遲較高。有時會封包損失。
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- 訊息交換 (Message Switching)
- 各交換節點須將收到的訊息儲存,等整個訊息完整接收後,再往下傳送到其他中間交換節點,直到傳送至目的點為止。
- 例如:電子郵件。
- 優點:成本費用較低、傳輸通道使用率高。
- 缺點:儲存需求高、延遲高。
xDSL 數位用戶線路(Digital Subsriber Line)
- 數位用戶線路(DSL;Digital Subscriber Line)
- 非對稱數位用戶線路(ADSL;Asymmetric Digital Subscriber Line)
- 上傳和下載提供不同速率,可以更有效率的發揮線路的傳送容量。
- 距離機房(communication office)愈遠速度愈慢。
- VDSL