v digital v Digitization sampling v Microsoft Digitization
- Slides: 49
v 所有訊息皆轉成數位(digital)的型態。 v 數位化(Digitization): 將類比信號取樣 (sampling)後轉換成 數位訊號。 v 可供電腦後續處理。 使用者互動 圖片來源:Microsoft 數位化流程 (Digitization process) 圖片來源:http: //kwave. sourceforge. net/doc/en/digitalization. html Page. 5
v 資料壓縮 (Data compression) § 將原始的檔案資料,透過各種不同的數學演算法處理後,使檔案 資料所需的儲存空間減少。(Data compression is to apply different mathematical algorithms to encode the original data to reduce the total data size. ) § 解壓縮(Decompression) ,將壓縮後的資料恢復原狀。 Run-length資料壓縮 範例(run-length compression) 圖片來源:林仁勇等譯,計算機概論, 2/e, 歐亞書局 Page. 7
v 常見資料壓縮:(Typical data compressions) § 文字壓縮: Zip, ARJ § 音訊資料壓縮(audio compression)-MP 3, AAC § 影像資料壓縮(image compression)-JPEG、GIF § 視訊資料壓縮(video compression)-MPEG 1, 2, 4, H. 264 Page. 8
Packet Switching, Circuit switching, Virtual circuit switching v Packet Switching: § data service § Minimum transmission (TX) time = 3τ + 5(T/3) (single path assumed) § What is TX time incurred using a datagram switch involving L hops and consisting of k packets? Page. 10
§ § § § Messages broken into smaller units (packets) Source & destination addresses in packet header Connectionless, packets routed independently (datagram) When a message arrives at the packet switch, the destination address is examined to determine the next hop. Packet may arrive out of order Re-sequencing maybe required at destination. Pipelining of packets across network can reduce delay, increase throughput Lower delay that message switching, suitable for interactive traffic Page. 11
v Circuit switching: § telephone service. § Minimum TX time = 6τ + T Page. 12
v Virtual Circuit Switching: § Integrated service Page. 13
§ Connection Setup § Packet transmission § Minimum TX time = 6τ + 5(T/3) Page. 14
§ Before the transmission of packets , it involves establishment of virtual circuit between source and destination § All packets follow the same path § Abbreviated header identifies connection on each link § Packets queue for transmission § Variable bit rates possible Page. 15
v 傳輸時間(TX time)比較 Page. 16
v 頻寬(bandwidth/B): § 載波的頻率範圍(HZ) § 傳送的數位資料量 (bits per second/ bps) v 頻 譜 效 益 (spectrum efficieny): bps/HZ Page. 21
v FDMA (frequency division multiple access):使用FDM多 方式的多重存取技術。(時域共用) v TDMA(time division multiple access):使用TDM多 方式 的多重存取技術。(頻域共用) v CDMA (code division multiple access) :使用CDM多 方 式的多重存取技術。(時、頻域共用) v SDMA (space division multiple access):使用SDM多 方 式的多重存取技術。(空間分割) Page. 34
Forward Error Connection (FEC) p FEC(N, k): Reed-Solomon Codes (RS)以(N, k)表示,是由 N個byte所組成,其中包括k個 data bytes與N-k個parity bytes,能夠提供 ü N-k /2個error correction或 ü N-k個erasure correction。 p 以FEC(8, 5) erasure code 為例 X X FEC decoder FEC encoder X Page. 38
v 1 G: 北 美 行 動 電 話 系 統 n 系統架構: AMPS • 行動台(mobile station, MS) v 類比調變 • 基地台(base station, BS) • 行動電話交換中心(mobile v 頻譜效率差 telephone switching office, v 抗雜訊能力差 MTSO) v 保密差 • 公用電話網路(public switching telephone network, PSTN) Page. 47
OSI 七層模型 v 實體層(Physical layer) v 資料鏈路控制層(Data link control layer) v 網路層(Network layer) v 轉運層(Transport layer) v 進程層(Session layer) v 展現層(Presentation layer) v 應用層(Application layer) Page. 49
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