1/21 Ethernet - IEEE Carrier Sense Multiple Access with Collision Detection (CSMA/CD)  All rights reserved. No part of this publication and file may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without prior written permission of Professor Nen-Fu Huang (

2/21 Ethernet - Typical CSMA/CD Network A C E BD F 同軸電纜 收發器電纜 終端器 收發器

3/21 Ethernet - IEEE Architecture 應用層 表達層 會議層 傳輸層 網路層 鏈結層 實體層 OSI 參考模式 媒介擷取控制 邏輯鏈結控制 LLC (IEEE 802.2) MAC PLS 實體層訊號處理 高層通訊協定 CSMA/CD 傳輸媒介 PMA MAU AUI MDI

4/21 Ethernet - CSMA/CD Protocol  Carrier Sense before transmission  Carrier Sense while transmission  Collision: Two or more stations transmitting simultaneously  Backoff: Random delay after collision  Deference: Defers transmission if channel is sensed busy  Collision Window (Slot time): Round-trip propagation delay time plus some carrier sense time. In IEEE 802.3, this value is defined to be 51.2 us.

5/21 Ethernet - CSMA/CD Collision Handling  Collision Signal is generated by Physical layer.  Jam signal (collision enforcement): To make sure that all stations involved in the collision will detect collision. A pattern of 32 bits.  Collision backoff and retransmission method (Truncated Binary Exponential Backoff Algorithm, BEBA): n : number of collisions experienced (n <= 16) k : Min (n,10) -- Truncation r : Random delay time (unit: slot time) between 0 <= r < 2 k

6/21 Ethernet - CSMA/CD Collision Handling  Slot time = 51.2 us.  Disadvantage of BEBA: Last-in-First-out effect: Stations with no or few collisions will have a better chance to transmit before stations that have waited longer.

7/21 Ethernet - IEEE Frame Format Preamble SFD DA SA LEN LLC PAD FCS 7 1 2, 6 2, 位元組 I/G U/L 15- 位元地址 46- 位元地址 I/G = 0 Individual Address （個別地址） I/G = 1 Group Address （群體地址） U/L = 0 Globally Administered Address （整體性地址） U/L = 1 Locally Administered Address （區域性地址） Preamble: ( ) for Synchronization SFD: Start Frame Delimiter ( ) DA: Destination Address SA: Source Address Length: Length of LLC-Frame LLC-Frame: Up to 1500 bytes PAD: Padding when LLC-Frame < 46 bytes FCS: Frame Check Sequence (CRC-32) MAC-frame size -- from DA to FCS Min 64 bytes to distinguish from collision Max 1518 bytes to prevent dominating bandwidth

8/21 Ethernet - Cyclic Redundancy Check (CRC32) Hardware Implementation X 0 X 1 X 2 X 3 X 4 X 5 X 6 X 7 X 8 X 9 X 10 X 11 X 12 X 13 X 14 X 15 X 16 X 17 X 18 X 19 X 20 X 21 X 22 X 23 X 24 X 25 X 26 X 27 X 28 X 29 X 30 X 31 MUX.. = AND = XOR = NOT MUX A B C O = A, C = 0 O = B, C = 1 輸出 輸入 控制訊號

9/21 Ethernet - Flow Chart 傳送訊框 組成訊框, i=1 線上有 訊號 傳送訊框 發生衝撞 送出擾亂訊號 傳送完畢 i > 16 Y Y Y N N N N Y i = i + 1 計算延遲時間 等待延遲時間 傳送成功 傳送失敗 接收訊框 開始接收 接收完畢 訊框過短 （衝撞） 位址辨識 成功 Y Y N N Y 訊框錯誤 接收成功 檢查碼 正確 尚有多餘 位元 對齊錯誤 長度欄位 內容正確 刪除訊框頭尾 長度錯誤 Y Y N N Y N

10/21 Ethernet - Collision Detection Window for Baseband CSMA/CD (=2a) B A a = 0.5, 傳輸時間 ＝ 1 t0t0 B A t 0 +a-  B A t B A t  衝撞 Ａ 開始傳送訊框 Ｂ 開始傳送訊框 Ｂ 偵測出衝撞 Ａ 偵測出衝撞

11/21 Ethernet - Cable Signaling (Manchester Encoding) Preamble Data Idle V V 0V Preamble Data Idle V 0V +0.7V 100ns 50ns Coaxial Cable Transceiver Cable

12/21 Ethernet - Network Configuration Example 1 (Single segment) A D B C E 同軸電纜 收發器電纜 終端器 收發器 同軸電纜區段（最長 500 公尺） （每區段最多接 100 個） （最長 50 公尺）

13/21 Ethernet - Network Configuration Example 2 (Two segments) D B C A 第一段同軸電纜 同軸電纜區段（最長 500 公尺） 第二段同軸電纜 G HI F E 同軸電纜區段（最長 500 公尺） 收發器電纜（最長 50 公尺） 訊號增益器

14/21 Ethernet - Network Configuration Example 3 (Five segments, maximum) C A B F D E G H J K I N L M 訊號增益器 半訊號增益器 半訊號增益器間電纜 （最長 1000 公尺） 區段 4 區段 1 區段 2 區段 3 區段

15/21 Ethernet - Architecture, Functions, and Implementation 傳送及接收 工作站界面 訊框包裝 鏈結管理 編碼及解碼 網路控制卡 收發器電纜 收發器 鏈結層 實體層 高層通訊協定 銅軸電纜

16/21 Ethernet - Calculation of Slot time (Example 3) 元件 編碼器 0.1us0.1us552.0us 收發器電纜 5.13ns/ 公尺 0300 公尺 300 公尺 3.08us 收發器（傳送路徑） 0.05us0.3us332.10us 收發器（接收路徑） 0.05us0.6us301.95us 收發器（衝撞路徑） 00.9us032.70us 同軸電纜 4.33ns/ 公尺 公尺 1500 公尺 12.99us 半訊號增益器間電纜 5.13ns/ 公尺 公尺 1000 公尺 10.26us 電纜驅動器 0.1us us 電纜接收器 0.1us us 訊號增益器（增益路徑） 0.2us0.4us201.20us 訊號增益器（衝撞路徑） 0.2us0.2us020.80us 載波感測 00.2us501.00us 衝撞偵測 00.2us051.00us 訊號上升時間 00.1us300.30us （至 70% 於 500 公尺處） 訊號上升時間（由 50%02.0us036.00us 至 94% 於 500 公尺處） 衝撞分割 00.2us010.20us 元件 穩定延遲 元件 啟動延遲 前進路徑 元件數 整體延遲 回程路徑 元件數 訊號來回傳遞最長延遲 46.38us

17/21 Ethernet - 10BaseT Ethernet Examples C A B F D E G H 集線器 (HUB) 無遮蔽式雙絞線 (UTP) 最長 100 公尺 10BASE2 界面 10BASE5 界面 10BASET F E G HC A B D 集線器（ＨＵＢ） 10BASE2 界面 10BASE5 界面 10BASET 串聯線

18/21 Ethernet - Half-duplex MAC  Ethernets have always used a half-duplex MAC.  Efficient bidirectional communications is effected by rapidly changing the direction of communication on half-duplex channel.  For Ethernet, this is not a problem due to stations can quickly arbitrate for the ability to send their frames.  However, the extension of the Ethernet MAC algorithm to gigabit data rates does strain the ability to efficiently operate in this mode.  When the arbitration time (equal to the round-trip delay, in the worst-case) approaches or exceeds the time to transmit a typical frame, the efficiency of the algorithm suffers.

19/21 Ethernet - Limitations of Half-duplex Operation  CSMA/CD implies an intimate relationship between the minimum length of a frame ( L, measured in bit-times, not absolute time) and the maximum round-trip propagation delay (2 a ) of the network: L > 2a  If we want to allow very long networks, we can make the minimum frame very long. Padding needs if a station wishes to send less data than this minimum frame. Reduces the transmission efficiency.  If we want to avoid the overhead of padding, we must reduce the extent of the network so that collision can be detected in all cases.

20/21 Ethernet - Limitations of Half-duplex Operation  For the original 10 Mbps Ethernet, a compromise was struck.  Minimum frame = 512 bits (64 bytes), not including the preamble and Physical Layer overhead.  Minimum data field = 46 bytes rarely imposes a significant padding overhead (IP header + TCP header = 40 bytes).  At 10 Mbps, 512 bit-times is 51.2us. Depends on the type of cable used and the network configuration, the extent of a 10 Mbps Ethernet can be on the order of from 2-3 Km. Preamble SFD DA SA LEN Data FCS bytes Minimum Frame Length (512 bits)

21/21 Ethernet - Network Extent  For a given minimum-length frame, the extent of a network scales inversely with data rate. 10Mbps 100 Mbps 1000 Mbps 10,000 m 1,000 m 100 m 10m ~ 2800m ~ 205m ~ 20m