【正文】 Physical Layer Propagation: UTP and Optical Fiber Chapter 3 Updated January 2020 XU Zhengchuan Fudan University 2 Orientation ? Chapter 2 – Data link, inter, transport, and application layers – Characterized by message exchanges ? Chapter 3 – Physical layer (Layer 1) – There are no messages—bits are sent individually – Concerned with transmission media, plugs, signaling methods, propagation effects – Chapter 3: Signaling, UTP, optical fiber, and topologies – Wireless transmission is covered in Chapter 5 3 Figure 31: Signal and Propagation Sender Receiver Transmission Medium Propagation Transmitted Signal Received Signal (Attenuated amp。 Distorted) A signal is a disturbance in the media that propagates (travels) down the transmission medium to the receiver If propagation effects are too large, the receiver will not be able to read the received signal 4 ? Test Your Understanding ? P 141 Data Representation 6 BinaryEncoded Data ? Computers store and process data in binary representations – Binary means ―two‖ – There are only ones and zeros – Called bits 1101010110001110101100111 7 BinaryEncoded Data ? NonBinary Data Must be Encoded into Binary – Text – Integers (whole numbers) – Decimal numbers – Alternatives (North, South, East, or West, etc.) – Graphics – Human voice – etc. Hello 11011001… 8 BinaryEncoded Data ? Some data are inherently binary – 48bit Ether addresses – 32bit IP addresses – Need no further encoding 9 Figure 32: Arithmetic with Binary Numbers Binary Arithmetic for Whole Numbers (Integers) (Counting Begins with 0, not 1) Integer 0 1 2 3 4 5 6 7 8 Binary 0 1 10 11 100 101 110 111 1000 “ There are 10 kinds of people— those who understand binary and those who don’t‖ 10 Figure 32: Arithmetic with Binary Numbers, Continued Binary Arithmetic for Binary Numbers 1 0 0 1 1 +1 +0 +1 +0 +1 +1 =0 =1 =1 =10 =11 Basic Rules 11 Figure 32: Arithmetic with Binary Numbers, Continued Binary Decimal 1000 8 +1 +1 =1001 =9 +1 +1 =1010 =10 +1 +1 =1011 =11 +1 +1 =1100 =12 Examples 12 Figure 33: Binary Encoding for Alternatives Encoding Alternatives (Product number, region, gender, etc.) (N bits can represent 2N Alternatives) Number of Bits In Field (N) 1 2 3 4 8 16 … Number of Alternatives That Can be Encoded with N bits 2 (21) 4 (22) 8 (23) 16 (24) 256 (28) 65,536 (216) … Each added bit doubles the number of alternatives that can be represented 13 Figure 33: Binary Encoding for Alternatives Bits Alternatives Examples 1 21=2 Male = 0, Female = 1 2 22=4 Spring = 00, Summer = 01, Autumn = 10, Winter = 11 8 28=256 Keyboard characters for . keyboards. Space=00000000, etc. ASCII code actually uses 7 bits 14 Powers of 2 Bits Alternatives 1 2 2 4 3 8 4 16 5 32 6 64 7 128 8 256 10 1,024 16 65,536 Each additional bit doubles the number of possibilities Start with one you know and double or halve until you have what you need ., if you know 8 is 256, 10 must be 4 times as large or 1,024. Memorize for 1, 4, 8, and 16 bits 15 Figure 33: Binary Encoding for Alternatives ? Quiz – How many flavors of ice cream can you represent in half a byte of storage? – How many bits do you need to represent 64 flavors of ice cream? – How many bits do you need to represent 6 sales districts? 16 Figure 34: ASCII and Extended ASCII ? ASCII Code to Represent Text – ASCII is the traditional binary code to represent text data – Seven bits per character ? 27 (128) characters possible – Sufficient for all keyboard characters (including shifted values) ? Capital letters (A is 1000001) ? Lowercase letters (a is 1100001) – Each character is stored in a byte ? The 8th bit in a byte normally is not used 17 Figure 34: ASCII and Extended ASCII, Continued ? Extended ASCII – Used on PCs – Uses a full 8 bits per character – 28 (256) characters possible – Extra characters can represent formatting in word processing, etc. ? Converters – TexttoASCII and TexttoExtended ASCII Converters are Readily Available on the Inter 18 Figure 35: Binary Coding for Graphics Image ? Pixels – 1. Screen is divided into small squares called pixels (picture elements) – 2. Each pixel has three dots—red, green, and blue. Sometimes a black dot too 3. JPEG stores one byte per color (24 bits total) This gives 256 intensity levels for each color or million colors overall (2563) 19 ? Test Your Understanding ? P 146 ? 3 c Signaling 21 Figure 36: Data Encoding and Signaling Data ―Now is the …‖ Male or Female Graphics Human Voice Binary