此贴长、有趣、信息量大。发报员按住键时收报员听到声音，放开键时声音消失。如果按键持续1ms代表0, 2ms代表1, ..., 65536毫秒代表65535，这个通信系统里的symbol就是16个位。它的问题：1. 发送65535这一个symbol耗时一分钟；2. 收报员哪能分清1ms和2ms？
Morse code is a method used in telecommunication to encode text characters as standardized sequences of two different signal durations, called dots and dashes or dits and dahs. Morse code is named after Samuel Morse, an inventor of the telegraph. In 1844, a skilled operator could send 4 characters per second (~40 words per minute, 0.021 kbit/s).
Morse code is a 变长码。The length of a dot is 1 unit; A dash is 3 units; The space between parts of the same letter is 1 unit; The space between letters is 3 units; The space between words is 7 units. 它不像哈夫曼码那样有不同的前缀，没有space的话没法区分..是I还是EE。
The Baudot code was the predecessor to the International Telegraph Alphabet No. 2 (ITA2), the most common teleprinter code in use until the advent of ASCII. Each character in the alphabet is represented by a series of five bits, sent over a communication channel such as a telegraph wire or a radio signal. The symbol rate measurement is known as baud, and is derived from the same name.
波特码是5位定长码。如果我们收到了不完整的一段，试上5次，依次丢掉0, 1, 2, 3, 4，头一个丢了，后面的还是完整的。十进制每位0~9十个数，二进制每位0~1两个数。一进制固定传0没有意义。波特码里英文字母不区分大小写。
The baud unit is named after Émile Baudot (1845-1903), the inventor of the Baudot code for telegraphy, and is represented in accordance with the rules for SI [国际单位制] units. That is, the first letter of its symbol is uppercase (Bd), but when the unit is spelled out, it should be written in lowercase (baud) except when it begins a sentence. It was defined by the CCITT (now the ITU) in November 1926. The earlier standard had been the number of words per minute. One baud was equal to one pulse per second, a more robust measure as word length can vary.
啥叫一个pulse? 比如两个人拉着一根绳，用抖绳子的幅度表示1, 2..., 8，每秒抖2下，窃以为pulse per second是2，bits per pulse是3，这是个调幅系统。如果手电前弄个塑料片，红0，绿1，蓝2, 白3，这是个调频系统(光是电磁波，不同颜色的光频率不同)，每个pulse 2个bits。
到了1940x，Teleprinter (50 baud)0.05 kbit/s 404 operations per minute, 6.7 operations per second，这机器可能比熟练的人速度慢。
波长乘频率=波速，真空中的光速是30万km/s。上图中VIS即Visual, 可见光。infra: 低于; ultra: 超过; 图左右颠倒下更顺。波长越短，每cm中的波数就越多。
频率越高，通信速度就越快，带宽越大吗？好像是的。语速越快，同样时间内说的话不是越多？疯狂地抖绳子不比慢慢抖传递的信息更多？那玩啥无线电，上gamma射线啊？首先它不好发射，其次它走直线，最后高能的会搞死人的。
下面列出历史上著名的和未来的一些标准的带宽。最后介绍RS-232 or a related standard, such as RS-485 or RS-422.
- GPS receivers typically NMEA 0183 at 4,800 bit/s 程序读进来的是个字符串
- Macintosh AppleTalk using RS-422 at 230.4 kbit/s
- Ethernet (10BASE-X) 10 Mbit/s 1.25 MB/s 1980 (1985 IEEE Standard)
- ADSL (G.dmt) ITU G.992.1 8192/1024 kbit/s 1024/128 kB/s 1999
- NG-PON2 (G.989) (fiber optic service) 40/10 Gbit/s 5/1.25 GB/s 2015
- GPRS (2.5G) 57.6 kbit/s 7.2 kB/s 28.8 kbit/s 3.6 kB/s
- DS1/T1 (and ISDN Primary Rate Interface) 1.544 Mbit/s 0.192 MB/s 1990 那时有T1=家有矿
- LTE Cat 14 3.9 Gbit/s 487 MB/s 1.5 Gbit/s 187 MB/s 5G不仅仅是带宽高，还有延时短，例如远程做手术。
- IEEE 802.11ax (aka Wi-Fi 6) 11 Gbit/s 1375 MB/s 2019
- PC3-8500 DDR3 SDRAM DDR3-1066 133 MHz 533 MHz 1.066 GT/s 68 Gbit/s 8.5 GB/s
- PC5-51200 DDR5 SDRAM DDR5-6400 800 MHz 3200 MHz 6.4 GT/s 409.6 Gbit/s 51.2 GB/s
- Apple 2 cassette tape interface 1.5 kbit/s 200 B/s 1977
- CD Controller (1×) 1.171 Mbit/s 0.146 MB/s 1988
- DVD Controller (1×) 11.1 Mbit/s 1.32 MB/s
- Blu-ray Controller (1×) 36 Mbit/s 4.5 MB/s 注意不是4.5Mbit/s，码率不高的1080p没意义
- ATA PIO Mode 4 133.3 Mbit/s 16.7 MB/s 1996
- NVMe over M.2 or U.2 (using PCI Express 4.0 ×4 link) 64 Gbit/s 7.876 GB/s 2017
- iSCSI over 100G Ethernet 100 Gbit/s 12.392 GB/s 2010
- FCoE over 100G Ethernet 100 Gbit/s 12.064 GB/s 2010
- Infiniband HDR 12× 600 Gbit/s 75 GB/s 2017
- AGP 1× 2.133 Gbit/s 266.7 MB/s 1997
- PCI Express 6.0 (×16 link) 1024 Gbit/s 126 GB/s 2020 得啥内存配它？
- NVLink 2.0 1.2 Tbit/s 150 GB/s 2017
- Infinity Fabric (Max. theoretical) 4.096 Tbit/s 512 GB/s 2017
- Thunderbolt 3 40 Gbit/s 5 GB/s 2015
- USB4 40 Gbit/s 5 GB/s 2019
- Serial MIDI 31.25 kbit/s 3.9 kB/s 1983
- HDMI 36.864 Mbit/s 4.608 MB/s audio
- DisplayPort 36.864 Mbit/s 4.608 MB/s audio
- HDMI 2.1 48 Gbit/s 6 GB/s [b] 2017 addio
- DisplayPort 2.0 (4-lane) 80 Gbit/s 10 GB/s 2019
串口
Operating systems usually create symbolic names for the serial ports of a computer, rather than requiring programs to refer to them by hardware address. Unix-like operating systems usually label the serial port devices /dev/tty*. TTY is a common trademark-free abbreviation for teletype, a device commonly attached to early computers' serial ports, and * represents a string identifying the specific port; the syntax of that string depends on the operating system and the device. On Linux, 8250/16550 UART hardware serial ports are named /dev/ttyS*, USB adapters appear as /dev/ttyUSB* and various types of virtual serial ports do not necessarily have names starting with tty. The DOS and Windows environments refer to serial ports as COM ports: COM1, COM2,..etc. Ports numbered greater than COM9 should be referred to using the \\.\COM10 syntax.
The number of data bits in each character can be 5 (for Baudot code), 6 (rarely used), 7 (for true ASCII), 8 (for most kinds of data, as this size matches the size of a byte), or 9 (rarely used). 8 data bits are almost universally used in newer applications. 通信的最小单位不是character更不是word，而是symbol, symbol一般是bit。
Parity is a method of detecting errors in transmission. When parity is used with a serial port, an extra data bit is sent with each data character, arranged so that the number of 1 bits in each character, including the parity bit, is always odd or always even. If a byte is received with the wrong number of 1s, then it must have been corrupted. However, an even number of errors can pass the parity check. 研究纠错码需要数学很好。
Stop bits sent at the end of every character allow the receiving signal hardware to detect the end of a character and to resynchronise with the character stream. Electronic devices usually use one stop bit. If slow electromechanical teleprinters are used, one-and-one half or two stop bits are required.
The data/parity/stop (D/P/S) conventional notation specifies the framing of a serial connection. The most common usage on microcomputers is 8/N/1 (8N1). This specifies 8 data bits, no parity, 1 stop bit. In this notation, the parity bit is not included in the data bits. 7/E/1 (7E1) means that an even parity bit is added to the 7 data bits for a total of 8 bits between the start and stop bits. If a receiver of a 7/E/1 stream is expecting an 8/N/1 stream, half the possible bytes will be interpreted as having the high bit set.
Flow control is used in circumstances where a transmitter might be able to send data faster than the receiver is able to process it. To cope with this, serial lines often incorporate a handshaking method, usually distinguished between hardware and software handshaking.
奈奎斯特
In signal processing, the Nyquist rate, named after Harry Nyquist, specifies a sampling rate. In units of samples per second its value is twice the highest frequency (bandwidth) in Hz of a function or signal to be sampled. With an equal or higher sampling rate, the resulting discrete-time sequence is said to be free of the distortion known as aliasing. Conversely, for a given sample rate, the corresponding Nyquist frequency in Hz is the largest bandwidth that can be sampled without aliasing, and its value is one-half the sample-rate. Note that the Nyquist rate is a property of a continuous-time signal, whereas Nyquist frequency is a property of a discrete-time system.
六级/考研单词: dash, successor, alphabet, advent, derive, accordance, pulse, robust, fibre, optic, ax, thunder, serial, audio, compute, hardware, abbreviation, data, seldom, detect, corrupt, electron, usage, interpret, transmit, distort, converse, correspond IT技术类英文比小说简单啊。
波特是engineer, 不是scientist. My fellow engineers, 加油！:-)
蓝光波长=3e8/500e-9=6e14=600THz. 量子纠缠说不定是因为斯猫波，它波长比gamma射线还短很多，斯猫粒子超小，光子对它来说内部是真空。微粒是它的琴键，你使劲看，也看，不，见！