Hayes AT指令集：从调制解调器标准到国际规范

Hayes AT指令集是由Dennis Hayes和Dale Heatherington于1981年为Hayes Smartmodem开发的命令语言，由一系列短文本字符串组成，用于实现拨号、挂断和参数设置等调制解调器操作1。该指令集包含基础命令集、扩展命令集、专有命令集和寄存器命令四类1，其中逃逸序列为三个加号(+++)加一秒暂停，用于从数据模式切换至命令模式1。

这套指令集后来成为大多数拨号调制解调器的标准，并经历了从300到28.8 kbit/s的多代扩展1。1995年，ITU-T建立了V.25 ter标准将其正式化，该标准于1998年更名为V.2501。根据V.250规范，调制解调器需要实现A、D、E、H、I、L、M、N、O、P、Q、T、V、X、Z、&C、&D、&F等命令1。

随着技术发展，AT指令集的应用范围不断扩大。GSM标准通过ETSI GSM 07.07(3GPP TS 27.007)定义了AT风格的命令扩展1。然而，Time Independent Escape Sequence(TIES)的实现方式存在安全隐患，可被利用进行拒绝服务攻击1。

The Hayes AT command set, developed by Dennis Hayes and Dale Heatherington for the Hayes Smartmodem in 1981, became the foundational language for dial-up modem communication.1 Comprising short text character strings, the command set enabled essential operations including dialing, hanging up, and parameter configuration.1 The system was organized into four distinct categories: basic commands, extended commands, proprietary commands, and register commands.1

The escape sequence of three plus signs (+++), followed by a one-second pause, allowed users to transition from data mode back to command mode during modem operation.1 Following its widespread adoption across the modem industry, the command set underwent multiple generations of expansion to support increasing data transmission speeds, ranging from 300 to 28.8 kilobits per second.1 In 1995, the International Telecommunication Union Telecommunication Standardization Sector (ITU-T) formally standardized the protocol as the V.25 ter specification, which was subsequently renamed V.250 in 1998.1 The V.250 standard mandated that compliant modems implement a specific set of commands: A, D, E, H, I, L, M, N, O, P, Q, T, V, X, Z, &C, &D, and &F.1

The Time Independent Escape Sequence (TIES) implementation introduced a notable security vulnerability that could be exploited for denial-of-service attacks.1 Beyond its use in traditional modems, the AT command format was adopted by the Global System for Mobile Communications (GSM) standard, which defined AT-style command extensions through ETSI GSM 07.07 (3GPP TS 27.007).1