调制解调器
维基百科，自由的百科全书
调制解调器是一个将数字信号调制到模拟载波信号上进行传输，并解调收到的模拟信号以得到数字信息的电子设备。 它的目标是产生能够方便传输的模拟信号并且能够通过解码还原原来的数字数据。根据不同的应用场合，调制解调器可以使用不同的手段来传送模拟信号, 比如使用光纤，射频无线电或电话线等。
使用普通电话线音频波段进行数据通信的电话调制解调器是人们最常接触到的调制解调器。在口語中，根據英語語音取首音節，很多人将电话调制解调器稱爲“貓”。
其他常见的调制解调器还包括用于宽带数据接入的有线电视电缆调制解调器，DSL调制解调器。 数字移动电话实际上也是一种无线调制解调器。电信传输设备是为了在不同的介质上远距离的传输大量信息，因此也都以调制解调器的功能为核心。其中，微波调制解调器速率可以达上百万比特每秒；而使用光纤作为传输介质的光调制解调器可以达到几十Gbps以上，是现在电信传输手段的骨干。
目录 |
[编辑] 历史
Modems起初是为1950年代的 半自动地面防空警备系统（SEGE）研制，用来连接不同基地的终端，雷达站和指令控制中心到 美国和加拿大的SAGE指挥中心。 SAGE运行在专用线路上,但是当时两端使用的设备跟今天的Modem根本不是一回事。 IBM是SAGE系统中计算机和Modem的供货商。几年后美国航空(American Airlines)的首席执行官(CEO)与IBM一位区域经理的一次会晤促使"mini-SAGE"这种航空自动订票系统。在这系统中，一个位于票务中心的终端连接在中心电脑上，用来管理机票有效性和时间。这个系统，叫做Sabre，是今天SABRE系统的早期原型。
1960年代早期，商业机算计的应用逐渐普及, 以及上述技术成果, 1962年 AT&T 发布了第一个商业化modem, Bell 103. Using用两个音调表示1和0的移频键控技术，103已经能够实现300 bit/s的传输速度。很短时间后继版本Bell 212就研制出来, 转移到更稳定的移项键控技术把数据速率提高到1200 bit/s。 类似Bell 201的系统用双向信号集在4对专用线路上实现了2400 bit/s。
贺氏智能Modem是一个重大的进步，1981年贺氏通讯研制成功。智能Modem是一个简单的300bpsModem，使用的是Bell103信令标准，内置了一个小型控制器，可以让计算机发送命令来控制电话线，例如摘机，拨号，重拨，挂机等功能。
在智能Modem之前，几乎所有的Modem都需要两个步骤来产生一个连接：第一步，人工在电话机上拨叫对方的号码，然后将听筒放在Modem附带的acoustic coupler里,一个用两个橡胶杯组成的用来在声音信号和电信号之间转换的设备。 使用智能Modem就不再需要acoustic coupler，而是直接将modem连接在标准电话线或插座上。然后电脑就能自动完成接通电话并拨叫号码的功能。这个改变极大的简化了bulletin board systems (BBS)的安装和使用。
到1980年代Modem的速率一直没有多大变化。美国一般使用一种与贝尔212类似的2400 bit/s的系统，而欧洲的系统稍有差别。到1980年代晚期大多数Modem都能支持当时所有的标准，2400bit/s逐渐普及。大量特定用途的标准也被加了进来，通常都是使用高速信道接受低速信道发送，典型的例子就是法国的Minitel 系统， 用户终端大部分时间都在接受信息。Minitel终端的Modem用1200bit/s接受数据75 bit/s发送命令反馈给服务器。
These sorts of solutions were useful in a number of situations where one side would be sending more data than the other. In addition to a number of "medium-speed" standards like Minitel, four US companies became famous for high-speed versions of the same concept. Microcom Systems introduced their MNP, Hayes their Ping Pong, USR had their HST protocol, and Telebit used software to increase performance. In all of these cases the high-speed line was set to 9600 bit/s, and the low-speed line to between 75 and 300 bit/s. Each company carved out a niche in the market, Telebit was huge in the universities due to their direct support of UUCP prototols in the modem itself, Microcom became common in commercial settings, and USR was huge among BBS operators (as they could download Fidonet messages more quickly), but the Hayes standard never caught on. In all of these cases there was a well defined high-speed and low-speed direction, but such a split was not so obvious for users who were uploading and downloading files in the same session, and these solutions were rarely used by them.
Operations at these speeds pushed the limits of the phone lines, and would have been generally very error-prone. This led to the introduction of error correction systems built into the modems, made most famous with Microcom's MNP systems. A string of MNP standards came out in the 1980s, each slowing the effective data rate by a smaller amount each time, from about 25% in MNP1, to 5% in MNP4. MNP5 took this a step further, adding compression to the system, thereby actually increasing the data rate - in general use the user could expect an MNP modem to transfer at about 1.3 times the normal data rate of the modem. MNP was later "opened" and became popular on a series of 2400 bit/s modems, although it was never widespread.
Another common feature of these high-speed modems was the concept of fallback, allowing them to talk to less-capable modems. During the call initiation the modem would play a series of signals into the line and wait for the remote modem to "answer" them. They would start at high speeds and progressively get slower and slower until they heard an answer. Thus two USR modems would be able to connect at 9600 bit/s, but when another user with a 2400 bit/s modem called in, the USR would "fall back" to the common 2400 bit/s speed. Without such a system the operator would be forced to have multiple phone lines for high and low speed use.
[编辑] 回声抑止
回声抑止是Modem设计种的另一个主要进步。一般来说电话系统要发送一个小的声音信号到讲话方的听筒，这样就告诉说话方声音正在传送。然而同样的信号会干扰Modem，使它分不清信号到底是自己发的还是对方发的。这也是为什么把频率分割频率给应答和原声；如果接受的频率跟自己相同就可以简单的忽略掉。通过改进电话系统可以允许更高的速度, 分离的可用电话信号带宽依然受到Modem强制半速限制。
Echo cancellation was a way around this problem. By using the phone system's timing, a slight delay, it was possible for the modem to tell if the received signal was from itself or the remote modem. As soon as this happened the modems were able to send at "full speed" in both directions at the same time, opening the market to a slew of 9600 bit/s bidirectional modems in the late 1980s. These earlier systems were not very popular due to their price, but by the early 1990s the prices started falling. The "breaking point" occurred with the introduction of the SupraFax 14400 in 1991, which cost the same as a 2400 bit/s modem from a year or two earlier (about $300US), but ran at the latest 14,400 bit/s rate (14.4 kbit/s) and also included fax capability. Over the next few years the speed increased to 28.8 kbit/s, then to 33.6 kbit/s, along with a slew of one-off non-standards like AT&T's 19.2 kbit/s system.
Modem设计的最主要的进步是1990年代晚期推出的56Kbps标准。这个标准类似于更早的1980年代被用户拒绝的高度/低速系统，但是随着互联网使用的普及稍微的牺牲一点速度换来使用性被逐渐接受。
[编辑] 价格
The pace of these introductions follows the computer market's fairly well. 1200 bit/s was introduced by Bell in the 1960s, but 300 bit/s remained the most common speed into the late 1970s due to the high cost of a 1200 bit/s modem. 1982年8月新加坡创新公司发布广告：99美元的"超低价" 300bps modem ，同时发布的1200 bps modems超过300美元。1988年2400bps Modems 低于300美圆, SupraModem 144 1992也是这个价格. 今天一个56k Modem一般低于300人民币, 大多数都有盒子，手册和品牌，还有软modems也可以找到，56K速率的价格低于100元人民币.
The original 300 bit/s system is considered the "ultimate fallback" today, although it's unlikely anything below 2400 bit/s would actually work.
[编辑] Description
[编辑] 窄带
今天标准的modem在1980年被命名为“智能modem”。主要又个重要的部分组成：模拟部分用来产生信号和操作电话；数字部分用来设置和控制。这两部分实际上集成进一块芯片，只是理论上还是分开来说明。
modem实际上工作在2种"模式", 数据模式时通过电话线发送或者接受数据给计算机；命令模式 时modem监听从计算机发来的命令数据并发送出去。一个典型的会话开始于经过自动假定设置的上电(一般是内置类型) ，然后Modem开始拨出一个电话号码用来连接远程的另一个Modem。连接成功之后Modem自动进入数据模式，可以收发数据了。当用户使用完了，逸出序列, "+++" 跟在大约1秒的暂停之后，Modem就转入命令模式，发送命令挂机。这样处理有一个问题就是。Modem并不真的知道一个字符串时命令还是数据。已知的解决方法例如，可以使用转义字符来解决连续+++的情况。
The commands themselves are typically from the 贺氏命令集, although that term is somewhat misleading. The original Hayes commands were useful for 300 bit/s operation only, and then extended for their 1200 bit/s modems. Hayes was much slower upgrading to faster speeds however, leading to a proliferation of command sets in the early 1990s as each of the high-speed vendors introduced their own command styles. Things became considerably more standardized in the second half of the 1990s, when most modems were built from one of a very small number of "chip sets", invariably supporting a rapidly converging command set. We call this the Hayes command set even today, although in this use the terminology is misleading: the command set in question has three or four times the numbers of commands.
The 300 bit/s modems used 移频键控 to send data. In this system the stream of 1's and 0's in computer data it translated into sounds which can be easily sent on the phone lines. In the Bell 103 system the originating modem sends 0's by playing a 1070 Hz tone, and 1's at 1270 Hz, with the receiving modem putting its 0's on 2025 Hz and 1's on 2225 Hz. These frequencies were chosen carefully, they are in the range that suffer minimum distortion on the phone system, and also are not 谐波s of each other. In early systems the choice of answer or originate was selected by a switch on the front of the modem, but as time went on the Smartmodems would assume originate if asked to dial, and answer if asked to answer the phone.
1200bit/s和更快的系统使用移相键控。 In this system the two tones for any one side of the connection are sent at the similar frequencies as in the 300 bit/s systems, but slightly out of phase. By comparing the phase of the two signals, 1's and 0's could be pulled back out, for instance if the signals were 90 degrees out of phase, this represented two digits, "1, 0", at 180 degrees it was "1, 1". In this way each cycle of the signal represents two digits instead of one, 1200 bit/s modems were, in effect, 600 bit/s modems with "tricky" signalling.
在这里波特和位每秒的区别得以实现 became real. 波特指的时系统的信号速率, 300bps的modem每个信号发送1位数据,所以数据速率和信号速率是一致的。1200 bit/s的系统就不是这样了,实际modems工作在600波特. 这导致了80年代BBS上一系列的激烈争论。
Increases in speed since then have invariably used more pairs of such low-speed signals for each side of the conversation, with 2400 bit/s modems using two pairs for each side, and 9600 bit/s eight pairs. There is a limit to how far this can be taken; eventually the signals will start to overlap and the system stops working. This limit is around 33.6 kbit/s (接近香农极限), which became possible due more to improvements in the underlying phone system than anything in the technology of the modems themselves.
Software is as important to the operation of the modem today as the hardware. Even with the improvements in the performance of the phone system, modems still lose a considerable amount of data due to noise on the line. The MNP standards were originally created to automatically fix these errors, and later expanded to compress the data at the same time. Today's v.42 and v.42bis fill these roles in the vast majority of modems, and although later MNP standards were released, they are not common.
With such systems it is possible for the modem to transmit data faster than its basic rate would imply. For instance, a 2400 bit/s modem with v.42bis can transmit up to 9600 bit/s, at least in theory. One problem is that the compression tends to get better and worse over time, at some points the modem will be sending the data at 4000 bit/s, and others at 9000 bit/s. In such situations it becomes necessary to use hardware flow control, extra pins on the modem-computer connection to allow the computers to signal data flow. The computer is then set to supply the modem at some higher rate, in this example at 9600 bit/s, and the modem will tell the computer to stop sending if it cannot keep up. A small amount of memory in the modem, a buffer, is used to hold the data while it is being sent.
几乎所有的现代modems都可以作为传真机使用。 Digital faxes, introduced in the 1980s, are simply a particular 图像格式 sent over a high-speed (9600/1200 bit/s) modem. 计算机中的传真软件可以转换任何图像为Modem可以发送的传真格式。这种软件开始曾经需要另外购买，现在已经十分的普遍了。
Winmodem或者软件调制解调器，软猫 是一个为Windows系统简化的Modem，它用软件实现了部分硬件的功能。再这种情况下计算机内置的声音硬件（声卡）用来产生Modem模拟部分的声音信号。 A small piece of hardware is then used to connect the sound hardware to the phone line. Modern computers often include a very simple 插槽, the communications/networking riser slot (CNR), to lower the cost of connecting it up. The CNR slot includes pins for sound, power and basic signalling, instead of the more expensive PCI slot normally used. Winmodems普遍的比传统modems便宜，因为他们集成了更少的硬件。 One downside of a Winmodem is that the software generating the modem tones is not that simple, and the performance of the computer as a whole often suffers when it is being used. For online gaming this can be a real concern. 另外的问题就是WinModems因为只能在过于依赖特定操作系统而缺乏灵活性，可能不会被其他操作系统(例如Linux)所支持,因为他们的制造商可能既不支持其他操作系统也不提供足够的技术支持和驱动程序。如果他们的驱动程序没有集成进系统的驱动程序库，后续的微软Windows系统可能也不会（或者不能很好的）支持Winmodem。
今天的现代语音Modem(ITU-T V.92标准) 已经十分接近公共电话网（PSTN）电话信道的香农信道容量（Shannon capacity）。它们都是 即插即用的 传真/数据/语音 modems (广播语音信息和录音，音频响应)。
[编辑] 无线“猫”
不同的无线调制解调器有不同的类型，不同的带宽和速度。 无线modems经常分成透明和智能两类。他们传送经过调制到载波频率上的信息，这样同时有很多无线通讯链路就可以在相同或者不同的无线频率工作。
透明modem运行类似于他们的表亲电话线Midem。典型的，他们是半双工的，这就意味着他们不能同时发送和接受数据。另外典型的特点是，透明modem循环可以收集位于分散位置而又不容易布设线路的地方的数据。透明modem一般用于公司有效地汇集数据。
智能modems内置介质访问控制器来避免因冲突和重发引起的未正确接受数据产生随机数据。智能modem因此较之普通透明Modem需要额外的带宽，特别的还要更多的无线频率带宽。IEEE 802.11标准包含了一个近程调制解调器标准，大规模应用于全球的网络。无线调制解调器被用于WiFI或者WiMax标准。
[编辑] 宽带
ADSL（非对称用户数字线） Modems也是一种调制解调器，俗称宽带猫主要区别在于它不止局限在普通电话使用的语音载波的频段。 现在的ADSL modems使用编码正交频分调变。.
线缆调制解调器也是一种调制解调器,它使用的是射频（RF）电视频道的一段载波频段。多个线缆modems可以使用一条电视电缆的相同频段，通过低水平介质访问协议来实现在同一通道共同工作。典型的“上行”和“下行”信号用 频分复用来隔离。
新型的宽带modems也开始普及了，例如 双路 卫星和电线调制解调器。
[编辑] 互联网访问
提起Modem很多人就会联想到网络访问。 A UCLA 2001年的调查显示81.3%的美国网民使用电话调制解调器，11.5%的网民使用线缆调制解调器， 数量远超过其他方法。
但是宽带服务的日趋完善给宽带调制解调器提供了广阔的市场空间。特别是ADSL在欧洲和亚洲的安装数量一直保持旺盛的增长。1Mbps以上的连接越来越多的取代窄带的拨号方式。
在中国，由于电话线路密集，为避免干扰，通讯电缆无法在一根电缆承载过多的ADSL信号。因此运营商提供了更多的接入方式，例如单纯的IP网络或者电力线等，甚至无线手机也加入到接入服务的竞争中。根据不同的接入协议，当前主要应用的是中国移动通讯的GPRS（30-50Kbps）和中国联通的CDMA1X（130Kbps）。他们的费用都比较廉价，在漫游服务上有区别。
[编辑] 参看
- 56K线路
- 数模转换
- 包价费用
- 调制 (调制技术列表)
- TCP/IP
- b:系列课程:调制解调器和AT命令 b:课程:串行数据通信 维基教科书
