1、英文资料及中文翻译 6TRANSMISSIONS OF DIGITAL DATA: INTERFACES AND MODEMS (From Introduction to Data Communications and Net Working, BehrouzForouzan) Once we have encoder our information into a format that can be transmitted, the next step is to investigate the transmission process itself. Information-process
2、ing equipment such as PCs generate encoded signals but ordinarily require assistance to transmit those signals over a communication link. For example, a PC generates a digital signal but needs an additional device to modulate a carrier frequency before it is sent over a telephone line. How do we rel
3、ay encoded data from the generating device to the next device in the process? The answer is a bundle of wires, a sort of minicommunication link, called an interface. Because an interface links two devices not necessarily made by the same manufacturer, its characteristics must be defined and standard
4、s must be established. Characteristics of an interface include its mechanical specifications (how many wires are used to transport the signal)； its electrical specifications (the frequency, amplitude, and phase of the expected signal)； and its functional specifications (if multiple wires are used, w
5、hat does each one do?). These characteristics are all described by several popular standards and are incorporated in the physical layer of the OSI model. 6.1DIGITAL DATA TRANSMISSION Of primary concern when considering the transmission of data from one device to another is the wiring. And of primary
6、 concern when considering the wiring is the data stream. Do we send one bit at a time, or do we group bits into larger groups and, if so, how? The transmission of binary data across a link can be accomplished either in parallel mode or serial mode. In parallel mode, multiple bits are sent with each
7、clock pulse. In serial mode, one bit is sent with each clock pulse. While there is only one way to send parallel data, there are two subclasses of serial transmission: synchronous and asynchronous (see Figure 6-1). Parallel Transmission Binary data, consisting of 1s and 0s, may be organized into gro
8、ups of n bits each. Computers produce and consume data in groups of bits much as we conceive of and use spoken language in the form of words rather than letters. By grouping, we can send data n bits at a time instead of one. This is called parallel transmission. Figure 6-1 Data transmission The mech
9、anism for parallel transmission is a conceptually simple one: use n wires to send n bits at one time. That way each bit has its own wire, and all n bits of one group can be transmitted with each clock pulse from one device to another. Figure 6-2 shows how parallel transmission works for n=8.Typicall
10、y the eight wires are bundled in a cable with a connector at each end. Figure 6-2 Parallel transmission The advantage of parallel transmission is speed. All else being equal, parallel transmission can increase the transfer speed by a factor of n over serial transmission. But there is a significant d
11、isadvantage: cost. Parallel transmission requires n communication lines (wires in the example) just to transmit the data stream. Because this is expensive, parallel transmission is usually limited to short distances, up to a maximum of say 25 feet. Serial Transmission In serial transmission one bit
12、follows another, so we need only one communication channel rather than n to transmit data between two communicating devices . Data transmission Parallel Serial Synchronous Asynchronous Sender Receiver We need eight lines s 8 bit synchronously The advantage of serial over parallel transmission is tha
13、t with only one communication channel, serial transmission reduces the cost of transmission over parallel by roughly a factor of n. Since communication within devices is parallel, conversion devices are required at the interface between the sender and the line (parallel-to-parallel). Serial transmis
14、sion occurs in one of two ways: asynchronous or synchronous. Asynchronous Transmission Asynchronous transmission is so named because the timing of a signal is unimportant. Instead, information is received and translated by agreed-upon patterns. As long as those patterns are followed, the receiving d
15、evice can retrieve the information without regard to the rhythm in which it is sent. Patterns are based on grouping the bit stream into bytes. Each group, usually eight bits, is sent along the link as a unit. The sending system handles each group independently, relaying it to the link whenever ready
16、, without regard to a timer. Without a synchronizing pulse, the receiver cannot use timing to predict when the next group will arrive. To alert the receiver to the arrival of a new group, therefore, an extra bit is added to the beginning of each byte. This bit, usually a 0, is called the start bit.
17、To let the receiver know that the byte is finished, one or more additional bits are appended to the end of the byte. These bits, usually 1s, are called stop bits. By this method, each byte is increased in size to at least 10 bits, of which 8 are information and 2 or more are signals to the receiver.
18、 In addition, the transmission of each byte may then be followed by a gap of varying duration. This gap can be represented either by an idle channel or by a stream of additional stop bits. In asynchronous transmission we send one start bit (0) at the beginning and one or more stop bits (1s) at the e
19、nd of each byte. There may be a gap between each byte. The start and stop bits and the gap alert the receiver to the beginning and end of each byte and allow it to synchronize with the data stream. This mechanism is called asynchronous because, at the byte level, sender and receiver do not have to b
20、e synchronized. But within each byte, the receiver must still be synchronized with the incoming bit stream. This is, some synchronization is required, but only for the duration of a single byte. The receiving device resynchronizes at the onset of each new byte. When the receiver detects a start bit, it sets a timer and begins counting bits as they come in. after n bits the receiver looks for a stop bit. As soon as it detects the stop bit, it ignores any received pulses until it detects the next start bit.
