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Signals and Protocols
Computers can communicate over a network in many ways and for many reasons, but a great deal that goes on in the networking process is unconcerned with the nature of the data passing over the network medium. By the time the data generated by the transmitting computer reaches the cable or other medium, it has been reduced to
signals that are native to that medium. These might be electrical voltages for a copper cable network, pulses of light for fiber optic, or infrared or radio waves. These signals form a code that the network interface in each receiving computer converts back into the binary data understood by the softwarerunning on that computer. The computer then interprets the binary code intoinformation it can use in a variety of ways. Of course there is a great deal more to this process than this description indicates, and there is a lot going on to make it possible for the e-mail you just sent to your mother to get reduced to electrical voltages, transmitted halfway across the country, and then reconstituted into text on her computer.
In some cases, a network consists of identical computers running the same version of the same operating system and using all the same applications, whereas other networks consist of many different computing platforms running entirely different software. It might seem that it would be easier for the identical computers to communicate than it would be for the different ones, and in some ways it is. But no matter what kind of computers the network uses and what software the computers are running, they must have a common language to understand each other. These common languages are called
protocols, and computers use many of them during even the simplest exchanges of network data. Just as two people must speak a common language to communicate, two computers must have one or more protocols in common to exchange data.
A network protocol can be relatively simple or highly complex. In some cases, a protocol is simply a code—such as a pattern of electrical voltages—that defines the binary value of a bit of data: 0 or 1. The concept is the same as that of Morse code, in which a pattern of dots and dashes represents a letter of the alphabet. More complicated networking protocols can provide a variety of services, including the following:
- <LI class=SmartList1>Packet acknowledgment. This is the transmission of a return message by the recipient to verify the receipt of a packet or packets. A packet is the fundamental unit of data transmitted over a LAN. <LI class=SmartList1>Segmentation. This is the division of a lengthy data stream into segments sufficiently small for transmission over the network inside packets. <LI class=SmartList1>Flow control. This is the generation by a receiving system of messages that instruct the sending system to speed up or slow down its rate of transmission. <LI class=SmartList1>Error detection. This is the inclusion of special codes in a packet that the receiving system uses to verify that the content of the packet wasn't damaged in transit. <LI class=SmartList1>Error correction. This is the generation by a receiving system of mess-ages that informs the sender that specific packets were damaged and must be retransmitted. <LI class=SmartList1>Data compression. This is a mechanism for reducing the amount of data transmitted over a network by eliminating redundant information.
- Data encryption. This is a mechanism for protecting the data transmitted over a network by encrypting it using a key already known by the receiving system.
In most cases, protocols are based on public standards developed by an independent committee, not a single manufacturer or developer. These public standards ensure that different types of systems can use them without incurring any obligation to a particular company. There are still a few protocols, however, that are proprietary, having been developed by a single company and never released into the public domain.
One of the most important things to remember about networking is that every computer on a network uses many different protocols during the communications process. The functions provided by the various protocols are divided into the layers that make up the Open Systems Interconnection (OSI) reference model, described in Lesson 2, later in this chapter. You might see references to Ethernet networks in books and articles, for example. Ethernet is a protocol running on those networks, but it is not the only protocol running on them. Ethernet is, however, the only protocol running at one particular layer (called the data-link layer). Some layers, however, can have multiple protocols running on them simultaneously.
Protocol Interaction
The protocols operating at the various OSI layers are often referred to as a
protocol stack. The protocols running on a networked computer work together to provide all of the services required by a particular application. Generally speaking, the services provided by the protocols are not redundant. If, for example, a protocol at one layer provides a particular service, the protocols at the other layers do not provide exactly the same service. Protocols at adjacent layers in the stack provide services to each other, depending on the direction in which the data is flowing. As illustrated in Figure 1.1, the data on a transmitting system originates in an application at the top of the protocol stack and works its way down through the layers. Each protocol provides a service to a protocol operating at the layer below it. At the bottom of the protocol stack is the network medium itself, which carries the data to another computer on the network.
Figure 1.1 The networking protocols running on a computer form a layered stack, with each protocol providing services to the protocol operating at the layer above or below it, depending on the direction of data flow
When the data arrives at its destination, the receiving computer performs the same procedure as the transmitting computer, except in reverse. The data is passed up through the layers to the receiving application, with each protocol providing an equivalent service to the protocol in the layer above it. For example, if a protocol at layer three on the transmitting computer is responsible for encrypting data, the same protocol at layer three of the receiving system is responsible for decrypting it. In this way, protocols at the various layers in the transmitting system communicate with their equivalent protocols operating at the same layer in the receiving system, as illustrated in Figure 1.2.
Figure 1.2 Protocols operating at the same layer in the stack on different systems can be said to communicate indirectly by providing complementary services
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آموزش Network + 
قسمت اول lessen 1 
