There must be a data path between the two computers, either directly or via a communication network. But more is needed. Typical tasks to be performed are :
1. The source system must either activate the direct data communication path or inform the communication network of the identity of the desired destination system.
2. The source system must ascertain that the destination system is prepared to receive data.
3. The file transfer application on the source system must ascertain that the file management program on the destination system is prepared to accept and store the file for this particular user.
4. If the file formats used on the two systems are incompatible, one or the other system must perform a format translation function.
It is clear that there must be a high degree of cooperation between the two computer systems. The exchange of information between computers for the purpose of cooperative action is generally referred to as computer communications. Similarly, when two or more computers are interconnected via a communication network, the set of computer stations is referred to as a computer network. Because a similar level of
cooperation is required between a user at a terminal and one at a computer, these terms are often used when some of the communicating entities are terminals.
In discussing computer communications and computer networks, two concepts are paramount:
1.Protocols
2.Computer-communications architecture, or protocol architecture
A protocol is used for communication between entities in different systems.
The terms "entity" and "system" are used in a very general sense. Examples of entities are user application programs, file transfer packages, data-base management systems, electronic mail facilities, and terminals. Examples of systems are computers, terminals, and remote sensors. Note that in some cases the entity and
the system in which it resides are coextensive (e.g., terminals). In general, an entity is anything capable of sending or receiving information, and a system is a physically distinct object that contains one or more entities. For two entities to communicate successfully, they must "speak the same language." What is communicated, how it is communicated, and when it is communicated must conform to some mutually acceptable conventions between the entities involved. The conventions are referred to as a protocol, which may be defined as a set of rules governing the exchange of data between two entities. The key elements of a protocol are
- Syntax. Includes such things as data format and signal levels.
- Semantics. Includes control information for coordination and error handling.
- Timing. Includes speed matching and sequencing.
Having introduced the concept of a protocol, we can now introduce the concept of a protocol architecture. It is clear that there must be a high degree of cooperation between the two computers. Instead of implementing the logic for this as a single module, the task is broken up into subtasks, each of which is implemented separately.
The file transfer module contains all of the logic that is unique to the file transfer application, such as transmitting passwords, file commands, and file records. There is a need to transmit these files and commands reliably. However, the same sorts of reliability requirements are relevant to a variety of applications ( e g , electronic mail, document transfer). Therefore, these requirements are met by a separate communications
service module that can be used by a variety of applications. The communications service module is concerned with assuring that the two computer systems are active and ready for data transfer and for keeping track of the data that are being exchanged to assure delivery. However, these tasks are independent of the type of network that is being used. Therefore, the logic for actually dealing with the network is separated out into a separate network access module. That way, if the network to be used is changed, only the network access module is affected. Thus, instead of a single module for performing communications, there is a structured set of modules that implements the communications function. That structure is referred to as a protocol architecture. In the remainder of this section, we generalize the preceding example to present a simplified protocol architecture. Following that, we look at more complex, real-world examples: TCPIIP and OSI.
A Three-Layer Model
In very general terms, communications can be said to involve three agents: applications,computers, and networks. One example of an application is a file transfer operation. These applications execute on computers that can often support multiple simultaneous applications. Computers are connected to networks, and the data to be exchanged are transferred by the network from one computer to another. Thus,the transfer of data from one application to another involves first getting the data to the computer in which the application resides and then getting it to the intended application within the computer.
With these concepts in mind, it appears natural to organize the communication task into three relatively independent layers:
1.Network access layer
2.Transport layer
3.Application layer
The network access layer is concerned with the exchange of data between a computer and the network to which it is attached. The sending computer must provide the network with the address of the destination computer, so that the network may route the data to the appropriate destination. The sending computer may wish to invoke certain services, such as priority, that might be provided by the network. The specific software used at this layer depends on the type of network to be used; different standards have been developed for circuit switching, packet switching, local area networks, and others. Thus, it makes sense to separate those functions having to do with network access into a separate layer. By doing this, the remainder of the communications software, above the network access layer, need not be concerned with the specifics of the network to be used. The same higher-layer software should function properly regardless of the particular network to which the computer is attached.
Regardless of the nature of the applications that are exchanging data, there is usually a requirement that data be exchanged reliably. That is, we would like to be assured that all of the data arrive at the destination application and that the data arrive in the same order in which they were sent. As we shall see, the mechanisms
for providing reliability are essentially independent of the nature of the applications.
Thus, it makes sense to collect those mechanisms in a common layer shared by all applications; this is referred to as the transport layer. Finally, the application layer contains the logic needed to support the various
user applications. For each different type of application, such as file transfer, a separate module is needed that is peculiar to that application.
Let us trace a simple operation. Suppose that an application, associated with SAP 1 at computer A, wishes to send a message to another application, associated with SAP 2 at computer B. The application at A hands the message over to its transport layer with instructions to send it to SAP 2 on computer B. The transport layer
hands the message over to the network access layer, which instructs the network to send the message to computer B. Note that the network need not be told the identity of the destination service access point. All that it needs to know is that the data are intended for computer B.
Let us say that the sending application generates a block of data and passes this to the transport layer. The transport layer may break this block into two smaller pieces to make it more manageable. To each of these pieces the transport layer appends a transport header, containing protocol control information. The combination of data from the next higher layer and control information is known as a protocol data unit (PDU); in this case, it is referred to as a transport protocol data unit.
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