Computer Network Technology and Application

Chapter 4 A Brief Introduction to Computer Networks (2)
2. Peripherals
On the network, in addition to computers, peripheral devices such as printers that serve users are also required.

3. Network equipment
In order to realize data transmission, network devices are indispensable on the network. The main devices currently used on the Internet include routing devices (responsible for the interconnection between different networks and routing of data transmission), switches (in the local network or communication The subnet provides computer interconnection and fast data transmission), HUB (only provides basic connection between terminal devices, as the price of switches decreases, HUB will be eliminated).

4. Network card
If a computer is connected to the local area network of a school or business, then this computer must be installed with a network card (network adapter), through which the network cable is connected, and data is received and sent.

5. Modem

The modem is what we usually call "Modem", which is generally used in the way of using telephone lines to surf the Internet, such as ADSL, ISDN, etc.

6. Transmission medium
A transmission medium refers to the wires that connect one network device or computer to another, and can be both wired and wireless.For example, the microwave of microwave communication is a wireless method, but at present, the wired method is more common, such as the twisted pair common in student dormitories.The characteristics of each line are different, and the environment used is also different. We will introduce it in detail in Chapter 4.

1.4.1 Understanding LAN
1. Definition of LAN
Local Area Network (LAN), also known as local network, is a computer network that connects scattered computer equipment through transmission equipment within a limited geographical area for high-speed data communication.

Local area network is widely used in enterprise office automation, enterprise management, industrial automation, computer-aided teaching, etc. For example, in an office, multiple computers are connected into a local area network to share printers and databases.

Local area network is also a common access method for us to access the Internet. For example, computers in student dormitories access the Internet through the campus network.

2. Main features of LAN

The main features of LAN are: small geographical range, generally within a dozen kilometers; fast speed; low bit error rate (bit error rate is the ratio of errors in the transmission process); easy to detect faults; easy to build a local area network ; The cost is relatively low.

1.4.2 Basic composition of LAN

A typical local area network consists of data exchange equipment, one or more servers and several workstations.On the one hand, the workstation provides users with local services, which is equivalent to a stand-alone use; on the other hand, the workstation can request services and access resources from the network system through the workstation to realize resource sharing.To set up a local area network, you must have a computer network card, network cable and switching equipment (commonly switches and HUB); if you want to access the Internet, you should also be equipped with a router at the exit.

Equipment and equipment connection lines (network cables).

1.5 Network System and Protocol

This section mainly explains several issues of the system structure, including protocols, protocol layering and related issues, and briefly introduces the system structures of OSI and TCP/IP.

1.5.1 Architecture for implementing an open network
1. Definition of Agreement

The computer systems we come into contact with not only have a variety of hardware configurations, such as CPU, memory, bus, etc. of minicomputers, workstations, and PCs. The settings are different, and the data format and data processing methods are also different.Moreover, the software such as the operating system that is equipped with the computer may also be different. In addition to Microsoft Windows, the commonly used systems currently include Linux and Unix operating systems, and different systems have different processing methods for data.In addition, there are many types of devices in the network, such as switches and routers. The former is responsible for data exchange, while the latter is responsible for path selection. The two have different processing modes for data.

In the face of these differences, it is necessary to establish and use data communication standards. A protocol is a collection of communication rules and agreements that both communication parties abide by.Multiple computer systems in different geographic locations interconnected through communication channels and devices must have a common language in order to make them work together to achieve information exchange and resource sharing.What, how, and when to communicate must follow certain mutually acceptable rules.

2. Composition of the agreement

(1) Semantics

Semantics are descriptions of the process between communicating parties.It specifies what control information needs to be sent, what actions to complete, and what responses to make.

(2) Grammar

Grammar is the overall content of usage rules.It is the format in which several protocol elements and data must be combined to express a complete content.

(3) Timing

Timing is the sequence in which events occur.For example, resources must be applied for first, and resources can only be used after approval.

3. Characteristics of the protocol

(1) Highly structured, sub-module approach.

(2) Divide the whole problem into several sub-problems.

(3) The sub-problems are relatively independent and interrelated, and the protocol is designed in a hierarchical manner.

(4) When designing the protocol, fully consider factors such as network topology, information volume, transmission technology, and data access methods.

4. Layering of architecture

This example illustrates how a complex system performs task division and function layering.

In the process of delivering and distributing letterhead, there are four types of roles: deliverer, post office, transport company, and reader, and their shared responsibilities and tasks are different.If the responsibilities or work of each type of role increases or decreases, it does not affect the responsibilities of other roles.

The entire postal delivery process can be divided into three levels. Different levels perform different tasks, and different levels are responsible for different tasks.

(1) Hierarchical structure of the network
① Entities and peer entities: The active elements of each layer are entities, which can be software, processes or hardware, that is, software and hardware that can communicate are communication entities; peer entities refer to the same layer corresponding to different computer systems or network devices entity.

②Real communication and virtual communication: In the network, the data that actually flows on the physical transmission medium (line) is called real communication; except for the physical medium, the communication between other peer entities is virtual communication, also That is, there is no real data transfer.

③Service data unit: The set of N-layer protocols in the two systems is called the N-layer protocol; the complete information carrier in each layer protocol is called the N-layer protocol data unit (Protocol Data Unit, PDU ).

④Interface and service access point: layer N-1 provides services for layer N, so there are interfaces between layers, and services are provided to the upper layer through the service access point (Service Access Point, SAP).

⑤ Service primitives: The specification statements used by the lower layer to provide services for the upper layer are called service primitives.

(2) The characteristics of the network architecture

① Use function as the basis for dividing levels.

② When the entities at the Nth layer implement their own defined functions, they can only use the services provided by the N-1 layer (lower layer).

③Layer N provides services to layer N+1. This service not only includes the functions of layer N itself, but also includes the functions provided by the lower layer services.

④ There are only interfaces between adjacent layers, and the specific details of the services provided are completely transparent (shielded) to the upper layer.

(3) The principle of network system layering

①The function of each layer should be clear and independent of each other.When the specific implementation method of a certain layer is updated, as long as the interfaces of the upper and lower layers remain unchanged, it will not affect the neighbors.

②The interlayer interface of the protocol must be clear, and the amount of information across the interface should be as small as possible.

③ The number of layers of the agreement should be moderate.If the number of layers is too few, the protocol of each layer will be too complex; if there are too many layers, the architecture will be too complex, making it difficult to describe and realize the functions of each layer.

From the above description, it can be concluded that the architecture is the overall framework and hierarchy, and the protocol is the specific completion of each level of tasks.

1.5.2 OSI architecture
The Open System Interconnection (OSI) basic reference model is a standardized open computer network hierarchy model formulated by the International Organization for Standardization (ISO), also known as the OSI reference model.

The OSI architecture defines a seven-layer protocol model for communication between peer layers, and serves as a framework to coordinate the standards for the functions of each layer; the OSI service definition describes the services provided by each layer, and the layer The abstract interface between layers and the service primitives used for interaction; the protocol specifications of each layer of OSI precisely define what kind of control information should be sent and what process should be used to interpret the control information.

It should be emphasized that the OSI reference model only provides a conceptual framework for the development of standards.But there are some protocols that have been borrowed into other network architectures.

(End of this chapter)