1. The concept of computer networks

Computer network: network for short, consisting of a number of nodes and links connecting these nodes. Nodes in a network can be computers, integrators, switches, or routers.

Networks can also be connected by routers, creating a larger network of computers called the Internet.

The network connects many computers together, while the Internet connects many networks together through routers. The computers connected to the network are often called hosts.

Computer network: is a decentralized, independent function of the computer system, through communication equipment and lines connected, by the functional software to achieve resource sharing and information transfer system.

It is important to note that there are many categories of computer networks:

(1) According to the scope of the network, it is classified: WAN, MAN, LAN, PAN.

(2) Network is classified according to users: public network and private network

2, computer network performance

Computer network performance generally refers to its several important performance indicators, as follows:

Rate of 2.1

Rate is the transmission rate of exponential data, also known as data rate or bit rate. Speed is one of the most important performance indexes in computer network.

2.2 the bandwidth

In computer networks, network bandwidth is the amount of data that can be transmitted in a unit of time (usually 1 second). For example, your telecommunications network is 100 megabits, meaning that the maximum transmission rate in a second is 100 megabits.

2.3 throughput

Throughput represents the amount of data passing through a network (or channel, interface) per unit of time.

Above three points, let’s take an example

  • A road can pass at most 100 cars per second (broadband is the equivalent of 100 cars per second).
  • And it’s not like 100 cars pass every second, if there’s 0 in the first second and 10 in the second… (But not more than 100 cars).
  • So we have 0 cars per second in 1 second, 10 cars per second in 2 seconds, 30 cars per second in 3 seconds, and we can’t say how much bandwidth there is, so we use throughput to say how much bandwidth there is in a given time.
  • Bandwidth refers to the maximum rate and throughput refers to the rate at a certain time. However, the throughput cannot exceed the maximum rate.

2.4 time delay

Delay is the time it takes for exponential data (packets/packets/bitstreams) to travel from one end of a network (or link) to the other. It’s in s. There are several kinds of delay:

(1) Transmission delay

It means that the time between when I’m talking to you, when I’m talking to you, and when I’m talking to you, that’s the transmission delay.

(2) Propagation delay

As shown in the GIF, the propagation delay is the time required from the first bit on the channel to the last bit reaching the host interface.

(3) Queuing delay

  • Packets pass through many routers as they travel over the network.

  • However, packets must queue up in the input queue for processing after entering the router.

  • After the router determines the forwarding interface, it has to queue up in the output queue to wait for forwarding, resulting in queuing delay.

  • The length of queuing delay often depends on the traffic of the network at that time. When the traffic of the network is large, queuing overflow will occur, which is packet loss.

(4) Processing delay

When a router or host receives a packet, it takes a certain amount of time to process it, such as analyzing the header of the packet, conducting the header error check, searching the routing table to select the quasi-sending interface for the packet, which results in processing delay.

(5) Round trip Time (RTT)

In computer networks, round-trip time is also an important performance metric, representing the total time elapsed between the time the sender sends data and the time the sender receives an acknowledgement from the receiver (which is sent immediately after receiving the data)

(6) Delay bandwidth product

It’s the propagation delay times the bandwidth

3. Computer network architecture



Formation of computer network architecture In order to make different architecture of the computer network can be interconnected, the international organization for Standardization ISO set up a special agency in 1977 to study this problem. They came up with a standard framework for trying to connect computers around the world, known as the Open System Interconnection Reference Model (OSI) or OSI. It seemed certain that the world would transform its computer networks to OSI standards in the near future.

By the early 1990s, however, the TCP/IP Internet was running successfully on a considerable scale around the world. As a result, today’s largest, worldwide TCP/ IP-based Internet does not use the OSI standard. OSI de facto INTERNATIONAL standard TCP/IP OSI and TCP/IP the structure and function of each layer has a five-layer protocol architecture OSI seven-layer protocol architecture is clear in concept and complete in theory, but it is both complex and impractical.

TCP/IP is a four-tier architecture, which has been widely used. When learning the principle of computer network, we often adopt a compromise method, integrating the advantages of OSI/IP and adopting a five-layer protocol architecture.

 

A. Application Layer The application layer is the highest level in the architecture. The task of the application layer is to complete a specific network application through the interaction between application processes. Application layer protocols define rules for communication and interaction between application processes. The process here is the program that is running on the host. No different application layer protocols are required for network applications that do not listen. There are many protocols in the Internet application layer, such as domain name DNS, HTTP protocol to support world Wide Web applications, SMTP protocol to support email, and so on. The data units at the application layer are called messgae.

B. Transport Layer The transport layer is responsible for providing a common data transfer service for communication between processes on two hosts. The application process uses the application-layer packets sent by the service. By generic, it means that multiple applications can use the same transport layer service, not for a specific network application. Because a host can run multiple processes at the same time, the transport layer has the functions of reuse and reuse. Reuse means that multiple application layer processes can simultaneously use the services of the lower transport layer. In contrast to reuse, the transport layer delivers the received information to the related processes of the upper application layer.

The transport layer mainly has the following two protocols:

• Transmission Control Protocol (TCP)- Provides a connection-oriented and reliable data Transmission service. The unit of data Transmission is segment.

• User Datagram (UDP). Protocol – provides a connectionless, best-effort data transfer service (without guaranteeing the reliability of data transmission) in the form of user datagrams. The differences between TCP and UDP protocols (as shown below)

C. Network layer The network layer provides communication services for different hosts on the packet switching network. When sending data, the network layer encapsulates the packet segments or user datagrams generated by the transport layer into packets or packets for transmission. In TCP/IP architecture, packets are also called IP datagrams, or simply datagrams, because the network layer uses the IP protocol. Another task of the network layer is to select the appropriate route, which is the packet passed down by the source host transport layer and can find the destination host through the router in the network.

D. Data Link layer The data link layer is commonly referred to as the link layer. As we know, data transmission between two hosts is always transmitted over a section of link, which requires the use of a special link layer protocol. When transmitting data between two neighboring nodes, the data link layer encapsulates IP datagrams handed over by the network layer into frames, and transmits frames on the link between the two neighboring nodes. Each frame contains data and necessary control information (such as synchronization information, address information, error information, etc.).

E. Physical Layer The unit of data transmitted at the physical layer is bit. When a sender sends a 1 or a 0, the receiver should receive the same 1 or 0, so the physical layer considers how much voltage represents “1” or “0”, and how the receiver recognizes the bit sent by the sender. The physical layer also determines how many pins the plug connecting the cable should have and how the pins should be connected. It is not, of course, a physical task to interpret what bits stand for.

Reference:

Juejin. Cn/post / 684490…