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君子可内敛而不可懦弱，面不公可起而论之。
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Chapter 1 Overview

#计网124
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This passage provides a summary of the first chapter of a book about computer networks. It discusses packet switching technology, important performance indicators, network architecture and terminology, applications and negative impacts of computer networks, and the overview of the internet. It also explains the three stages of internet development and the role of standardization. The passage further explains circuit switching and packet switching methods, the classification of computer networks, and various performance metrics. It concludes by discussing utilization rate and packet loss rate as metrics for network performance and how they indicate congestion.

Chapter Highlights#

(1) Packet switching based on store-and-forward technology.

(2) Important performance metrics of computer networks (bandwidth, delay, round-trip time, utilization, etc.).

(3) Architecture and related terminology of computer networks.

1.1 Computer Networks in the Information Age#

1.1.1 Various Applications of Computer Networks#

1.1 Common Applications of Computer Networks

1.1 Common Applications of Computer Networks

1.2 Negative Impact of Computer Networks

1.2 Negative Impact of Computer Networks

1.2 Overview of the Internet#

1.2.1 Network, Internet, and the Internet#

1. Network#

  • A network is composed of multiple nodes and the links that connect these nodes, as shown in Figure 1.3. The nodes in a network can be computers (laptops, desktops, servers, etc.), network devices (hubs, switches, routers, etc.), or other devices with network functionality (network printers, network cameras, IoT devices, etc.). The links in a network can be wired or wireless.
    image

2. Internet#

  • The Internet is composed of multiple networks and the routers that connect these networks. If we ignore the interconnections, the Internet can be seen as a larger network, hence it can also be called a "Network of Networks".

3. Internet#

  • The various communication devices (such as smartphones, tablets, laptops, servers, network printers, etc.) connected to the Internet are called hosts, while routers are dedicated devices used for network interconnection and are generally not referred to as hosts.
    image
  • In summary, we can summarize the differences and relationships between networks, the Internet, and the Internet as follows:
       Networks are interconnected by nodes and links, multiple networks are interconnected by routers to form the Internet, and the Internet is the largest network in the world today.
  • Note:
    • internet means the internet and is a generic term referring to a network formed by interconnecting multiple computer networks, which can use any communication protocol as the communication rule between these networks.
    • Internet refers to the Internet and is a specific term that specifically refers to the largest and most open computer network in the world, which is formed by interconnecting numerous networks and routers. The communication rule between these networks must use the TCP/IP protocol suite.

1.2.2 Development of the Internet#

1. Three Stages of Internet Development#

First Stage (1969)

  • The first packet-switched network ARPANET was established, which developed from a single network ARPANET to the Internet.
  • In the mid-1970s, research on interconnecting multiple networks.
  • In 1983, the TCP/IP protocol became the standard protocol for ARPANET, marking the birth of the Internet.

Second Stage (1985)

  • In 1985, NSF built NSFNET (backbone network, regional networks, and campus networks) around six large computer centers.
  • In 1990, the ARPANET mission was completed and officially shut down.
  • In 1991, the U.S. government handed over the operation of the Internet backbone network to private companies and began charging units connected to the Internet.

Third Stage (1993)

  • In 1993, NSFNET was gradually replaced by several commercial Internet backbone networks; government agencies are no longer responsible for Internet operations, which are operated by various Internet service providers (ISPs).
  • In 1994, the World Wide Web (www) technology promoted the rapid development of the Internet.
  • In 1995, NSFNET ceased operation, and the Internet became fully commercialized.

Internet Service Providers (ISPs)

2. Standardization of the Internet#

The standardization work of the Internet plays an important role in its development, and an important characteristic of its standardization is that it is oriented towards the public. The Internet Society (ISO C) is an international organization responsible for the comprehensive management of the Internet and promoting its development and use worldwide. The formal standardization of the Internet goes through the following four stages:

  1. Internet Draft (not yet an RFC document).
  2. Proposed Standard (becomes an RFC document from this stage).
  3. Draft Standard.
  4. Internet Standard.

3. Components of the Internet#

Edge:
  Composed of all the hosts connected to the Internet. This part is used for direct communication (data, audio, and video transmission) and resource sharing.

Core:
  Includes a large number of networks and connections. The routers of these networks form the core, which provides services (connectivity and switching) to the edge.

1.3 Three Switching Methods#

Circuit Switching#

  • Circuit switching refers to the method of connecting telephone lines through telephone exchanges.
  • From the perspective of resource allocation, switching is a way to dynamically allocate transmission line resources.
  • Circuit switching includes three steps:
    1. Connection establishment (resource allocation).
    2. Conversation (continuous occupation of communication resources).
    3. Connection release (returning communication resources).

Circuit Switching

  • The line transmission efficiency of circuit switching is usually low when transmitting computer data.

Packet Switching#

  • Sender: Construct, packetize, and send packets.
  • Router: Buffer and forward packets.
  • Receiver: Receive packets and reconstruct messages.

Packet Switching

Comparison of Circuit Switching, Message Switching, and Packet Switching#

Comparison of Three Switching Methods

1.4 Definition and Classification of Computer Networks#

1 Definition of Computer Networks#

  • There is no unified standard for the exact definition of computer networks.

  • The simplest definition is: A computer network is a collection of interconnected autonomous computers.

    • Interconnected: Refers to computers communicating with each other through wired or wireless means.
    • Autonomous: Refers to independent computers with their own hardware and software, capable of running and operating independently.

  • A more precise definition is: Computer networks are mainly composed of generic, programmable hardware interconnections, which are not specifically designed to achieve specific purposes (such as transmitting data or video signals). These programmable hardware can transmit various types of data and support a wide range of applications that are constantly growing.

    • The hardware connected to computer networks is not limited to general-purpose computers, but also includes smart hardware such as smartphones.
    • Computer networks are not only used for data transmission but also support various applications (including various applications that may appear in the future).

2 Classification of Computer Networks#

  • Classified by switching technology:

    • Circuit-switched network
    • Message-switched network
    • Packet-switched network

  • Classified by users:

    • Public network
    • Private network

  • Classified by transmission medium:

    • Wired network
    • Wireless network

  • Classified by coverage area:

    • Wide Area Network (WAN)
    • Metropolitan Area Network (MAN)
    • Local Area Network (LAN)
    • Personal Area Network (PAN)

  • Classified by topology:

    • Bus network
    • Star network
    • Ring network
    • Mesh network

1.5 Performance Metrics of Computer Networks#

  • The performance of computer networks can be measured from multiple aspects, and the commonly used performance metrics are as follows:

1. Rate:#

  • A bit is the unit of data quantity in a computer and also the unit of information quantity in information theory. A bit is a binary digit, either 1 or 0.
  • Rate refers to the speed at which hosts connected to a computer network transmit bits over a digital channel, also known as bit rate or data rate.

Rate

2. Bandwidth:#

  • In analog signal systems, bandwidth represents the frequency range occupied by various frequency components in a signal. Unit: Hz (KHz, MHz, GHz).
  • In computer networks, bandwidth represents the capacity of a network communication line to transmit data, that is, the "maximum data transfer rate" from one point to another in a unit of time.
  • In fact, there is a close relationship between the two descriptions of "bandwidth". The wider the "frequency bandwidth" of a communication line, the higher the "maximum data rate" of the data it transmits.

3. Throughput:#

  • Throughput represents the amount of data passing through a network (or channel/interface) per unit of time.
  • Throughput is limited by the bandwidth or rated speed of the network.

4. Delay:#

  • The time it takes for data to travel from the start of the network to its destination.

Delay

  • Rate, Bandwidth, Throughput, and Delay

Summary

5. Delay-Bandwidth Product:#

  • Delay-Bandwidth Product = Propagation Delay x Bandwidth.
  • If the sender continuously sends data, by the time the first bit of the sent data is about to reach the destination, the sender has already sent a number of bits equal to the delay-bandwidth product.
  • The delay-bandwidth product of a link is also known as the "bit-based link length".

6. Round-Trip Time (RTT):#

  • The time it takes for information to travel bidirectionally on the Internet, also an important performance metric.

Round-Trip Time

7. Utilization:#

  • Channel utilization represents the proportion of time a channel is utilized.
  • Network utilization is the weighted average of the channel utilization of the entire network.

Utilization

8. Packet Loss Rate:#

  • The ratio of the number of lost packets to the total number of packets transmitted during transmission, reflecting the network congestion situation.
  • Packet loss rate can be divided into different types, including interface packet loss rate, node packet loss rate, link packet loss rate, path packet loss rate, network packet loss rate, etc.
  • Packets can be lost in two main situations:
    1. Packets are discarded by nodes due to errors during transmission: During transmission, packets may be corrupted due to noise, interference, etc., which prevents nodes from correctly decoding the packets and therefore discards them.
    2. Packets are discarded when they reach a full packet-switching device: If the transmission rate exceeds the processing capacity of a switch, incoming packets may be queued in the switch's buffer for processing. If the buffer is full, new packets will be discarded. In high communication loads, this situation can lead to network congestion.
  • The packet loss rate can reflect the degree of network congestion, and different packet loss rates correspond to different congestion situations:
    • The path packet loss rate is 0 when there is no congestion.
    • The path packet loss rate is 1% to 4% during mild congestion.
    • The path packet loss rate is 5% to 15% during severe congestion.
  • Delay-Bandwidth Product, Round-Trip Time, Utilization, and Packet Loss Rate

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