1.1 The role of computer networks in the information age

  • Network refers to the “three networks”, namely telecom network, cable TV network and computer network.
  • It has become the world’s second largest network after the global telephone network

1.2 the Internet

1.2.1 overview

  1. Node: computer, hub, router, network, etc
  2. Network: Consists of a number of nodes and links connecting these nodes
  3. Internet (network of networks) : Networks are connected by routers to form a larger network
  4. Internet: The world’s largest Internet, the computer connected to the Internet is called the mainframe

The network connects many computers together, and the Internet connects many networks together!

1.2.2 Development stages of the Internet

  1. From a single network ARPANENT to the Internet
  2. The Internet consists of three levels: backbone, regional and campus
  3. The Internet has become a multi-level ISP structure
    • ISP: Internet service provider, such as China Telecom and China Mobile
    • According to the service coverage and IP address number, ISPs are divided into three levels: trunk ISP, regional ISP and local ISP
    • There are five levels of access to the Internet today:
      • Network access point NAP
      • National Backbone Network (Backbone ISP)
      • In the ISP
      • Local ISP
      • Campus network, enterprise network or PC Internet access users

1.2.3 Standardization of Internet

The development of formal standards for the Internet goes through four stages:

  • Internet Draft — Not an RFC document at this stage.
  • Proposed Standards — an RFC document from this stage.
  • Draft Standards
  • Internet Stanard

1.3 Composition of the Internet

The structure of network includes network edge, network core and access network

1.3.1 Network Edge

Above we learned what hosts are, and because they are at the edge of the Internet, they are called end systems.

  • Host = end system
  • Hosts are sometimes further divided into two categories: clients and servers

1.3.2 access network

A network that physically connects end systems to their edge routers

1.3.3 Network core

In the network core of the Internet, the router plays a special role. The router is a key building block for packet switching, and its job is to forward received packets.

  1. Circuit switching

    Two telephones can be connected to each other only by a pair of wires.

    N telephones are connected in pairs with N(n-1)/2 pairs of wires. When the number of telephones is large, the number of wire pairs required by this connection is proportional to the square of the number of telephones.

    Circuit switching features:

    • Circuit switching must be connection-oriented.
    • There are three stages of circuit switching: establishing a connection, communicating, and releasing a connection

    Disadvantages of circuit switching:

    • Computer data is emergent.
    • This results in low utilization of communication lines.

  2. Packet switching

    As the number of telephones increases, switches are used to complete the task of switching across the network.

    Meaning of switching:

    • Transfer – To connect telephone lines by transferring them from one telephone line to another.
    • From the perspective of communication resource allocation, “switching” is the dynamic allocation of transmission line resources in a certain way.

    Packet switching principle:

    • At the sending end, the packet is divided into shorter and fixed-length data segments.

    • A header is added before each data segment to form a group.

      • The header of each group contains control information such as the address.
      • The node switch in the packet switching network forwards the packet to the next node switch according to the address information in the header of the received packet.
      • With this store and forward method, the final group can reach the final destination.

    • Packet switching networks use “packets” as data transmission units.

    • Each packet is sent in turn to the receiver (assuming the receiver is on the left).

    • After receiving the packet, the receiving end strips the header and restores the packet.

    Advantages:

    • Efficient dynamic distribution of transmission bandwidth, the communication link is occupied segment by segment.
    • Flexibility in grouping as transmission unit and routing lookup.
    • Quickly send packets to other hosts without first establishing a connection; Fully use the bandwidth of the link.
    • Reliable and perfect network protocol; The adaptive routing protocol makes the network have good survivability.

    Disadvantages:

    • Queuing is required when grouping nodes are stored and forwarded, which will cause a certain delay.

    • The header that the packet must carry, which contains the essential control information, also imposes some overhead.

1.3 Classification of computer networks

  1. Different definitions of computer networks:

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

  2. From the network switching function classification:

    • Circuit switching
    • Message switching
    • Packet switching
    • Mixed exchange

  3. Classify from the scope of the network

    • WAN (Wide Area Network)
    • Local Area Network (LAN)
    • MAN (Metropolitan Area Network)
    • Access Network (AN)

  4. By the users of the network:

    • Public Network
    • Private Network

1.4 Main performance indicators of computer network

  1. Bandwidth: Synonymous with the highest data rate that a digital channel can transmit, in bits per second, or B /s (bit/s).

    • Kilobits per second, kilobits per second (10^3 b/s).
    • Megabits per second, megabits per second (10^6 b/s)
    • Gigabits per second, which is gigabits per second (10^9 b/s).
    • Terabytes per second, which is 10 to the 12th b per second.

    Note:

    • In computing, K = 2^10 = 1024, M = 2^20, G = 2^30, T = 2^40.
    • The width of the signal Narrows with the increase of bandwidth on the time axis.

  2. Delay or latency

    Total delay = transmission delay + propagation delay + processing delay

    • Transmission delay (Transmission delay) The time required for data blocks to enter the transmission media from nodes during data transmission.
      • Channel bandwidth The rate at which data is transmitted over a channel. Often called the rate at which data is transmitted over a channel.

    • Propagation delay The time it takes for an electromagnetic wave to travel a certain distance in a channel.

    • Processing The time it takes a delay switching node to do some of the necessary processing for store and forward.
      • The delay experienced by packet queuing in node cache queue is an important part of processing delay.
      • The length of the processing delay often depends on the amount of traffic in the network at the time.
      • Sometimes the queuing delay is used as the processing delay.

    Round-trip delay RTT

    Round-trip Time (RTT) Indicates the total delay from the Time when the sender sends data to the Time when the sender receives an acknowledgement from the receiver (the receiver sends an acknowledgement immediately after receiving data).

1.5 Computer network architecture

  1. A five-tier protocol architecture

    Computer 1 sends data to computer 2:

    Benefits of layering:

    • The layers are independent of each other.
    • Good flexibility.
    • Structurally separable.
    • Easy to implement and maintain.
    • Can facilitate standardization work.

  2. Entities, protocols, services, and service access points

    • An entity is any hardware or software process that can send or receive information.
    • A protocol is a collection of rules that control the communication between two peer entities.
    • Under the control of the protocol, communication between two peer entities enables this layer to provide services to the next layer.
    • To implement this layer protocol, you also need to use the services provided by the lower layer.
    • Service users in this layer see only the service and not the underlying protocol.
    • The protocol below is transparent to the service users above.
    • The protocol is “horizontal”, that is, the protocol is the rule that controls communication between peer entities.
    • The service is “vertical”, that is, the service is provided from the lower layer to the upper layer through the layer interface.
    • A Service Access Point (SAP) is a place where entities at two adjacent layers of a system interact.

  3. Connection-oriented and connectionless services

    • Connection-oriented Services
      • The connection-oriented service has three phases: connection establishment, data transfer and connection release.
    • Connectionless service (connectionless)
      • Communication between two entities does not require a connection to be established.
      • It’s an unreliable service. The service is often described as “best effort delivery” or “best effort.”

  4. OSI versus TCP/IP architectures