Computer Network Notes - Computer Network Architecture

1. Computer network architecture

1.1 Overview of Computer Networks

1.1.1 Concept of computer network

Simple definition: A computer network is a collection of interconnected, autonomous computer systems.

Broad view: A computer network is a system that can realize remote information processing or a system that can further achieve resource sharing.

Resource sharing perspective: A computer network is a collection of autonomous computer systems interconnected in a way that can share resources with each other.

User transparency perspective: A computer network is a network operating system that automatically manages resources for users. It can call the resources that users need. The entire network is as transparent to users as a large computer system.

Autonomous Computer: A computer capable of managing, configuring, and maintaining itself

1.1.2 Computer network composition

Physical composition

Computer network: three parts: hardware, software, and protocols.

Hardware: host + communication processor + communication line + switching equipment.

Software: including software for resource sharing and various tool software that are convenient for users to use.

Protocol: Rules, standards established for the exchange of data in a network.

Working style composition

Computer Network: Edge Part, Core Part

Edge part: consists of all hosts connected to the Internet for direct use by users for communication and resource sharing.

Core part: It consists of a large number of networks and routers connecting these networks, providing connectivity and switching services to the edge part.

Functional composition

Computer network: communication subnet, resource subnet

Communication subnet: It is composed of various transmission media, communication equipment and corresponding network protocols. It provides data transmission, exchange and control capabilities for the network and realizes data communication between networked computers.

Resource subnet: It consists of the main terminal and various software resources and information resources. It is responsible for the data processing business of the entire network and provides users with various network resources and services.

1.1.3 Functions of computer networks

• data communication:The most basic and important functions. Including connection control, transmission control, error control, flow control, routing, multiplexing and other functions.
• Resource sharing: including data resources, software resources and hardware resources.
• Distributed processing: When the computer system is overloaded, the processing tasks are transferred to other computer systems in the network for processing, and idle computer resources are used to improve the utilization of the entire system.
• Comprehensive information processing: centralized or hierarchical processing of data scattered in computers around the world.
• Load balancing: distribute work tasks evenly to each computer in the computer network.
• Improve reliability: Each computer in the computer network can replace each other through the network.

1.1.4 Classification of computer networks

• Distribution scope: wide area network, metropolitan area network, local area network, personal area network
• Topology: star network, bus network, ring network, mesh network
• Transmission technology: broadcast network, point-to-point network
• User: public network, private network
• Data switching technology: circuit switching network, message switching network, packet switching network

1.1.5 Computer network standardization public work and related organizations

Standardization steps: Internet drafts, proposed standards, draft standards, Internet standards

Related organizations: ISO, ITU, IEE

1.2 Computer network architecture and reference model

1.2.1 Computer network hierarchical structure

Why layer?

When the system structure is large, layering can better implement the management mechanism and improve the efficiency of the entire system. But don’t layer too much, otherwise it will cause a waste of resources.

• Entity: Any hardware or software process that can send or receive information. An entity is a specific software module.

• Peer layer: The same layer on different machines.

• Peer entities: Entities on the same layer.

1.2.2 Agreement

A rule that controls how two peer entities communicate,is horizontal

Protocol components: semantics, syntax, synchronization.

• Semantics: the interpretation of the meaning of the elements that make up the protocol,say what
• Syntax: the structure or format of data and control information,how to say
• Synchronization: Specifies the execution sequence of events.

1.2.3 Interface

Interfaces are also called service access points. Starting from the physical layer, each layer provides service access points upwards, that is, services cannot be provided without interfaces.

• Service data unit (SDU): Service data unit of layer n - n-SDU
• Protocol Control Information (PCI): Protocol control information for layer n - n-PCI
• Interface control information (ICI): Interface control information for layer n - n-ICI
• Protocol data unit (PDU): n-th layer service data unit + n-th layer protocol control information, n-SDU+n-PCI=n-PDU, n-PDU=(n-1)-SDU,Data unit transmitted between peer entities at the same layer
• Interface data unit (IDU): n-th layer service data unit + n-th layer interface control information, n-SDU+n-ICI=n-IDU,Data unit transmitted between adjacent interfaces

1.2.4 Services

The lower layer calls functions provided by the upper layer.Services are vertical

Those functions that are "visible" to higher-level entities are called services.

• The nth layer entity must not only use the services of layer n-1, but also provide services of this layer to layer n+1. This service isThe sum of services provided by the nth layer and the layers below it. The top level provides services to users.
• The upper layer can only use the services of the lower layer through the interface of the adjacent layer, but cannot call the services of other layers. That is, the implementation details of the services provided by the lower layer are transparent to the upper layer.

Transparency: Users only know what functions are available, but not how to implement them.

connection-oriented services

When two communicating parties communicate, a communication line is established in advance: establishing a connection, using the connection, and releasing the connection.

advantage:

• Reliable information flow, information reply confirmation

shortcoming:

• Occupy communication channel

Connectionless oriented services

When the two parties communicate, there is no need to establish a communication line in advance. Instead, each packet (message group) with a destination address is transmitted to the line, and the system selects a route for transmission.

advantage:

• Does not occupy communication channels

shortcoming:

• The information flow may be lost and the information may not be confirmed by reply.

There is an answering service

After receiving the data, the receiver gives the corresponding response to the sender.

no answering service

The receiver does not automatically respond after receiving the data.

Reliable service

The network has error detection, error correction, and response mechanisms to ensure that data is transmitted correctly and reliably to the destination.

unreliable service

The network cannot guarantee that the data is transmitted to the destination correctly and reliably. The network can only be as correct and reliable as possible.

1.2.5 ISO/OSI reference model and TCP/IP model

5-layer structural model

• Application layer (user to user)
  • Task: Provide an interface between the system and users
  • Functions: File transfer, access and management, email services
  • Protocols: FTP, SMTP, POP3, HTTP
• Transport layer (application to application, process to process)
  • Transmission unit: message segment (TCP) or user datagram (UDP)
  • Task: Responsible for communication between two processes in the host
  • Function
    • A service that provides reliable transport for end-to-end connections
    • Provide management services such as flow control, error control, and service quality for end-to-end connections.
  • Protocol: TCP, UDP, ARQ
• Network layer (host to host)
  • Transmission unit: datagram
  • Hardware implemented: Router
  • Task
    • Encapsulate the message segments transmitted from the transport layer into packets
    • Select appropriate routes so that packets from the transport layer can be delivered to the destination host
  • Function
    • Provide services for the transport layer
    • Packing and unpacking
    • Routing
    • congestion control
  • Protocols: ICMP, ARP, RARP, IP, IGMP
• Data Link Layer (Link Layer)
  • Transmission unit: frame
  • Hardware implemented: switches, bridges
  • Task: Assemble IP datagrams transmitted from the network layer into frames
  • Function
    • Link connection establishment, teardown, detachment
    • Frame delimitation and frame synchronization
    • error detection
  • Agreement: PPP, HDLC
• physical layer
  • Transmission unit: bit
  • Hardware implemented: hub, repeater
  • Mission: Transparently transmit bitstream
  • Function: Provide data transmission path for data terminal equipment

Replenish

OSI model

• Application layer
• presentation layer
• session layer
• transport layer
• Network layer
• data link layer
• physical layer

TCP/IP architecture

• Application layer

• transport layer

• Network layer

• data link layer

• physical layer

• session layer

  • Function: Establish, maintain and release user-oriented connections between two nodes, manage and control sessions, and ensure reliable transmission of session data.

• presentation layer

  • Function: Responsible for processing the representation format of information exchanged between two communication systems with different internal data representation structures (Data format conversion), providing necessary data compression and decompression functions for data encryption and decryption as well as improving transmission efficiency.

Advantages of layering

1. The layers are independent of each other
2. Good flexibility
3. structurally separable
4. Easy to implement and maintain
5. Can promote standardization work

The difference between OSI model and TCP/IP model

3 main concepts: service, interface, protocol.
The protocol is well hidden.
It was produced before the invention of the protocol.
There are 7 layers in total.
Network layer: connection and connectionless.
Transport layer: only connection-oriented.

There is no clear distinction between services, interfaces, and protocols. It
was created after the invention of the protocol.
There are 4 layers (not 5) in total.

Network layer: only connectionless
Transport layer: connection-oriented and connectionless

1.2.6 Computer network performance indicators

• Latency: The time it takes for data to travel from one end of a network or link to the other, sometimes called delay or delay.

  • Sending delay (or transmission delay): the time it takes for a host or router to send data, that is, the time it takes from sending the first bit of the data frame to the time the last bit of the frame is sent. Therefore, the sending time transmission delay

    Sending delay = data frame length (bit)/sending rate (bit/s)

  • Propagation delay: the time it takes for electromagnetic waves to propagate a certain distance in the channel.

    Propagation delay = channel length (m)/propagation speed of electromagnetic waves on the channel (m/s)

  • Processing latency: The time it takes for a host or router to process a packet after it is received.

  • Queuing delay. When a packet enters the network for transmission, it has to pass through many paths. However, after the packet enters, it is queued in the input queue to wait for processing. After the router determines the forwarding interface, it also needs to be queued in the output queue to wait for forwarding. This is Queuing delay occurs.

    Total delay = sending delay + propagation delay + processing delay + queuing delay

• Delay-bandwidth product Delay-bandwidth product is also known as the link length in bits.

  Delay-bandwidth product = propagation delay x bandwidth

• Round trip time: The total time elapsed from the time the data is sent to the time the sender receives the confirmation message from the receiver (the receiver sends the confirmation immediately after receiving the data).

• Utilization:Including channel utilization and network utilization

  • Channel utilization refers to how many percent of the time a certain channel is used (with data passing through it). The utilization rate of a completely idle channel is zero.
  • The network utilization is the weighted average of the channel utilization of the entire network. However, it should be noted that the higher the channel utilization rate and network utilization rate, the better, because the higher the utilization rate, the longer the data forwarding delay in the router will be.

Practice questions

The following two conclusions are correct or incorrect!

A network with a bandwidth of 1Mbit/s and a network with a bandwidth of 1kbit/s has a much higher propagation rate of bit streams on the link. (×)

A network with a bandwidth of 1Mbit/s has a much greater data transfer rate than a network with a bandwidth of 1kbit/s. (×)

The wider the bandwidth, the faster the speed at which one bit is sent, rather than the faster the propagation speed of each bit through the data link.

The data transmission rate is determined by the total delay (total delay = sending delay + propagation delay + processing delay + queuing delay), while bandwidth only determines the sending delay