Computer network study notes

Computer network study notes
Chapter 1 Computer Network Overview

1. The three major networks are telecommunications networks, cable television networks and computer networks.

2. Computer networks are interconnected, autonomous, and equal collections of computers, which mainly include hardware, software, and protocols. It consists of several nodes and links and computers. Nodes can be computers, routers, hubs, switches, etc.

3. Functions of computer network: data communication, resource sharing, distributed processing (multiple computers work on different parts of the same task together), improving reliability (one computer cannot go to the next one), and load balancing.

4. The function of the Internet Exchange Point IXP is to allow two networks to connect directly and exchange packets without the need for a third-party intermediary (the third network) to achieve it.

5. NSFNET three-level computer network (backbone network, regional network, campus network (enterprise network)).

6. The first stage of the Internet development process is the process from a single network ARPANET to the Internet. The second stage is characterized by the establishment of a three-tiered Internet. The third stage is characterized by the formation of a three-tiered ISP Internet (current) / Statutory standard OSI de facto standard TCP / IP protocol

7. The architecture of a computer network is a collection of layers of a computer network and their protocols.

8. Formulating formal Internet standards goes through three stages
   Internet Draft-Not a RFC (Request for Comments) Document Proposed Standard-Becomes an RFC Document
   Internet standards-Each standard is assigned a number STD xx, and a standard can be associated with multiple RFC documents.

9. The Internet is divided into the edge part (resource subnet, which is directly used by users) and the core part (communication subnet, which provides services for the edge part). The communication method at the edge is client-server C / S and peer-to-peer connection P2P. There are three types of core switching technologies: circuit switching (nodes do not use the “store-and-forward” method, but send and receive at any time), message exchange, and packet exchange (current). The host is at the edge of the network. Its main role is to process information.
   The router is in the core part, and its main function is to perform packet switching according to the store-and-forward method (divide a message into several groups for transmission). The advantage of packet switching is that it is efficient, flexible, fast, and reliable. The disadvantage is that packets need to be queued when they are stored and forwarded at each node, which will cause a certain delay.

10. Communication between computers refers to communication between two different processes on two computers.

11. What are the advantages of a computer network architecture as a layered structure / basic principles of layering
    The layers are relatively independent and flexible, can be separated in structure, easy to implement and maintain, and easy to standardize.

12. In general, the functional implementation requirements of each layer
    Error control Flow control (mainly to coordinate the speed matching problem between the sender and receiver)
    Segmentation and reassembly (data) Multiplexing and demultiplexing connections Connection establishment and release

13. The protocol is horizontal (the two peer entities communicate through the protocol (the communication intermediary is the SAP service access point) to serve the upper layer), and the service is vertical (the lower layer serves the upper layer through the interface and can only be unidirectional , And then the lower layer indirectly provides the services provided by its lower layer to its upper layer by encapsulating the service content provided by the lower layer of this layer).

14. To see if a computer network protocol is correct, you must check very carefully whether this protocol can cope with various abnormal situations.

15. The basic idea of ​​TCP / IP four layer protocol
    The core idea is network interconnection. A unified virtual logical network is established at the transport layer and network layer to shield all physical network differences and achieve network interconnection.

16. Performance indicators of computer networks 1. Speed ​​The speed in network technology refers to the data transmission rate, which is also called the data rate or bit rate. The unit of rate is bit / s (bit per second) (or b / s, sometimes written as bps, or bit per second). When referring to the speed of the network, it often refers to the rated or nominal speed, not the speed at which the network actually runs.

17. Bandwidth originally refers to the bandwidth of a signal. The bandwidth of a signal refers to the frequency range occupied by various frequency components contained in the signal. The unit of bandwidth in this meaning is hertz (or kilohertz, megahertz, gigahertz, etc.). In computer networks, the highest data rate that a channel can pass is called bandwidth, and the unit is bps, which is the highest speed supported by the network device. Throughput represents the actual amount of data bps that passes through a network in a unit of time.

18. Delay refers to the time required for data to be transmitted from one end of the network to the other.
    (1) The transmission delay is the time required by the host or router to send a data frame, that is, the time from the first bit of the transmitted data frame to the completion of the last bit of the frame. Therefore, the transmission delay is also called the transmission delay. The formula for sending delay is: sending delay = data frame length (bit) / sending rate (bit / s)
    (2) The propagation delay is the time it takes for an electromagnetic wave to travel a certain distance in the channel. The formula for calculating the propagation delay is: propagation delay = channel length (m) / large propagation rate of electromagnetic waves on the channel (m / s). The propagation rate of electromagnetic waves in free space is the speed of light. That is 3.010 ^ 8m / s.
    The transmission delay occurs in the transmitter inside the machine and has nothing to do with the length of the transmission channel. The propagation delay occurs on the transmission channel medium outside the machine, regardless of the transmission rate of the channel. The longer the signal is transmitted, the greater the propagation delay.
    (3) Processing delay (4) Queueing delay Packets need to pass through many routers during network transmission. But after entering the router, the packet must be queued in the input queue first. After the router determines the forwarding interface, it must be queued in the output queue to be forwarded. This creates queuing delays. The length of the queuing delay depends on the network traffic at that time. When the network’s traffic is heavy, queue overflow will occur, causing packets to be lost, which is equivalent to infinite queueing delay.
    Total delay = sending delay + propagation delay + processing delay + queuing delay delay bandwidth product = propagation delay s bandwidth bps = bit

19. Round-trip time RTT In many cases, information on the Internet is not only transmitted in one direction but is interactive in two directions. Therefore, we sometimes need to know how long it takes for a two-way interaction to occur at once. (From the time the sender sends the data until the sender receives the acknowledgement from the receiver, which is RTT)

20. Utilization rate includes channel utilization rate and network utilization rate. Channel utilization indicates the percentage of time a channel is being used (with data passing). The fully idle channel utilization is zero. Network utilization is a weighted average of channel utilization across the network. Channel utilization is not as high as possible. This is because, according to the theory of queuing theory, when the utilization rate of a channel increases, the delay caused by the channel also increases rapidly. Excessive channel or network utilization can cause very large delays. Network current delay = Network idle delay / (1-Utilization)

21. Computer network architecture
    The data units transmitted by the physical layer, data link layer, and network layer of the @OSI reference model are bits, frames, and packets, respectively. Physical layer: activates, maintains, and deactivates mechanical, electrical, functional, and process characteristics between communication endpoints. This layer provides a physical medium for the upper layer protocol to transmit data.
    Data link layer: The data link layer provides reliable transmission over unreliable physical media. The role of this layer includes: physical address addressing, data framing, flow control, data error detection, retransmission, etc.
    Network layer: The network layer is responsible for routing data packets between subnets. In addition, the network layer can also implement functions such as congestion control (through active queue management AQM), and internetworking.
    Transport layer: The first end-to-end, host-to-host level. The transport layer is responsible for segmenting the upper layer data and providing end-to-end, reliable or unreliable transmission. In addition, the transport layer has to deal with end-to-end error control and flow control issues.
    Session layer: The session layer manages the session process between hosts, that is, it is responsible for establishing, managing, and terminating sessions between processes. The session layer also utilizes inserting checkpoints in the data to achieve data synchronization.
    Presentation layer: The presentation layer transforms the upper layer data or information to ensure that one host application layer information can be understood by another host’s application program. Data conversion at the presentation layer includes data encryption, compression, and format conversion.
    Application layer: Provides an interface for operating systems or network applications to access network services.
    @ 网桥 is to connect two LANs with different physical layers, different MAC sublayers, and different speeds. For example, 10MB / S and 100MB / S LAN. Because it has a storage conversion function. A network card is a piece of hardware that receives information from a computer, converts information, and temporarily stores information. It is an interface that passes the received information to the upper layers, such as (CUP). A gateway is essentially the IP address of a network leading to other networks. The gateway is the post and telecommunications office. All the information must be packed, sealed, and addressed here before it can be sent and received. The network card is the equipment, that is, the post box of the post and telecommunications office, your mailbox. The bridge is the postman, but he is only responsible for it. A town (LAN) is not responsible for the WAN. Repeater: It is a network interconnection device with single function. Working in the physical layer is what you see and see. It is mainly used to strengthen the signal. Generally used when the signal is transmitted far away, which is equivalent to an amplifier. Hub: A hub (HUB) is a basic device in a data communication system. It is a hardware device that does not require any software support or requires very little management software. It is widely used in various occasions. The hub works in a local area network (LAN) environment. Like a network card, it is applied to the first layer of the OSI reference model, so it is also called a physical layer device. The hub uses electrical interconnections. When the environment for maintaining the LAN is a logical bus or ring structure, it is entirely possible to use the hub to build a physical star or tree network structure. In this respect, the hub acts as a multi-port repeater. In fact, a hub is actually a kind of repeater. The only difference is that the hub can provide more port services, so the hub is also called a multi-port repeater. Router: The router works in the third layer of the OSI model, which is the network layer. The router uses the “logical” network address (ie, IP address) defined at the network layer to distinguish between different networks, to achieve network interconnection and isolation, and to maintain the independence of each network. Routers do not forward broadcast messages, but restrict broadcast messages to their respective networks. Data sent to other networks are first sent to the router, and then forwarded by the router. When selecting a route, the router only needs to consider the IP address of the destination station, and does not need to consider the physical address of the destination station. To connect a local area network to the Internet, the preferred device is a router. Switch: The LAN switch has many ports, each port has its own dedicated bandwidth, and can connect to different network segments. The communication between the ports of the switch is simultaneous and parallel, which greatly improves the information throughput. In order to further improve performance, each port can also be connected to only one device Bridge: The bridge acts on the data link layer of the OSI architecture, and it can distinguish the MAC address of the packet. In actual operation, the bridge will compare the packet location of the received data with its known network segment address. If the packet address is not in the same network segment, it will forward the data. The repeater works at the physical layer of OSI. The repeater has the function of amplifying a signal, and it is actually a signal regeneration amplifier. Therefore, the repeater is used to extend the length of the LAN segment and drive long-distance communication.

22. Data transmission between layers (Peer-to-peer entity refers to the entity that executes the peer-to-peer protocol. An entity is an abstract concept that can be understood as a program that executes a protocol.)

23. The main topological structures in computer networks are star, ring, tree, bus, and network.

24. There are two basic error control codes, namely error detection codes and error correction codes. One type of error detection code widely used in computer networks and data communications is the CRC code / cyclic redundancy code, and other types of error detection codes. These are Hamming codes and parity codes.

25. According to the geographical distribution of the network, the computer network can be divided into four types: local area network (LAN), metropolitan area network MAN, wide area network (the wide area network currently used basically uses a mesh topology) WAN and personal area network PAN. According to the users of the network, it can be divided into public networks and private networks. According to the communication technology, it is divided into broadcast network and point-to-point network. Access network (AN), local access network or resident access network. A network from the user’s end to the first router of the Internet does not belong to the core part of the Internet, nor does it belong to the edge of the Internet.

26. In a system that uses electrical signals to express data, there are two types of data: digital data and analog data.

27. The three types of wired media commonly used in computer networks are coaxial cable, twisted pair, and optical fiber-the best resistance to electromagnetic interference.

28. What are the three elements of a network protocol? What does each mean?
    A: Network protocol: A rule, standard, or convention established for data exchange in the network.
    (1) Grammar: The structure or format of data and control information.
    (2) Semantics: that is, what kind of control information needs to be issued, what actions are completed, and what responses are made.
    (3) Synchronization: a detailed description of the sequence of event realization.

29. Symbol: When using a time-domain waveform to represent a digital signal, it represents the basic waveform of different discrete values.
    The communication between the computer and the printer is simplex communication. What is simplex communication?
    Refers to the working mode that messages can only be transmitted in one direction.
    Half-duplex communication: The communication and both parties can send information, but the two parties cannot send at the same time (of course they cannot receive at the same time). This communication method is one party sends the other party to receive, and then the other way around.
    Full-duplex communication: Both parties of a communication can send and receive information at the same time.

30. Round-trip time: The total elapsed time from when the sender sends data until the sender receives a confirmation from the receiver (the receiver sends the confirmation immediately after receiving the data). Sometimes, the round-trip time also includes processing delays, queuing delays, and sending delays when forwarding data at the intermediate nodes of the network.
    Baseband signal: The signal from the source, that is, the signal of the basic frequency band. Data signals like computer output representing various text or image files are baseband signals.

31. A protocol is a rule or convention for communicating between peer peer entities at different nodes.

32. When two computers use a telephone line to transmit data signals, the necessary equipment is a modem.
    What is a modem? Cat.
    A modem is a piece of computer hardware that translates a computer’s digital signals into analog signals that can be transmitted along a normal telephone line. These analog signals can be received by another modem at the other end of the line and translated into a computer-readable language. . This simple process completes the communication between the two computers.

33. The English abbreviation of Internet Service Provider is ISP Internet Service Provider.

34. The main purpose of computer interconnection is resource sharing. 32. The Internet was formerly ARP