Computer Network review

1. Computer Network Type:
A. From the perspective of network service management: peer-to-peer network, customer/server network, and hybrid network.
B. Operating System: Windows Network, UNIX network, and Linux network.
C. network scale: Lan (802.3, 802.4, 802.5, 802.11), Metropolitan Area Network (802.6), Wan (TCP/IP, digital communication technology), and Internet.
D. network topology: Bus (broadcast, Ethernet), ring, Star (ring), tree (Internet), mesh, hybrid
2.802.11: a wireless LAN Standard originally developed by IEEE, which is mainly used to solve the wireless access between users and user terminals in office LAN and campus network. The business is mainly limited to data access, the maximum speed is 2 Mbps. 802.11b and 802.11a
3. Lan 802 standard: the IEEE 802 series standard is the LAN and man technical standard developed by the IEEE 802 LAN/man standard committee. Among them, the most widely used are Ethernet, licensing ring, and wireless LAN. Each of these criteria is under the responsibility of a specialized working group of the Committee.
4. WAP: Wireless Application Protocol.
5. GPRS: general packet radio service. General grouping wireless services. Compared with the circuit switching technology adopted by GSM, the group switching technology is adopted.
6. network topology: the connection mode of hosts in the network.
7. Computer Network: a resource sharing system composed of a series of devices (network nodes) connected by transmission media.
8. Mobile Network: when a user accesses the network, the Access Point of the current network is wider than that of the wireless network. Wireless or wired.
9. Wireless Network: access the network through wireless channels.

10. Information: A person's understanding of the existence or motion of things in the real world.
11. Data: the representation of certain attributes of a thing after normalization. Stored on the media.
12. Signal: the specific physical performance of data, with a definite physical representation.
13. Digital Communication: the process of data transmission between different computers.
14. Communication System Model: source, channel, sink, and noise.
15. Analog Signal-> digital signal-> analog signal: Compile the decoder. Pulse code modulation.
16. Digital signal-> analog signal-> digital signal: modem. Modulation> Demodulation
17. Advantages and Disadvantages of analog transmission: narrow spectrum and high channel utilization; disadvantages: signal attenuation and noise interference.
18. Advantages and Disadvantages of digital transmission: low signal distortion and low channel utilization.
19. Analog channel bandwidth: The amplitude on the channel frequency response curve is the width between the two frequencies at the center of the channel (1/2. W = f2-f1, determined by the physical characteristics of the channel.
20. digital channel bandwidth: the maximum data rate that the channel can reach.
21. bitrate (baud rate): number of bitrate transmitted over a channel per unit of time. Unit: Porter. It is also called modulation rate.
22. nequest theorem: If the channel bandwidth is W, the maximum bitrate is B = 2 W (baud ).
23. Bit Rate (BPS): The amount of information transmitted on the channel per unit of time (BPS) is called the data rate.
24. Signal-to-Noise Ratio: ratio of average signal power to noise power.
25. Bit Error Rate: the probability that binary code elements are mistakenly transmitted in the transmission system.
26. Manchester encoding: data 1 is represented by 0 in the first half and E in the second half. DATA 0 is represented by E in the first half and 0 in the second half. It is used in 10 Mbps Ethernet and wireless paging encoding.
27. Check Manchester encoding: using the differential encoding technology, each intermediate hop is changed, but the interval starts with 1 hop and 0 hop.
28. Alternate anti-transcoding: 1 is represented by E and-E levels, and 0 is 0. Common PCM: pulse coding modulation.

29. modulation: Carries digital signals with a certain frequency carrier, amplitude modulation (Amplitude Shift key ask), frequency modulation (Frequency Shift key PSK), and phase modulation (Phase Shift key PSK)
30. analog data is encoded as digital signals: pulse code modulation PCM; Incremental modulation DM.
31. Pulse Code Modulation: Sampling (at twice the maximum clock speed of the signal), quantization, coding

32. Circuit Switching: a temporary dedicated physical channel is provided between devices that require communication. For example, the traditional public telephone network. Suitable for data with high real-time requirements.
33. Circuit Switching types: analog switching (Air Separation switching) and digital switching (time division switching ).
34. Packet Exchange: (storage and forwarding) for data communication.
35. Packet group exchange: the packet is exchanged in groups.
36. multiplexing: in a data transmission system, two or more data sources are allowed to share the same transmission medium, just as each data source has its own channel. For example, public exchange Telephone Network (PSTN ).
37. OFDM: cable TV system, Broadband LAN, TDM, WDM, and CDM ): separate original signals with different codes.
38. Multiple Access: identifies communication objects (such as multiple access cellular systems) through channels. A channel can only accommodate one user for calls.
39. Code Division Multiple Access: CDMA; Code Division Multiple Access: FDMA; Time Division Multiple Access: TDMA; Synchronous Code Division Multiple Access: TD-SCDMA

40. Communication errors: Random Errors Caused by Hot noise and sudden errors caused by impact noise.
41. parity code: add one digit after the seven ASCII codes to make the number of "1" in the code word an odd or even number.
42. Optical Code: redundant data bit.
43. CRC verification of cyclic redundancy: verification is implemented using cyclic codes. It is a data transmission error detection function that performs polynomial calculations on data and attaches the obtained results to the end of the frame. The receiving device also executes similar algorithms to ensure the correctness and integrity of data transmission.

44. baseband transmission: A transmission mode that does not shift the baseband signal spectrum. The signal to be transmitted that is not modulated by the carrier is called a baseband signal. The band occupied by the signal is called a baseband. The ratio of the high frequency to the low frequency of the baseband is generally greater than 1.
45. band transmission: band transmission first transforms (modulated) the baseband signal into a analog signal (called a band signal) with a high frequency range for transmission in a simulated channel ), then, the band signal is transmitted in the analog channel.
46. Circuit exchanging: the process is similar to that of telephone exchange. In line switching, before two computers exchange data through the Communication Subnet, you must first establish an actual physical line connection in the Communication Subnet.
47. datagram: The basic unit of data transmitted over the network. It contains a header and the data itself. The header describes the data destination and its relationship with other data.
48. virtual circuit: a bidirectional and transparent logical transmission channel established between the logical or physical ports of two terminal devices through a group exchange network.

 

49. Architecture of computer networks: functional division, hierarchical division, and network structure of computer networks.
50. reason for hierarchical design: The computer network is divided into several layers by function. In this way, a higher level of network design is based on a lower level, it also provides necessary service functions for a higher level, which can break down complex problems and facilitate processing. Each layer uses the services provided by the next layer to provide services for the previous layer, in this way, you do not have to worry about the structure and implementation details of the next layer. As long as the lower layer and the interfaces of the previous layer do not change, the changes in structure and implementation details will not affect the previous layer, in this way, the independence between layers is maintained.
51. Network Protocol: Rules and standards established to achieve data exchange in the network, used to describe the information exchange process between processes. It consists of syntax, semantics, and exchange rules.
52. Interface: an interface exists between adjacent layers. It defines the original operations and services provided by lower layers to higher layers.
53. OSI: Open System Interconnect reference model: Open System Interconnection Reference Model.
54. Layer 7 of OSI: application layer, presentation layer, Session Layer, transmission layer, network layer, data link layer, and Physical Layer
55. Physical Layer: involves transmitting original bit streams on the channel.
Data link layer: enhances the physical transmission of original bits so that the network layer is displayed as a line without errors. Solve the Problems Caused by frame destruction, loss, and repetition.
Network Layer: controls the operation of subnets. Including route selection, congestion control, accounting, and network addressing
Transport Layer: Creates communication between processes and processes. It provides a traffic control mechanism at the transport layer.
Session Layer: Manage conversations.
Presentation Layer: defines the syntax and semantics of the transmitted information.
Application Layer: contains a large number of application protocols that are commonly needed.

56. TCP/IP protocol family: physical network interface layer (LAN), network layer (IP, ARP), transmission layer (TCP, UDP), application layer (FTP, DNS, HTTP, SMTP)
IP Address: provides non-connected datagram transmission services. End-to-End grouping and sending functions, addressing functions for identifying network numbers and host node numbers, data segmentation and re-assembly, and routing selection functions.
ICMP: embedded with the IP protocol, which meets the requirements for reporting errors.
ARP Protocol: parses the physical address of the peer.
TCP: Transmission Control Protocol (connection oriented)
UDP: User Datagram Protocol (unreliable and connectionless protocol)
FTP: File Transfer Protocol
SMTP: email Protocol
TELNET: virtual terminal Protocol
57. Main questions about the OSI reference model:
A. unreasonable layering.
B. Related Service definitions and protocols and their complexity.
C. Some functions are repeated and some are not included
D. completely ignore the connectionless service and connection protocol.
D. The relationship between computers and communications is hardly mentioned.
58. Problems with the TCP/IP model:
A. There is no concept of clearly distinguishing between services, interfaces, and protocols.
B. It is completely unavailable.
C. The network layer is just an Interface
D. There is no distinction between the physical layer and the data link layer (application layer, transport layer, network layer, and network interface layer)
59. Differences between TCP/IP and OSI reference models:
Similarities: the concept of independent protocol stack and layer functions are similar.
Differences: A. Different layers: Layer 4/Layer 7
B. Services, interfaces, and Protocols are differentiated.
C. OSI is not biased towards any specific protocol and there are non-conforming service specifications. TCP/IP is a description of the protocol and matches the protocol well.
D. In the transport layer, OSI only supports connection-oriented protocols, and TCP/IP supports connection and no connection.

60. Application layer objective: to share files between two users.
70. Customer/Server mode:
How servers work: cyclic servers and concurrent servers
71. Services provided by the transport layer for the application layer: implemented based on Socket socket.
Connection-oriented Transmission Control Protocol (TCP): a connection-oriented reliable byte stream transmission layer service, it is used to provide a reliable, end-to-end byte stream communication in an unreliable network. Handshake, traffic control, and error correction. HTTP, FTP, and SMTP.
Non-connection Datagram Protocol (UDP): no connection unreliable transmission protocol.
72. Socket: the cornerstone of network communication, which is equivalent to the socket at both ends of network communication.
Stream socket; datagram socket
Socket program design process:
A. Create a Socket socket ()
B. Bind the socket to the local address ()
C. Listen to int listen ()
D. Wait for the socket accept () to be connected ()
E. Establish connection int connect ()
F. Send data int send ()
G. receive data int Recv ()
H. Close the connection int Shutdown ()
I. Disable socket int closesocket ()
73. Common application layer protocols:
FTP: File Transfer Protocol
SMTP and POP3: email protocol, simple mail protocol, and Post Office Protocol.
TELNET: virtual terminal Protocol
HTTP: hypertext transfer protocol, the foundation of WWW (World Wide Web.
DNS: domain name Protocol
74. Host identification in the Network: physical address (MAC), IP address and domain name address (DNS)
Physical address: MAC address. The address naming mechanism is used for internal communication in the physical network, which is represented by six groups of hexadecimal numbers.
IP Address: a CIDR address defined by shielding physical addresses of different networks. It consists of 32-bit binary digits.
Domain Name System: (Network Name identification system established by hierarchical naming mechanism): local place name. group name. Outlet name
75. domain name resolution: the problem of domain name address-to-IP address ing is implemented from top to bottom, mainly including recursive resolution and iterative resolution. The customer/Server mode is used.
Domain Name Server: software, complete domain name-IP ing
76. The familiar port used by the server is port 53.
77. WWW (World Wide Web): The most common service in the network, used for data transmission between two hosts, based on the customer and server mode.
WWW File Format: HTML file, hypertext file.
Location of the www html file: the URL (Uniform Resource Locator.
WWW communication transmission mode: HTTP Hypertext Transfer protocol.
WWW port: Port 80
78. Uniform URL resource positioning character: Uniform Resource Locator. It is a concise representation of the resource locations and access methods that can be used on the Internet.
The general URL format is: Protocol: // host address: Port/path/file name.
79. Hypertext Markup Language HTML: hypertext markup language, the standard format language of WWW.
80. Hypertext Transfer Protocol (HTTP): defines how to communicate between the WWW client and the server, and how to send and respond to requests. Although the transport layer protocol TCP is used, HTTP is a connectionless protocol.
81. FTP: File Transfer Protocol. It adopts the Client/Server mode and is a connection-oriented protocol. Control listening port 21. Data transmission uses Port 20.
82. Composition of the email system: User proxy, message transmission proxy (email server), and transmission protocol.
83. Simple Mail Transfer Protocol-SMTP (server-client), used to send emails between the email server and the client to the mail server. It adopts the TCP protocol and the Client/Server structure. Port 25.
84. Multi-purpose Internet Mail Extension protocol-mime (Multipurpose Internet Mail Extensions): Mail supplement protocol. The sender converts non-ASCII characters to ASCII characters and restores them at the receiver.
85. Email Address: username @ host name.
86. Post Office Protocol POP (Post Office Protocol): used to allow the client to obtain users' emails from the server. Port 110 is used.
87. Internet Message Access Protocol-IMAP (Internet Message Access Protocol)

88. Transport Layer-the core of the entire protocol hierarchy. Reliable and cost-effective data transmission between the source host and the target host is required to shield users from lower-Layer Networks and provide unified services. Provides end-to-end (process-to-process) transmission. The main task is to deliver network-layer data to the right application.
89. Parameters for evaluating the service quality at the transport layer:
Latency: Connection latency and transmission latency.
Probability of connection establishment failure: the possibility of connection establishment failure within the maximum connection establishment delay period.
Throughput: the number of user data bytes transmitted per second.
Residual Error Rate: measure the percentage of lost or disordered messages to the total number of sent messages.
Priority: the connection priority.
Restoration:
90. Transport Layer programming scheme: port numbers are used to identify application processes. It is represented by 16-bit binary numbers.
91. Traffic Control: control the data sending traffic at the sending end. The most common protocol-Sliding Window Protocol: maintains a data sending and receiving window at the receiving end of the data.
92. Transport Layer Protocol: connection-oriented transmission protocol and connectionless transmission protocol (unreliable, implemented using services provided by the network layer ).
<B> connection-oriented transmission protocol: </B>
93. three-Step handshake (connection establishment): add the serial number to the connection request signal sent by the client; the server sends a response signal after receiving the connection request, the response signal includes the request sequence number and the server's specified connection sequence number. After receiving the response signal, the client responds to the response signal and establishes a connection.
94. fault recovery technology: After a connection is established between the customer and the server, the data transmission before the interruption is resumed, mainly relying on the on-site storage technology.
95. On-site storage: After a network failure occurs, the customer or server uses time-out to discover a fault and saves the data related to the current connection before closing the connection. After the connection is established again, the data can be used to restore the connection.
96. Port Number:
FTP: 21
Telent: 23
SMTP: 25
DNS: 53
HTTP: 80
POP3: 110
97. three-segment handshake mode for TCP connection establishment: the sender (client) sends a SYN segment to the receiver (server), specifying the port number of the server to which the client intends to connect and the initial serial number (ISN ); the server sends back the SYN packet segment containing the server's initial serial number as the response. At the same time, the response sequence number is set to the customer's ISN plus 1 to confirm the customer's Syn packet segment; the customer sets the response sequence number as isn plus 1 of the server to confirm the SYN packet segment of the server, thus establishing a complete TCP connection. A-> (syn, seq = x)-> B; B-> (syn, seq = Y, ACK = x + 1)-> A; ACK = Y + 1
98. The release of the TCP connection adopts the four-step handshake mode: when one party completes its data transmission task, it sends a fin to terminate the connection. When a fin is received, it must notify the other end of the application layer of which data transmission has been terminated.
99. TCP traffic control adopts the Sliding Window Protocol and the sliding window protocol uses the credit window protocol.
100. TCP error message segment discovery and correction: the TCP protocol only uses a positive response rather than a negative response. Therefore, errors are discarded directly, but no information is sent, so that the sender waits for timeout.
101. UDP adds port and simple error detection functions on the IP datagram service to implement data transmission from processes to processes.

102. Network Layer tasks: Define a unified global addressing solution, find the shortest path in a complex network, and provide services for the upper layer.
103. in the actual network, the application layer and the transmission layer generally adopt the TCP/IP protocol. The physical layer and the data link layer generally adopt the data communication network protocol or LAN protocol, and the network layer includes two parts: the network layer of the TCP/IP protocol (responsible for editing application data, path finding and data delivery) and the network layer of the communication network (addressing, addressing, and ).
104. Role of the network layer: Provides communication networks for the transmission layer to ensure that the data group can correctly reach the receiving host from the sending host, that is, provide end-to-end communication (communication between the host and the host ). The transport layer provides process-to-process communication.
105. virtual circuits: connection-oriented services; compared with connectionless datagram. These are two types of services at the network layer.
106. Network Layer programming scheme: hierarchical digital naming mechanism.
107. broadcast address: All IP addresses with host numbers "1"
108. Limited broadcast address: IP address with 32-bit full value of 1.
109. "0" Address: 0.0.0.0
110. Local Network: All IP addresses with host numbers "0. /
111. IP address (inter-network protocol address ):
Class A address: 0 + 7-bit network ID + 24-bit host ID
Class B address: The 10 + 14-bit network logo number + 16 indicates the host ID
Class C address: 110 + 21-bit network logo number + 8-bit host number
112. subnet mask: implements the division and Management of subnets in a network. The two-level structure of the IP address is changed to the three-level structure "network ID + subnet ID + host ID ".
Value definition: In a 32-bit mask structure, the mask value for all network identifiers is "1", and for all host identifiers, the mask value is "0 ",
Class A: 255.0.0.0; Class B: 255.255.0.0; Class C: 255.255.255.0;
113. Address Resolution: from IP address to MAC address. The Address Resolution Protocol ARP (Address resoultion protocol. Address Resolution is completed through ing.
Reverse Address Resolution RARP: from MAC address to IP address.
114. ARP Address Resolution Protocol: ARP obtains the physical address of the host or other nodes by sending broadcast requests.

115. data link layer: data transmission between directly connected computers.
Basic Service: transmits data from the network layer of the source machine to the network layer of the target host.
116. Frame -- transmission unit of the data link layer.
117. Point-to-point network-> WAN technology (group switching)
Broadcast Network-> LAN technology (grouping broadcast)
118. IEEE 802.3 general network media access control protocol CSMA/CD and physical layer technical specifications. (Ethernet Protocol)
IEEE 802.11 Wireless LAN media access control protocol and its physical layer Technical Specifications
119. data link layer:
Logical Link Control Layer (LLC): provides unified data link layer services to senior management.
Media Access Control Layer (MAC): defines media-related control algorithms.

120. Ethernet (baseband Local Data Communication Network) protocol:
121. CSMA (carrier-based multi-channel access) Principle: before sending data, you must first listen on the channel to see if there is any carrier signal sent by another station. If yes, it indicates that the channel is busy. Otherwise, the channel is idle and determined based on the predefined policy:
A. If the channel is idle, whether to send it immediately
B. If the channel is busy, continue listening?
122. CD (conflict detection): detects conflicts while listening. Once a conflict is detected, the transfer is canceled and all users on the network are notified. Then, try again after a period of time.
123. ethernet frame structure: Leading (8B)-> Destination Address (6B, MAC address)-> source address (6B)-> type (2b)-> data (46b-1500b) -> frame verification sequence (4B)

124. The data link layer of 802.3 is the same as that of for LLC, Mac, and LLC.
125. Why CSMA/CD is not fully used in:
A. The ability to detect conflicts requires the ability to send and receive at the same time, but it is very expensive to implement this function in a wireless LAN.
B. More importantly, even if it detects a conflict and does not listen for a conflict at the time of sending, a conflict still occurs on the receiver.
126. CSMA/CA in: the carrier listens for multiple access channels and avoids collision.

127. ATM: asynchronous transmission mode, used for Broadband Integrated Service Digital Network (BISDN ).
128. Current TCP/IP problems: the IP address space is short, the route table is large, difficult to manage, lack of memory, and lack of support for mobile devices.
129. DHCP: Dynamic Host Configuration Protocol
130. IPv6: Internet Protocol version 6 (Internet Protocol version 6)
Features:
A. uses a 128-bit address structure and has a larger address space.
B. The address structure uses a globally unique hierarchical address based on the prefix to maintain a small route table and efficient trunk network routing.
C. automatic configuration of Network Interfaces
D. service type differentiation of Data Types
E. multicast takes precedence over broadcast during routing.
F. built-in authentication and encryption
131. Tunneling Technology: used when two IPv6 hosts need to communicate through an IPv4 network.

132. network interconnection devices:
NIC:
Repeater: a repeater is responsible for transmitting information by bit on the physical layer of the two nodes to complete signal replication adjustment and amplification, thus extending the network length.
Hub: concentrator, multi-port Repeater
Switch: a central node device that uses unshielded twisted pair wires to build frame exchange data communications required by a star network. Work on the second layer of OSI.
Interconnection devices:
Bridge: a device that connects networks on the data link layer, working on the MAC Sub-layer. Used for LAN interconnection.
Router: a device that connects multiple networks at the network layer. Transfers data by frame between two local networks.
Gateway: a device that connects multiple networks at a high level. It is used to perform layer-1 to layer-7 conversion for two different networks.
133.5-4-3 rule: One Ethernet can have up to five network segments and four reconnections. Only three of them can be connected to computer terminals.
134. intrusion Detection: a reasonable supplement to the firewall is to collect and analyze the information from several key points in the computer network, A mechanism to check whether there are violations of security policies and signs of attacks on the network or system. The second gate after the firewall can monitor the network without affecting network performance, so as to provide real-time protection for internal attacks, external attacks, and misoperations.
135. firewall: The only egress of information between different networks or network security domains. It can control network information flows according to network security policies and has strong anti-attack capabilities. It provides information security services, infrastructure for network and information security.

Ethernet: A baseband LAN specification created by Xerox and jointly developed by Xerox, Intel, and Dec. The Ethernet network uses CSMA/CD (Multi-Channel Access and Conflict Detection Technology for carrier monitoring) and runs on multiple types of cables at a rate of 10 Mb/s. Ethernet is similar to the IEEE802 · 3 standard.

It is not a specific network, but a technical specification.
Ethernet is the most common communication protocol standard used by local area networks. The standard defines the cable type and signal processing method used in the LAN. Ethernet is 10 ~ 10 Base T Ethernet is the most widely used Ethernet technology because of its low cost, high reliability and 10 Mbps speed. The direct expansion of wireless Ethernet can reach 11 Mbps. Many manufacturing suppliers can use common software protocols for communication, which is the best open.
The Internet is a network of networks ). It connects physical networks of different types, sizes, and geographic locations to a whole using TCP/IP network protocol. It is also an international collection of communication networks, integrating modern communication technology and modern computer technology, integrating various information and information resources in various departments and fields, thus forming an information resource network shared by online users.