Overview of computer communication and network Chapter 1

Chapter 1 Overview

1. How many stages can the development of computer networks be divided? What are the characteristics of each stage?

A: The development of computer networks can be divided into the following three stages.

(1)Terminal-orientedComputer Communication Network: The computer is the center and Controller of the network, and terminals are distributed around the central computer, with a hierarchical star structure, each terminal shares the hardware and software resources of the host through communication lines. The main task of the computer is batch processing. After the emergence of a time-sharing system in 1960s, it has interactive processing and batch processing capabilities.

(2)Computer-computerNetwork. Group exchange networks for interconnection of multiple computers: systems such as IBM's SNA network and DEC's DNA network are relatively independent and there is no unified standard. The group switch at the intermediate node is responsible for the storage forwarding and Route Selection of the group. This greatly improves the utilization of communication lines and is suitable for burst computer data.

(3)Build a computer network architecture: To make computer networks of different architectures interconnected, ISO proposes a standard framework for interconnection of various computers into networks worldwide-OSI, a basic reference model for interconnection of open systems .. In this way, as long as the OSI Standard is followed, a system can communicate with any other system located anywhere in the world and following the same standard.

 

2. Briefly describe the key points of group exchange.

A: group exchange is essentially in the"Storage-forwarding. It has the advantages of circuit switching and packet switching. In a group exchange network, data is divided into many small pieces of data by a certain length-grouping. Transmits data in short groups. When a group is switched, each group is marked with a header and transmitted dynamically over a physical line. Each intermediate exchange node in the path transmits data groups in sequence according to the order in which each packet is received. It stores the received data groups in the memory of the switch node, next, we will forward the data in the network. One station and one station will "store-Forward" in turn, and finally reach the receiving end, remove the grouping header from the receiving end, and re-assemble the data fields into complete packets in order.Compared with circuit switching, group switching features higher circuit utilization and lower transmission latency and better interaction.The main advantages of the group exchange network are:
① Efficient. In the packet transmission process, the transmission bandwidth is dynamically allocated, which occupies the communication link segment by segment.

② Flexible. Each node is intelligent, and each group selects a route for forwarding independently.

③ Fast. Groups are used as transmission units. Before communication, you can send groups without establishing a connection. The network uses high-speed links.

④ Reliable. Perfect Network Protocol; distributed multi-route Communication Subnet.

 

3.Compare the main advantages and disadvantages of circuit switching, packet switching and group switching in multiple aspects.

Answer: (1) When a computer terminal communicates with each other, one party initiates a call. When the switch is established, the other party receives the call signal from the initiator and agrees to establish a communication connection, both parties can communicate with each other.This circuit is used by both parties throughout the communication process.Exclusive to a physical line. It features high real-time performance, low latency, and low switching device costs. However, it also results in low line utilization, long circuit connection time, and low communication efficiency,Different types of end users cannot communicate with each other. Circuit Switching is more suitableLarge amounts of information, long packets, frequently used between fixed users.

(2) message exchangeStored in switch memory.When the output circuit is requiredIdleAnd then send the message to the receiving switch or terminal.It transmits data over the network in the "Storage-forwarding" mode. The advantage of message exchange is thatHigh relay circuit UtilizationMultiple users can transmit data on one line at the same time, and terminals with different rates and procedures can communicate with each other. However, its shortcomings are also obvious. Stores and forwards packets,High network transmission latencyAnd occupies a large amount of switch memory and external memory. When a packet is re-sent, the amount of re-transmission data is large, which cannot meet the requirements of users with high real-time performance. Packet Exchange is applicableShort message transmission and low real-time requirementsCommunication between network users, such as public telegraph network.

(3) group exchange is developed on the basis of "Storage-forwarding. It has the advantages of circuit switching and packet switching. Group switch used onlineDynamic Reuse TechnologyTransfer by a certain lengthSplit into many small pieces of data-grouping. Transmits data in short groups. When a group is switched, each group is marked with a header and transmitted dynamically over a physical line. Each intermediate exchange node in the path transmits data groups in sequence according to the order in which each packet is received. It stores the received data groups in the memory of the switch node, next, we will forward the data in the network. One station and one station will "store-Forward" in turn, and finally reach the receiving end, remove the grouping header from the receiving end, and re-assemble the data fields into complete packets in order. Group exchangeIt is higher than the circuit utilization rate of circuit switching and is not exclusive to the line.;It has lower transmission latency than message exchange, better interaction, and smaller data volume during re-transmission,Fast, efficient, and flexible.

4. What types of computer networks can be classified?

A: From the networkScopeClassification: Wan, man, lan, and pan;
Classification from the scope of use of the network: public network and private network;
Slave NetworkExchange FunctionClassification: Circuit Switching, packet switching, and group switching.

5. What components does a computer network consist?

A: A computer network should have three main components:

(1)Several hosts that provide services to users;

(2) OneCommunication Subnet, It consistsDedicated Node switchAndCommunication links connecting these nodesComposed;

(3) A seriesProtocolThese protocols are used for communication between hosts or between hosts and subnets.

 

6. What are the three elements of network protocols? What does it mean?

A: to exchange data in a computer network in an orderly manner, you must abide by rules agreed in advance. These rules, standards, or conventions established for data exchange in the network are called network protocols. A network protocol consists of the following three elements:

(1)SyntaxThe structure or format of data and control information;

(2)SemanticsThat is, the control information, actions, and responses that need to be sent;

(3)SynchronizationThat is, a detailed description of the event Implementation sequence.

 

7. The advantages of layered processing methods for network protocols are demonstrated.

Answer: advantages: (1) mutual communication between layersIndependent, A certain layer canUse the services provided at the next layer without knowing how the services are implemented.. (2) Good flexibility. When a layer changes, as long as its interface relationship remains unchangedThe above or below layers are not affected. (3) The structure can be separated and the layers can be implemented using the most appropriate technology. (4) ease of implementation and maintenance. (5) promoting standardization.

 

8. This article describes the key points of the principle network architecture with five-layer protocols, including the main functions of each layer.

Answer: considering the advantages of OSI and TCP/IP, a principle architecture is adopted. Main functions of each layer:

Physical Layer-Physical Layer task isTransparently transmits bit streams. The physical layer also determines the definition and connection method of the connector.

Data link layer-the task at the data link layer isData in frame units is transmitted without errors on the link between two adjacent nodes.. Each frame includes data and necessary control information.

Network Layer-the task at the network layer isSelect an appropriate route so that the Group sent by the sending station can find the correct route, route one site at a time in the network, and finally deliver the route to the target station.

Transport Layer-the task at the transport layer isProvides a reliable end-to-end transmission service between network processes on two end-to-end nodes for communication.To make them invisible to the details of data communication below the transport layer.

Application Layer-the application layer is directlyYour application process provides various application services.

 

9. TryTCP/IPAndOSITo compare the architecture. Discuss their similarities and differences.

Answer: (1) the similarities Between OSI and TCP/IP are that they both adopt a hierarchical structure and are all hierarchical by function.

(2) Differences Between OSI and TCP/IP:

① OSI is divided into seven layers: physical layer, data link layer, network layer, transmission layer, Session Layer, presentation layer, and application layer. TCP/IP is divided into four layers: network Interface Layer, Internet layer, transmission layer, and application layer.

② There is a strict call relationship between the OSI layers and the communication between two (n) layersMust pass through the next layer (N-1) layer entity, not beyond the levelAnd TCP/IP can bypass the adjacentThe next layer directly uses the services provided by lower layersTherefore, it reduces unnecessary overhead and improves the Protocol efficiency.

③ OSI has a model and a protocol, which is suitable for theoretical discussion. TCP/IP has a protocol and a model, which has been widely used.
 

10. The following terms are explained:

Protocol stack, entity, peer layer, service primitive, Service Access Point (SAP), PDU, SDU, customer, server, customer/Server mode.

A: Protocol Stack: the protocols in the protocol stack are distributed across several layers. They are painted together like a stack. Therefore, they are vividly called protocol stacks.

Entity-represents any hardware or software process that can send or receive information.

Peer layer-any two nodes communicate at the same level (for example, on the 4th layer of the two systems). It seems that data (that is, data units plus control information) is directly transmitted to each other, that is, the so-called "peer layer" communication. Protocols at different layers are actually the rules for data transmission between peer layers.

Service primitives-the upper layer must exchange commands with the lower layer to use the services provided by the lower layer. These commands are called service primitives in OSI.

Service Access Point (SAP) ---- in the same systemA logical interface for interaction (information exchange) between two adjacent entities.

PDU-the Unit of data transmitted between peer layers.

SDU-the service data unit, the unit of data exchanged between the layer and the layer.

Customers and servers ----The customer and server refer to the two application processes involved in the letter.. The customer first initiates a service request to the server, which is the service requester. The server then provides the service to the customer, which is the service provider.

Customer/server model ----Most network applicationsProgramWhen writing, assume that one end is the customer and the other end is the server,The purpose is to allow servers to provide specific services to customers. The client is the caller, and the server is the called party.

 

11.What are the characteristics of connection-oriented and connectionless services?

A: connection-oriented services must be established before data exchange,Pre-reserve resources, The connection should be terminated after the data exchange ends,Release reserved resources. The connectionless service can communicate between two entities without establishing a connection. Resources are dynamically allocated during data transmission without pre-reservation.

 

12.What is the difference between a protocol and a service? What is the relationship?

A: The Protocol is horizontal and the service is vertical. 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 by the lower layer and the upper layer through the interface between layers.. Protocol and service relationship: under the control of the protocol, communication between two peer entities enables the current layer to provide services to the next layer. To implement this layer protocol, use the services provided at the following layer. The lower layer provides services to the upper layer, the upper layer calls the services provided by the lower layer, and the upper and lower layers exchange information with service primitives.