Network refers to “ Three networks ”, Telecommunication network , Cable TV network and computer network . The computer network is the fastest growing and plays a central role .

The two most important characteristics of computer network :

1. data communication : Computer network enables Internet users to exchange information with each other , It's like these users' computers can all be connected to each other . It is the most important function of computer network .

2. resource sharing : It can be information sharing , Software sharing , It can also be hardware sharing .

A computer network consists of several nodes and links connecting them . Networks can also be interconnected through routers , Form a computer network with greater coverage , It's called the Internet , The Internet is “ Network of networks ”. Computers connected to the network are often called hosts .

internet( Internet ) Is a common noun , Generally refers to a computer network formed by the interconnection of multiple computer networks .

Internet( internet , Internet ) Is a proper noun , Refers to the world's largest , Open , A specific Internet formed by the interconnection of many networks , use TCP/IP Protocol group as the rule of communication .

The Internet now adopts the packet switching technology of store and forward , And the third floor ISP structure .

Standardization of computer network

All Internet standards are based on RFC Published on the Internet in the form of . The formulation of formal standards for the Internet goes through the following three stages :

1. Internet draft     This stage is not yet RFC file .

2. recommended standard     This stage began to become RFC file

3. Internet standard

Composition of computer network

In terms of working mode : Computer network is divided into edge part and core part . The edge part consists of all connected to the Internet , Composition of host for direct use by users , Used for communication and resource sharing . The core part consists of a large number of networks and routers connecting these networks , It provides connectivity and switching services for the edge .

The communication modes between end systems at the edge of the network can be divided into two categories : customer - Server mode (C/S mode ) And equivalence (P2P mode ):

1. customer - Server mode : The customer is the service requestor , The server is the service provider .

2. Peer to peer connection mode : When communicating, the two hosts do not distinguish which is the service requestor and which is the service provider . As long as both hosts are running P2P Software , They can have equal , Peer to peer communication .

The router plays a special role in the core of the network , It is the key component of packet switching , The task is to forward the received packets , Packet switching is the most important function of the core part of the network .

1. Main characteristics of circuit switching : Establish connection - conversation - Release connection . It is connection oriented . During the whole time of the call , Two users always occupy end-to-end communication resources .

2. Main characteristics of packet switching : Packet switching adopts store and forward technology . Generally, the whole block of data to be sent is called a message . Before sending the message , First, the longer message is divided into smaller equal length data segments , After adding the necessary control information to form the header , Form a group . The router received a packet , Store it for a while , Check its head , Lookup forwarding table , According to the destination address in the header , Find the appropriate interface and forward it , Switch packets to the next router . On which link does the packet transfer occupy the communication resources of this link .

Comparison of three kinds of exchange

Definition of computer network :

Definition of resource sharing viewpoint : A plurality of autonomous computer systems with independent functions distributed in different places are connected through communication equipment and lines , Under the management of fully functional network software and protocol , System aiming at resource sharing in network .

Classification of computer networks :

Classification according to the transmission technology used by the network :

Type of communication channel : Broadcast communication channel and point-to-point communication channel .

The corresponding computer network can be divided into : Broadcast network and point-to-point network .

Classify according to the geographical coverage of the network : LAN , Man and WAN .

Classify according to the topology adopted by the network : Bus type , Star type , Ring and mesh .

Classification according to network users : Private and public networks .

Performance index of computer network

rate : The rate in the network refers to the rate at which the host connected to the computer network transmits data on the digital channel , Also known as data rate or bit rate , Unit is “ Bits per second ”(bit/s or b/s).

bandwidth : Bandwidth indicates the ability of the communication line of the network to transmit data , It can be transmitted by digital channel “ Maximum data rate ” Synonyms for , Unit is “ Bits per second ”(bit/s or b/s).

throughput : Represents the amount of data passing through a network in unit time .

time delay : The total time it takes to transfer data from one end of the network to the other . Total delay = transmission delay + Propagation delay + Processing delay + Queuing delay .

transmission delay : The time required by the node to push all bits of the packet to the transmission link , That is, starting from the first bit of the transmission packet , The time required to complete the transmission of the last bit of the packet .

Propagation delay : It takes time for electromagnetic waves to travel a certain distance in the channel , That is, the time required for a bit to propagate from one end of the link to the other .

Processing delay : When the host or router receives the packet, it takes some time to process it .

Queuing delay : Packets must wait in the input queue for processing after entering the router , After the router determines the forwarding interface , Also queue up in the output queue for forwarding .

bandwidth delay product : If the sender sends data continuously , When the first bit sent is about to reach the end , Indicates the number of bits sent by the sender . bandwidth delay product = Propagation delay × Channel bandwidth .

round trip time RTT: Indicates that the sending end starts sending data , The total time it takes for the sender to receive the acknowledgement from the receiver .

Utilization rate : If order D0 Indicates the delay when the network is idle ,D Indicates the current delay of the network ,U Indicates network utilization , that D=D0  /(1-U)   
therefore , High channel or network utilization will cause very large delay .

Computer network architecture

agreement : Rules established for data exchange in a network , Standards or conventions are called network protocols , It's horizontal . The format of the exchanged data and related synchronization issues are clearly specified . Network protocol is mainly composed of the following three elements :

1. grammar : Structure or format of data and control information .

2. semantics : What control messages need to be sent , What actions are completed and what responses are made .

3. synchronization : Detailed description of event implementation sequence .

Benefits of Tiering :

1. The layers are independent .

2. Good flexibility .

3. Structurally separable .

4. Easy to implement and maintain .

5. Can promote standardization .

Each layer of computer network and the collection of its protocols are the architecture of the network . The architecture is abstract , The implementation is concrete , Is really running computer hardware and software .

Architecture with five layer protocol

Computer network architecture

1. application layer : The task of application layer is to complete specific network applications through the interaction between application processes . The protocol of application layer defines the rules of communication and interaction between application processes . We call the data unit of application layer interaction as message .

2. Transport layer : The task of transport layer is to provide general data transmission services for the communication between two host processes . The transport layer mainly uses the following two protocols :

Transmission control protocol TCP: Provide connection oriented , Reliable data transmission service , Its data transmission unit is message segment .

User datagram protocol UDP: Provide connectionless , Best effort data transmission service ( The reliability of data transmission is not guaranteed ), Its data transmission unit is user datagram .

3. network layer : The network layer is responsible for providing communication services for different hosts in the packet switching network . When sending data , The network layer transmits the message segments or user datagrams generated by the transport ship in encapsulated component groups or packets . stay TCP/IP In the system , Due to the use of network layer IP agreement , So grouping is also called IP Datagram , Datagram for short .

Another task of the network layer is to select the appropriate route , Enable the packets transmitted from the transport layer of the source host to find the destination host through the router in the network .

The Internet is connected by a large number of heterogeneous networks through routers . The network layer protocol used by the Internet is a connectionless internet protocol IP And many routing protocols , Internet layer or Internet layer IP layer .

4. data link layer : When transferring data between two adjacent nodes , The data link layer is handed over from the network layer IP Datagrams are assembled into frames , The frame is transmitted on the link between two adjacent nodes . Each frame includes data and necessary control information . The control information enables the receiver to detect whether there is an error in the received frame .

5. physical layer : The unit of data transmitted on the physical layer is bit . The physical layer does not add control information to the frames transmitted from the data link layer .

entity , agreement , Services and service access points

When studying information exchange in Open Systems , Entity represents any hardware or software process that can send or receive information .

The protocol is to control two peer entities ( One or more entities ) A collection of rules that communicate .

Under the control of the agreement , The communication between two peer-to-peer entities enables this layer to provide services to the upper layer . To implement this layer protocol , You also need to use the services provided by the following layer .

A protocol is a rule that controls the communication between peer entities , yes “ Horizontal ”. Services are provided from the lower layer to the upper layer through inter layer interfaces , yes “ Vertical ”. Not all functions completed in one layer are called services , Only those entities that can be higher “ visible ” The function of is called service .

Entities of two adjacent layers interact in the same system ( Exchange of information ) Place , Commonly referred to as a service access point .

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