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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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