<> one , Overview of computer network

<>1. Basic concepts of computer network

<>1.1 Computer network definition

Use communication equipment and communication link or communication network , Independent calculation and system connected but with different bit values . However, in order to exchange information and follow a certain protocol .
Generally speaking : A computer network consists of several nodes and links connecting them .

<>1.2 Definition of agreement

Protocol is a protocol that needs to be followed in data exchange between protocols of network communication entities
It consists of three parts ( Three elements ):
(1) grammar ( Certain rules of writing )
(2) semantics ( Meaning of transmission data )
(3) sequential ( Event implementation sequence )

<>1.3 The function of computer network

1. Hardware resource sharing
2. Software resource sharing
3. information resources sharing

<>1.4 Classification of computer network

1. By coverage : Personal network , LAN , Metropolitan area network , Wan
2. By topology : Star shape , Bus type , annular , reticular , Tree , Hybrid topology, etc

3. By exchange mode : Circuit switched network , Packet switching network and packet switching network
4. By network user attribute : Public network , Private network

<>2. Computer network structure

<>2.1 Network edge

It consists of all the hosts on the Internet ( End system ), For ordinary users , Used to access various resources

<>2.2 Network core

It is composed of routers on the Internet , Most of them provide services for the network edge

<>2.3 Access network

Linking network edge and network core , Part way :
1. Telephone dialing
2. Asymmetric subscriber data line ADSL
3. Hybrid fiber coaxial cable HFC
4. LAN
5. Mobile access network

<>3. Data exchange technology

<>3.1 Data exchange concept

Data exchange (Data Switching) It refers to multiple data terminal devices (DTE) between , The process of establishing a temporary data communication interconnection path for any two terminal devices .
Communication subnet : Switching node and transmission medium

classification : Circuit switching , Message exchange , packet switching

<>3.2 Circuit switching

Establishing temporary dedicated lines for information exchange , Only for two users
advantage : High real time performance , The real delay and delay jitter are small . Therefore, it is suitable for the transmission of real-time and mass data .
shortcoming : Low channel utilization , Single transmission rate

<>3.3 Message exchange

In terms of message , Storage between nodes of exchange unit — Transmission by forwarding

advantage :
1. No connection is required
2. High line utilization
3. Flexible use
shortcoming :
It is not suitable for real-time business

<>3.4 packet switching

Storage is also used - Forwarding technology , Divide data into groups for transmission

advantage :
1. The exchange speed is fast
2. Reliable and efficient transmission
3. Switching devices need low storage capacity
shortcoming :
1. Packet length and delay time
2. Packet length and bit error rate

<>3.5 Mixed exchange

Circuit switching and packet switching are used simultaneously in a computer network , It's called hybrid switching

<>3.6 compare

<>4. Computer network performance

<>4.1 rate

The amount of data transmitted per unit time in a network , Also called data rate or bit rate . The basic unit is bits per second (bit/s)

<>4.2 bandwidth

It refers to the bandwidth of a signal , The unit is hertz . In computer network , It refers to the ability to transmit data in a network

<>4.3 time delay

It refers to the time of data from one node to the next .

1. transmission delay ( Transmission delay ): Time from the beginning to the end of sending data frame
2. Propagation delay : The time it takes for an electromagnetic wave to travel a certain distance in a channel
3. Processing delay : Processing time of packet message received by host or router
4. Queuing delay : When entering a node , Waiting time for this node to finish processing its own tasks

<>4.4 throughput

The actual amount of data per unit time through a network
It is often used to measure the actual data transmission capacity of the network

<>4.5 bandwidth delay product

bandwidth delay product = Propagation delay x bandwidth

<>4.6 round trip time RTT

Time taken for data to and fro

<>4.7 Utilization rate

Channel utilization : What percentage of the time in a certain channel is data passing through
Network utilization : Weighted average of channel utilization in the whole network

<>4.8 Packet loss rate

To a large extent, it can reflect the degree of network congestion
Packet loss rate = Total number of packet loss / Total number of groups

<>5. Computer network architecture

definition : The level of computer network and the set of protocols

<>5.1OSI reference model ( Basic reference model of Open Systems Interconnection )

<>5.2TCP/IPC reference model

<>5.3 Five layer reference model

At present, the Internet generally uses five layer model , Because it's better than the other two ways : Relatively simple and functional

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