Class A:
Nyquist theorem (Nyquist criterion)
V = the number of different symbols
In any channel, symbol transmission rate is capped
The wider the channel frequency band, transmission can be performed efficiently with a higher symbol rate
To increase the transmission rate of the data, it is necessary to try to make each symbol carries more bits of information - a modulation method requires diverse
Shannon's theorem
The larger the SNR, the higher limit of the transmission rate information
For a given transmission bandwidth and a certain signal to noise ratio limit, the information transmission rate is determined
As long as the transmission rate information is less than the limit of the channel transmission rate, to find a way to achieve transmission undifferentiated
The actual channel transmission rate can reach a lot lower than it
From one side surface, the number of bits corresponding to one symbol is limited
Coding: the data is converted into a digital signal
NRZ NRZ coding
Manchester encoding
Differential Manchester encoding
4B / 5B encoding
Circuit Switched
Two nodes to establish a
dedicated physical communication path
. This path has been occupied during the entire data transmission. Into connection establishment, data transfer, connection release.
advantage
Communication
delays Small
Orderly
transfer
There is no conflict
Adapt to a wide range
And real time
Simple control
Shortcoming
Establish a
connection for a long time
Line
Exclusive
Inflexible
: as long as the two sides to establish any communication channel in a little out of order, it is necessary to re-establish a dial-up connection
Difficult to normalize
Message switching
Unit of exchange is the
message
advantage
Without establishing a connection
Dynamic allocation
line
Improving line
reliability
Increase line
efficiency
It provides multi-target service
Shortcoming
Forwarding delay
Network nodes need to have
a larger buffer
Packet switching
:
limiting the upper limit of each transmission data block
,
large logical data is divided into small data blocks
, together with the necessary control information, configuration packet
Advantages (angle: latency, utilization, storage, error)
No setup delay
High line utilization
Simplifies storage management
(with respect to the packet exchange)
Expedited transfer
Reduces the probability of errors
and
the amount of retransmission data
Shortcoming
There is
transmission delay
Need to transmit
additional information
classification:
Connection-oriented virtual circuit
: combines the advantages of circuit-switched and packet data, the establishment of
virtual circuits logically connected
Three phases: virtual circuit
establishment
, data
transmission
and VC
release
Setup and teardown time overhead required
. Short packet is too much to high for a long time frequent data exchange efficiency
After the connection is established, it is determined that the transmission path
It provides
a reliable
communication function
A node failure, can cause destruction of VC
Packet header does not include the destination address
Link may be a plurality of virtual circuits
Connectionless datagram
Without establishing a connection
The maximum extent possible to deliver
Complete address of the packet to be transmitted end and a receiving end
Forwarding delay
Strong ability to adapt to failures
Improve network throughput
High resource utilization
中继器：转发器。将信号整形并放大再转发出去
原理是
信号再生
工作在
物理层
“5-4-3规则”：5段通信介质，4个中继器，只有三段可以挂计算机。
不能互联两个物理层不同的网段（不是存储转发设备）
集线器（Hub）：
多端口中继器。
工作在物理层
半双工状态
B级：
基带：基本频带，指传输变换前所占用的频带，是原始信号所固有的频带
基带传输:在传输时
直接使用基带数字信号(
不转换为模拟信号，即
不调制)
传输介质：
UTP双绞线：两根采用一定规律
并排绞合的相互绝缘的
铜导线组成。
STP屏蔽双绞线：UTP加一个金属丝编织成的屏蔽层
同轴电缆：由内导体、绝缘层、屏蔽层和塑料外层组成
50Ω同轴电缆（基带同轴电缆）：用于局域网
75Ω同轴电缆（宽带同轴电缆）：用于有线电视
物理层接口的特性：（
机械电工养了一只乌
龟）
机械特性：物理连接的边界点。引脚的数目
电气特性：信号的电压高低，传输速率，距离限制
功能特性：某一电平的电压的意义
规程特性：工作规范与时序关系
C级：
数据是指传送信息的实体
信号是指数据的电气或电磁表现
数据传输方式：并行传输，串行传输
数字数据（数字信号）：取值仅允许有限的几个离散数值的数据（或信号）
模拟数据（模拟信号）：连续变化的数据（或信号）
码元：一个固定时长的信号波形来表示一位k进制数字。这个时长称为码元宽度
信道：模拟信道和数字信道；无线信道与有线信道
基带传输：基带信号将数字信号1和0用两种不同的电压标识，送到数字信道上传输
宽带传输：宽带信号将基带信号进行调制后形成频分复用模拟信号，然后传送到模拟信道上传输
单工通信，半双工通信，全双工通信
码元传输速率：单位是波特
信息传输速率（信息速率，比特率）：单位是比特/秒（b/s）
调制：数据变换为模拟信号的过程
ASK幅移键控
FSK频移键控
PSK相移键控
QAM正交振幅调制
模拟数据编码为数字信号——脉码调制PCM
采样定理（奈奎斯特定理）：采样频率是原始信号的2倍
模拟数据调制为模拟信号——频分复用FDM
多模光纤（利用光的全反射）：近距离传输
单模光纤（光纤直径减少至一个光波长度）：适合远距离传输
无线电波：无线手机通信，无线局域网
微波、红外线和激光
