计算机网络笔记:Physical Layer
date
Jun 21, 2021
slug
computer-networks-physical-layer
status
Published
summary
Time has left its curse upon this place
type
Post
tags
计算机网络
Notes for VE489, SU2021.
Digitization
SNR
- Nyquist: Perfect reconstruction if sampling rate
- Quantizer: 映射到个值上,动态范围
- ,. 用得到。()
- 所以用的位数越多 SNR 越大,但是信道的幅度是有上限的,增加到一定程度就无法和噪声区分开了(In the absence of noise, the bit rate can be increased without limit by increasing m)。
- bit rate:
Shannon Theory
- 在信道上进行无差错传输的理论最大传输速率,小于这个速率都可以找到零差错传输的办法。
和确定
- Given a channel bandwidth (baseband) of 20 MHz, if the SNR of the channel is 25 dB, then what is the maximum transmission rate that can be achieved in this channel for reliable communications?
- 信道质量
- bit rate
- BER
差错控制
Internet Checksum
(所有16-bit Word求和 mod ,然后取负)
放在 header 里,每个 router 检查一次。
Hamming Code
- 3/7, 4/15, 5/31, 6/63
m=3 例子
- 令
(这个设计可以改?对应的就是矩阵变一下)
- ,是 parity check matrix
- 认定没有传输错误