# SDH (Synchronous Digital Hierarchy)

Please read the book "The ComSoc Guide to Next Generation Optical Transport SDH, SONET, OTN" before you get started the following read.

通信网的发展历史(The history of communication network):

1. PDH (Plesiochronous Digital Hierarchy)
2. SDH (Synchronous Digital Hierarchy)
3. DWDM (Dense Wavelength Division Multiplexing)
4. OTN (Optical Transport Network)
5. PTN (Packet Transmission Network)

## SDH (Synchronous Digital Hierarchy)

特点: 统一比特率、光接口，网管能力强，有充足的开销比特，采用同步复用，分插灵活。

> STM-N: Synchronous Transport Module of level N

**STM-1**： 155 Ｍb/s

**STM-4**： 622 Ｍb/s

**STM-16**： 2488 Ｍb/s or 2.5Gbit/s

**STM-64**： 9953 Ｍb/s or 10Gbit/s

> 它被用在铁路等专网。(为了稳定，`专用网络`一般使用落后技术)
>
> 话说自从我学了这个专业，才发现`网络`不仅指`The world wide network`

![](/files/-Lf5heEbxsguZQX25AcR)

1. 最上面一横排是`骨干层`，由`大型车站`组成，使用`OTN/DWDM`，速度可达`40x10G`
2. 第二排是`汇聚层`，由`小型车站`或`铁路局`组成，使用`SDH`，速度可达`2.5G (STM-16)`
3. 最后一排以及各种圆圈是`接入层`，表示一段线路(因为线路上才有大量`基站`)，使用`SDH`，速度可达`622 Mbit/s (STM-4)`


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