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本节概述
传输速率与带宽对应表如下，注：此表为全双工通信速率且没考虑编码方式
详细说明
我们在接触PCIe设备时会接触到一个词，传输速率，比如8GT/s。或者有人问你设备的带宽多少？通信速率多少？
Width（带宽）：通常用x1、x2、x4、x8等表示，表示该链路由几条lane组成。
Speed（速率）：通常用2.5GT/s、5GT/s、8GT/s，表示每条lane上的传输速率。PCIe1.0为2.5GT/s，PCIe2.0为5GT/s，PCIe3.0为8GT/s，
GT/s —— Giga transation per second
（千兆传输/秒），即每一秒内传输的次数。重点在于描述物理层通信协议的速率属性，可以不和链路宽度等关联。
Gbps —— Giga Bits Per Second （千兆位/秒）。GT/s 与Gbps 之间不存在成比例的换算关系。
PCIe 链路吞吐量计算方法：
吞吐量 = 速率 * 带宽 * 2方向(全双工) * 编码方式
例如：PCI-e2.0 协议支持 5.0 GT/s，即每一条Lane 上支持每秒钟内传输 5G个bit；但这并不意味着 PCIe
2.0协议的每一条Lane支持 5Gbps 的速率。为什么这么说呢？因为PCIe 2.0 的物理层协议中使用的是 8b/10b 的编码方式。
即每传输8个bit，需要发送10个bit；这多出的2个bit并不是对上层有意义的信息。那么， PCIe 2.0协议的每一条Lane支持 5 * 8 / 10
= 4 Gbps 的速率。以一个PCIe 2.0 x8的通道为例，该链路的吞吐量=5GT * 8 * 2 * 8/10 = 64 Gbps = 8GB/s。
同理，PCI-e3.0 协议支持 8.0 GT/s, 即每一条Lane 上支持每秒钟内传输 8G个Bit。而PCIe 3.0 的物理层协议中使用的是
128b/130b 的编码方案。 即每传输128个bit，需要发送130个bit。那么， PCIe 3.0协议的每一条Lane支持 8 * 128 / 130
= 7.877 Gbps = 984.6 MB/s 的速率。以PCIe 3.0 x8的通道为例，该链路的吞吐量= 8GT * 8 * 2 * 128/130
= 126.031 Gbps = 15.754 GB/s。
