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Pause Threshold for Long Distance Links长途链路的暂停阈值
This section uses the following basic concepts: 本节使用以下基本概念：
Bit Time (BT): It is the time taken to transmit one bit. It is the reciprocal of the bit rate. For example, BT of a 10 GbE port is 1/10,000,000,000 seconds or 0.1 nanoseconds and that of 100 GbE port is 0.01 nanoseconds. 它是传输一个比特所需的时间。它是比特率的倒数。例如，10 GbE 端口的 BT 为 1/10,000,000,000 秒或 0.1 纳秒，100 GbE 端口的 BT 为 0.01 纳秒。
Pause Quanta: It is the time taken to transmit 512 bits. In other words, it is 512 BT. 它是传输 512 比特所需的时间。换句话说，就是 512 BT。
Inter-Frame Gap (IFG): Ethernet has an inter-frame gap of 12 bytes. It takes 96 BT for transmission. 以太网的帧间间隔为 12 个字节。传输需要 96 个字节。
Preamble and Start Frame Delimiter (SFD): An Ethernet frame starts with 7 bytes of preamble and 1 byte of SFD. These 8 bytes are typically not counted in an Ethernet frame size of 1522 bytes. Collectively, the preamble and SFD take 64 BT for transmission. 以太网帧以 7 个字节的preamble和 1 个字节的 SFD 开始。这 8 个字节通常不计入 1522 字节的以太网帧大小中。前言和 SFD 的总传输时间为 64 BT。
As Figure 7-2 explains, the headroom on a traffic receiver should be large enough to accommodate frames during the following delay factors. 如图 7-2 所示，流量接收器的headroom应足够大，以容纳下列延迟因素下的帧。
Figure 7-2 Worst-case delay for calculating the headroom for PFC.
1. Delay due to transmission of a frame of maximum length (D-Max-Frame-Len) 传输最大长度帧时产生的延迟 (D-Max-Frame-Len): The queue on the traffic receiver reached the Pause Threshold. But, at the traffic receiver, a frame just started transmission. A Pause frame does not preempt the transmission of another frame. Hence, the transmission of the Pause frame is delayed until the other frame is transmitted. This delay accounts for the maximum frame length of all the traffic classes. Considering the maximum frame length of 9216 bytes, this delay can be up to 73728 BT (9216 x 8). Adding 96 BT for IFG and 64 BT for preamble and SFD, the total delay can be up to 73888 BT. 流量接收器上的队列达到了暂停阈值。但在流量接收器上，一个帧刚刚开始传输。暂停帧不会抢先传输另一个帧。因此，暂停帧的传输会延迟到其他帧传输完毕。这一延迟占所有流量类别的最大帧长。考虑到最大帧长为 9216 字节，这一延迟可达 73728 BT（9216 x 8）。再加上 IFG 的 96 BT 以及preamble和 SFD 的 64 BT，总延迟可达 73888 BT。
2. Delay due to transmission of Pause frame (D-Pause) 传输暂停帧（D-暂停）导致的延迟: The size of a Pause frame is 64 bytes, and therefore it takes 512 BT for transmission. Adding 96 BT for IFG and 64 BT for preamble and SFD, the total delay is 672 BT. 暂停帧的大小为 64 字节，因此传输需要 512 BT。加上 IFG 的 96 BT 和preamble及 SFD 的 64 BT，总延迟为 672 BT。
3. Interface delay for transmitting the Pause frame (D-Intf) 传输暂停帧的接口延迟 (D-Intf): This is the delay caused by the lower layers while transmitting and receiving a frame. Ethernet standards define the upper value of this interface delay. For example, maximum delays of 8192 BT for 10 GbE, 6144 BT for 25 GbE, 24576 BT for 40 GbE, 122880 BT for 100 GbE. These values include transmit and receive delay. There are different implementations at the same Ethernet speed, and each may have a different interface delay. For further details, refer to IEEE Standard for Ethernet 802.3 and search for delay constraint. 这是低层在传输和接收帧时造成的延迟。以太网标准规定了该接口延迟的上限。例如，10 GbE 的最大延迟为 8192 BT，25 GbE 为 6144 BT，40 GbE 为 24576 BT，100 GbE
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