I/O Interfaces & Buses

Which describes the credit-based flow control used in PCI Express?

  1. AThe receiver advertises its free buffer space as credits, and the sender transmits packets only within those credits
  2. BThe sender transmits regardless of the receiver, and overflowing packets are discarded
  3. CUsers are billed according to the amount of data sent
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Answer: A. The receiver advertises its free buffer space as credits, and the sender transmits packets only within those credits

Sending a packet into a full receive buffer would waste effort in discarding and resending it. With credits, the receiver tells the sender how much more it can accept, the sender transmits only within that amount, and the receiver returns credits as it processes packets and frees buffer space. Overflow never occurs, so there is no discarding and resending.

  • A. Correct. Buffers never overflow.
  • B. The mechanism exists precisely to prevent overflow.
  • C. It has nothing to do with financial credit.
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More I/O Interfaces & Buses questions

  1. In AI systems with many GPUs, why are dedicated high-speed GPU-to-GPU links (such as NVIDIA NVLink) used in addition to PCI Express?
  2. In PCI Express, sending many packets that each write 64 bytes yields effective bandwidth well below the theoretical figure. What is the main reason?
  3. Which statement is correct about DMA using a ring of descriptors in memory, as widely used by network cards and NVMe SSDs?
  4. Which statement is correct about PCI Express link power saving (the L0s and L1 states of ASPM)?
  5. Why have high-speed connections between chips and boards moved from parallel transmission over many wires to serial transmission over a few fast wires?