1.4 Delay, Loss, and Throughput

• How do packet delay and loss occur?

  • Packets queue in router buffers waiting for turn for transmission; arrival rate to link (temporarily) exceeds output link capacity
  • Packet loss occurs when memory to hold queued packets fills up
• Processing delay is the time it takes routers to process the packet header. It checks for bit errors and usually these delays are less than a milliseconds.
• Queuing delay is the time waiting at output link for transmission. It depends on congestion level of router.
• Transmission delay is the amount of time required to transmit all of the packet's bits into the link.
• Transmission delay is the packet length (bits) divided by the link transmission rate (bps)
• Propagation delay the time required to propagate from the beginning of the link to router B.
• The propagation delay is the distance between two routers divided by the propagation speed.
• If arrival rate (in bits) to link exceeds transmission rate (capacity) of link for a period of time:

  • packets will queue (buffered), wait to be transmitted on link
  • packets can be dropped (lost) if memory (buffer) fills up
• Lost packet may be retransmitted by previous node, by source end system or not at all.
• What affects queuing delay?

  traffic arrival rate (La), speed of outgoing link (R), nature of arriving traffic (uniform or bursty)
• If the traffic intensity (La/R) is ~0, the average queuing delay is small.
• If the traffic intensity (La/R) is 1, the average queuing delay is large.
• If the traffic intensity (La/R) is greater than 1, then there is more traffic than the network can handle. The average queuing delay is infinite.
• Throughput is the rate (bits/time unit) at which bits are transferred between sender/receiver