The Delay-Bandwidth Product

1. The Delay-Bandwidth Product “Keeping the pipe full”

2. Effective Data Rate • The nominal data rate determines the transmit time for a frame or other atomic unit of transmission • But users measure the performance of their applications and essentially compute their own effective data rate • If the network is idle between frames then clearly the application will not have an effective data rate close to the nominal data rate • This was illustrated to some extent with the gaps created by access delay, or even the increased interval between frames that might result from queuing

3. The problem • For a protocol to roughly achieve the nominal data rate of the slowest link it traverses it must “keep the pipe full” on that slowest link • It must avoid creating time gaps when the network is idle • This can be a problem for protocols that depend on feedback to continue • Suppose we send 1500 bytes and wait for a 100 byte acknowledgement (ACK) in order to send more data (a simple, reliable protocol) • Consider LAN and WAN example networks with some typical characteristics

4. LAN and WAN examples

5. Notional diagram for the WAN

6. It hurts when I do that… • Then don’t do that! This “Stop and Wait” reliable protocol is not a good choice for a WAN • Other reliable protocols buffer the data they send so that then can resend data if no ACK is received – but in the mean time they can keep sending while waiting for ACKs • How much data does the sender need to buffer? • It must buffer each bit of data while waiting for the ACK for that data to return • All the additional data sent while waiting for the first ACK must also be buffered to wait for its respective ACK • The answer is the “delay-bandwidth product” • This is actually the “delay-data-rate product”

7. The WAN example with the pipe full

8. Delay Data-rate Product • The round-trip delay is the time it takes between sending some data and receiving its ACK • Divide that delay by the duration of a bit and we would have the number of bits that could be sent during that delay period • But the duration of a bit is the inverse of the data rate • So multiply by the data rate instead of dividing by the bit duration

9. Summary • The round-trip delay times the data rate of the slowest link is the amount of data that must be “in-flight” or “on-the-wire” to avoid idle time while waiting for an ACK • Sometimes the one-way delay is used to compute a one-way delay-bandwidth product • In the WAN situations where delay bandwidth product is significant it is usually “good enough” to just use the propagation delay and then double or triple the result to deal with the potential for queuing delay, etc. • We will see in another lesson that bandwidth and data rate are often numerically similar