What is Latency? And why is it such an issue?

Latency in the context of a bus journey
  • Consider a man’s journey home. The bus journey only takes 10 minutes but buses run only once every hour.
  • Question: The man leaves work at 5pm at what time will he get home?  
  • Answer: If the workman leaves work at 5pm and immediately catches a bus he will be home at 5:10pm. But if he has just missed the previous bus he will have to wait an hour for the next bus and be home at 6:10pm.  
  • His wife at home knows that if she has to wait more than 1hr 10mins  for him to travel home then something has gone wrong.  
  • A return journey can take from 20min to 2hrs 20min.

What is Latency?

Measuring Latency on a bus Journey

  • Latency is a measure of time delay experienced in a system. In the man’s journey the latency is between 10 mins and 1hr 10mins. His wife will happily wait 1hr 10mins for him but after this his time is out.  
Typical Latency
PCI card <1ms 1 char time at 9600 baud
USB ~16-75ms  16-75 char times
Local Network  5-20ms  5-20 char times
Internet 100-1,000ms  100 – 1,000 char time

Latency Variations

  • Latency is not a fixed quantity, it varies, in our example it has a two values that are a factor of 7 in range, 10mins or 1hr 10mins.
  • Often other external influences also effect the latency. Imagine that there is a lot of traffic on the road and the bus journey may no longer take 10 minutes.
  • Bus systems such as USB and Ethernet share the interface with other connected devices just as the workman’s bus shares the road withother traffic. The quoted latency for USB and Ethernet assume no delays due to external traffic on their buses due to other devices.
  • User serial programs have to be written to take account of real world latency

This is part 2 of a 3 part series:

Part 1: Brief History of Serial Communications

Part 3: The Software Challenge

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