Simple Delay

Simple Delay
Simple delay causes packets to arrive later than might be expected. There are several contributing
factors to simple delay:
Laws of physics delay
The
laws of physics delay
is caused by the fact that data cannot be
propagated through either copper or fiber instantaneously, or even at anything like the speed of
light. In fact, about 60 percent of light speed in copper, and not much faster in fiber, is the norm.
The good news is that this delay is standard.
Data traveling across copper for a distance of 100 meters takes about 0.5 microseconds
to arrive. This might seem very small, almost insignificant. But for
data traveling at 100Mbits/second, this is a delay of about 50 bits!
Understanding the Fundamentals of QoS
645
Serialization delay
Serialization delay
is caused by store-and-forward devices such as
switches and routers having to place data onto an outgoing interface. The greater the interface
speed, the less time it takes to place the bits on the interface. So, the higher the speeds, the less
serialization delay. This is obviously unpredictable, because varying frame/packet sizes will
result in different delays.
Processing delay
Processing delay
is caused by the router or switch having to make a forwarding
decision. This is again variable and unpredictable, because it may depend upon the processing
overhead on that device at the moment of search, the internal buffer architecture and load, internal
bus load, and the searching algorithm in use. There may be some statistically measurable average,
but that’s no good for individual packets.
Output buffer priorities
Output buffer priorities
are the final stage of the delay. Should a
buffer become full, then the mechanism for discarding may be simple tail-drop, or something
more complex such as Random Early Discard. And if the queuing method is FIFO, then that
favors larger frames/packets, whereas if we implement sophisticated queuing, we must always
remember that putting one data stream at the front of the queue is bound to result in another
stream being at the back.
There are other components contributing to the total delay, but for the purposes
of the BCMSN exam, they can be ignored. A quick search for “serialization
delay” on the Web, however, reveals several educational sites with further
information for the adventurous reader.