Hello Jonn,
the link speed is the rate at which bits can be sent over the link.
A 256 kbps serial link is capable of sending 256000 bits per second.
In QoS scenarios the link speed is considered when calculating the delay budget: one component of delay is the serialization delay that is a fixed time given a packet size and a link speed.
A packet of 100 bytes = 800 bits has a serialization delay over 256 kbps of :
800 / 256000 = 3,125 mseconds
this shouldn't be a critical time
However a packet of 1500 bytes = 8*1500 bits has a serialization delay of 46.8 msec
The problem may arise if a VoIP packet has to wait for a long packet transmission: it can make the difference between a delay of 3,5 msec or a delay of 50 msec this can cause delay variation = jitter that is an impairment for VOIP codec and decodec that are good in dealing with fixed delay.
So in very slow link like a 64 kbps in order to have an upper limit to delay for VOIP packets big data packets are fragmented in order to avoid to have a VOIP packet to wait for whole data packet to be transmitted but interleaving (multiplexing over time ) unfragmented VOIP packets with data fragments.
This allows for reduction of jitter and better VOIP quality.
Note: that is not enough to have the VoIP packet placed in a priority queue if the data packet arrives some microseconds before and the priority queue is empty it is transmitted by the scheduler. LFI (link fragmentation and interleaving) allows to have a VOIP packet to wait not the whole long data packet to be transmitted but just for the first chunk fragment of it minimizing jitter.
if you have a 10 Mbps link you don't need LFI link fragmentation and interleaving
Hope to help
Giuseppe
