Everything about Head-of-line Blocking totally explained
Head-of-line blocking (HOL) is a phenomenon that appears in buffered telecommunication network switches. A switch is usually made of buffered input ports, a switch fabric and buffered output ports. Because of the
FIFO nature of the input buffers and switch design, the switch fabric can only switch the packets at the head of the buffer per cycle. HOL arises when packets arriving at different input ports are destined for the same output port. If the HOL packet of a certain buffer at the input can't be switched to an output port because of contention, the rest of the packets in
that buffer are blocked by that Head-of-Line packet, even if there's no contention at the destination output ports for those packets. The phenomenon may have severe performance-degrading effects in input-buffered systems.
Effect on switch throughput
This phenomenon limits the throughput of switches to 58,6% [InputVersus Output Queueing on a Space-Division Packet Switch; Karol, M.; Hluchyj, M.; Morgan, S.;, IEEE Transactions on Communications, Volume 35, Issue 12, Dec 1987 Page(s):1347 - 1356 ].
Overcoming HOL
One way this drawback is overcome is by using
Virtual Output Queues.
Packet reordering
HOL can significantly increase packet reordering - Jon C. R. Bennett, Craig Partridge, Nicholas Shectman, Packet reordering isn't pathological network behavior, IEEE/ACM Transactions on Networking (TON), v.7 n.6, p.789-798, Dec. 1999. In the paper, it gives detail diagram and description how HOL occurs and results.
Further Information
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