Cell Relay Archive

[murray@pa.dec.com (Hal Murray)]

> Can anyone tell me what's the meaning of head-of-line blocking?

Suppose you are building a switch or router and you decide
to store packets in a FIFO rather than in RAM.

Suppose each link has a separate FIFO and the packet on the
front of the FIFO for link A wants to go out link B
and the next one wants to go out link C.

But suppose Link B is busy with another packet, say from link C.

If link C (outgoing side) is idle, then you could send the
second packet in the FIFO.  But you can't do that because
it's a FIFO and you can only access the first packet.

That's head of line blocking - packets behind the first
one get blocked if the first one needs a resource that is busy.

-- 
These are my opinions, not necessarily my employers.