Allied Telesis AT-S62 - Manual (Page 102)

Chapter 6: Port Parameters
Section I: Basic Operations 102
The possible settings for the duplex mode are Full-duplex and
Half-duplex.
7 - HOL Blocking Prevention Threshold
Head of line (HOL) blocking is a problem that occurs when a port
on a switch becomes oversubscribed. An oversubscribed port is
receiving more packets from other switch ports than it can
transmit in a timely manner.
The problem an oversubscribed port can create is that it can
prevent other ports from forwarding packets to each other. This is
because ingress packets on a port are buffered in a First In, First
Out (FIFO) manner. If the head of an ingress queue consists of a
packet destined for an oversubscribed port, the ingress queue will
not be able to forward any of its other packets to the egress
queues of other ports.
A simplified version of the problem is illustrated in Figure 24. It
shows four ports on a switch. Port D is receiving packets from two
ports—50% of the ingress traffic on Port A and 100% of the
ingress traffic on Port B. The result is that not only is Port A unable
to forward packets to Port D because the latter’s egress queues
are filled with packets from Port B, but it is also unable to forward
traffic to Port C because its ingress queue has frames destined to
Port D that it is unable to forward.
Figure 24 Head of Line Blocking
Port A
C C C C D D D D
Ingress Queue
Port B
D D D D D D D D
Ingress Queue
Port C
Egress Queue
Port D
D D D D D D D D
Engress Queue
100%
50%
50%