Allied Telesis AT-S62 - Manual (Page 113)

AT-S62 Management Software Menus Interface User’s Guide
Section I: Basic Operations 113
6 - Duplex
This selection is used to set the duplex mode of a port. The option only
appears when option 4 - Negotiation is set to Manual. The possible
settings are:
Full Full-duplex
Half 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%