Allied Telesis AT-S62 - Manual (Page 102)

Chapter 6: Port Parameters
Section I: Basic Operations 102
Figure 23 Head of Line Blocking
The HOL Limit parameter can help prevent this problem from
occurring. This parameter sets a threshold on the utilization of a
port’s egress queue. When the threshold for a port is exceeded,
the switch signals other ports to discard packets to the
oversubscribed port.
For example, referring to the figure above, when the utilization of
the storage capacity of Port D exceeds the threshold, the switch
signals the other ports to discard packets destined for Port D. Port
A drops the D packets, enabling it to once again forward packets
to Port C.
The number for this value represents cells. A cell is 64 bytes. The
range is 1 to 61,440 cells. The default is 61,440.
8 - Flow Control
Sets flow control on the port. This option applies only to ports
operating in full-duplex mode.
A switch port uses flow control to control the flow of ingress
packets from its end node.
A port using flow control issues a special frame, referred to as a
PAUSE frame, as specified in the IEEE 802.3x standard, to stop the
transmission of data from an end node. When a port needs to stop
an end node from transmitting data, it issues this frame. The frame
instructs the end node to cease transmission. The port continues
to issue PAUSE frames until it is ready again to receive data from
the end node.
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%