Magnum P62F-Series Ethernet Switches Installation and User Guide (11/01)
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result of indiscriminate cut-through forwarding is greater traffic congestion, especially at peak
activity. Since collisions and bad packets are more likely when traffic is heavy, the result of
store-and-forward operation is that more bandwidth is available for good packets when the
traffic load is greatest.
To minimize the possibility of dropping frames on congested ports, each Magnum
P62F Ethernet Switch dynamically allocates buffer space from an 1MB memory pool, ensuring
that heavily used ports receive very large buffer space for packet storage. (Many other switches
have their packet buffer storage space divided evenly across all ports, resulting in a small, fixed
number of packets to be stored per port. When the port buffer fills up, dropped packets result.)
This dynamic buffer allocation provides the capability for the maximum resources of the
Magnum P62F unit to be applied to all traffic loads, even when the traffic activity is unbalanced
across the ports. Since the traffic on an operating network is constantly varying in packet
density per port and in aggregate density, the Magnum P62F Ethernet Switches are constantly
adapting internally to provide maximum network performance with the least dropped packets.
When the P62F Switch detects that its free buffer queue space is low, the Switch sends
industry standard (full-duplex only) PAUSE packets out to the devices sending packets to cause
“flow control”. This tells the sending devices to temporarily stop sending traffic, which allows a
traffic catch-up to occur without dropping packets. Then, normal packet buffering and
processing resumes. This flow-control sequence occurs in a small fraction of a second and is
transparent to an observer. See Section 4.6 for additional details.
Another feature implemented in Magnum P62F Ethernet Switch is a collision-based
flow-control mechanism (when operating at half-duplex only). When the Switch detects that its
free buffer queue space is low, the Switch prevents more frames from entering by forcing a
collision signal on all receiving RJ-45 half-duplex ports in order to stop incoming traffic.
The latency (the time the frame spends in the Switch before it is sent along or
forwarded to its destination) of the P62F Ethernet Switch varies with the port-speed types, and
the length of the frame is a variable here, as it is with all store-and-forward switches. For 10
Mb-to-10 Mb or 10 Mb-to-100Mb or 100Mb-to-10 Mb forwarding, the latency is 15
microseconds plus the packet time at 10Mb. For 100Mb-to-100Mb forwarding, the latency is 5
microseconds plus the packet time at 100Mb.
