Magnum 4K220 Switches Installation and User Guide (10/04)
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enabling all of the available bandwidth to be used for valid information.
While other switching technologies such as "cut-through" or "express" impose
minimal frame latency, they will also permit bad frames to propagate out to the Ethernet
segments connected. The "cut-through" technique permits collision fragment frames,
which are a result of late collisions, to be forwarded to add to the network traffic. Since
there is no way to filter frames with a bad CRC (the entire frame must be present in
order for CRC to be calculated), the 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 4K220 Switch dynamically allocates buffer space from a 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 may result.) This dynamic buffer allocation provides the
capability for the maximum resources of the Magnum 4K220 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 4K220 Switches are constantly adapting internally to
provide maximum network performance with the least dropped packets.
When the 4K220 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 4K220 Switches 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 half-duplex ports in order to stop incoming
traffic.
