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Magnum 6KQE Managed Edge Switch Installation and User Guide 02/09
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2.2.2 Frame Buffering and Flow Control
Magnum 6KQE’s are store-and-forward switches. Each frame (or packet) is
loaded into the Switch’s memory and inspected before forwarding can occur. This
technique ensures that all forwarded frames are of a valid length and have the correct
CRC, i.e., are good packets. This eliminates the propagation of bad packets, 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 which 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.
When the Magnum 6KQE 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 it packets to cause “flow control”. This tells the sending devices to
temporarily stop sending traffic, which allows the traffic to catch-up 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.
Another feature implemented in the Magnum 6KQE 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.