Frame Buffering And Flow Control - GarrettCom Magnum 6K32 Hardware Installation And User's Manual

Magnum 6K32 Managed Switch
fulfills its role in support of QOS, giving packet processing priority to priority tagged
packets according to the 802.1p standard. In addition to hardware support for QOS, the
MNS software (R2) supports two priority queues that can be shared across the eight
levels of defined packet priorities for application-specific priority control by the user
through software configuration settings.
2.2.8

Frame Buffering and Flow Control

Magnum 6K32'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 6K32 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.
Another feature implemented in Magnum 6K32 Switches is a collision-based
flow-control mechanism (when operating 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 to stop incoming traffic.
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