GarrettCom 6K32TRC - Manual (Page 24)

Magnum 6K32T & 6K32TRC Managed Switch Installation and User Guide 05/07)
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2.2.7 Packet Prioritization, 802.1p QOS
Quality of Service means providing consistent predictable data delivery to
users from datagram paths that go all across a network. As a LAN device, the Magnum
6K32T & 6K32TRC can do its part to prevent any QOS degradation while it is handling
Ethernet traffic through its ports and buffers.
The Magnum 6K32T & 6K32TRC switching hardware supports the IEEE
802.1p standard and 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 (R3) 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 6K32T & 6K32TRC’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 6K32T & 6K32TRC 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.