Magnum ITS
Blade
Hardened Switches for 2070’s Installation and User Guide (02/03)
8
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 ITS
Blade
Hardened Switch dynamically allocates buffer space from its 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 ITS
Blade
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 ITS
Blade
Hardened Switches are
constantly adapting internally to provide maximum network performance with the least dropped
packets.
When the ITS
Blade
Hardened 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 ITS
Blade
Hardened 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
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