GarrettCom 4K-Series Manual De Usuario

Magnum 4K8 Switches      Installation and User Guide      (10/04) 

 

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The Magnum 4K8 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 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 4K8 Switches dynamically allocates buffer space from an 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 result.) This dynamic buffer allocation provides the 

capability for the maximum resources of the Magnum 4K8 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 4K8 Switches are constantly adapting internally to provide 

maximum network performance with the least dropped packets. 

When the 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.