GarrettCom DS60s Installation And User Manual

GarrettCom DS60s Installation And User Manual Manual pdf 16 page image

Magnum DS50 / DS60s Dual Speed Hubs Installation and User Guide (06/02)The up-link operates the same, whether Port # 1 is connected to either a 100Mbps or 10Mbpsdevice.When the up-link port is used to cascade two DS50 / DS60 hubs, the auto-sensingfeature of the DS50 / DS60 will cause the connecting link to operate at 100Mbps speed.2.3 Frame Buffering and LatencyThe Magnum DS60’s 6th port is a store-and-forward switch. Each frame (or packet)is loaded into the Switch’s memory and inspected before forwarding can occur. This techniqueensures that all forwarded frames are of a valid length and have the correct CRC (i.e., are goodpackets). This eliminates the propagation of bad packets, enabling all of the availablebandwidth to be used for valid information.While other switching technologies such as "cut-through" or "express" imposeminimal frame latency, they will also permit bad frames to propagate out to the Ethernetsegments connected. The "cut-through" technique permits collision fragment frames, which area result of late collisions, to be forwarded to add to the network traffic. Since there is no way tofilter frames with a bad CRC (the entire frame must be present in order for CRC to becalculated), 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 isheavy, the result of store-and-forward operation is that more bandwidth is available for goodpackets when the traffic load is greatest.To minimize the possibility of dropping frames on congested ports, Magnum DS60’sSwitched port dynamically allocates buffer space from an 1MB memory pool, ensuring thatheavily used ports receive very large buffer space for packet storage. (Many other switcheshave their packet buffer storage space divided evenly across all ports, resulting in a small, fixednumber 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 to theDS60’s 6th port to be applied to all traffic loads, even when the traffic activity is unbalancedacross the ports. Since the traffic on an operating network is constantly varying in packetdensity per port and in aggregate density, the Magnum DS60’s Switched 6th port is constantlyadapting 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 sendsindustry 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 allowsa traffic catch-up to occur without dropping packets. Then, normal packet buffering and12www GarrettCom com. .