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5th Oct 2009, 7:08 PM #1
Cisco Sem 3 Chapter 3 - help/advice/explanations needed!
Just revising Cisco Sem 3 Chapter 3 atm (well, just finished Section 1 just now) and there's a few things that I just don't get -
When a host connects to a switch port, the switch creates a dedicated connection. When two connected hosts communicate with each other, the switch consults the switching table and establishes a virtual connection, or microsegment, between the ports.
The switch maintains the virtual circuit (VC) until the session terminates. Multiple virtual circuits are active at the same time. Microsegmentation improves bandwidth utilization by reducing collisions and by allowing multiple simultaneous connections.
Can someone please explain exactly what this means? As these two paragraphs just go completley over my head...
Store and Forward
In this type of switching, the entire frame is read and stored in memory before being sent to the destination device. The switch checks the integrity of the bits in the frame by recalculating the cyclic redundancy check (CRC) value. If the calculated CRC value is the same as the CRC field value in the frame, the switch forwards the frame out the destination port. The switch does not forward frames if the CRC values do not match. The CRC value is located within the frame check sequence (FCS) field of an Ethernet frame.
This "CRC" check thing, what the hell is the point of it exactly, cos as far as I can see, the switch recieves data that It then re-reads, and if the re-read data matches the old data it forwards it through the port but if it doesn't, it doesn't forward it through the port? Huh?!
Any help on this would be appreciated so much as I'm just getting so frustrated that the above paragraphs seem to make little or no sense to me at all, and they should... cos the information I'm meant to be digesting seems so simple!
Thanks in advance.Tom Simpson
My Top Three: "The War Games", "The Caves of Androzani" & "Time-Flight".
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