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Thread: Which route will the signal take

  1. #1
    handscombmp
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    Which route will the signal take

    This has been bugging em for sometime now and i've never really bothered to investigate it till today.
    I really want to see which route the signal takes as i wonder it could be taking the long way could decrease my transfer rate.

    I've currently go a my main PC, Server & Xbox all connected to a switch. This switch is in turn connected to my modem/router via one cable. At the router i have my another two pc's connected.

    Now say my main pc wants to talk to the server i assume the switch has the brains to instead of sending all the way to the router and back it just goes through the switch and directly to the server.

    I ask this as how i see it if it's going to the router it's having to share the bandwith as it travels there and back decreasing the transfer times.

  2. #2
    Does he need a reason? Funkstar's Avatar
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    Re: Which route will the signal take

    Quote Originally Posted by handscombmp View Post
    Now say my main pc wants to talk to the server i assume the switch has the brains to instead of sending all the way to the router and back it just goes through the switch and directly to the server.
    The data will only flow between the devices that are in communication. The switch will make sure the data goes to and from the PC and server directly without involving the router.

    The router is only really routing information between your network and the outside network, hence the routing part of it.

    You could have a dozen PC all connected to a switch with no other network hardware and still get it to work. You would need to asign static IPs or set up a DHCP server on one box (not dificult) but there rally isn't anything to it.

    I think this is what happens: the very first request from PC1 is sent to the switch, which then polls each socket in turn asking for a response, once it finds the port with the correct response (i.e. the device with the correct IP replied) it remember which socket that is and only sends trafic for that MAC address to that port. There could be several MAC addresses on one port (a nested switch) and the same MAC address answering to mulitple IPs (virtual servers).

    Ethernet is complicated, but it is fantastic at what it does, and at this level it is all plug and play too. it's amazing how they got something so complicated on paper to be so easy in practice

    A good way to test all this is if you are using a gigabit switch and the two PCs are gigabit. The transfer will be aproaching gigabit speeds (note you never get the full speed unless using very exotic equipment). if it were going via the router you would be limited to the 100mbit of that link (assuming it has a 100mbit switch in there).

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  4. #3
    handscombmp
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    Re: Which route will the signal take

    Thanks funkstar.

    That roughly what i thought (though not to that level of detail )

    I have to agree with you on how fantastic it is and it works just on plug and play.

    Why most thinks can't work like that is beyond me

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    Does he need a reason? Funkstar's Avatar
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    Re: Which route will the signal take

    Quote Originally Posted by handscombmp View Post
    That roughly what i thought (though not to that level of detail )
    Thats not detail. You can do whole degrees on this stuff. I sometimes wish I'd done that at uni instead of SompSci.

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    Re: Which route will the signal take

    Just to clear up the above......when a switch doesn't know what interface a particular device is connected to it floods the frame out of all interfaces apart from the one it received the frame on. Every time a switch receives a frame it takes the source MAC address and puts it in the forwarding information base (FIB) along with the associated interface (if it's already there it updates the aging timer). The next time a frame is received from the same source address the switch will do a look up, find the entry in the FIB and send the frame out the correct interface. When the aging timer expires the entry is removed from the FIB and the process starts again.

    Bear in mind your average home switch is a layer 2 device, it has no knowledge of IP addresses and works only with MAC addresses. A protocol called Address Resolution Protocol (ARP) is used to get from an IP address to a MAC address and is pretty simple in operation

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    The late but legendary peterb - Onward and Upward peterb's Avatar
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    Re: Which route will the signal take

    Indeed as Funky Monkey says.

    IP addressing is only used when you are connecting to another network (or sub network) via the router (which is a layer 3 device) If the computer raising a connection request doesn't recognise the packet as going to a device within its own (sub) network through the switch, it will send that packet to the default gateway - the router. That router will then forward the packet - using the IP address - to the network router it thinks will be able to find the destination. In the case of a home router - the gateway - there will only be one forwarding address - to the ISP gateway.
    Last edited by peterb; 17-07-2009 at 06:30 AM.
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  8. #7
    handscombmp
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    Re: Which route will the signal take

    Ok i'm sorry to bring this thread up again but it seemed to make sense as it's bascially on the same lines (and it was my thread in the first place )

    Yets say i've now got three homeplugs. The Acess Point (ap) homeplug is plugged into the router (which a pc is plugged into in turn).
    Homeplug one has a HTPC connected to it and homeplug two is connected to a switch which has a pc,xbox and server plugged into it.

    Now say the HTPC wants to access something from the server does it now have to go all the way to the ap homeplug to the router then back through the ap homeplug all the way to the htpc.
    Or can it go from homeplug two straight to homeplug one.

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    Re: Which route will the signal take

    In the majority of cases if devices are in the same subnet and are connected by switches then traffic will take the most direct route there. If they are in a different subnet then traffic will need to be routed from one subnet to the other so will go via a router. In your case I'm assuming everything is in the same subnet so traffic will go direct.

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