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broadcast v collision

Discussion in 'Networks' started by philbenson, Dec 3, 2007.

  1. philbenson

    philbenson Byte Poster

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    Morning all,

    I'm reading up about Ethernet networks in terms of broadcast domains and collision domains. To me this seems to be two names to describe the same thing.

    If I have a single network segment connected by a simple hub then quite clearly all hosts on that segment can communicate with each other directly using the MAC address and so we have by definition a broadcast domain.

    Now in the same segment, there is nothing then to prevent two hosts attempting to transmit at the same time thus producing a collision - in which case we also have a collision domain. I understand that when this happens, a JS or Jam Signal is sent out by the two hosts which caused the collision in order to prevent a repeat collision when the data is resent.

    I know that broadcast packets cannot pass through routers and so if we have a router connecting two broadcast domains, collisions can occur locally within each segment but it is not possible for collisions to occur between packets from different segments since the router is acting as a barrier.

    Am I right here and broadcast and collision domains basically the same thing or is there a distinction between the two?
     
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  2. simongrahamuk
    Honorary Member

    simongrahamuk Hmmmmmmm?

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    Colision domains are where a packet can collide with another packet. In theory on a switched network you should not get any collisions because packets are only sent out of the appropriate switch port (layer 2). Hubs on the other hand can cause collisions because packets are sent out of every single port except the on the the packet was received on.

    By placing switches into the network the (theoretical) number of collision domains is increased, but since there would only be one link (PC - Switch Port) collisions should not occur.

    As switches do not filter out broadcasts you still have a single broadcast domain, this is only broken up when a Layer 3 switch or a router is put inplace to seperate the broadcast domain.

    Thats my understanding of it, and I could well be wrong, but since no one else has yet had a go with an answer I though I'd try! :biggrin
     
  3. hbroomhall

    hbroomhall Petabyte Poster Gold Member

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    I'd say that was pretty much spot on.

    In addition, the use of UTP rather than co-ax means that even on the PC to switch link (for example) each end can transmit at the same time (a.k.a. full-duplex), so you won't be getting collisions there either!

    Harry.
     
    Certifications: ECDL A+ Network+ i-Net+
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  4. philbenson

    philbenson Byte Poster

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    Ok many thanks for that.

    So basically then collisions can occur in any broadcast domain then because a hub - as a layer 2 device - sends out a packet received on one port to all others. Whereas switches prevent collisions because they "learn" the MAC addresses of the hosts connected to each port and therefore know which port to forward the packet onto.

    I assume then that if you connected a hub with multiple clients to a single port on a switch, those hosts connected to the hub would still represent a broadcast domain with the potential for collisions again

    Is this right?
     
    Certifications: MCP, MCP+I, MCSE, MCSA, MCTS
    WIP: CCNA(?)
  5. simongrahamuk
    Honorary Member

    simongrahamuk Hmmmmmmm?

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    You still have a single broadcast domain right up until you hit a router or other layer 3 device regardless of hub or switch.

    What you have is a single collision domian on the link that goes from the switch to the hub, everything on the hub side of that link is in a single collision domain. Once you get to the switch collisions should no longer occur because it is 'intelligent' and each switch port represents its own collision domain.

    Hope that makes sense. 8)
     
  6. BosonMichael
    Highly Decorated Member Award

    BosonMichael Yottabyte Poster

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    Well, unless you implement VLANs on a switch... but that's somewhat complicated, and not encountered until you get to CCNA-level studies. :)
     
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  7. philbenson

    philbenson Byte Poster

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    Hi guys and sorry to keep on about this but I have found a diagram on this website

    http://164.109.41.59/category.php?filename=wp-31_network_fundamentals.xml

    Now if you scroll down the page you will find a diagram illustrating a switch with various ports connected either to hubs or single servers or clients. This is clear enough and I can appreciate that each port on the central switch represents a collision domain in itself, but I would have thought that each of the hubs also makes up a separate broadcast domain for their connected hosts.

    Obviously the term domain in this context is different to the one used to describe an Active Directory domain.
     
    Certifications: MCP, MCP+I, MCSE, MCSA, MCTS
    WIP: CCNA(?)

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