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some sort of questions

Discussion in 'Networks' started by kobem, Apr 12, 2007.

  1. kobem

    kobem Megabyte Poster

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    general network ones

    1- in ethernet there is a jam signal event that stops
    other transmitting hosts sending data at the same time.

    thing confuses me is that if jam signal prevents , so how
    can collisions occur on the media?

    .............................

    2- one of the things cause congestion is
    "low bandwidth" but there is something i didn't figure out about it. Packets go in order and even if bandwidth is low since they go in order(using tcp) , it mustn't have congestion.

    where is the mistake?


    3- think two routers connecting to each other
    with a serial cable and one of them has
    1500 kbit bandwidth and other has 56000 kbit

    how can this be?
    (there is only one cable and it must have only one
    bandwidth i think)
     
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  2. Phoenix
    Honorary Member

    Phoenix 53656e696f7220 4d6f64

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    Hi mate

    Ethernet is a CSMA/CD system not a CSMA/CA system

    The Jam signal is sent after a collision has occured by the host intending to re transmit, this causes all other hosts to back off for a random period before transmitting

    Adapters do 'listen' briefly for traffic on the line before transmitting but there are chances that two can transmit at the same time even though it seemed clear for both, the chances of this obviously increase as the network gets busier

    hope that helps
     
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  3. kobem

    kobem Megabyte Poster

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    2- one of the things cause congestion is
    "low bandwidth" but there is something i didn't figure out about it. Packets go in order and even if bandwidth is low since they go in order(using tcp) , it mustn't have congestion.

    where is the mistake?


    3- think two routers connecting to each other
    with a serial cable and one of them has
    1500 kbit bandwidth and other has 56000 kbit

    how can this be?
    (there is only one cable and it must have only one
    bandwidth i think)
     
    Certifications: CCNA
  4. hbroomhall

    hbroomhall Petabyte Poster Gold Member

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    Mostly congestion is where many stations all want to talk at once. Because of the 'back-off' from collisions this means that the network spends a lot of time waiting rather than sending.

    For any link the *speed* settings at each end must be the same. However - speed and bandwidth are not quite the same. If one device was a very slow device you could set a high speed on its interfaces, but the actual rate of information would be less.

    Harry.
     
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  5. kobem

    kobem Megabyte Poster

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    i didn't understand exactly , so for the question
    must both routers have same bandwidth or not?

    2- one of the things cause congestion is
    "low bandwidth" but there is something i didn't figure out about it. Packets go in order and even if bandwidth is low since they go in order(using tcp) , it mustn't have congestion.
    where is the mistake?
     
    Certifications: CCNA
  6. Tinus1959

    Tinus1959 Gigabyte Poster

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    @2.
    Picture yourself a big room with people who are trying to make a speech. To get the attention they stand up and tap their spoon to a glas. If at the same moment someone else taps the glas to, they both sit down and wait for a random amount of seconds.
    It is clear that in a room with just 10 people all wanting to speak just 5 times an hour the traffic is less dense that in a room with 4000 people trying to speak 50 times an hour.

    @3
    On a serial link between two routers, one end is the DTE, the other end is the DCE
    The clock rate is set at the DCE side. 56000 is a normal value.
     
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  7. hbroomhall

    hbroomhall Petabyte Poster Gold Member

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    They must be running at the same *speed* (in fact one end defines the speed as Tinus says) but the *bandwidth* of the link will be the bandwidth of the slowest device.

    Harry.
     
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  8. Theprof

    Theprof Petabyte Poster Forum Leader

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    Basically you dont have to have both routers with the same speed of the interface.

    Example if you have a router that can transmit let say at 10mbps and another router transmitting at 5mbps the data being transfered will only be transefring at the 5mbps. The speed of the transfer is always going to be transfered at the slower speed if the routers have a different speed of each interface. likewise if both routers have 10mbps interface then the data is going to be transfered at 10mbps.
     
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  9. kobem

    kobem Megabyte Poster

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    think that you have routers and 100 Mbps
    ethernet cable between them

    so 100 Mbps is only one thing
    and if it is only one , why can routers have bandwidth values
    differently?

    ...........
    and

    for instance "low bandwidth" causes congestion.
    But there is something i don't understand , if packets go
    in order
    (all packets will go at the same speed), how can congestion occur?
     
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  10. hbroomhall

    hbroomhall Petabyte Poster Gold Member

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    Because the link isn't the only thing that can limit bandwidth. You *may* be limited by the boxes at either end.

    Because too many talkers are trying to use it. As Tinus said, 2 people talking in a room can talk quickly. 100 people talking means that you shout, and still miss stuff.

    Harry.
     
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  11. Bluerinse
    Honorary Member

    Bluerinse Exabyte Poster

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    Packets leave a *single* interface in order but there may be *many* interfaces talking on the same network and that is how congestion and collisions occur.
     
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  12. kobem

    kobem Megabyte Poster

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    ok , i figured some sort of it out but

    i asked you this and couldn't get the answer

    you said : congestion occurs because of many interfaces put
    the packets on the road but WHY "LOW BANDWIDTH"
    IS AN ISSUE?

    if many interfaces are available , it also occurs
    with "HIGH BANDWIDTH" i think ?
     
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  13. kobem

    kobem Megabyte Poster

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    with boxes did you mean router devices ?


    2-and last , look at this

    if i have 100 Mbps ethernet cable

    100 Mbps is link speed or bandwidth ?
     
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  14. hbroomhall

    hbroomhall Petabyte Poster Gold Member

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    Yes.

    Link speed. The actual bandwidth achieved depends on congestion and the general pattern of useage. And also on things like whether the link is full-duplex or not.

    Harry.
     
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  15. nugget
    Honorary Member

    nugget Junior toady

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    You might also like to check out the warriors of the net video here.

    It gives a good introduction to what happens to packets and maybe having it visually explained might help to understand it a bit better.
     
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  16. BosonMichael
    Highly Decorated Member Award

    BosonMichael Yottabyte Poster

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    Is it too late for the [​IMG] :morebeer :popcorn ?
     
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  17. kobem

    kobem Megabyte Poster

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    hey bosonmichael , are you brain beacon michael?

    hey hbroomhall , you said it is link speed

    i ask last things : think dsl and my isp gives me 256 Kbps

    so here 256 Kbps is link speed or bandwidth , if it
    is link speed what is bandwidth for me?

    second : i asked before but repeating it.

    congestion is seen on "low bandwidth" much compared
    to high bandwidth why?
     
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  18. hbroomhall

    hbroomhall Petabyte Poster Gold Member

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    It would be nice if it was 256kbits/sec as well. But many ISPs either traffic-shape, or throttle, or just don't have the resources, so stories of poor bandwidth are common.

    If I understand the question correctly - because it is much easier to overwhelm a slow system than it is a fast system.

    Harry.
     
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  19. BosonMichael
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    BosonMichael Yottabyte Poster

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    Yep - see here: link
     
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