some sort of questions

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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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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

 

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)

 

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.

 

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?

 

@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.

 

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.

 

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.

 

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?

 

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.

 

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.

 

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 ?

 

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 ?

 

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.

 

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.

 

Is it too late for the ?

 

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?

 

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.

 

Yep - see here: link