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1024 maximum Addressable nodes

Last response: in Networking
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February 7, 2013 4:44:06 AM

I was told that Star Topology can only support 1024 maximum addressable nodes on a single segment, ie a 1024 port switch, Now i realize how crazy of a switch it would be to have 1024 ports but my real question is Why is the limit on a star topology 1024. Now I have googled the question and came up with tons of links that say the maximum supported is 1024 but nothing explains why? Also I understand that you can use multiple switches and exceed that 1024 but why 1024 per segment?

So to reiterate, Why can a star topology only support 1024 maximum addressable nodes per segment, and if this is not true please explain where this myth come from.
Thank you for your help
Nick

Example of the maximum
http://books.google.com/books?id=UD0h_GqgbHgC&pg=PA199&...

http://quizlet.com/14002418/ch-5-key-terms-1-cint-121-n...

Best solution

February 7, 2013 6:34:54 AM

It has to do with the algorithm that is used for collision detection (CDMA/CD) when sending data packets over Ethernet, and really doesn't have anything to do with a star topology per say, other than the fact that 10BaseT UTP Ethernet cabling is the most commonly used cabling for a star topology. The algorithm is called a truncated binary exponential backoff algorithm.

1024 is basically the maximum number of attempts that the network will try to send out a packet before the transmission is reported as failed (1023 actually, with the first transmission not being considered as a retransmission, and retransmission only occurring if there is a collision between two packets on a network). In reality though, this never comes into play because we utilize switches these days, thus eliminating the structure of all computers being on the same segment.

Basically though, the way it works is that you have what's called a slot time, which is the maximum amount of time it takes for one electrical pulse to travel between the two farthest nodes on a segment, multiplied by 2, is used in conjunction with another variable that acts as a counter, starting at 10 and going down to 0 (known as the ceiling), to randomize the time sequence of sending out a data packet so that the continuation of two or more computers transmitting a data packet at the same time and those data packets colliding doesn't reoccur indefinitely (which would happen if the time sequence wasn't best randomized for each computer).

1024 nodes on a single segment is like making sure that each computer gets a single chance to see if they were the intended recipient for the data packet (one transmission for each computer).

I hope this clears things up for you! :) 
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February 7, 2013 7:06:26 AM

So if I am understanding correctly 1023 is the maximum slot times to wait before a transmission but if there was 1025 node on the network , on of the nodes would not have a time to transmit because no matter what delay it transmitted on the slot time would already be assigned? So that is why 1024 is the maximum for not star-topology but rather the CSMA/CD algorithm over Ethernet?
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February 7, 2013 7:10:57 AM

You got it.

Edit: 0 <= r < 2^k,

where r is the random time delay, k = min (n, 10), and n being the transmission #.
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February 7, 2013 7:13:47 AM

Best answer selected by ngouterm.
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February 7, 2013 7:15:45 AM

Fantastic, I am currently working on my net+ and my teacher did not know the answer and I could not just let it go at that the way star works because it did not make sense to me because star topology is an idea only limited by the media. Thank you for explaining it so clearly.
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February 7, 2013 7:20:36 AM

ngouterm said:
Fantastic, I am currently working on my net+ and my teacher did not know the answer and I could not just let it go at that the way star works because it did not make sense to me because star topology is an idea only limited by the media. Thank you for explaining it so clearly.


Glad I could help. Now, go tell your teacher off! :D 


...though, the Network+ is nowhere near that technical. It's mostly device rolls (router, hub, switch, etc.), OSI model, subnetting, IP to binary converting and general topology questions. Good luck with it!
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