India Broadband Forum

cool_techie_tvm · 02-16-09, 11:18 AM

Here is the basic topology of an ADSL connection.

Subscriber telephone line <--> Exchange DSLAM <--> NIB (national internet backbone)

Between the telephone line and the DSLAM the contention ratio is always 1:1. It means that we are not sharing the line with anyone. The contention policy is always set between the DSLAM and the NIB. Usually 50:1 is set for residential purposes and 20:1 is set for business users. The link between DSLAM and NIB is always of the order of 10+ mbps (do not know the exact value implemented by BSNL, just using an arbitrary value to specify a high bandwidth line). This value is usually dependent upon the number of users getting connected to the particular exchange.

Like mentioned earlier, let the bandwidth of the link between DSLAM and NIB be 10 mbps.
10mbps = 10*1024 kbps = 10240 kbps.
The default download speed provided by BSNL is 2048 kbps. Divide 10240 kbps by 2048 kbps, we get 5. In turn it means only 5 subscribers will be able to get 1:1 contention ratio, which proves to be cumbersome for large ISPs like BSNL. Now here is where contention ratio of 50:1 comes into play.

When a CR of 50:1 is implemented, the math is like this ---> 50:1 x 5 = 250*. Now it is self explanatory. When 1:1 CR was implemented, only 5 users could get 2048 kbps out of a 10240 kbps line. But when a 50:1 CR was implemented 250 users get to access the same line at 2048 kbps. I repeat, 250 users get to access the original 10240 kbps line at 2048 kbps.

The rest of the math is easy. Imagine 250 users competing for 2048 kbps on a 10240 kbps line. The network will get congested in no time and this is what happens for some users in some particular circles.

The only way to avoid this is to increase the capacity/bandwidth of the back end line connecting the DSLAM and the NIB.

I hope the explanation was lucid enough, if in any doubt, please feel free to respond.

*For those of u who forgot the math of ratios take 50:1 as 50/1. So 50:1 x 5 = 250 !! Capiche?!!

Gary4gar · 02-16-09, 11:47 AM

Very informative post and Nicely explained!

If i am not wrong, there a AC aka BRAS in between DSLAMs at exchange and core Gigabit routers at NIB.

cool_techie_tvm · 02-16-09, 12:00 PM

True. I didnt want to get into the core of matters, hence plainly breezed through the architecture

Gary4gar · 02-16-09, 01:03 PM

Quote:

Originally Posted by cool_techie_tvm

True. I didnt want to get into the core of matters, hence plainly breezed through the architecture

I wish you were working for BSNL, then it would have been best ISP in the world

vikramjb · 02-16-09, 07:23 PM

Quote:

Originally Posted by Gary4gar

I wish you were working for BSNL, then it would have been best ISP in the world

True man

. Thanks for detailing it for me cool techie a.k.a praveen

So theoretically speaking, when you say 250 users can be handle by one server right ? So we have another 2 servers we can support another 250 * 2. In total it should be 750. If these 750 users need optimal browsing experience, either the servers need upgrade or does the network line needs upgrade or do we reduce the number of users on a server ?

Who decides which server to connect ?, Is it related to the sales man problem that i miserably failed to learn in one of Data Structure & Analysis

cool_techie_tvm · 02-16-09, 07:28 PM

The number of users cannot be controlled. For that BSNL has to stop issuing connections but since broadband is easy money, they wont stop issuing newer connections. The new network, NIB 2 (117.xx.xx.xx) is designed to handle high speed data transfers and shortly the current NIB 1 users (59.xx.xx.xx) and NIB 2 subscribers will be bridged to form one single network. These are the plans of BSNL, do not know when they will be implemented.

So ISPs usually upgrade the back end infrastructure.

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02-16-09, 11:18 AM	#21
cool_techie_tvm Amor vincit omnia Join Date: Dec 2007 Posts: 2,682 Blog Entries: 4 Rep Power: 4	Here is the basic topology of an ADSL connection. Subscriber telephone line <--> Exchange DSLAM <--> NIB (national internet backbone) Between the telephone line and the DSLAM the contention ratio is always 1:1. It means that we are not sharing the line with anyone. The contention policy is always set between the DSLAM and the NIB. Usually 50:1 is set for residential purposes and 20:1 is set for business users. The link between DSLAM and NIB is always of the order of 10+ mbps (do not know the exact value implemented by BSNL, just using an arbitrary value to specify a high bandwidth line). This value is usually dependent upon the number of users getting connected to the particular exchange. Like mentioned earlier, let the bandwidth of the link between DSLAM and NIB be 10 mbps. 10mbps = 101024 kbps = 10240 kbps. The default download speed provided by BSNL is 2048 kbps. Divide 10240 kbps by 2048 kbps, we get 5. In turn it means only 5 subscribers will be able to get 1:1 contention ratio, which proves to be cumbersome for large ISPs like BSNL. Now here is where contention ratio of 50:1 comes into play. When a CR of 50:1 is implemented, the math is like this ---> 50:1 x 5 = 250. Now it is self explanatory. When 1:1 CR was implemented, only 5 users could get 2048 kbps out of a 10240 kbps line. But when a 50:1 CR was implemented 250 users get to access the same line at 2048 kbps. I repeat, 250 users get to access the original 10240 kbps line at 2048 kbps. The rest of the math is easy. Imagine 250 users competing for 2048 kbps on a 10240 kbps line. The network will get congested in no time and this is what happens for some users in some particular circles. The only way to avoid this is to increase the capacity/bandwidth of the back end line connecting the DSLAM and the NIB. I hope the explanation was lucid enough, if in any doubt, please feel free to respond. *For those of u who forgot the math of ratios take 50:1 as 50/1. So 50:1 x 5 = 250 !! Capiche?!!

02-16-09, 11:47 AM	#22
Gary4gar Gold Member Join Date: Apr 2007 Location: Jaipur Posts: 775 Blog Entries: 1 Rep Power: 3	Very informative post and Nicely explained! If i am not wrong, there a AC aka BRAS in between DSLAMs at exchange and core Gigabit routers at NIB.

02-16-09, 12:00 PM	#23
cool_techie_tvm Amor vincit omnia Join Date: Dec 2007 Posts: 2,682 Blog Entries: 4 Rep Power: 4	True. I didnt want to get into the core of matters, hence plainly breezed through the architecture

02-16-09, 07:28 PM	#26
cool_techie_tvm Amor vincit omnia Join Date: Dec 2007 Posts: 2,682 Blog Entries: 4 Rep Power: 4	The number of users cannot be controlled. For that BSNL has to stop issuing connections but since broadband is easy money, they wont stop issuing newer connections. The new network, NIB 2 (117.xx.xx.xx) is designed to handle high speed data transfers and shortly the current NIB 1 users (59.xx.xx.xx) and NIB 2 subscribers will be bridged to form one single network. These are the plans of BSNL, do not know when they will be implemented. So ISPs usually upgrade the back end infrastructure.