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STS1 / OC-1 circuit bandwidth

Posted on 2011-04-26
Medium Priority
Last Modified: 2012-06-27
I'm having latency/congestion problems with my Level 3 circuit where I have a 200 MB CIR. Level 3 says we were going above our CIR even though my interfaces usually showed somewhere near 120 Mbps outbound (to Level 3). I asked the question of whether my 200 MB was 200 MB up and 200 MB down or whether it was 200 MB total. They said it was 200 MB up and 200 MB down. ok - great.

But when I brought up the option to upgrade my service they mentioned that they are only provisioned at my location with four STS1 circuits and that it would take time to expand their services there. Now, from what I can tell four STS1's have a BW of 51.84 Mbps each. And that is total - not 51 MB up and 51 MB down.

Can anyone tell me if I'm wrong here? If I have a T1 for a circuit, then I am getting 1.54 MB total. Are STS1 circuits somehow different?

Question by:ascendmax26
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LVL 17

Expert Comment

ID: 35468886
You are correct that if they have four STS1 circuits at your site, then you only have a total of about 200 MB - not the 400 MB that is implied by the statement they made to you.  All 200 MB can be up or down, but that is the total.  It is like a T1 for purposes of your analogy.  
Do they use a different measurement timeslice or method that would account for the differences in throughput statistics?  Average over 5 mins versus peak load, for example?

Accepted Solution

jon1966 earned 2000 total points
ID: 35470246
For an STS-1 you have 51.84 Mbit/s transmit and 51.84 Mbit/s receive, so in essence Level 3 is correct.

Is Level 3 multiplexing 4 STS-1's together or sending independently or encapsulating into ATM and bonding multiple circuits?
LVL 17

Expert Comment

ID: 35471273
I was under the impression that if send and receive are kept separate, for example, using a send fiber pair and a receive fiber pair like with most OC-X implementations, you can get the "full duplex" effect that provides the doubled throughput.  However, with some installations of the TDM clocking used in converting to the STS1 copper link, the throughput is limited to the line rate rather than maintaining the full capacity for send and receive simulataneously.  It may help to know the termination hardware.  Furthermore, many carriers use rate limiting techniques (so they can sell you more bandwidth).  And as mentioned - how Level 3 treats the bandwidth once it hits their side of the interface is critical as well.  

Expert Comment

ID: 35471520

Technically correct, but the carrier is going to sell the circuit as full duplex with two strands.  I do believe an end to end description is required, but safe to say that if the CIR is 200Mbit/s that you should come close to that.  SNMP data on the CER interface and stress testing is required.  

I suspect the handoff is an STS-1 multiplexer to coax or similar.  Customer equipment or configuration policies may also affect the throughput.

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