PTP Data Transfer Rate !!

Products PTP
1

Is there any approved mathematical relation to calculate the data transfer rate for any PTP link given the aggregate capacity??

Customers think that a link of an aggregate capacity of 100 Mbps (1:1) will result in 50Mbps data transfer rate up and down.

2

Hi Mohamed,

That's an interesting question. The one-way capacity for PTP 500/600/650/700 depends on adaptive modulation mode, channel bandwidth, link optimization (IP or TDM), link symmetry and range. If Link Symmetry is set to Adaptive, the capacity is also a function of traffic loading.

The aggregate capacity is the sum of the two individual one-way capacities.

The exact calculation of capacity is of course quite complex, and this is not really a calculation you can do for yourself. We do provide the capacity at zero range in the PTP 650 User Guide for all of the above configuration options, together with a set of graphs showing how capacity depends on link range.

Also, the LINKPlanner includes the very same calculation and includes the range-related component.

Back to your original question, and assuming the same modulation mode in each direction, the aggregate capacity is divided between the two one-way capacities based on the symmetry. So if the symmetry setting is (say) 3:1, the capacity is 3/(3 + 1) = 0.75 in one direction and 1/(3 + 1) = 0.25 in the other direction.

Hope this helps!

Mark

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3

Hi Mark,

Thanks for your replay. I think that my question wasn't clear enough.

For a PTP650 link working with default lite capacity and a CH. W. of (10 MHz) assuming a perfect radio channel conditions and a symmetry of (1:1) the resulting maximum aggregate data rate is (28 Mbps -> 14 Mbps Up / 14 Mbps Down).

I got a call from one of our customers compalining of slow connection and when I check the link I found that it is working perfectly with no issues.

He is pushing a file from one end to the other with a transfer rate of around (1.6 Mbps) which is almost the same value when using iperf or NetStress throughput tester.

Advise please.

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4

Mohamed,

If the link is working perfectly with no issues, you might start asking the following questions:

How does the customer define, “slow”?

How is the customer measuring throughput?

Is the customer reporting 1.6 Mega bits per second, or 1.6 Mega Bytes per second? (1.6 Mega Bytes per second is ~12.8 Mega bits per second)

Is anyone else using the link?

Are there any nearby wireless devices using the same channel as the PTP 650?

Is the customer directly connected to the PTP 650, or are there other devices in-between? (Wi-Fi client, Wi-Fi access point, ePMP/PMP540 subscriber, ePMP/PMP 450 access point, etc.)

I hope this helps!

Dave

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5

Hi David,

The link is connecting a branch site to a main site in which a daily traffic is pushed uplink from baranch to main site.

The customer transfers files between (2) PCs directly connected to both ends of the link and gets the transfer rate from the operating system.

The connection is working perfectly but the problem is that he thinks that as the link shows (14 Mbps Up and Down) he must get a transfer rate of (14 Mbps Up and Down) on the screen while moving the files.

I suggested to change the link symmetry ratio allocating more traffic for uplink.

Thanks for your notes.

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6

Hi Mohamed,

As David points out, iPerf can report measurements as bits/s or Bytes/s depending on the -f option.

If iPerf is reporting 1.6 MBytes/s then this is actually 1.6*1.024*1.024*8 = 13.42 Mbits/s, which is very close to the expected link capacity.

Please check the bits/bytes measurement before we start to think of more complicated solutions.

Thanks, Mark

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7

Hi Mark,

Thanks for following up. I got the idea, and I believe that it is in Bytes not in bits. I’ll pass that to our customer.

Thanks.

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