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# How to calculate the max  transmission speed?

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Which's the maximun bytes rate i can send trough a 128kbps link? it's as simple as 128kbps/8?

Thanks
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Commented:
It certainly is 16 kilobytes per second is the maximum however this relies on a perfect system, which obviously does not exist.
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Commented:
The answer to the question you've posed is 16,384 bytes. But that does not mean that you can transfer a data file that is 16,384 bytes in one second which has nothing to do with whether or not the system is "perfect". In fact, jsbennison's answer (no offense intended) incorrectly implies that a 128kbps link really isn't a 128kbps link. If you were to measure the number of bits transmitted on a saturated 128kpbs link, you'd find that in fact 128k bits are being transferred per second. I believe he is referring to the difference between the clock rate of the link and the data rate of the link: 128kbps is the clock rate.

To determine the data rate of a link, at a minimum, you must consider the protocols being used, link configuration, and propagation delay. For example, if you FTP a file across a 128kbps link, there are lots of factors which will effect the data transfer rate such as transmit and receive buffer size, MTU, window size, whether or not "delayed ACK" is set, TCP segment size just to name a few.

Give more detail and I'll qualify the answer a little more.

Good luck.
Steve

Commented:
OK, there you go some details:
We're using a 128kbps link to connect 2 LAN's where all the servers are in LAN1; we check the link's speed doing PING's and normally it takes 20ms (PING's from router to router, from LAN1 workstation to LAN2 workstations, etc., etc.). But sometimes the speed just go down and a PING can take 3000ms ! We already detect some causes like large e-mail attachments transmissions or large files copies, other times we don't detect any "obvious" cause. We had been monitoring the link use with MRTG software a here's the origin of my question: When we check the MRTG graphics the "peaks" match with the 3000ms PING's (in our recodrs) but the "Bytes per second" scale never pass from 500 bytes. So if the 128kps link suppose to transmit 16Kbytes per second, why it comes down with a 500Bytes transmission? The users in LAN2 connect to the mail and applications servers in LAN1; servers OS are NT4 and HP-UX; and we're using just TCP/IP protocols.

Thanks,
Pitagoras
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Commented:
Assuming you've not tweaked MRTG to do something different: MRTG takes readings of octets in/out every 5 minutes, so the graphs will never show peaks of 128kb unless the line is saturated for a full 5 minute period.

Think of it this way.

1. Time=0 and octets out=0, MRTG takes a reading
2. I transfer 16384 octets of data in appx. 1-2 seconds
3. Time=5 minutes and octets out = 16384
4. MRTG calculates: 16384 bytes of data were transmitted over a 5 minute period equalling 16384 bytes divided by 300sec giving about 55 bytes per second which is the value MRTG would graph.

MRTG is not a good tool for looking at traffic patterns as granular as minutes and seconds. Also rememeber that ICMP traffic is pretty low on the data totem pole and can give misleading readings about the health of a network if other things (e.g. TCP-oriented traffic) are going on.

I apologize in advance if this sounds condescending and forgive the technical shorthand but if you transfer a 12.8kb file over a 128kbps link you aren't using 10% of the link for one second, you are using 100% of the link for 1/10th of a second (sorta kinda).

If you supply even more detail, I promise to spew even more bilge!

Good luck.
Steve

Commented:
In theory 16,384 bytes is what you expect in reality highly unlikely.
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Commented:
skate033 . . . clock rate vs. data rate . . .

Commented:
Stevej, your comment clear some things to me, but i still have dudes: If my MRTG graphics (sorry but i haven't other tool by now) it's a flat line around 210Bytes (not just in a 5 min. period but all the day), then i can say i'm transfer 210bytes all the time, it's that means i'm transfer 63Kbytes per second?

Thanks,
Pitagoras

P.D. If you can tell me another way (instead ping's or mrtg) to check the use of the line, i'll appreciate so much.
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Commented:
If MRTG shows a flat line -- a COMPLETELY FLAT LINE -- then it probably isn't working.

What type of routers are you using?

Search the internet for a tool called getif.exe. It's a freeware tool and simple to use. Since you have SNMP on the routers you can use this tool to take readings of the input and output rate of the interfaces at intervals that you specify - one second, 5 seconds, etc. It will give you a more granular view of the data traffic.

You can e-mail me at steve.jennings@verizon.com and I will email this program to you if you like. It's small.

Good luck.
Steve

Commented:
Well, is not a flat line, i decribe it in a wrong way: the MRTG graphic have peaks and variations, for example, everyday around 8:00 AM when all users in LAN2 are turning on their computers and doing logon in the mail server theres's always a peak; then around 9:00 AM the graphic come down and, almost all the time, remains "stable" the rest of the day. When i say "stable" i mean the average use (reported by MRTG) is the same along the day.

Thanks,
Pitagoras
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Commented:
I should have asked you all these questions first:

What kind of routers are you using?

Do you have filters (access control lists) on the routers at either end of the WAN link?

Is this a fractional T-1 point-to-point link or a frame relay PVC?

If this is a frame relay PVC, is 128kbps the clock rate of the PVC or the clock rate of the physical link?

About the pings . . . are you pinging from WAN interface to WAN interface or from a workstation on the LAN across the WAN (128kb link) to a workstation on the far end LAN?

Are you also collecting data for MRTG on the LAN interface?

Does the MRTG graph for the far end WAN interface look the same (with input and output inverted) as the near end WAN interface?

Good luck.
Steve

Commented:
Sorry for the delay. Here are some answers:

1. I'm using Cisco 2500 routers and Newbridge 2703 DTU's in both points.
2. I have no filters or access list restrictions
3. It's a fractional T1 point to point con
4. I'm pinging from WAN to WAN interface and from workstation to workstation. Same results.
5. I'm collecting data from the router ethernet interface.
6. I'm just collecting from one end. I'm not collecting info from the far end.

Thanks.
Sr Manager Cloud Networking Ops
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Commented:
Ok. Cisco and Newbridge. Aren't those Newbridges kind of old? And no filters to consume CPU and slow down the router. No frame relay network congestion because it's point-to-point. And now I'm confused. I thought you were taking i/o readings from the 128kbit serial interface, but your note says "ethernet".

So to recap: On occasion ping time goes to 3000ms from w/s to w/s AND from WAN i/f to WAN i/f. And MRTG reflects this. That would seem to eliminate your LAN or the far end LAN from being the culprit.

Are there high rates of physical errors or retransmits when the pings take 3000ms? Have you checked the Newbridge DTU to see whether it shows physical errors? Have you looked at the serial interface stats on the near and far end Ciscos to see if they show high error input or output values? . . . like CRC or alignment errors? or framing errors on the DTU? Anything that would indicate physical layer problems?

I'd be taking MRTG samples from near and far end ethernet and serial ports. By the way, these slowdowns don't occur every 5 minutes do they? (har)

Good luck.
Steve

Commented:
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Commented:
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