In article m,
David Nebenzahl wrote:

OK, this question is totally out of idle curiosity. No customers' jobs
depend on it. No actual electronic repair issues are involved.

Like a surprising number of people, I still have dial-up Internet
access. (Yeah, I know, I'm living in the Stone Age.) So I'm quite
familiar with various connections speeds. I also can observe my network
traffic on my firewall's control panel (I use Sygate, a freebie, which
I'm quite happy with).

What I don't understand is why network traffic, at least as reported by
Sygate, is so choppy. On a good day, I get a "fast" connection, meaning
48 kbps, or maybe even (gasp!) 49.2; that's the fastest speed I ever get.

What I see, invariably, is something like a triangular waveform, with a
period of about a second, where the transmission speed varies from
(usually) 4.4 and 5.9 K (I assume this is bytes, not bits, per second,
but whatever). The speeds never change, at least not much. With a faster
connection, it just stays at the higher speed longer, which flattens out
the peaks of the "waveform".

Why is this? I remember hearing that sending packets down a telephone
wire is a "bursty" business; is that part of it?

There are several aspects to the communication process which can cause
this sort of "burstiness".

One is the phone line itself (the modem-to-modem transmission). Modem
connections these days usually use V.42 error correction technology.
The data being sent is broken up into frames or packets (which may
have separation points that have nothing to do with the underlying
TCP/IP packets). The receiving modem will validate a checksum or CRC
on each frame, and if it's bad (e.g. as the result of some line noise,
corrupting a bit in in the frame) the modem will request that its peer
retransmit the entire frame. The data won't be delivered from the
modem to your PC until it has been successfully received (with the
correct CRC) and you'll observe a brief "stutter" in reception
during each such error-correction... often followed by a burst of
rapidly-delivered data, as those frames which were in transit behind
the damaged one are delivered almost instantly.

TCP is also a burst-prone technology. The sending system will
gradually increase the rate at which it transmits packets (and the
number of packets it transmits without waiting for an acknowledgement)
until something goes "sproing" (that is, a packet is lost due to
errors, or discarded because some router in the path decides that its
buffers are full). The loss of a packet causes the receiving system
to send back a "Hunh? Please resent", or to not send back an
acknowledgement... and these will require retransmission of the lost
packet. Net result is that the net transmission speed varies, as a
result of packet loss and network congestion levels.

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
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