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My (Rogers) cable connection has been pretty bad recently (channels 3 and 10 are particularly fuzzy—it’s analog, not digital cable). Not surprisingly, this has caused my cable modem to drop out and have to reestablish a connection a couple of times since it started. The poor connection of course means higher corruption (not necessarily dropped per se) which causes the TCP/IP stack to have to retransmit packets more often. Reduction of bandwidth throughput aside, I got to wondering if it increases the actual bandwidth usage. That is, if there is a high error rate on the line causing packets to have to be retransmitted:

  1. Does this increase a bandwidth monitoring program’s numbers?
  2. Does the ISP count the retransmitted packets toward the monthly cap?

Based on what I remember from my university networking courses and common sense, I have a feeling that the answer to both questions is yes, but I cannot reliably measure the first, and have no authoritative answer for the second. I’m wondering if maybe the retransmitted packets are acknowledged as being duplicates and thus not counted somewhere along the line.

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Close the question for “too-localized” or “not-constructive”‽ Seriously‽ How the heck is asking how a poor network signal affects transfer counts not constructive or too localized? It is a general computer-network related question that is quite valid and informative. At least if the votes to close were to migrate it to Server Fault, then it would rational and makes sense since it fits better there (I didn’t thought of asking there), but too-localized and not-constructive‽ Ridiculous! I’m just glad that Seth managed to get his good answer in. –  Synetech Mar 27 '12 at 22:27
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3 Answers

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There are really two questions here.

First, "does a high drop rate increase bandwidth usage?" If you define bandwidth as speed, bits per second, then the answer is no. TCP/IP will slow down far out of proportion to the rate of dropped packets. In other words, a 1% loss rate is going to reduce speed by a lot more than 1%. But if you define bandwidth by total bytes sent, then the answer is yes. But not by much more than the loss rate. Note that if your loss rate were more than about 5%, TCP would likely cease to function at all.

Second, "does a bad cable connection cause a high packet drop rate?" The answer is, not so much. A bad physical connection will affect the carrier signal between the cable modem and the upstream receiver. You've seen this in the modem itself dropping the connection. The details depend on the technology involved, but generally speaking the two modems are going to slow down the data rate and/or perform frame recovery at the physical layer before outgoing data reaches the IP layer where its being counted. Incoming data will get counted, but most of it will be slowed down and/or corrected at the physical layer. Some frame loss may result in IP layer packet loss and retransmissions at the IP layer. But as noted above, this will only marginally increase total bytes received and then only if you are trying to download the same total bytes as you would have with a good connection.

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I thought it was obvious, but I mean total bytes sent, not speed; that’s why I specifically asked about bandwidth monitoring programs and the total-bytes-transferred field in the Task Manager. That said, this is a wonderful answer. It is clear and concise and fits what I remember about network architecture. Further it fits the observation I’ve had recently of my speeds being lower than they should. I guess the speed is reduced to accommodate the lowered signal quality. Good thing you said this before I got around to yelling at my ISP for throttling after they promised to stop. :-D –  Synetech Mar 27 '12 at 22:23
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How Rogers measure this is a question for them. getting the answer may be a challenge

based on the issues with cable, I would have them look at that as it is a servcie issue. High error rate and the bad TV reception would indicate a problem.

Check any splitters you may have and make sure they are new and properly rated.

It took a number of calls to fix an issue I had but it was their issue.

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I’m not worried about fixing it right now. The picture and sound are clear enough, and I’m too lazy-tired on Saturday afternoon to bother fiddling with cables and splitters right now. Yes, obviously a specific ISP may measure things differently than another, but I’m looking for a network-architecture level answer. If a packet is dropped (i.e., not completely transmitted), would it count at my end? at the sending end? at some point in the middle? What if it is transmitted completely, but corrupt? Do routers, switches etc. count each and every packet regardless of successful transmission? –  Synetech Mar 24 '12 at 21:27
    
If a node counts packets for some reason, it will count any packet that passes through it, then forget about the package while keeping the number. It will not count packets that don't pass through it. This is not related to "consumption" of bandwidth at end nodes for billing purposes, that is arbitrarily determined by your ISP. Bookkeeping is not a central role for most network architecture. –  Eroen Mar 25 '12 at 6:07
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Generally, routers don't know if a packet is being retransmitted. The end points are responsible for tracking which packets have been successfully transmitted, and which need to be retransmitted. Ease of getting retransmission counts and rates varies among operating systems.

I would expect Rogers and any other ISP to charge you for retransmitted packets sent by your system(s). If the Cable Modem should retransmit any packets it couldn't send completely, and I would not expect you to be charged for those retransmissions. The Rogers support desk should be able to connect to your Cable Modem and determine if the quality of your line. (If you have a cable box, they should also be able to do the same for it.)

EDIT: I would expect Rogers or any other ISP to charge you for all data transferred between your system to their cable modem. This may include retransmissions resulting from problems with your line. Many retransmission result in more than one packet being retransmitted.

The cable modem perform retransmissions on their own if they detect that they were unable to correctly perform a transmission. They also generate some traffic of their own to maintain their connection. Neither of these sources of traffic should be charged.

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> I would expect Rogers and any other ISP to charge you for retransmitted packets sent by your system(s). If the Cable Modem should retransmit any packets it couldn't send completely, and I would not expect you to be charged for those retransmissions. Those two lines seem to contradict each other. I take it you mean that the ISP won’t count it if the modem is faulty, but what about the Networking tab in Task Manager? Does it count them in it’s total-bytes-transferred field? Does the router count it it it’s bytes-transferred field? –  Synetech Mar 25 '12 at 3:47
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