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Why is High Speed Cable so SLOW ??

Posted on 2003-02-24
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I have a high speed networking question.

Currently I'm a Rogers High Speed Internet cable subscriber and am experiencing unbareably slow speeds.  HowStuffWorks.com has provided some insight as to the bottleneck I'm experiencing but hasnt gone into enough detail to satisfy my needs.

Can you recommend any books/websites which better lay out the behind-the-scenes networking  problems experienced when using internet access through cable.   I want to know what is causing the problem and what are the possible solutions one could employee to resolve it.

HowStuffworks.com has mentioned a 6 MHZ channel which a neighborhood shares and because of this there is only so much bandwidth that can be used.  Once the limits have been reached, I suppose, more collisions occur on the medium for the neighborhood and so one is waiting longer to send/receive a message.   Can the situation be likened to a traffic jam without any traffic lights??
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Question by:aniston
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21 Comments
 
LVL 7

Expert Comment

by:Mikealcl
ID: 8012188
hehe, welcome to broadband home of over subscription.

their is really nothing you can do about it.  if the bottleneck isnt in your neighbor hood it will be somewhere else.  best to just get a provider that you know has a good reputation, or a dedicated buisness connection.

even with a buisness connection you have to deal with problems, like sending information threw chicago " the black hole of the internet"  my ping goes up 100x when it gets to chicago...

Mikeal
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by:stevenlewis
ID: 8012936
>Can the situation be likened to a traffic jam without any traffic lights??
 It really isn't collisions, but the avoidence there of, each will have to wait until it senses the network is clear, and when you are the only one on, there is no wait, when there is a lot, it has to wait, as do the others
It's more like a busy street, and a side street, and a stop sign, and your data is waiting at the stop sign, waiting a spot on the street
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magarity earned 200 total points
ID: 8013271
No, they actually are collisions.  When the segment is open, multiple clients can decide to transmit.  When this happens at the same time, the packets corrupt one another.  When no response is received then each client waits a random number of milliseconds before retrying.  Hopefully the second time the random wait numbers are far enough apart that both transmissions take place sucessfully.

A digital cable TV channel can handle 25Mbits/sec of data, as I recall.  Most providers dedicate two such channels for cable internet.  This provides 50Mbits/sec.  Then they typically limit each cable modem to a maximum of 3Mbits/sec though the device is capable of 10.  So, 50/3 = 16.66.  But notice in the fine, fine print that they only promise 10 times the speed of a 28.8 modem.  So, 16.66/.0288 = 578 people who can share this connection IF everything is perfect and there are no collisions.  This of course doesn't happen, but if they claim to have done this math and sold 578 houses on the same segment then this will really slow things down when everyone uses it at the same time.  Getting over a couple of hundred people on a coax LAN is just begging for loads of collisions.  There's a reason why the business world ripped the coax out of the walls and replaced it with twisted pair.  Too bad, because cable modem that isn't oversold is a great idea and can work quite well.

I don't know what operating system you are using.  If available, you can check the system monitor for the 'collisions' statistic in the network section.

I suspect there is simply some bandwidth hogs on your local circuit.  Probably several kids with filesharing P2P programs like KaZaa.  Those are notorious for using up resources on the network.  They also are usually left to run 24/7.  While cable modem can be nice if done properly, I recommend a switch to DSL if at all possible.  Most DSL providers have transfer amount caps that limit the amount of P2P traffic going through their servers.  Mine allows 20GB/month.  More than enough for me, though I could use that up in a hurry with a P2P.
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by:stevenlewis
ID: 8013352
magarity , so cable is  CSMA/CD and not CSMA/CA?
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by:magarity
ID: 8014636
For anyone who is interested, the two acronyms refer to collision detection and collision avoidance, respectively.  Collision avoidance tries to lock the channel by a broadcast.  This introduces overhead and still results in collisions of the broadcasts.  Collision detection is what I've described in my first post; it is just rules to deal with collisions.

Cable modems do make use of CSMA/CA but collision avoidance is not perfect so there are still plenty of collisions in heavily used networks.
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Author Comment

by:aniston
ID: 8015321
magarity, wonderful info!

Can you recommend any terrific "books"/websites so that I may get a better understanding and appreciation for the problem which broadband users face?!  I really do want to look into this problem closer.

From your comment it seems that if there was better management of the traffic, thru a digital traffic light, there could be more "efficiency".  Tell me, once traffic gets through the neighborhood segment does it simply spill off into a common highway of other segments or do we all get dumped into the backbone?

BTW I have Windows 2000.  Where exactly do i go to find the System Monitor to check out the "collisions" ??
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by:magarity
ID: 8015560
""
if there was better management of the traffic, thru a digital traffic light, there could be more "efficiency"
""

No, not at all.  This creates a lot of overhead by itself.  

The problem:
How to share one wire which can only handle one transmission at a time among a lot of users.

Using the CSMA/CA scheme:
1.  A network card has a packet to send.
2.  Card monitors wire until there appears to be no activity.
3.  Card shouts to everyone on the wire: "I'm getting ready to send!"
4.  Card monitors wire to see if there is any activity.
5.  Card sends packet or goes back to step 1
6.  Card waits for a standard reply that its packet was recieved by the intended recipient.  If no response, go to step #1.

Problems occur at steps 3 and 5.  If two cards shout at the same time then both assume the wire is free at stage 5.

How to get around this:
Replace step 3 with:
3.  Card X sends to a traffic cop, "may I please have the wire so I can send?"  Traffic cop shouts to everyone, "Hey, everybody, card X is going to send, so shut up!"  Card X sends.  Traffic cop shouts to everyone "OK, card X is finished!"

There is STILL room for collision here.  To completely lock it down, traffic cop would need to get replies from all the other cards, "OK, I'll be quiet while card X sends."  As a side note, this extended version is similar to how USB works, which is why it's a lousy format.  Anyway, you can see that to send a 512 BYTE TCPIP packet, there would be an absurd amount of overhead!  A basic web page is dozens, if not hundreds depending on the graphics, of packets in size.  You can imagine how perfecting the traffic flow would really just make it an even worse logjam.

So the cards just use the "shout to everyone and then assume its OK to send" method.  This introduces a small amount of overhead and it helps a lot but it isn't perfect.  Perfect would create more hassle than its worth.

Once the packets arrive at the neighborhood switchbox, the yare typically put on a fiber optic cable going to a central router.  This varies widely from cable company to cable company but the neighborhood to central is much more efficient (efficient also equals expensive, which is why they don't have a better scheme all the way to your wall).

You may be able to monitor the packet collisions by opening the performance monitor.  Start>programs>administrative tools>performance monitor.  Add the network interface object and select collisions.  Depending on your cable modem's model, you may or may not be able to see anything meaningful.
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Author Comment

by:aniston
ID: 8015880
Oh i see.  I guess what gets me is the 50mb/sec bandwidth -- I'm thinking all those in a neighborhood are taking a slice of the 50mb/sec pie at the same time (ie. 500 users would each get 1mb/sec but if there were too many users the speed for each would drop significantly) rather than a single user "shouting out" into a open channel and it is this "waiting" to shout out that is causing the slow downs.  Interesting....if I'm interpreting what youre saying correctly.  :P

I'm ordering in books: How Networks Work and Understanding Networks.  Any others you'd recommend for understanding the broadband bottelnecks and networking fundamentals??
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by:magarity
ID: 8015958
Studying coax cable networking might get you a job with the cable company but it is nearly useless anywhere else.  Any modern business environment with a computer network will use twisted pair networking which is very different.  I've seen a very few places still using old in-wall coax because they are too cheap and too unconcerned with performance to replace it.  The way around the bottleneck of coax networking is to replace it.
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by:tomh3307
ID: 8016288
Well first of all, You people deffinaly have no clue, The amount of bandwidth availible to all the cables modems is much much larger than you all might think, When you are talking about QAM 64 which is the downstream modulation used in most, if not all digital plants that support DOCSIS 1.0 , the downstream max is 37 Mbps, QAM 16 on the upstream gives around 10Mbps, a Cisco 7246 has 4 blades 6 upstream ports per blade and one downstream for those 6 up ports.  with 6 up ports of 250 customers thats 1500 customers MAX, If you look on cisco's website they have an equation that will help you figure out how many customers is too many for what level of use at any given point, This is information that is NOT overlooked by your cable provider.  its all about bandwidth management, You can never design a network expecting to handle everybody downloading a full tilt all the time, there is'nt a network on the planet that can handle that.  Did you know the phone companys are coming out with XDSL which models after a cable plant?  I bet you did'nt know that.  Thats because the bandwidth they provide on DSL sucks, If you want a 5 Meg connection you need fiber or 3 T'1 a cheap cable modem can do that in a breeze.  



Cisco spec is no more than 250 customers per upstream port.  However even with this number if everybody was downloading at the same time which almost never happens everybody still gets decent speeds.  

Most phone companys cant handle more than 33% of all thier customers on the phone at the same time,  and that is OLD technology.  

Next of all almost all speed issues reguarding cable modem connections 80% of the time computer issues 10% Signal strenth issues 5% of the time Abuse related issues 5% of the time actuall capacity issues.  

Cable modems do NOT use CSMA/CD for transmission, thats 802.3 people, Ethernet uses it.  Modems are DOCSIS. Completely different standard.  

For all of you people that have no clue,

http://www.cablemodem.com/specifications/

Look at Docsis 1.0, not many providers use 1.1 yet, and 2.0 is supported by most CMTS vendors yet.

At least know what you are talking about instead of bashing cable technology, DSL has the same capacity issues that cable does.  

Bottom line, DSL Is good but the technology is old and going to be phased out eventually,


BTW when troubleshooting slow speeds, make sure all your proxy settings are empty, no auto config script or detect settings make sure thats all unchecked, shut off file sharing<-- This kills speeds most times(Kazaa,Bearshare,Morpheous),Try turning off any firewalls see what happens, Run a tracert, see if the reply times will tell you anything, Clear your temporary internet files, Set you browser to check for new page at every visit and make sure you cache is set at either 0 or 1.  Try another computer if possible see if it yields the same results.  

At least try to do something, try some testing instead of "assuming cable sucks" you dont even know how it works.  

Peace out.
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by:magarity
ID: 8017369
"Well first of all, You people deffinaly have no clue"

What a special way to make new friends.  Buzz off, please.
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LVL 41

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by:stevenlewis
ID: 8021120
tomh3307, all very nice and infomative, but I have a dsl (768 kbit), and a small network, and my wife uses kazaa, and my speeds aren't "unbearable".
It is also common knowlege that as the # of users go up, the speed for an individual goes down, or is that an urban myth?
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by:Mikealcl
ID: 8021400
i think tom works for a cable company.....

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by:stevenlewis
ID: 8021432
Could you pull around back LOL
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by:magarity
ID: 8022353
Dear tomh3307,
You wrote "You people deffinaly have no clue  ...  Cable modems do NOT use CSMA/CD for transmission" and then provided a handy link to the DOCSIS specification.  Which you CLEARLY DID NOT EVEN READ.

PAGES 6  THROUGH 9 CLEARLY STATE, FOR THOSE OF US WHO DO KNOW HOW TO READ:

"The Cable Modem to Customer Premise Equipment Interfaces (CMCI) are described in this
specification  ...  The Internet Protocol (IP) version 4 standard MUST be passed transparently through the CMCI. The CMCI MUST support both IEEE 802.3 and DIX Ethernet."

As everyone, EXCEPT IGNORANT JERKS LIKE YOU, knows 802.3 includes ISO/IEC 8802-3 which is, in fact, CSMA/CD.

SO BEFORE YOU MAKE A TOTAL ASS OF YOURSELF, NEXT TIME LEAVE THE TECHNICAL DISCUSSIONS TO THE NETWORK PROFESSIONALS.

YOU CAN'T EVEN SPELL "DEFINITELY" OR "COMPANIES"

idiot
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by:stevenlewis
ID: 8022387
Now folks, let's not let this deteriorate into a flame war :~)
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by:tomh3307
ID: 8023095
Okay, When people were talking about communicating with the Headend or CMTS they were speculating it uses CSMA/CD, That is the method of transmission between your computer to the computer yes, some people may use USB, however most modems need a 802.3 port,  I was addressing peoples speculation of a Cable modem using CSMA/CD to communicate with the head end and the "lag" that will cause.  The channel is deffianty shared between all users.  

Even DSL providers have to manage thier bandwidth, they can't handle all of thier customers downloading or uploading at full tilt 24/7  Its just the way it is,  

More good info
http://www.cisco.com/warp/public/109/max_number_cmts.html

P.S. So sue me if I cant speel
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Expert Comment

by:virtualfreak
ID: 8048371
I think you have enough people commenting on the technical info so I won't, but I just wanted to address the second part of your question which was is there anything you can do about the "slow speed".  Whether or not you have always experienced it since installation or whether it has just started, you may want to call the cable company and have them come out and perform an attenuation test to see if they can tweak your signal strength by reducing any interference.  Sometimes cables spliters in the house or other devices can interfer and cause degraded service.  That was what was causing my constent slow speed until they installed a filtering device.  This helped but I still found that there were peek congestion times that slowed downloading.  If you are big on downloading files, GetRight is a nice program to help decresae download times.
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by:tomh3307
ID: 8051523
Agreed, you should have the signal strength verified.  Also have you tried the troubleshooting steps I outlined above? such as clearing cache, tring pings and tracerts and stuff? if so post some results.
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by:tomh3307
ID: 8051575
Also to address Steven's question of bandwidth utilization, yes it actually is urban myth.  The answer is it depends.  It depends on your setup.  lets ues 802.3 for example.  If you are using a hub you effectivly divide your bandwidth between the # of users.  If you install a switch, all of those users have dedicated bandwidth to the backbone. (assumeing their computers are directly wired to a switch port)  Then each user has a 10 or 100 Mbps connection.  Then speed is only reialnt upon the backbone.  CMTS's do not operate as a hub, the operate similar to switchs.  they use packet switching between VLAN's to divide bandwidth.  

So to answer your question is it depends, on what type of network you have setup as to how responsive it is to bandwidth utilization.  If you have 10 users hooked up to a 3548XL that has a 100BaseTX connection to the backbone, that would mean that if they were all downloading at the same time, it would get slow.  100% utilization is not good.  Well if they were rate limited at 1Mbps, then it doesnt matter if they were all downloading at the same time or not.  They would get the same bandwith all the time, you could even add quite a few users(50-60) before bandwidth would become an issue.  
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by:stevenlewis
ID: 8051630
Ah, Tom, but there's the rub
>they use packet switching between VLAN's to divide bandwidth
bandwidth is a finite quantity, and as long as the cumulative use exceeds the available bandwidth, slowdowns must occur. Even with a gigabit network, using a gigabit switch, if demand exceeds avalable bandwidth slowdowns will occur. As for cable companies over selling bandwidth, that never happens (and airlines never overbook flights LOL). Now when cable first came out, it may have been more prevelant to experience heavy usage slow downs, and I'm sure they ahve been addressing the problem, I am also sure that it can be (and still is in some cases) an issue. I have had clients run spped tests at peak times, and then at off peak times, and have noticed a marked increase in speed during off peak times. Granted some is due to the usuall internet congestion during peak times, but I do compare the results with my own speed tests during the same period, and I don't experience the degree of slowdown that the cable users experience. In some cases, during off peak hours, my clients report speed far exceeding my speeds, but during peak periods, they experience slower speeds. Even when I worked for the isp, if we over sold our connection to the backbone (a leased T1 thru a larger ISP), during peak usage, we experienced slow downs.
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