Hey guys, figured I'd do something halfway helpful for once. So, today we have a tutorial on dechipering Cable Modem signal levels and logs, and how cable broadband works in general (Note this is not the same as DSL signals, they are completely different).
First, to understand this, you need to have a basic understanding of the basic system behind cable broadband, and the protocol used to get internet to your house.
Lets start with that.
It can seem pretty complicated at first, but I'll do my best to keep it as everyman as possible.
So, at the heart of every cable system is the hybrid fiber-coaxial network, as shown in the simplified (terribly drawn) diagram below.
There are several major components of interest here.
The first is the Cable Headend. this the main distribution point for cable TV and internet services for a fairly broad area. The headend has two main duties. It serves as the initiation point for Cable TV. This is where the satellite dish to receive programming from the orbiting satellites that bring the programming to the network come in. In addition to local feeds from local broadcasters. It also handles all requests for VOD (video on demand) content, when you order a movie on your cable box.
The other duty is housed in something call the CMTS, or Cable Modem Termination System. When your modem connects to the network, this is the thing it is communicating with. The CMTS handles all inbound/outbound data to the WAN, and the larger internet. It negotiates the connection with your modem. It also handles the private network for VoIP Phone service provided by the cable company.
The headend itself houses a multiplexer, it essentially stuffs both the incoming/outgoing signal from the CMTS and Tv portions into the Fiber Optic trunk lines. These Trunk lines feed a distribution network, which consists of several hubs, which split the network further.
From the hub, it's off to your local fiber-coax node. The fiber node handles the transition from fiber network, for internet, and also houses digital-analog converter for analog cable channels in areas that still receive it. to the coaxial cable lines that go to individual homes/businesses. The fiber node generally feeds 50-500 homes in an area. This section of cable from the node to the home is referred to in common jargon as the "last mile". Fiber is fancy interesting and all, but of most interest to us is the DOCSIS segment.
DOCSIS, or Data Over Cable System Interface Specification, is designed by a company called CableLabs. It essentially is the protocol which handles how your modem interfaces with the fiber node, and from there the CMTS.
DOCSIS uses many different channels on a single node, which allows for load balancing, and many devices to connect. A single DOCSIS channel allows for roughly 40mbps of bandwith. DOCSIS 2.0 and earlier allow only a single channel to be used. Newer DOCSIS 3.0 (and future 3.1) specifications allow for channel bonding (up to 8 in DOCSIS 3) allowing faster internet speeds over the cable system.
Most people who have cable internet have probably at some point in the past had internet problems. You might have talked to a phone technician, or even a field technician who talks about "signal levels". This may have meant nothing to you, and you just nodded along as they talked, wondering what they were on about.
Well, I'm here to help decipher those in an easy to understand (hopefully) manner.
So with mode cable modems, you can go into your browser, and see the signal levels, by going to 192.168.100.1 .
This what it looks like on a Motorola SB6121 (DOCIS 3.0, 4x channel bonding capable)
Link to larger version > http://i.imgur.com/api6CD7.png
This page might look confusing, but it's actually fairly simple.
Let's start with the top section first.
This is the downstream section. This means this contains all the stats on the "downstream" section of the signal, meaning the flow of information to your modem from the Node and CMTS.
The first 2 lines are pretty straightforward.
Channel ID, and the frequency. These are DOCSIS channels, the ones being used by this modem at this time, in a bonded scenario. This model of modem can handle 4 channels simultaneously, with the frequencies measured in Mhz (537, 543, 555 and 561Mhz).
The next line down is Signal to noise ratio. This is 1 of the 3 most important metrics for connection health. This is essentially how "clean" the signal is, measured in dBmV or Decibel-Millivolts. IE how much usable info, vs junk info. The higher this number, the better. As a general rule, any number above 32-34 is ideal. Below that and you might start to have connection issues. Below 29/30 and you are probably having lots of trouble. As you can see, mine are 37, 38, 38, and 38. That is very good. Anything 40 or higher is pretty rare, though would be excellent.
The next line is Downstream modulation, this is the specific symbol (data rate, essentially) being used by that channel. In 99% of cases, it will probably be QAM256.
Next line below that is downstream power level. This is the 2nd of the 3 most important signal metrics. This is the strength of the signal to your modem, from the Node, after it has passed through all the coax lines outside, the tap, the splitters in your house, and all the coax inside. This is a relative number. 0 is ideal. The further away from 0, the worse it is. Spec is -16 to 16. However, ideally, especially for DOCSIS 3.0, you want between -8 and +8. Mine are 3,2,3,3. So that is very very good. This will fluctuate with all sorts of factors. Temperature, as cable has a higher resistance when hot.
If you've ever looked at a splitter, you've probably seen a number on the outputs. On a 2-way splitter, it's usually -3.5dbmv. Meaning that if the strength of the signal coming into the house is +10dbmv, after passing through one splitter, the signal to both outputs is now +6.5dbmv. This decreases with more and more cable, and more splitters. It adds up. So if you start with 0 at the house, and have splitters, the strength can quickly drop into bad/weak levels.
If this is causing connectivity problems, a broadband tech will generally remove splitters, or try to simplify the layout (remove number of splitters), and possibly use an amplifier to boost the signal strength.
Next section is upstream. This is the section with all the information relating to the "upstream" signal, IE what the modem is doing to get it's signal to reach the Node with a reliable connection.
The first 2 lines are the same as with downstream. Channel ID and frequency, again measured in Mhz.
The next line is ranging service ID. This isn't really important, but does tell you which channels are in use. In my modem, the upstream is physically bonded, but only 1 channel is actually being utilized for data.
The next line is symbol rate. This essentially is the speed with which the modem is talking to the node. Either 2.56 or 5.12MSymbols/s is common. In most cases, it doesn't really matter which is in use until you start hitting higher upload speeds. Even at 2.56MSymbols/s I'm still acheiving my full rated upload of 5mbps.
Next line is the 3rd and arguably, the most important (and most often to cause problems) metric. The upstream power level. This is the power level at which your modem is transmitting (ie "ranging") to establish reliable connection with the Node. This metric is by far the most likely to be bad, and cause connection problems. Essentially, in most cases anything below 53-54 is acceptable, but under 50 is ideal. Most modems top out at 55dBmV. Meaning if you are at 55, the modem is transmitting at maximum power, and that means something is wrong. too many splitters, or bad lines. Being at max can mean that your connection might drop, and the "ranging" will fail, causing loss of connectivity.
the modem will then try to re-range, and regain connectivity. This will generally result in a trouble call, with a broadband tech visit needed to help resolve the problem.
There is a low end too- generally anything above 35 is recommended. Any lower and you can start experiencing connectivity loss. This an extremely uncommon problem, usually the problem is transmit power too high. Mine is around 44 most of the time, which is pretty much perfect.
Next line i upstream modulation, again similar to the downstream. Ranging status is exactly what it sounds like. The status of the connectivity on that upstream channel to the node.
The last box essentially tells the number of errors in your connection. Obviously the more unerrored codewords, the better. Mine is a very healthy connection, with very few errored codewords.
The last important page is the logs page.
http://i.imgur.com/gRLipze.png
These are the internal logs in the modem. As you can see, on the 15th we had some "problems" An amplifier at the street level for our neighborhood had gone bad, and a tech was replacing it. During that time our connection was lost, and the modem was still trying to reestablish connection.The 17th has a T4 time out, this isn't always bad, it's an occasional hiccup in the network. The 18th was a power outage, see cable modem reboot due to power reset.
Most of the time, this page isn't much help to the end user, unless you know what you are looking for. But it is an important page to know what you are looking at.
Hopefully this will help you better understand your modem, and the system which brings the service into your house.
Let me know if you have any questions or if I made a mistake.
First, to understand this, you need to have a basic understanding of the basic system behind cable broadband, and the protocol used to get internet to your house.
Lets start with that.
It can seem pretty complicated at first, but I'll do my best to keep it as everyman as possible.
So, at the heart of every cable system is the hybrid fiber-coaxial network, as shown in the simplified (terribly drawn) diagram below.
There are several major components of interest here.
The first is the Cable Headend. this the main distribution point for cable TV and internet services for a fairly broad area. The headend has two main duties. It serves as the initiation point for Cable TV. This is where the satellite dish to receive programming from the orbiting satellites that bring the programming to the network come in. In addition to local feeds from local broadcasters. It also handles all requests for VOD (video on demand) content, when you order a movie on your cable box.
The other duty is housed in something call the CMTS, or Cable Modem Termination System. When your modem connects to the network, this is the thing it is communicating with. The CMTS handles all inbound/outbound data to the WAN, and the larger internet. It negotiates the connection with your modem. It also handles the private network for VoIP Phone service provided by the cable company.
The headend itself houses a multiplexer, it essentially stuffs both the incoming/outgoing signal from the CMTS and Tv portions into the Fiber Optic trunk lines. These Trunk lines feed a distribution network, which consists of several hubs, which split the network further.
From the hub, it's off to your local fiber-coax node. The fiber node handles the transition from fiber network, for internet, and also houses digital-analog converter for analog cable channels in areas that still receive it. to the coaxial cable lines that go to individual homes/businesses. The fiber node generally feeds 50-500 homes in an area. This section of cable from the node to the home is referred to in common jargon as the "last mile". Fiber is fancy interesting and all, but of most interest to us is the DOCSIS segment.
DOCSIS, or Data Over Cable System Interface Specification, is designed by a company called CableLabs. It essentially is the protocol which handles how your modem interfaces with the fiber node, and from there the CMTS.
DOCSIS uses many different channels on a single node, which allows for load balancing, and many devices to connect. A single DOCSIS channel allows for roughly 40mbps of bandwith. DOCSIS 2.0 and earlier allow only a single channel to be used. Newer DOCSIS 3.0 (and future 3.1) specifications allow for channel bonding (up to 8 in DOCSIS 3) allowing faster internet speeds over the cable system.
Most people who have cable internet have probably at some point in the past had internet problems. You might have talked to a phone technician, or even a field technician who talks about "signal levels". This may have meant nothing to you, and you just nodded along as they talked, wondering what they were on about.
Well, I'm here to help decipher those in an easy to understand (hopefully) manner.
So with mode cable modems, you can go into your browser, and see the signal levels, by going to 192.168.100.1 .
This what it looks like on a Motorola SB6121 (DOCIS 3.0, 4x channel bonding capable)
Link to larger version > http://i.imgur.com/api6CD7.png
This page might look confusing, but it's actually fairly simple.
Let's start with the top section first.
This is the downstream section. This means this contains all the stats on the "downstream" section of the signal, meaning the flow of information to your modem from the Node and CMTS.
The first 2 lines are pretty straightforward.
Channel ID, and the frequency. These are DOCSIS channels, the ones being used by this modem at this time, in a bonded scenario. This model of modem can handle 4 channels simultaneously, with the frequencies measured in Mhz (537, 543, 555 and 561Mhz).
The next line down is Signal to noise ratio. This is 1 of the 3 most important metrics for connection health. This is essentially how "clean" the signal is, measured in dBmV or Decibel-Millivolts. IE how much usable info, vs junk info. The higher this number, the better. As a general rule, any number above 32-34 is ideal. Below that and you might start to have connection issues. Below 29/30 and you are probably having lots of trouble. As you can see, mine are 37, 38, 38, and 38. That is very good. Anything 40 or higher is pretty rare, though would be excellent.
The next line is Downstream modulation, this is the specific symbol (data rate, essentially) being used by that channel. In 99% of cases, it will probably be QAM256.
Next line below that is downstream power level. This is the 2nd of the 3 most important signal metrics. This is the strength of the signal to your modem, from the Node, after it has passed through all the coax lines outside, the tap, the splitters in your house, and all the coax inside. This is a relative number. 0 is ideal. The further away from 0, the worse it is. Spec is -16 to 16. However, ideally, especially for DOCSIS 3.0, you want between -8 and +8. Mine are 3,2,3,3. So that is very very good. This will fluctuate with all sorts of factors. Temperature, as cable has a higher resistance when hot.
If you've ever looked at a splitter, you've probably seen a number on the outputs. On a 2-way splitter, it's usually -3.5dbmv. Meaning that if the strength of the signal coming into the house is +10dbmv, after passing through one splitter, the signal to both outputs is now +6.5dbmv. This decreases with more and more cable, and more splitters. It adds up. So if you start with 0 at the house, and have splitters, the strength can quickly drop into bad/weak levels.
If this is causing connectivity problems, a broadband tech will generally remove splitters, or try to simplify the layout (remove number of splitters), and possibly use an amplifier to boost the signal strength.
Next section is upstream. This is the section with all the information relating to the "upstream" signal, IE what the modem is doing to get it's signal to reach the Node with a reliable connection.
The first 2 lines are the same as with downstream. Channel ID and frequency, again measured in Mhz.
The next line is ranging service ID. This isn't really important, but does tell you which channels are in use. In my modem, the upstream is physically bonded, but only 1 channel is actually being utilized for data.
The next line is symbol rate. This essentially is the speed with which the modem is talking to the node. Either 2.56 or 5.12MSymbols/s is common. In most cases, it doesn't really matter which is in use until you start hitting higher upload speeds. Even at 2.56MSymbols/s I'm still acheiving my full rated upload of 5mbps.
Next line is the 3rd and arguably, the most important (and most often to cause problems) metric. The upstream power level. This is the power level at which your modem is transmitting (ie "ranging") to establish reliable connection with the Node. This metric is by far the most likely to be bad, and cause connection problems. Essentially, in most cases anything below 53-54 is acceptable, but under 50 is ideal. Most modems top out at 55dBmV. Meaning if you are at 55, the modem is transmitting at maximum power, and that means something is wrong. too many splitters, or bad lines. Being at max can mean that your connection might drop, and the "ranging" will fail, causing loss of connectivity.
the modem will then try to re-range, and regain connectivity. This will generally result in a trouble call, with a broadband tech visit needed to help resolve the problem.
There is a low end too- generally anything above 35 is recommended. Any lower and you can start experiencing connectivity loss. This an extremely uncommon problem, usually the problem is transmit power too high. Mine is around 44 most of the time, which is pretty much perfect.
Next line i upstream modulation, again similar to the downstream. Ranging status is exactly what it sounds like. The status of the connectivity on that upstream channel to the node.
The last box essentially tells the number of errors in your connection. Obviously the more unerrored codewords, the better. Mine is a very healthy connection, with very few errored codewords.
The last important page is the logs page.
http://i.imgur.com/gRLipze.png
These are the internal logs in the modem. As you can see, on the 15th we had some "problems" An amplifier at the street level for our neighborhood had gone bad, and a tech was replacing it. During that time our connection was lost, and the modem was still trying to reestablish connection.The 17th has a T4 time out, this isn't always bad, it's an occasional hiccup in the network. The 18th was a power outage, see cable modem reboot due to power reset.
Most of the time, this page isn't much help to the end user, unless you know what you are looking for. But it is an important page to know what you are looking at.
Hopefully this will help you better understand your modem, and the system which brings the service into your house.
Let me know if you have any questions or if I made a mistake.
