• Comcast DNS website
• How do I change what DNS server my computer uses?
• How do I get a different IP address?
• How the CMTS determines your modem's transmit power
• What is CRAN?
• What is HFC (Hybrid Fiber Coax)?
• What is QAM?
• What ports does Comcast block?

Provided via website: Comcast DNS
This site provides you with the Comcast DNS server status, IP addresses, and troubleshooting tools.

• DNS Cache Check

• Comcast DiG

• Sandia Labs DNSVIZ

• Comcast WHOIS

For ref: Domain Name System (DNS)

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by Johkal
last modified: 2013-03-31 19:52:45

Note: Instructions below are for Windows 2000, XP and 2003 users only. Additionally, if you are behind a router, you may have to change DNS settings using the router configuration utility. Please consult your user manual.

To change DNS servers:

• Right click on the My Network Places icon on the desktop and select Properties. (If the icon is not available, go to Start menu | My Computer | My Network Places | View Network Connections.)

• Locate the network connection that is associated with your Internet connection. This may be labeled something like "Local Area Connection." If you have more than one connection to choose from, be sure you determine the right one before proceeding.

• Right click on the appropriate connection and select Properties.

• In the list that appears under the General tab, double click on Internet Protocol (TCP/IP).

• In the window that appears, you want to select the option to Use the following DNS server addresses...

• At this point you will want to enter at least one of your personal preferred DNS servers.
A complete list of servers can be found here: Comcast DNS servers Click the tab of choice to see the type of DNS server you are seeking.

• Press Ok out of all windows until you are back to the Network Connections window. You can now open your web browser and browse to a website to see if it connects faster.

*This FAQ is based on user knowledge from a volunteer core of BroadbandReports' members. This FAQ in no way constitutes official information from Comcast or any of its affiliates.

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by draven edited by sortofageek
last modified: 2011-08-14 13:22:06

Because of the way DHCP works, it is possible to change your IP address by changing your MAC (media access control) address. Most routers have a MAC clone function which is useful for letting customers who’s ISP registers them by the modem, input a NIC’s MAC in the router. Keep in mind that after changing the MAC in your router, you will need to restart your modem to clear it’s CPE MAC Address table.

Most NICs also offer a MAC cloning function similar to that of a router that can be accessed through device manager: >Your NIC->Properties->Advanced->Locally Administered Address. Change the value to a 12 digit long hexadecimal code and restart your computer.
Some NICs won't let you change the MAC. In such a case, see the following: »www.klcconsulting.net/Change_MAC_w2k.htm
This link has some very useful information; including a program that can allegedly change the MAC: “regardless of whether manufacturers allow this option or not”.

*This FAQ is based on user knowledge from a volunteer core of BroadbandReports' members. This FAQ in no way constitutes official information from Comcast or any of its affiliates.

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by Nerdtalker edited by Johkal
last modified: 2011-03-18 17:36:18

When a cable system is set up for two-way, it is swept. Sweep is the process of verifying that basically all of your amps in the field have the same signature as the node. You are basically trying to re-create the node out to the last amp. The node is the fiber to cable converter that every neighborhood has. Once the amps in a node have all been swept, they are known to have unity gain. All of the amp chips in the return are set and the amount of RF needed to transmit through them to the headend are the same at the input to the amp chip. Now, where it differs is at the taps and other system passives. Ultimately, you are setting your system so that a preset amount of RF will equal 0 dBmV at the CMTS. The CMTS wants to see the RF signals from the modems all at the same dB level. The CMTS cannot discriminate the signals from modems if the values are all different. It is here that the CMTS can dictate the amount of RF needed from each modem. So, when a DOCSIS modem is synchronizing, it first gets the forward info, it then ranges upstream. Ranging basically consists of calls to the CMTS where the modem says, "can you hear me?"... this goes on until the CMTS acknowledges the modem. Once the CMTS can hear the modem, it fine tunes the modems transmit power and the modem can then use DHCP to get an IP, grab a config file, time of day, and boom... your modem is synchronized. The modem receives maintenance packets on a regular interval to adjust the transmit power. This happens OFTEN. (the packets).. The return transmit power should NEVER move more than a dB. If you are looking at the 192.168.100.1 page and your transmit power moves more than a dB or two, you have a problem (usually a loose fitting). Now, how do you compute what the return should be? Easy. First you need to know the unity gain value. Then you need to know the tap value you feed from. (you'll need to know all of the passives in the plant to your tap too. Good techs can figure this out with a print). Then, you take the unity gain figure, add it to the tap value, and that should give you the transmit value at the tap. Most drops to the house under 200 feet should lose no more than 2 dB. Then add your splitter loss and you will have your transmit power value.

Let me make this example. Lets say you live at a house by a tap that is right by an amp. Lets say the tap is a 23 value. Lets use 16 as the unity gain figure (its not 16 in a lot of systems). 16 + 23 = 39 dBmV. So, if the modem was hooked up at the tap, the 192.168.100.1 page should say the transmit is 39 dBmV. Now, add 1 or 2 for the drop to the house (cable) and then add splitter loss. Lets say the modem is hooked up to a two way in the house box outside and you have a two way inside with a tv next to the modem. Do the math... 39 + 1 + 3.5 + 3.5 = 47 dBmV. Your modem should be transmitting 47 dBmV in this scenario.

So, for those of you who have return problems (like really high) The best test you can do is go to the ground block where the cable reaches the house and plug the modem in there. (We have test equipment to do all of this but for you the modem at the house block will work)... You'll need a LONG cat5 cable for this but. Get the modem working right out of the house box with no house cable or splitters. If your modem transmit power is over 45dBmV at the ground block, you probably have a unity gain problem that the maintenance people need to fix. If it is lower than that, start doing some math and look at the splitters in the way. Remember when you add or remove splitters, the transmit value in the 192.168.100.1 page will move roughly the same amount. All of this assumes that you have good wire and solid fittings.

Thanks to TechnoScott

*This FAQ is based on user knowledge from a volunteer core of BroadbandReports' members. This FAQ in no way constitutes official information from Comcast or any of its affiliates.

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by Big_D edited by Johkal
last modified: 2008-11-01 14:56:44

The Converged Regional Area Network, or CRAN allows all voice, video, and Internet traffic to go over a single network with redundant 10 GigE networks. The speed of the connections are very fast. All devices on the Comcast network, regardless if it is a cable modem, digital cable box, VoIP modem, or whatever are connected to each other (from demarc of the home and outward) on a redundant 10 GigE optical network. Such a network can provide network speeds far in excess of what Verizon's Fios offers with little upgrade by Comcast should they want to offer equivalent speeds.

All areas are being converted to the CRAN. The most apparent thing you will notice when you are switched, is the additional hops. These hops have little to no effect on speeds or latency. The good news is; by keeping the traffic more internal, it reduces cost to Comcast and allows the subscriber to put a less detrimental affect on the network.

Two threads for review on CRAN:

How can I check if I am on Comcast CRAN or not?

How can I tell if I am on the CRAN?

*This FAQ is based on user knowledge from a volunteer core of BroadbandReports' members. This FAQ in no way constitutes official information from Comcast or any of its affiliates.

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by mbernste edited by Johkal
last modified: 2008-11-01 14:59:38

HFC is the network used by Comcast and other cable providers to provide High Speed Internet.




Please see this Wikipedia entry for more information.

*This FAQ is based on user knowledge from a volunteer core of BroadbandReports' members. This FAQ in no way constitutes official information from Comcast or any of its affiliates.

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by Big_D edited by Johkal
last modified: 2008-11-01 14:59:52

QAM, or Quadrature Amplitude Modulation, is the modulation scheme used in cable plants. Basically, QAM is how the modem encodes digital information to be sent over the RF interface of the modem.

64QAM and 256QAM are two very common modulation schemes used in the downstream channels for cable modem or digital cable plants. 16QAM and QPSK are two modulation schemes commonly used for the upstream channels in a cable plant.

“In digital applications, the modulating signal is generally quantised in both its in-phase and 90° components. The set of possible combinations of amplitudes, as shown on an x-y plot, is a pattern of dots known as a QAM constellation.” Wikipedia explanation

The dots mentioned in the cited description are also called decision points. When the modem plots these points, each will fall into the boundary of a decision area, which corresponds to a sequence of 0s and 1s. This is called the QAM constellation.


QAM Constellation



The number before QAM (for example 16QAM) refers directly to the number of decision points in the constellation. Meaning, 256QAM has 192 more decision points than 64QAM. The number before QAM is also always a power of two. (EG. 2^6=64QAM)

There are several trade offs to this:
•The more Decision points, the more throughput per channel.
•The more Decision points, the more susceptible the channel is to noise (as the decision boundaries get smaller to accommodate more decision points, it becomes harder to distinguish which boundary the point was intended to lie in)


More Decision points=More Bandwidth=Higher susceptibility to noise or poor signal (the same applies inversely as well)



BER



So what does this mean to me?
Ideally, the customer shouldn't notice any difference between a 64QAM or a 256QAM modulated downstream channel.

Despite the fact that 256QAM means more possible throughput per downstream channel, that doesn't necessarily equate to the customer seeing faster speeds. At best, it'll enable the cable company to either:

•Deliver the same number of customers incrementally faster speeds.
-or-
•Deliver a larger number of customers the same speeds as before.


Modulation Schemes and Speed table



*This FAQ is based on user knowledge from a volunteer core of BroadbandReports' members. This FAQ in no way constitutes official information from Comcast or any of its affiliates.

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by Nerdtalker edited by Johkal
last modified: 2008-11-01 15:00:10

Comcast currently blocks ports 25, 68, 135-139, 445, 520, and 1080.

It is questionable whether some of these ports are presently blocked.
See this topic: Blocked ports

Also, see this Comcast FAQ link.

*This FAQ is based on user knowledge from a volunteer core of BroadbandReports' members. This FAQ in no way constitutes official information from Comcast or any of its affiliates.

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by Big_D edited by Johkal
last modified: 2010-04-08 18:48:22