Comcast Repair Checklist
Comcast internet and voice service is an excellent value. The Comcast outside plant is extensive and complex. For best service, review this page and work with Comcast to maintain a satisfactory level of service. Be a scientist and calmly and rationally troubleshoot your issues.
Modem coax cabling must be perfect
- The cable modem must be installed next to your router in your computer area/room and on a UPS.
- The cable drop from the mainline to your modem must be RG6 or RG11 cable.
- Remove old HBO filters from the tap plate used in the days of an analog cable TV.
- Replace the splitter between outside cable and cable modem with a quality two-way splitter (as pictured) with a favored power port for the cable modem.
- If you have two modems, they must have separate connections to the main line tap plate. They can not both exists on the same RG6 coax building drop.
- Extremely cold days can cause "cable suck-out" which causes the cable center conductor to "suck out of it's connector. Google "suck-out" for details.
- We discourage sharing of cable with TV or cable dialtone boxes as this is a point for noise and power loss.
- Fix the temp mainline splitter installed when your neighbor got Comcast and the 3rd party installer split your tap and cut your signal in half.
We require business grade Internet service
- SMC D3G cable modem is required as it has faster electronics than the 8014 and thus allows for higher speeds.
- Business grade service has a higher level of response and technical support.
- Business grade service allows a static IP. Static IP allows us to monitor your service.
- If sharing the Comcast with any data, choose one up from the base package.
Verify signal levels in your D3G modem
Dn power > 0db, Dn SNR > 32db, Up power < 42 db
Comcast service delivery is like municipal water service delivery. It's a system of interconnected large and small diameter pipes. Comcast states that a working "water pressure" range is -15db to +15db with lower than -11db as being in the "yellow range". Unofficially, Comcast insiders state it should be at 0db or higher for trouble free performance. The picture below shows a Comcast SMC8014 cable modem diagnostic page with very low downstream power but acceptable (SNR) signal to noise ratio.
Good SNR is especially important if the downstream power can not be improved. Inside cabling defects such as a loose connection to a TV can cause poor SNR. this. On critical or new cable modem installs, we will discourage connection of cable TV until performance is proven. It is the customer's responsibility to correct any deficiencies on interior cabling.
You can't measure upstream SNR but you can extrapolate it by the upstream power the Comcast CMTS(head-end) commands the modem to transmit to push it's signal back to Comcast.
Best we have seen is down db = 0 to 3db, down snr > 37db, up db < 35.5. This is dialed in and seen on new mainlines to a building.
This is the view from your D3G diagnostic page
Verify signals with Comcast (This gets a trouble report made)
Above modem performance must be within normal bounds for a week according to Comcast Scout or Grandslam systems.
Typical Scout Report health Check Report
These reports show data marked as green, yellow or red. This report is all green.
|IP Address of Modem||188.8.131.52|
|Current CMTS Status||ACTIVE|
|Device Make/Model||SMC Networks SMCD3G-CCR|
|Device Up Time||104 days 9:14:35.05|
|Docsis Capability||DOCSIS 3.0|
|Downstream Signal Quality (SNR)||35.0|
|Modem Lost Syncs||1|
|Receive Power Level (downstream)||0.1 Dbmv|
|Receive Power Level (upstream)||2.8 Dbmv|
|Serial Number of the Device||H29421C3C4|
|Transmit Power Level (upstream)||48.0 Dbmv|
|Upstream Signal Quality (SNR)||32.0|
|Current Device Status||6(Online)|
|End of Life Status||No|
|Modem Boot File||d11_m_smcd3gccr_bcistarter_c05.cm|
Ground the cable shield OUTSIDE the building per National Electrical Code section 800-33
- Cable Grounding. The metallic sheath of communications cables entering buildings shall be grounded as close as practicable to the point of entrance or shall be interrupted as close to the point of entrance as practicable by an insulating joint or equivalent device.
- The grounding conductor shall not be smaller than No. 14. The grounding conductor shall be run to the grounding electrode in as straight a line as practicable.
- Bonding of Electrodes. A bonding jumper not smaller than No. 6 copper or equivalent shall be connected between the communications grounding electrode and power grounding electrode system at the building or structure served where separate electrodes are used. Bonding together of all separate electrodes shall be permitted.
Modem performance issues
- Comcast modems share frequencies on the coax mainline with their neighbors before the signals are aggregated to neighborhood coax-to-fiber hub. At times, modems may shift to less congested frequencies. This switch should happen within 50 ms but in an abnormal condition, this switch can take up to a minute. During that time, no data is transmitted. Any VPN's will drop. Consider replacing the modem.
- Go to firewall settings and disable "Gateway Smart Packet Detection".
Mainline tap problems
Picture of a cable mainline connected to a mainline amplifier and to the right a two position mainline tap plate.
Two RG-6 coax drops come off the tap plate. One has a blue tag and the other has a yellow tagged HBO channel filter.
In this picture, installing a new (third) connection for a new customer would require the independent installer to first call the cable company to convert the two (2) position tap plate to a four (4) position tap plate, then the installer would need to make a second trip.
Or, to get his "paid per the job" install done, the installer may install a 2-way indoor splitter and then call the office to get the main line tap plate changed. On the later scenario, the customer who gets split, may be forced into a marginal power level condition. Plus, they will have two unexpected service interruptions. One when split, the second when un split. They may also have marginal power levels and thus marginal service in between splits.
Picture of four position mainline tap with one unused position properly
capped to prevent RF intrusion. Orange RG6 direct burial cable is used. The
cable may turn yellow with age and exposure to the sun's UV rays.
Inspect your mainline tap. Make sure no coax splitters (or HBO filters) are used to deliver service due to a lack of open mainline tap ports.