Cable Modem Troubleshooting Tips


The flashing lights

Cable modem data activity light keeps flashing

Most cable modems have an LED labelled Cable or Data or D/S, which indicates when data arrives from the cable network. Some users get worried that this LED flashes even when their PC is doing nothing or is switched off. If they are running a firewall, they are puzzled that the firewall does not register any traffic. There is no need for concern: some essential data (DHCP and ARP protocols) is broadcast into the cable network and arrives at every cable modem whether your PC needs it or not. You need not worry that someone is trying to hack into your PC just because the data light is flashing.

If the rate of flashing goes up when your PC is online (compared to when your PC is switched off or disconnected), but you do not know of any reason why your PC should be generating network traffic, then that might be a source of concern.

Surfboard 3100 or 4100

A Surfboard cable modem has five lights on the front panel, and two on the back panel associated with the ethernet socket. The ones on the front panel are:

The ones on the back of the 3100 are:

The ones on the back of the 4100 are:

Power: When the Surfboard powers up or re-boots, the Power light flashes while it carries out internal self-diagnosis. If it never comes on at all, check the mains supply to the cable modem (it is easy for the little mains plug to come loose within the socket on the back of the modem). If the Power light never goes steady, the cable modem is faulty, and should be exchanged for a new one. When the Power light does steady, the cable modem is ready to try using the CATV cable.

Receive: Next the cable modem begins searching all frequencies on the cable for a channel which looks as if it is carrying the cable modem downstream data. The Receive light flashes while it is searching - this can take several minutes if the cable modem is new or after a hard reset. The cable modem remembers in non-volatile memory the frequency it last successfully used, and it tries that channel first: if that works, this search process will take only a few seconds. If the Receive light never goes steady (and you have to watch it for a long time), then there is a serious problem with your CATV feed, or the local UBR is not working: check that your CATV cable is properly screwed in to the back of the cable modem. If this condition persists, contact your local ISP's Customer Service. When the cable modem has locked on to the downstream data frequency, the Receive light goes steady.

Send: Once the Receive light has gone steady, the Send light starts to flash. The cable modem listens to the downstream data stream. From time to time the UBR sends information out on the downstream channel, telling all newly online cable modems on what frequency to transmit upstream data. When it receives that information, the cable modem tries to announce its presence on the appropriate upstream channel. If the UBR hears the announcement, a set of negotiations (ranging requests) take place during which the cable modem gets allocated a specific minislot (time slot allocation) on the upstream channel, and the cable modem is commanded to adjust its upstream transmit signal level until the UBR hears the right level (so that all cable modems on the same network present about the same signal level to the UBR). If the Send light never goes steady, it is possible that there is a problem so severe with your CATV feed that the UBR cannot hear your cable modem announcing itself: contact your ISP for service. When finally the UBR grants the cable modem permission to access the network, the Send light goes steady.

Online: Once the Send light has gone steady, the Online light starts to flash. The cable modem, having been granted access to the network, broadcasts a DHCP request, sending its own HFC MAC address as data. If this HFC MAC address has been registered with the DHCP server (that's what is supposed to have happened when you registered your HFC MAC address), the DHCP server replies, granting an IP address to the HFC interface of the cable modem (this IP number will be in the or range). If the DHCP server rejects the DHCP request, the cable modem gives up and re-boots, starting with the flashing Power light above. This behaviour (reboot after online light flashed for a short period) is characteristic of your MAC address not having been correctly registered with the DHCP server for your area: call your ISP for help. Having acquired an IP number, the cable modem is now able to initiate TCP/IP requests. It calls again with a TFTP protocol request for a configuration file: the configuration file will be downloaded to the cable modem. See Cable Modem configuration file below for more about this file. Once all this has finished, the Online light goes steady, and all four green lights will now be steadily lit.

The cable modem is now fully online to the network, but there might be a delay of a few minutes before it is ready to offer service to your PC. For instance, it might need to download a firmware update for itself. If you tend to keep your cable modem powered off when not in use, you should wait a while after all green lights have gone steady on the cable modem before powering up your PC, otherwise your PC will make DHCP requests to the cable modem before the cable modem is ready to respond. If this happens, the PC will fail to obtain connectivity to the network.

Activity: this light flashes yellow if the cable modem is receiving or transmitting data on the CATV cable.

ACT: [3100] this light on the back of the cable modem flashes when the cable modem is receiving or tranmistting data on the ethernet link to your PC.

LINK: [3100] this light on the back of the cable modem lights steadily green when the ethernet socket sees a connection from a live ethernet socket on your PC. If your PC is running, and ethernet enabled, the LINK light should be lit. If it is not lit, then either there is an ethernet cable fault, or ethernet networking is disabled in your PC. This is your problem to fix, not your ISP's.

LINK/ACT: [4100] lights steadily when an ethernet link has been made, and flashes when data is being transferred.

100: [4100] indicates that the ethernet link has been sensed to be 100 Mbps.

3Com Tailfin lights

Blinking short off, long orange Power-on self-test in progress
Steady orange Power-on self-test failed
Blinking short off, short orange Searching for downstream channel
Blinking long orange, short green   Acquisition negotiations in progress
Blinking short off, short green Denied permission to go online
Steady green Online and operational

3Com CMX lights

Terayon TeraJet 210 lights

ntl:home 100/120 (Ambit) lights

For the sequence of LEDs as the modems boots up, see

Pace digital set top box lights

The centre LED of three flashes while the STB is trying to establish two-way communication with the cable network, and is lit steadily when two-way communication is available. The cable modem service will be available when this LED is steadily lit.

CATV = Communal Antenna Television
UBR = Universal Broadband Router = CMTS coupled with a router.
CMTS = Cable Modem Termination Shelf (or System).
DHCP = Dynamic Host Control Protocol.
TFTP = Trivial File Transfer Protocol.
STB = Set Top Box.

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