Many reasons may exist for why your Comcast connection may be slow. During peak times of the day when traffic over the cable lines is the heaviest, everyone's connections can suffer. It is also possible that someone is using your connection without permission if you have a wireless router, or perhaps you are maximizing your bandwidth unintentionally by downloading or uploading. In any case, it can be difficult to determine the cause of a slow connection. If you are unable to determine the cause of your connection's sluggishness, conduct an online speed test and contact Comcast.
Neighborhood Node Connection
Unlike DSL, all cable connections are routed through what is typically referred to as a neighborhood node. If the node itself is being over-utilized, your Internet connection could slow down drastically. This type of connection issue is especially common at peak times of data transfer. Peak times can vary by area but are typically between 7 p.m. and 2 a.m. If you notice that your connection slows down around a particular time, this may be the cause. Another possible cause could be a promotion. Comcast has been known to run promotions offering bandwidth upgrades at discounted rates. If you noticed a severe slowdown in your connection during a promotion, it may be that too many people in your neighborhood took advantage of the promotion and are placing too high a load on the neighborhood node.
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Your connection could be slow because someone else is using it without your authorization. To determine this, shut down all your PCs and then check the lights on the router to see if it shows any active connections. If it does, there is a good chance that either your router is not password-protected or that someone has hacked the router's password and is using your bandwidth. If you determine that this is the case, log into your router and change the password. Once the password has been changed, you will have to reset the passwords saved for your wireless connections on all your PCs.
Another common problem that causes slow connections is when upload or download streams get overloaded. Your connection moves in two directions, up and down. Your download stream is the data coming from the Internet to your PC, while the upload stream is the data going from your PC to servers across the Internet. Your connection could be slow because you are using the maximum amount of bandwidth available to you to download a file, and therefor you have very little bandwidth available for anything else, like surfing the Internet.
On the other hand, although the majority of the traffic involved in Internet browsing happens over your downstream, you must have an upstream available through which to request the data. Most Comcast contracts allow for about half as much upstream as downstream. If someone is downloading something from you, (for instance, if you are sending a torrent), and you have no available upstream, the connection will move extremely slow (even if you're using none of your downstream), because you will not be able to transmit requests for data.
Another common cause of slow connections is malicious software like spyware and viruses that hijack your connection to transmit large quantities of data. To test for this, get an up-to-date virus and malware scanner and run a full system scan on your computer. Remove any viruses or other malware discovered by the scanner, reboot your system and retest your connection.
Another cause of slowness could be a physically degraded connection or signal interference across a line. If a cable is physically damaged, it will suffer data loss, which will significantly slows your connection down. Most coaxial cable is shielded to prevent signal interference across the wire. If your connection runs through a section of unshielded cable, some form of interference could also be causing data loss. If this is the case, contact Comcast.