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Physical Layer
- Interference.
- ISPs don’t want to talk about it, but events like drop wires swinging in the wind can affect a DSL or cable modem connection. Cable broadband networks are also susceptible to radio interference – your connection will get a little worse when somebody is operating a blender or microwave oven.
- Aged Infrastructure.
- Coaxial copper wires in the are also aging, similar to the telephone copper wires. Some coaxial networks were built in the 1970s. Coaxial cable networks exhibit signs of aging sooner than telephone copper networks because the wires act like a huge antenna, and older networks attract so much interference and noise that it become harder to transmit the signals through the wires.
- Distance Limitation.
- Unamplified signals are not generally transmitted more than about 2.5 miles over a coaxial network from a network node.
- This limitation is based mainly on the number of amplifiers needed on a single coax distribution route.
- Amplifiers are needed to boost the signal strength for coaxial distribution over a few thousand feet. Modern cable companies try to limit the number of amplifiers on a coaxial route to five or less since adding amplifiers generally reduces broadband speeds.
- Unamplified signals are not generally transmitted more than about 2.5 miles over a coaxial network from a network node.
Upstream Bandwidth
- Bandwidth Limitations.
- All bandwidth is not the same. For example, the upload bandwidth in a cable company network uses the worse spectrum inside the cable network – it uses the frequency that is most susceptible to interference.
- This never mattered in the past when customers cared about download bandwidth, but an interference-laden 10 Mbps upload stream is not going to deliver a reliable 10 Mbps connection.
- Frequency Allocation.
- Most cable systems deploy upload broadband using the frequencies on the cable system between 5 MHz and 42 MHz. This is a relatively small amount of bandwidth and it also sits at the noisiest part of cable TV frequency.
- Recall the days of analog broadcast TV and analog cable systems when somebody running a blender or a microwave would disrupt the signals on channels 2 through 5.
- Cable companies are now using these same frequencies for upload broadband. The DOCSIS 3.0 specification assigned upload broadband to the worst part of the spectrum because before the COVID-19 pandemic almost nobody cared about upload broadband speeds.
- Before the pandemic, the upload link was mostly used to send out attachments to emails or backup data on a computer into the cloud. These are largely temporary uses of the upload link and are also considered non-critical – it didn’t matter to most folks if a file was uploaded in ten seconds or five minutes. However, during the pandemic, all of the new uses for uploading require a steady and dedicated upload data stream. People now are using the upload link to connect to school servers, to connect to work servers, to take college classes online, and to sit on video call services like Zoom. These are critical applications – if the broadband fails then the user loses the connection. The new upload applications can’t tolerate best effort – a connection to school either works or it doesn’t.
- Recall the days of analog broadcast TV and analog cable systems when somebody running a blender or a microwave would disrupt the signals on channels 2 through 5.
- Most cable systems deploy upload broadband using the frequencies on the cable system between 5 MHz and 42 MHz. This is a relatively small amount of bandwidth and it also sits at the noisiest part of cable TV frequency.
Data Over Cable Interface Specification (DOCSIS)
- One limitation of a DOCSIS network is that the standard does not allow for symmetrical data speeds, meaning that download speeds are generally much faster than the upload speeds. This is an inherent design characteristic of DOCSIS 3.0 and 3.1 where no more than an eighth of the bandwidth can be used for upload.
- DOCSIS 3.0 standard allows bonding together enough channels to create broadband speeds as fast as about 250 Mbps download.
- In a digital conversion a cable company compresses video signals and puts multiple channels into a slot that historically carried only one channel.
- This is why over the air (OTA) high definition television (HDTV) and over the top (OTT) content delivered via the Internet has a superior signal quality (picture) in comparison to video delivered via cable video programming.
- In a digital conversion a cable company compresses video signals and puts multiple channels into a slot that historically carried only one channel.
- DOCSIS 3.1 theoretically allows all of the channels on the network to be used for data and which can produce broadband speeds as fast as 8–10 Gbps if a network carried only broadband and had zero television channels. Since there are still a lot of TV channels on a cable network, most cable companies have increased the maximum broadband speeds to between 500 Mbps and 1 Gbps using DOCSIS 3.1.
- DOCSIS 4.0 will allow for symmetrical gigabit data speeds. This will require even more empty channel slots on a cable network and the new standard assumes that cable company will increase total system bandwidth of the network to at least 1.2 GHz of bandwidth. The gear needed to upgrade to DOCSIS 4.0 will not hit the market for at least two or three years.
- Most of the big cable companies have already said they are not interested in upgrading immediately to the new standards since the upgrades are expensive.
- Cable companies will ultimately face a big decision, because if they are going to upgrade to DOCSIS 4.0 they also might instead consider the leap to fiber.
- Most analysts think that upgrade is likely decades away, but most think that cable companies will eventually migrate to fiber.
- Cable companies strive to minimize capital costs and are likely to milk the current networks for as long as possible, to the detriment of the consumer.
- Fiber.
- Cable companies are only going to able to provide speeds above 1 gigabit per second by implementing another round of expensive upgrades.
- There is a lot of speculation in the industry that cable companies would upgrade to fiber-to-the-home (FTTH) rather than make such an upgrade. Unfortunately, if large cable providers ever decide that fiber is its future, the company is going to upgrade major metropolitan markets long before upgrading secondary markets.
- Cable companies are only going to able to provide speeds above 1 gigabit per second by implementing another round of expensive upgrades.
Content derived from: "Feasibility Report for a Community Network - Falmouth, MA" | Douglas Dawson, Owner, CCG Consulting (posted )
Read more: "The Case for Fiber to the Home, Today: Why Fiber is a Superior Medium for 21st Century Broadband" | Electronic Frontier Foundation (posted )