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The quality of the channel often does not permit the use of OFDM until distribution amplifiers are replaced. Unfortunately this is a totally different issue for

The quality of the channel often does not permit the use of OFDM until distribution amplifiers are replaced. Unfortunately this is a totally different issue for the upstream and downstream directions as well, which is why several carriers, including Comcast in many markets, currently do OFDM downstream and only QAM upstream. The downstream issue has, on the whole, always been easier to fix due to the design of the outside plant, which is why 1gbps down has become a standard cable offering in DOCSIS 3.1 markets. In the case of upstream channels they are constrained by a variety of different equipment, some of which predates the use of QAM for all metadata. Comcast is beginning a (presumably long) process of transitioning away from conventional cable television and to completely OTTd television service, which is essentially IPTV over DOCSIS, branded as Flex. This seems to be their long-term strategy and will allow a radically higher degree of flexibility in how they run their infrastructure. I would expect this to become the norm over the next ten years or so as DOCSIS is increasingly viewed as the primary purpose of the cable infrastructure.

nimbius 6y ago
Fascinating! I had no idea there was a delineation between upstream and downstream encoding albeit its a fairly obvious condition in retrospect thinking of the upload caps.
- rayiner 6y ago
  It’s a fairly obvious distinction considering that you’re dealing with a system that was initially designed for one way communication. The cable system is not an Ethernet network with multiple pairs of conductors for each direction, all hooked up to a switch. It’s effectively a single shared bus, which back in the day was shared between hundreds of houses. Remember, it was initially designed to just get OTA signals into TVs over cable.
  - jcrawfordor 6y ago
    It's also important to remember that fiber internet, in the sense people are usually talking about, is fundamentally the same. PON (passive optical networks) are passive in that a single fiber is shared between multiple houses using passive splitters. This means that the fiber is a shared medium and upstream and downstream, and multiple users, must all be managed based on TDM. In the widely used GPON, Gigabit PON, that shared medium operates at a gross rate of 1gbps as the name implies. The exact same issues of coordinating the shared medium between users exists for fiber last-mile technologies but is generally better managed because fiber carriers don't have nearly as much legacy to contend with and so universally use more modern scheduling and allocation methods. From a practical perspective, any method of last-mile internet delivery will probably rely on a shared medium. The cost of "home run" cable routing is just too high, the telephone network which was originally structured this way has invested a great deal to move away from it wherever possible. There is no widely-used last mile internet technology today which does not have bandwidth contention between users. Fortunately, in practice, a 1gbps gross shared medium is usually sufficient to offer "gigabit symmetric" service to a neighborhood of subscribers without complaints, although there is some debate about whether or not popular 4K video streaming will change this.

nimbius 6y ago

Fascinating! I had no idea there was a delineation between upstream and downstream encoding albeit its a fairly obvious condition in retrospect thinking of the upload caps.

rayiner 6y ago
It’s a fairly obvious distinction considering that you’re dealing with a system that was initially designed for one way communication. The cable system is not an Ethernet network with multiple pairs of conductors for each direction, all hooked up to a switch. It’s effectively a single shared bus, which back in the day was shared between hundreds of houses. Remember, it was initially designed to just get OTA signals into TVs over cable.
- jcrawfordor 6y ago
  It's also important to remember that fiber internet, in the sense people are usually talking about, is fundamentally the same. PON (passive optical networks) are passive in that a single fiber is shared between multiple houses using passive splitters. This means that the fiber is a shared medium and upstream and downstream, and multiple users, must all be managed based on TDM. In the widely used GPON, Gigabit PON, that shared medium operates at a gross rate of 1gbps as the name implies. The exact same issues of coordinating the shared medium between users exists for fiber last-mile technologies but is generally better managed because fiber carriers don't have nearly as much legacy to contend with and so universally use more modern scheduling and allocation methods. From a practical perspective, any method of last-mile internet delivery will probably rely on a shared medium. The cost of "home run" cable routing is just too high, the telephone network which was originally structured this way has invested a great deal to move away from it wherever possible. There is no widely-used last mile internet technology today which does not have bandwidth contention between users. Fortunately, in practice, a 1gbps gross shared medium is usually sufficient to offer "gigabit symmetric" service to a neighborhood of subscribers without complaints, although there is some debate about whether or not popular 4K video streaming will change this.