[pip08456]

Originally Posted by Chrysalis

so this here.

http://www.cableforum.co.uk/board/35230501-post2.html

VM are not actually using scdma on docsis2 then? sound slike a mode that would help latency. It does seem VM are barely utilising docsis2, no qam64 and no scdma.

ATDMA is a direct evolution of DOCSIS 1.x physical layer (PHY), which uses TDMA multiplexing. DOCSIS 1.x upstream PHY uses a frequency division multiple access (FDMA)/TDMA burst multiplexing technique. FDMA accommodates simultaneous operation of multiple radio frequency (RF) channels on different frequencies. TDMA allows multiple cable modems to share the same individual RF channel, because it allocates each cable modem its own time slot in which to transmit. TDMA is carried over in DOCSIS 2.0, with numerous enhancements. SCDMA is a different approach, in which up to 128 symbols are transmitted simultaneously via 128 orthogonal codes. SCDMA multiplexing allows multiple modems to transmit in the same time slot. Both ATDMA and SCDMA provide the same maximum data throughput, although one might perform better than the other under specific operating conditions.

and

In order to get high throughput with SCDMA, the modems must all be time aligned within a fraction of the symbol rate. Otherwise, the “S” (synchronous) part of CDMA fails, and the data from one modem corrupts the data from other modems. The result is packet loss. The timing resolution is measured in nanoseconds. There are issues when you measure things in nanoseconds across a distance of 40 km (a short network) or up to 320 km (a long network):

minute changes in fiber path distance, caused by temperature (expansion and contraction of the glass itself)

expansion of the coaxial network (which is why every span has an expansion loop)

the fact that the speed of light also changes with temperature, in both fiber and coaxial line (Velocity of Propagation as a percentage of speed of light)

Every 1 second, an SCDMA modem must be time-aligned, if the modem is more than 20 km from the headend, even if less than half of that network is overhead plant. This represents at least 60 to 80 percent of the cable modems for most multiple service operators (MSOs).

If the hybrid fiber-coaxial (HFC) network is 100 percent underground (including the fiber), the modems are less than 10 km from the headend, and the temperature is very constant for a given day. Then the modems can be time-aligned less often.

Apparently, timing alignment had become a major problem with the modems of some vendors in general. They lose synchronization with the downstream and do not realize it, and then transmit at the wrong time. Therefore, the modem transmits at a time that is reserved for another modem and causes packet loss both for itself and for the other modem. The packet loss for all modems vanishes when only the bad modems are removed from the network.

source