Yes, KB.. "some," perhaps a great deal. A quick read of the passage might even leave one with an even grander impression.
But the fact remains that the coax loop to the house is not entirely a passive consideration in the overall design. Nor is the coaxial section in HFC where dwdm is being applied in the example discussed.
The cable loop and modem specs result from the radio frequency analog signals which must conform to the RF spectrum plan as adopted by CableLabs in the CMTS and DOCSIS models. These translate to a low-bandwidth dole to the upstream direction for data services, due to the way that the directionality of the spectrum splits occur, and the fact that cable was never intended to do this sort of thing from the time of its earliest design. Instead, it's been an afterthought.
These upstream limitations exist in the lower end of the cable spectrum (between 5 MHz and 40 MHz). Note that the new interactive services and digital TV services which will use STBs instead of cable modems, which are beginning to roll out now, are situated in the higher end of the spectrum, typically between 500 MHz and 800 MHz.
This means that only a sparse amount of bandwidth is allocated in the upstream direction under the original DOCSIS plan (in niches below 40 MHz which are not used for telephony, and which are ingress/noise free).
That's why we must keep the number of users on any given segment low, and at some point combine them onto a DWDM or other form of multiplexing link towards the head end or hub, upstream.
In review, the upstream problem has more to do with the limited amount of spectrum which has been allocated to the upstream direction in the original spectrum plan. The DWDM fix comes into play only after segmentation of the local loop has already taken place, and the dwdm channels carry the individual user segments between the neighborhood field node and the head end.
The key to optimization, however, for as long as coax RF is going to be used icw the current spectrum plan, is to segment the plant into as small a number of users per group as is economically feasible.
FAC |
