Originally Posted by
bfdtv 
Verizon is relatively unique among cable systems in their design.
They have two SuperHeadEnds (SPEs), one for redundancy. These SPEs aggregate all digital cable content using fiber, dozens of big ugly dishes (BUDs), and a warehouse full of receivers, SD MPEG-2 encoders, and commercial insertion equipment.
The SPEs send the MPEG-2 video via SONET to video hubs (VHOs) in each region. I don't know the current count, but I believe there are now somewhere between 7 and 13 VHOs. The VHOs modulate the MPEG-2 channels from the SPEs into RF (QAM) for delivery to the home. A section of RF (QAM) spectrum is reserved for locals. The VHOs receive the local networks from broadcast affiliates in the region (typically via fiber) and modulate them onto the reserved QAM channels. The combined QAM signal is sent via fiber to each Verizon central office in the region with guide data.
Your local office receives that modulated QAM video from the VHO and combines it with voice and high-speed data. From there is sent to your home via fiber.
Verizon has 135 6MHz channels available between the CO and your home. The problem, as I alluded to above, is that the VHOs and COs evidently have the hardware to support only 103 of those 135 channels today. Depending on the VHO, anywhere from 54 to 63 of those channels is allocated to QAM, with the rest allocated to analog. The oldest VHOs may have 54 slots available for QAM while newer VHOs may have 63.
More comprehensive upgrades are apparently required to upgrade the VHOs and COs to use the remaining 32 channels, so for now, Verizon is working to eliminate the analog channels. It's not a matter of just switching off an analog channel and switching on a QAM channel because both use separate equipment at the VHO and CO; hence, new equipment must be installed in every VHO and CO.
Each QAM slot provides a minimum of 38.8 Mbps usable. Verizon currently allocates 7-9 SD channels per QAM and 2 HD channels per QAM. The average FiOS system probably uses 52-53 QAM slots today.
Verizon's first step may be to make all VHOs and COs capable of using 63 QAM slots, because the newer VHOs are already capable of that. Verizon rolls out channels nationally, not regionally, so some in newer FiOS markets are probably stuck waiting for older markets to upgrade their equipment.
Updated Friday, October 19
Verizon responded to this post on Engadget with,
Ben, there is no "technological limitation in our video hubs and central offices. In our fiber system it is just a matter of adding new equipment to increase capacity. That's certainly true.
The problem is that the equipment upgrades necessary require many months of work. In some cases, building upgrades are required because Verizon didn't include storage space for that equipment in their plans. Verizon is relying on contractors outside the company to perform many of these upgrades, which slows the process. It is possible some timetables have changed recently, but as of three months ago, Verizon engineering did not expect to have the capacity necessary to add most of the new DirecTV HD channels for another year (i.e. 4Q 2008). This information was supplied by employees working in the FiOS TV engineering department under Frank B.
There is now talk that Verizon will eliminate the analog channels by June or July, which would free up capacity for about 80 new HD channels. Whether management will authorize the resources necessary to make this happen is unknown.
