How does FiOS (Verizon's fiber-to-the-home offering) work? I'm asking for information beyond the Wikipedia article.
It states there are three optical channels, one for data up, one for data down, one for TV.
Telephony (POTS) uses the data up/down channels. Is it a form of VOIP? I believe I saw mentioned the telephone hardware is simply a one-line version of equipment that telephone co's already use to bring fiber from a CO to a cabinet, and copper POTS the rest of the way to a neighborhood of customers. What protocol does this use?
It mention they multiplex up to 32 customers' fibers onto a single fiber optically. Does this cause contention for limited IP bandwidth like what happens with a neighborhood of active cable modem users? How do they multiplex the "data up" channel so that multiple subscribers don't try to "transmit" their data at once?
It mentions "video on demand" uses the data down channel, not the TV channel. Does this count against the IP bandwidth allowed? What protocol is this?
The fiber carries three wavelengths (or colors, if you prefer) of light. This is what you are calling "channels." Two are transmitted by the network toward the subscriber (downstream) by a device at the central office called an optical line termination (or OLT). The other is transmitted by the subscriber premise equipment to the network (upstream) by a device called an optical network termination (or ONT).
Note that ONTs are further classified by the type of building they are used in. An ONT at a typical house is called a "single family unit" or ONT:SFU. This is what most people mean when they say ONT. An ONT at an apartment building is called a "multiple dwelling unit" or ONT:MDU. This is often shortened to MDU. Further, business users may use these or other special purpose ONTs depending on their location and service needs.
Anyway, back to the wavelengths. In FiOS, one downstream wavelength is dedicated to TV services while the other is phone/data. Since we're talking fiber here, each wavelength can carry a huge amount of information. In fact, a single wavelength can carry the same information you'd find on an
870Mhz coax cable system. And, this is exactly what FiOS does with the TV wavelength - they put an entire set of cable channels across it. The ONT receives this signal and turns into a multi-channel RF over coax signal that feeds the TVs and set-top boxes in the home. Note this is pretty much the same way that cable TV works, except that the fiber to coax conversion happens outside the home.
The other downstream wavelength is a 622Mbps ATM data flow. Since ATM was designed to carry both voice and data, this makes sense. For each phone line assigned to the ONT, a full 64Kbps of bandwidth is allocated. This is not VoIP. There is no compression It is basically an extension of the digital phone network to the ONT. The ONT acts like a mini-CO for phone service. The subscriber's internet data flows over this link as well, using a different ATM link and class of service than the phone(s). The rate is controlled by the OLT.
The upstream channel is a 155Mbps ATM data flow. Of course, the phone service has a portion of this bandwidth dedicated to it. The subscriber's upstream internet data flows through this link as well. This link is shared with up to 32 users on a fiber. The OLT is in charge of granting transmit permission to the ONTs. Phone service is guaranteed by ATM classes of service as well as proper network engineering. Internet data is best effort up to the subscriber's transmit speed.
On a cable TV system, the set-top boxes transmit back to the head end using RF energy below the lowest received channel. Since FiOS can't do this (the TV wavelength is one way), the set-top boxes transmit using IP data over the upstream channel. This is one of many reasons why the boxes have to be connected to the data router (note bi-directional data between the router and boxes is carried over coax in the home, but this is different than how cable does it).
You asked about VOD, and yes, it is carried over the downstream ATM wavelength as IP data. I don't know for sure, but I'd be very surprised if it counted against the user's internet downstream data rate. It most certainly is carried using a separate ATM class of service than "regular" data. As such, it would be trivial to separate it for speed management purposes. Further, I doubt Verizon would want VOD users complaining of picture problems when high download traffic is going on at the same time.
FiOS is a BPON. The "P" means passive, which means there is nothing powered between the OLTs and ONTs. When a fiber is split to serve up to 32 users, this is done with unpowered equipment. In areas with FiOS, you will often see big boxes on the telephone poles a few feet off the ground. These are the splitters. For each fiber connected to the OLTs, they can provide up to
This is pretty much what I meant by asking if the phone service was a mini version of the fiber-POTS devices the telcos have in cabinets between a neighborhood and the CO. "ATM" was what I was looking for and wanted to ask about.
I take it that this is part of ATM and is old hat for the likes of Verizon. A form of time multiplex.
It's been a while since I've looked at the details; but yea, that's basically it.
The finer points that I don't recall are the details of "time slots." My vague recollection is they are not at all like time slots on a TDM wire (T1, E1, ...). I seem to remember they are dynamically allocated and there is no concept similar to "frames."
Verizon ONTs have an interesting piece of optical hardware called a triplexer. This device splits the two received wavelengths to the data and video parts of the box while merging the upstream transmit wavelength onto the network fiber. Similar to a diplexer in an RF system, but because it handles three wavelengths it's gets the "tri" moniker.
On the video path, the optical energy is feed to a sensor that converts the light to an electrical current. This is feed to a trans-impedance amplifier. The output of this is a full spectrum RF signal that feeds the coax in the home with broadcast video.