On 10/26/2012 4:12 AM, Jean-Francois Mezei wrote:
*** Wavelengths (ok, this is a newbie question :-)
It was my understanding that lasers were on/off devices which transmitted bits very fast. But I suspect I am very wrong on this because this doesnt seem to support RFoG deployment by cable, nor the Verizon FIOS which allocates some spectrum to TV and some to data.
Anyone have a good pointer on how the lasers in an FTTH/GPON function ? Are they able to produce a wide range of "colours" and modulate signals ? Or are they really a bitstream with a single narrow colour and it is the hardware at both ends which converts between multi frequency broadcast signals and light pulses ? Ok, the first thing to understand is that there are a lot of FTTH technologies and they really work very differently from each other. I'll describe three of the more common ones, but these are far from the only FTTH technologies.
RFoG (RF over Glass) is simply taking the DOCSIS HFC network to the extreme where each home has its own fiber node in the nid. In this case there are generally only two wavelengths active on the glass, one for the upstream side and one for the down stream. Inside of those wavelengths things are further sub-divided into 6 Mhz (8 for EuroDOCSIS) wide channels on the downstream and 6.4 Mhz on the upstream. (3.2 if you're running modems older than DOCSIS 2.0). You can run RFoG as an overlay on an existing GPON installation and in that case the DOCSIS signalling is encapsulated inside of the PON layer 1 and layer 2 transmissions, but in my experience this is relatively uncommon. http://en.wikipedia.org/wiki/RFoG GPON (Gigabit Passive Optical Networking) is a technology that allows for multiple wavelengths to be sent down a strand (16/32/64) which is then split by a passive prism (no power or brains) to serve the households attached to that splitter. This is a pure data delivery mechanism so in many cases TV line ups are done as IP streams, however you can work with specific vendors to have the TV channels delivered separately in a normal MPEG2/4 stream that "normal" digital set top boxes can handle once its been converted to RF in the home. (This is the approach that Verizon took). Its also common to keep some amount of copper in the loop and instead of running fiber all the way to the home its run to a cabinet in a neighborhood and from there VDSL2 (with vectoring if possible) is the local distribution loop. This gets rid of the need for batteries in customer's homes and doesn't create a big drop in speed. This can only be done within about 4000 feet of the cabinet. http://en.wikipedia.org/wiki/Gpon Active Ethernet is yet another way of delivering services over a fiber plant and it requires the least amount explanation since it really works very similar to how ethernet works in normal enterprise networks and its a baseband technology. Having said that it can (and often is) combined with either course or dense WDM (Wave Division Multiplexing) to help keep the fiber count down and that does use multiple wavelengths. http://en.wikipedia.org/wiki/CWDM
*** ONT Selection
Are there standards similar to DOCSIS where in theory, any ONT *should* work on any GPON system ? (with carriers only qualifying a limited number of models on their system). Or does the selection of the OLT and line cards determine a narrow set of ONTs that are compatible ?
Or does it get down to each carrier getting custom made ONTs for the system they are deploying ?
In general you find out from your OLT vendor what ONTs they work with other than their own. If you're in an open network scenario you'll have to get this from the infrastructure owner.
Do most ONTs on the market generally allow end user access to some of the config/status data to help in debugging problems ? or are they generally inaccessible by users ?
They seldom if ever allow end user access. You should find out exactly what technology is being used, since some have decent stat reporting (DPoE) via SNMP while others use TR-069.
In the case of Bell Canada, I have obtained the following 2 images:
http://www.vaxination.ca/temp/ont1.jpg outside equipment http://www.vaxination.ca/temp/ont2.jpg indoors equipment
The ONT appears to be the alcatel-lucent equipment indoors. Would this mean that the outdoor equipment really acts only as a demarc which would be just a passive optical connector to join the patch cord to indoors CPE with the optical cable coming from the telephone pole ?
You'll have to get the details on this from the infrastructure owner, there are tons of options for operators to choose from on indoor versus outdoor using the same technology.
From what I have seen, Verizon FIOS uses an "all-in-1" ONT that is outdoors and provides ethernet, POTS phone and coax for TV services. So each carrier appears to have different approaches. (and Verizon also has spectrum dedicated to TV, spectrum to data and some to voice, which looks like a hybrid RFoG system).
*** IPTV and packet priority/QoS/rate-limit
In the case of Bell Canada, they have an IPTV service and they use twin PPPoE sessions on twin VLANs. One is the data connection to/from the BRAS, and the other is the IPTV stream from the Mediaroom data centre. (in the second picture, this would be handled by the black equipment on the left which would be the router that does the PPPoE sessions and outputs ethernet data and MoCA)
In their VDSL2 environment, the DSLAM enforces QoS to give IPTV packets priority, reducing data througput to ensure it fits within the limited copper speeds.
In an FTTH system, how would this be accomplished ? Would QoS and rate limiting be done by the ONT (since it is the new choke point where subscription speed is imposed), or would the OLT rate limit each end user's total bandwidth and prioritise IPTV packets to ensure it all fits whn it gets to the ONT's rate-limit ?
Based on what I heard, those on Bell's FTTH have their IPTV streams fit within their subscription speed, so if they only subscribe to 16mbps service, and use 7mbps for IPTV, they only have 9mbps left for data.
It _CAN_ be done by the ONT/OLT together, but that's a configuration/deployment choice.
*** POTS Service
From a POTS service point of view, is it easy to specify that customer X's SIP traffic goes to server A, while customer Y's traffic goes to server B ? (aka: allow CLECs to provide VoIP service to end users)
Does the ONT get the SIP server destination when it is provisioned or does it talk to the OLT and it is the OLT which routes voice traffic to the SIP server assigned to that customer ?
Again, this one just depends on how the underlying carrier built it. Having said that if its not prioritized SIP traffic (unless the operator prioritizes all RTS data) it will simply be IP traffic that will go to where ever the SIP endpoint (phone, ATA, etc) is programmed to send it. If that route crosses the Internet WAN connection it can have QoS issues.
**** Installation costs
If a carrier has been installing FTTH systems for about 2 years now, would its installation costs have become fairly stable by now, or would they still be going down as the carrier optimises its processes and has more trained crews ? How long does it generally take before they have stable installation costs ?
It goes down some past the first two years but generally the biggest drop is during the first 12-16 months and those are process and crew experience related while the costs that drop after that tend to be driven by CPE costs dropping.
From the information I have gathered, Bell uses Corning Flexnap on the telephone poles to connect lines to individual homes. Those are custom made at the factory for each street block (based on distances where each line to home will be). Is this fairly common method ? Or is it considered a more expensive method used by carriers with fewer trained employees ?
If a carrier is using Flexnap to save on cable termination/connection manhours, would they also order pre-cut fibre cable runs between the flexnap on the telephone pole and each home ? Or would those be cut and terminated on the field ?
And out of curiosity, in a Flexnap setup, what happens if a couple of fibre strands between two poles are damaged by a squirrel ? Does this mean that they have to manually splice the lines from the affected homes into spare strands on the cable ?
I don't know enough about Flexnap to comment.
I appreciate any guidance/information you may provide. The more information I can provide to the regulator, the more informed a debate there can be and thus a better regulatory decision.
Jean-François Mezei Vaxination Informatique Montréal, Canada
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