NTL - how far from fibre?

[ian@huth]

NTL cable provides both TV and broadband. This is carried over fibre and coax. Where does the changeover from fibre to caox take place and what hardware is used to make this transition? What is the maximum length of coax that can be used and is this maximum the same for TV and broadband?

[Graham F]

AKAIK its fibre up until the street cab, this is where the change over happens. Not sure abt the rest, soz!

[ian@huth]

Originally Posted by Graham F

AKAIK its fibre up until the street cab, this is where the change over happens. Not sure abt the rest, soz!

There are different types of street cabs though, I doubt if they all have fibre to them.

[Graham F]

i was under the impression and i could be wrong its only the last little bit, ie from cab to house that isn't fibre could be wrong though

[SMHarman]

I think it is fiber to the big three door cabs, from then out it is coax. I didn't see any fiber in the two door one at the end of my drive and the smaller flip top ones don't have any in them.

[Escapee]

Originally Posted by ianathuth

NTL cable provides both TV and broadband. This is carried over fibre and coax. Where does the changeover from fibre to caox take place and what hardware is used to make this transition? What is the maximum length of coax that can be used and is this maximum the same for TV and broadband?

A simple answer to your question is the fibre ends at the Nodal cabinet, the nodal cabinet usually feeds an area passing an average of 500 Homes. The Nodal cabinet then uses coax in both direction (forward and return path) to feed Distribution Amplifier cabinets and Distribution points.

The nodal amplifier contains an optical receiver that converts the optical signal back to an RF electrical signal that is the same as the one leaving the headend, it also contains an optical transmitter to take the return path RF from your set-top or cable modem and convert it into an optical signal to send back to the ubr.

One area thats probably not realised by many is between the headend and the node, is a hubsite. This is a building containing racks of equipment, and the optical forward signal in the case of TV is converted from optical to RF, and then back to optical to feed the nodes. The return path signals are also converted back to RF at the hubsite and split to various equipment for processing. (ubr, intersect etc) some frequencies/services are fed to another optical transmitter to be yet again converted and sent back to the headend.

So to put it into context the RF Television signals, both analogue and digital leaving the headend modulators undergo the following before they reach to customer.

RF fed to headend optical transmitter, down a bibre to the hubsite. Fed to an optical receiver for conversion back to RF, this signal is then fed to an optical transmitter and fed via splitters to individual nodes. The node then converts this optical signal back to RF to be distributed via DA/DP cabinets.

The return path/US is just the reverse process..................

[ian@huth]

Thanks for that Escapee. How long can the coax be after it leaves the nodal cabs?

[SMHarman]

Interesting to see that to get Fibre to the Home the additional runs needed are not long (by comparison to say BT).

[andrew_wallasey]

A nodal cab is those big ones you see about 5 foot tall round and about with the doors on the front and side.

[andrew_wallasey]

Originally Posted by ianathuth

Thanks for that Escapee. How long can the coax be after it leaves the nodal cabs?

Depends on what size of coax is used.

[altis]

There's a description with pictures here:
http://homepage.ntlworld.com/draig.goch/

Note: this site is Firefox unfriendly!

[ian@huth]

Originally Posted by altis

There's a description with pictures here:
http://homepage.ntlworld.com/draig.goch/

Note: this site is Firefox unfriendly!

Forgot about that site. Thanks for the Firefox warning, good job there is an option to open in IE in the right click menu.

Like that bit in the "connection from the home to the headend" page which talks about allowing future fibre to the home upgradability. Will it ever?

Could extra fibre be run from the nodal cabinet back to the hub and headend to allow VSDL over the NTL network?

[Escapee]

Originally Posted by andrew_wallasey

Depends on what size of coax is used.

Spot on, also there are 2 seperate issues here. The coax between node and DA/DP and the coax from DA/DP to the customer.

Most of us are now aware that distance from the home to the cabinet and level available at the tap port affect what coax is used for the drop cable to the customer. Also remember that attenuation at higher frequencies will also affect the choice of cable, to put it into simple terms a customer with x amount of level with a cabinet outside his front door compared to a customer with x amount of level and a far away cabinet would need different cable. The customer with the cabinet directly outside would not be affected as much with slope attenuation as frequency increases.
(The type of cable used has very little affect on attenuation at the return path/US frequencies because the frequencies are low)

The choice of cable between cabinets is a similar situation, the original ntl areas can generally use around 2 or 3 different sizes, and the choice is more to do with distance between cabinets than levels. The Distribution amplifier not only has to amplify the signal but also has to compensate for the slope by amplifying more at higher frequencies. The amplifiers have a device known as an "cable equaliser" plugged in to compensate for the cable loss. The frequency response of the equaliser would be exactly the opposite of the cable it is compensating for. Another point to consider is how many times they are splitting the signal from the Node/DA cabinet to feed other cabinets, every time you split in the real world you will lose aalmost 4dB. Sometimes the split is not equal as one cabinet may be much closer than the other to be fed, in this case they could use a directional coupler. If it was say an 8dB directional coupler there would be about 1.5dB loss in the through leg and 8dB on the tap leg. All of this would depend on the planning and topology of the area, but generally cabinets further away would have larger hardline coax (750 or 860 series) to keep the slope attenuation within a manageable figure, and cabinets closer would use smaller hardline (500 series) for cost efficiency.

The HFC network is far more complex to understand than first appears, there are lots of other consideratiuons for derating amplifiers in cascade, cascade noise figures, and thats without even thinking about all the return path headaches.

[andrew_wallasey]

Originally Posted by Escapee

<Snip>

The 750/860 are absolute *******s to pull in arghh!

[andrew_wallasey]

I wonder if anyone has got hardline as their drop cable because they are so far from the cab.