26-11-2016, 12:00 PM
Virgin Media's broadband is provided in a completely different manner to all other broadband offerings - the question is why?
In these days of increasing FTTH, many would say that VM is inferior because it is only FTTC - which is correct but misleading.
Unlike broadband over telephone wires, a customer at the extreme range of the distribution network gets exactly the same level of service as the customer outside whose house the fibre is terminated. Again, why?
The VM network started of as a number of analogue cable TV distribution systems, long before broadband internet was even thought of. The important thing to remember that even new plant extensions built today follow exactly the same principles and specifications.
When cable TV on a nationwide basis was first introduced, it was a condition of the licence that the performance of the networks must follow a British Standard which specifies end-of-line worst case signal to noise and distortion figures. In other words, quality of service is guaranteed throughout the network. Originally, of course, these networks were analogue, carrying TV signals in 8MHz wide channels. As has happened to broadcast TV, these have been replaced by digital muxes in the same 8MHz wide channels - in fact the payload of a cable mux is higher than its broadcast brother because the guaranteed network quality means that less stringent error correction is required leaving more bits for data!
Broadband internet uses the same 8MHz wide channels - in fact, on a spectrum analyser, you can't tell which channels are carrying internet data and which are carrying TV data. Originally, there was only one mux for broadband for a network segment serving up to 19,200 homes but, as take up - and speeds - increased, those blocks were gradually segmented into smaller groups, typically as small as 2,400 homes - although I've worked on networks that could, if required, easily be segmented into 600 home blocks. In addition, the number of channels carrying internet data has increased enormously from the original one channel.
Increasing development means that these 8MHz channels can be bonded together to provide much higher data speeds than can be carried in a single channel.
So, the main difference is that wherever on the VM network you are connected, you will get the advertised speed whereas on services provided over BT lines, an advertised 'up to' many Mbs service may only crawl out of the end of the line at 1Mbs - if you are lucky!
One area where a phone line service can score - if you are lucky enough to get the high advertised speeds - is the speed at which data can be uploaded, which could be a critical factor for some people.
The cable network covers 5 - 750MHz. Some can work up to 865MHz but, for the sake of uniformity, this extra bandwidth is not, to my knowledge, used.
This band is split into two sections - downstream and upstream. In the early days, the upstream channels only had to carry pay per view requests and very little else and was usually restricted to 30MHz with the downstream starting at 50MHz. The traffic requirement has changed out of all recognition over the years and increased to levels that nobody in the early days would have thought possible - no different to telephone circuits, if you think about it!
The majority of the traffic is, of course, downstream TV and internet traffic and this has to take the lion's share of the bandwidth but the split has been revised so that the upstream path now extends to 65MHz with the downstream starting at 85MHz (the 20MHz gap is for the sharp cut off diplex filters that keep the two paths separate at the input and output of each amplifier.) Much more efficient transmission methods are now employed to get optimum data capacity on the upstream path but, even if the crossover point was to be nudged up even further, there is no way that upload speeds can even remotely approach those of the downstream bandwidth.
In these days of increasing FTTH, many would say that VM is inferior because it is only FTTC - which is correct but misleading.
Unlike broadband over telephone wires, a customer at the extreme range of the distribution network gets exactly the same level of service as the customer outside whose house the fibre is terminated. Again, why?
The VM network started of as a number of analogue cable TV distribution systems, long before broadband internet was even thought of. The important thing to remember that even new plant extensions built today follow exactly the same principles and specifications.
When cable TV on a nationwide basis was first introduced, it was a condition of the licence that the performance of the networks must follow a British Standard which specifies end-of-line worst case signal to noise and distortion figures. In other words, quality of service is guaranteed throughout the network. Originally, of course, these networks were analogue, carrying TV signals in 8MHz wide channels. As has happened to broadcast TV, these have been replaced by digital muxes in the same 8MHz wide channels - in fact the payload of a cable mux is higher than its broadcast brother because the guaranteed network quality means that less stringent error correction is required leaving more bits for data!
Broadband internet uses the same 8MHz wide channels - in fact, on a spectrum analyser, you can't tell which channels are carrying internet data and which are carrying TV data. Originally, there was only one mux for broadband for a network segment serving up to 19,200 homes but, as take up - and speeds - increased, those blocks were gradually segmented into smaller groups, typically as small as 2,400 homes - although I've worked on networks that could, if required, easily be segmented into 600 home blocks. In addition, the number of channels carrying internet data has increased enormously from the original one channel.
Increasing development means that these 8MHz channels can be bonded together to provide much higher data speeds than can be carried in a single channel.
So, the main difference is that wherever on the VM network you are connected, you will get the advertised speed whereas on services provided over BT lines, an advertised 'up to' many Mbs service may only crawl out of the end of the line at 1Mbs - if you are lucky!
One area where a phone line service can score - if you are lucky enough to get the high advertised speeds - is the speed at which data can be uploaded, which could be a critical factor for some people.
The cable network covers 5 - 750MHz. Some can work up to 865MHz but, for the sake of uniformity, this extra bandwidth is not, to my knowledge, used.
This band is split into two sections - downstream and upstream. In the early days, the upstream channels only had to carry pay per view requests and very little else and was usually restricted to 30MHz with the downstream starting at 50MHz. The traffic requirement has changed out of all recognition over the years and increased to levels that nobody in the early days would have thought possible - no different to telephone circuits, if you think about it!
The majority of the traffic is, of course, downstream TV and internet traffic and this has to take the lion's share of the bandwidth but the split has been revised so that the upstream path now extends to 65MHz with the downstream starting at 85MHz (the 20MHz gap is for the sharp cut off diplex filters that keep the two paths separate at the input and output of each amplifier.) Much more efficient transmission methods are now employed to get optimum data capacity on the upstream path but, even if the crossover point was to be nudged up even further, there is no way that upload speeds can even remotely approach those of the downstream bandwidth.
