Technical Questions

I'm having the XXL 50Mb service installed in about 10 days time and I have some questions, mostly out of curiosity but also for future reference (ie troubleshooting). I've tried to research a little by myself, but most documents explaining cable broadband are lacking detail and not specific to Virgin Media UK. Any help on the following thoughts/questions is appreciated!

1 - ACKS. The 50Mb service currently provides for up to 1.5Mb upload. If the full 50Mb download speed is being utilised, I'm assuming most (if not all) of the 1.5Mb available upload speed would be required for TCP Acknowledgements. How much (if at all) does the number of ACKS required vary from protocol to protocol? Is there a list or a formula that can be applied? How likely is it that upload speed will be increased to 10Mb? Will this be a free upgrade?

2 - TAS. There appears to be a provision within the DOCSIS specification for a system called TAS (TCP ACK Suppression). As I understand it, this system allows for TCP ACK requirements to be based upon time between ACK transmissions and a scaling ACK requirement based on download speed, instead of just based on the amount of data. The document I read is here: 'DOCSIS per sub throughput optimization' - Have I interpreted it correctly? Do Virgin make use of this feature? If so, how much of an effect does it have on various protocols? Comparison graphs with the feature enabled v disabled or comparing a similar speed download on ADSL vs Cable would be very interesting!

3 - QoS. According to this Wikipedia page, DOCSIS 1.1 and above allows for QoS. How does this work and do Virgin implement it?

4 - Contention. Exactly at what point does contention come into play? I would image at a local level (on the copper cabled parts?) and across Virgin's backbone. At what point is data on the DOCSIS 3.0 50Mb service separate from the other services? And which of these networks is considered the least contended? Is there a way I can look up the local router/server I'm to be connected to and discover its load and general status?

5 - Signal/Noise. How big a role does signal and noise play on the copper cabling? Is it comparable to ADSL and FTTC technologies, where the overall speed of your connection can be affected, or is it more of a digital cliff - it either works at 50Mb or it doesn't work at all/connection flaps up and down. My house has a cable point in every room, could I plug the modem into any of these sockets without concerns of noise or interference? I see people mention attenuators - What do these do?

6 - Bugs. After reading through posts on here and on Virgins community forums there appear to be a number of issues affecting some customers. The most concerning one appears to be a DHCP bug with the VMNG 300 modem, where if the connection drops and resumes, the modem fails to pass along updated information to the router. The router thinks nothing has happened and you remain connected to the router wirelessly but can not access the internet. A modem reboot is required to fix the problem. I read of some users requiring several reboots in one day, and others in 'good conditions' requiring a reboot every other day or so. I remember the early days of ADSL where with a good ISP (Zen in my case), one could maintain a connection for months at a time - Is this sort of reliability now just a dream? About 2 years ago, whilst doing some freelance IT work, I noticed that some clients cable connections would require a modem and router reboot quite often. Is this a common or known issue on Cable in general?

Another question about bugs - What are Virgins tolerances on attainable download speeds? If I'm on the 50Mb product, would it be considered an issue if I were to only obtain 25Mb (Assume a test set up - Ethernet connection to modem, modem plugged into 'master' socket, etc�) on any speed test or test file download from a high bandwidth server? Would this kind of problem warrant an engineer call out or would Virgin dismiss it as contention/normal 'up to' service?

Cheers for reading and for any information you can offer.

Edited by deleted (Wed 24-Feb-10 16:44:49)

In reply to a post by DoktorEddie:

I'm having the XXL 50Mb service installed in about 10 days time and I have some questions, mostly out of curiosity but also for future reference (ie troubleshooting). I've tried to research a little by myself, but most documents explaining cable broadband are lacking detail and not specific to Virgin Media UK. Any help on the following thoughts/questions is appreciated!

1 - ACKS. The 50Mb service currently provides for up to 1.5Mb upload. If the full 50Mb download speed is being utilised, I'm assuming most (if not all) of the 1.5Mb available upload speed would be required for TCP Acknowledgements. How much (if at all) does the number of ACKS required vary from protocol to protocol? Is there a list or a formula that can be applied? How likely is it that upload speed will be increased to 10Mb? Will this be a free upgrade?

2 - TAS. There appears to be a provision within the DOCSIS specification for a system called TAS (TCP ACK Suppression). As I understand it, this system allows for TCP ACK requirements to be based upon time between ACK transmissions and a scaling ACK requirement based on download speed, instead of just based on the amount of data. The document I read is here: 'DOCSIS per sub throughput optimization' - Have I interpreted it correctly? Do Virgin make use of this feature? If so, how much of an effect does it have on various protocols? Comparison graphs with the feature enabled v disabled or comparing a similar speed download on ADSL vs Cable would be very interesting!

3 - QoS. According to this Wikipedia page, DOCSIS 1.1 and above allows for QoS. How does this work and do Virgin implement it?

4 - Contention. Exactly at what point does contention come into play? I would image at a local level (on the copper cabled parts?) and across Virgin's backbone. At what point is data on the DOCSIS 3.0 50Mb service separate from the other services? And which of these networks is considered the least contended? Is there a way I can look up the local router/server I'm to be connected to and discover its load and general status?

5 - Signal/Noise. How big a role does signal and noise play on the copper cabling? Is it comparable to ADSL and FTTC technologies, where the overall speed of your connection can be affected, or is it more of a digital cliff - it either works at 50Mb or it doesn't work at all/connection flaps up and down. My house has a cable point in every room, could I plug the modem into any of these sockets without concerns of noise or interference? I see people mention attenuators - What do these do?

6 - Bugs. After reading through posts on here and on Virgins community forums there appear to be a number of issues affecting some customers. The most concerning one appears to be a DHCP bug with the VMNG 300 modem, where if the connection drops and resumes, the modem fails to pass along updated information to the router. The router thinks nothing has happened and you remain connected to the router wirelessly but can not access the internet. A modem reboot is required to fix the problem. I read of some users requiring several reboots in one day, and others in 'good conditions' requiring a reboot every other day or so. I remember the early days of ADSL where with a good ISP (Zen in my case), one could maintain a connection for months at a time - Is this sort of reliability now just a dream? About 2 years ago, whilst doing some freelance IT work, I noticed that some clients cable connections would require a modem and router reboot quite often. Is this a common or known issue on Cable in general?

Another question about bugs - What are Virgins tolerances on attainable download speeds? If I'm on the 50Mb product, would it be considered an issue if I were to only obtain 25Mb (Assume a test set up - Ethernet connection to modem, modem plugged into 'master' socket, etc�) on any speed test or test file download from a high bandwidth server? Would this kind of problem warrant an engineer call out or would Virgin dismiss it as contention/normal 'up to' service?

Cheers for reading and for any information you can offer.

1 - Not all of it, I munch about 800kbit. It's about both protocol and number of connections that protocol opens but the 1.75Mbit upstream is enough. Upstream will be increased but unsure if it will be to 10Mbps and if a 10Mbps option becomes available unsure if this will be free of charge.

2 - Yes they do. http://focus.ti.com/general/docs/bcg/bcgprodcontent.... The modems VM provide use the TI Puma 5 chipset.

3 - It allows an operator to open up individual service flows on a modem and assign a different level of service to each, giving a service flow priority access to bandwidth or guaranteeing it bandwidth in the case of a VoIP call. This would be similar to an ATM link where one can have multiple PVCs with different levels of service inside a single VP. Virgin do not use this, each modem has a single service flow which is strictly best effort.

4 - The DOCSIS 3 service is separated at the local headend / hubsite physically, before that it sits on different frequencies to the legacy network but shares the same physical plant. You share 3 or 4 downstream channels with a number of other physical areas (nodes) and a single upstream channel with your own node, the other nodes that share downstream with you have their own upstream channel you cannot access. Each downstream channel is at present capable of 50Mbps of usable throughput, each upstream channel approximately 8.8.

5 - Signal:Noise is an aggregate of the conditions from the CMTS card, the line card in the Cable Modem Termination System you connect to, to your modem, including the coaxial and optical runs. There is a digital cliff, a brief period of degraded performance as FEC struggles to keep the BER clean then it falls off the cliff.

On the upstream path your transmissions are combined with the rest of your physical area / node, and any noise and ingress that may be getting onto the network will potentially affect signal quality. Proactive network maintenance to keep it sealed and remove sources of noise has to be done on the return (upstream) path of the network, downstream pretty much looks after itself.

6 - Reliability has always been my bugbear though this was due to local network issues more than software bugs. A few customers have reported disconnects however this has not been replicated by the equipment manufacturers yet and is under investigation. There is a free trial so suck it and see would be my recommendation.

VM's network is a standard HFC construction - http://en.wikipedia.org/wiki/Hybrid_fibre-coaxial

Edited by deleted (Thu 25-Feb-10 11:38:06)

is #2 where they prioritise acks (or just all small packets)?

In reply to a post by Chrysalis:

is #2 where they prioritise acks (or just all small packets)?

The modem acts as a TCP optimiser. No prioritising of traffic the modem actually does TCP proxying using cumulative acknowledgement and selective acknowledgement to reduce ack requirement.

Edited by deleted (Fri 26-Feb-10 21:35:58)

Thanks for the detailed reply Ignitionnet, the wikipedia link is interesting too.

You said that the upload is 1.75Mb not 1.5Mb (which is what Virgin advertise), is this higher figure based on real-world tests? Is there a reason for the difference such as advertising rules/overheads for example?

I have high hopes for a working connection from Virgin, as we have no BT line and no desire to pay the installation for one. There's also 4 people in the house, each with a laptop, 2 games consoles and 3 wifi equipped smart phones - Hopefully the 50Mb connection will cope well.

Thanks again,

In reply to a post by DoktorEddie:

Thanks for the detailed reply Ignitionnet, the wikipedia link is interesting too.

You said that the upload is 1.75Mb not 1.5Mb (which is what Virgin advertise), is this higher figure based on real-world tests? Is there a reason for the difference such as advertising rules/overheads for example?

Hi,

They add a bit of 'fluff' to the connection so that real world speeds appear to be closer to the speed they sell. It's to get the speed test results closer to 50/1.5 to avoid people complaining that their 50M service will only run at a max of 47 or whatever.

From my own 50Mbps modem:

Primary Downstream Service Flow
SFID : 1259
Max Traffic Rate : 53000000 bps

Primary Upstream Service Flow
SFID : 1258
Max Traffic Rate : 1750000 bps


Those are the parameters I'm connected to the network with and that's the standard 50M profile 53 down, 1.75 up.