This is a very good question! Symbol rate and bandwidth are directly related to one another. Also, the higher the symbol rate the higher the data rate (think faster data speeds), so we ideally we want big symbol rates, but that means big bandwidths. There are a couple of problems with big bandwidths. The upstream has a limited usable bandwidth and the larger the channel bandwidth the greater the chance it will be impacted by noise or other upstream impairments such as group delay.

So what is the relationship between symbol rate and bandwidth? As the symbols are passed through the root-raised cosine filter in the cable modem to filter away high frequency components, it is up-sampled with a resulting factor of 1.25 (rrc shaping alpha = 0.25). So the bandwidth is 1.25x the symbol rate. A symbol rate of 2560 ksym/sec is then 2560 ksym/sec * 1.25 = 3.2 MHz.

A number of studies have been down which show that two 3.2 MHz channels running at 64 QAM are much more resilient to RF impairments than one 6.4 MHz (5120 ksym/sec) channel at 64 QAM. Why? The biggest impairment is group delay which has a substantial impact over a large bandwidth channel like 6.4 MHz.

So to answer your question succinctly, the most important thing which will affect the symbol rate on HFC is RF impairments as the symbol rate increases. The smaller the symbol rate, the less impact by RF impairments; similarly the larger the symbol the more impact by RF impairments.

-Brady

I’m a new guy for DOCSIS, and I have a question about the symbol rate. The symbol rate is increased a lot from DOCSIS1.x to DOCSIS2.0, and I want to know what is the most important thing which will affect the symbol rate on HFC? Both for downstream and upstream.

Thanks,
Daswang.

Thank you! I plan to keep reading.

If your modem is running 64-QAM in the upstream, then your operator supports at least DOCSIS 2.0 (maybe even DOCSIS 3.0). It is not uncommon for an operator to run a 64-QAM modulation with a low symbol rate. Why? Because using the higher modulation with a lower symbol rate gives higher data rates in the same bandwidth with reasonably good impairment immunity when using the DOCSIS 2.0 pre-equalization and adaptive signal cancellation available in the DOCSIS 2.0 and higher chip sets.

So your your cable operator is doing the right thing, making the most out of their upstream. I would imagine that they likely have some upstream issue to still resolve before they can go full 64-QAM, 6.4 MHz, but until then they are using a smaller bandwidth (3.2 MHz). It is also likely, as you suggest that they are using A-TDMA where the legacy modem transmit data in 16-QAM on the same channel, which is perfectly normal. The DOCSIS 1.1 modem just uses a different burst descriptor (IUC) in order to do this.

-Brady

There were actual pre-DOCSIS deployments in the mid-90′s, but these were done without using an inter-operable specification. The first DOCSIS 1.0 spec was developed in the late 90′s (released in 1999 I believe), but actual deployments to subscribers did not occur until about 1990. It took awhile before the last non-DOCSIS LanCity modems where taken out of service. So the cable upstream was used for data since in a non-telephony method since at least 1994, but I bet there is a geek out there who can date it back even before then.

-Brady

Thanks for the all the great HFC information on your sight. I mainly work on metro ethernet & SONET but still get allot of CMTS work at my job and this site has really helped.

My question:

When did cable providers first get the chance to offer cable modem returns on the cable network without relying on the telco return architecture? Based off my research it appears this was achieved with the launch of 1.1.

Thanks

I’m trying to figure out what docsis version my provider supports.
Weird thing is, the symbol rate as I can see at my modem diag page, is 2.560Msym. So that should team up with 16QAM for docsis 1.1.
But my upstream modulation is 64QAM.
So maybe I have docsis 2.0?
The only reason my provider is not doing 5.12 Msym could be that they have still a lot of docsis 1.1 modems in the field.
Is this technically possible? I mean, combine 64QAM with 2.560Msym?

Cheers,
Arthur.

Thanks

For the upstream, the DOCSIS 3.0 RFI specifies the upstream signal accuracy as being within 50 ppm (parts per million) over an operating temperature range of 0C to 40C for 5 years from the day the cable modem was manufactured. So how do we translate 50 ppm to frequency? Simple, just use this equation:

delta f = (center freq * x ppm) / 1e6

So if your cable modem is transmitting at a center frequency of 35 MHz
Your x ppm = 50

Then your allowed cable modem error (or delta f) is:

delta f = (35 MHz * 50 ppm) / 1e6 = 1,750 Hz = 1.75 KHz (error)

Then your minimum frequency could be 34.998250 MHz and your maximum frequency could be 35.001750 MHz and you would still be within the DOCSIS specification.

For the downstream accuracy, the DOCSIS specification just defines this with an straight forward spec. of +/- 30 kHz. So no math involved.

Hope that helps.

-Brady

If the isn’t anything in Docsis about it, is there any controlling document that specifies what the frequency resolotion can be?

Same question applies for the downstream as well.