US Cable Support > Comcast HSI > 4 Upstream Channels

4 Upstream Channels

5-42mhz is the sub band......take into account diplex filter rolloff in amps, and your at 5-37ish.

Then subtract the 5-18 garbage you cannot avoid, so your left with 18-37mhz. 19 usable mhz worth of space.

6.4mhz per 64qam, 3.2 for a 16qam, and dont forget the box driver!!!

so approximately 16mhz is used for 3 channels, 2 64s and a 16....and a couple db for the qpsk box driver and youre at 18mhz.

can you throw another 16qam down in the garbage of 5-18? sure, but no modem will use it...
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I'm better than you!

reply to JeepMatt
As gar187er said, fitting 4 upstream HSI QAM carriers in that 18 to 37 MHz usable spectrum area is a challenge.

However, in some field trials back in 2010, Comcast had managed to obtain an upstream data rate of 75 Mbps by bonding two 6.4MHz-wide 64QAM channels with two 3.2MHz-wide 16QAM ones in that space (»www.lightreading.com/document.as···lr_cable). I suppose this is how they figure they will be able offer the 65 Mbps upload speed for the new 305/65 "Platinum" HSI tier.

Looking through all the modem signal screen captures posted in Johkal's »Evidence of Upstream Channel Bonding - Post Here stickied thread, the majority of the systems do just show having the usual two 64QAM channels along with one (legacy DOCSIS 1.X) 16QAM one, at the time they were captured.

However, there are actually a few systems that already show 4 upstream channels, similar to Comcast's field trials. In those few cases, a second 16QAM channel had been placed just inside in the sub-20MHz area, where the greater noise level tolerance of a 16QAM carrier was utilized.

Two notable exceptions to that 4-channel scheme can be seen, however, where in the first (Johkal's) post, all 4 upstream carriers were placed between 22 and 37 MHz, and in another case from Hillsborough/San Mateo, CA (»Hillsborough/San Mateo, CA), where they managed to fit three 64QAM channels (at 23.7, 30.6 and 37 MHz) and one 16QAM one (at 18.9 MHz). That arrangement should definitely support that new 65 Mbps upload speed.

FWIW, might it also be that the local engineers decide how many return QAM's to implement to satisfy the current upload capacity needs of any given local system, so it may be deliberate decision ?

reply to telcodad

Re: 4 Upstream Channels

reply to EG

That's sad. Hope that the headend techs are well trained and qualified.

HE Techs cant make calls like adding channels, and what not....all of those decisions are done at region/division....
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I'm better than you!

Figured that, thanks gar187er !

reply to gar187er
In Motorola systems, box return is usually around 8 - 16 Mhz. Though in SA/Cisco plants, I've never seen a DHCT return lower than 18 MHz. Weird. Wonder if the Moto aloha protocol is more robust with noise than SA's DAVIC? Eh, hopefully one day they will all just run on DOCSIS and that space can be free'd up.

reply to JeepMatt
An article on the CED Magazine site today about some of the ways cable operators can increase upstream data speeds:

Options abound for boosting cable upstream speeds
The return path lags behind downstream speed, but cable is working on its game plan.
By Mike Robuck, CED Magazine - August 20, 2012
»www.cedmagazine.com/articles/201···m-speeds

reply to JeepMatt

reply to JeepMatt

reply to JeepMatt
But can you get more than 27.5 dB upstream SNR on Older 750 MHz Antec nodes?

reply to JeepMatt
Same here, 8 DS chans, 2 US chans.

reply to telcodad

Theoretically, it shouldn't matter which direction, but in the real world, I suppose there could be certain types of signal distortions that may be unique to the upstream path that may cause a difference (and require a slightly better SNR to achieve the same error rate performance).

As gar187er said in that same thread:

Isn't the lower frequency band of the return path subject more to noise than the higher forward band ?