Comments on: DOCSIS and Cable Modems – How it works :: UCD

By: Charter Cable Channels

Charter Cable Channels — Fri, 04 Nov 2011 05:57:08 +0000

Hello! I want to say that this post is amazing, nice written and come with almost all important infos. I’d like to peer more posts like this .

By: Brady

Brady — Tue, 08 Dec 2009 15:25:30 +0000

Sathish,

The MAC address you identified, 01E02F000001 is called out in Annex A of the DOCSIS specification. This is defined using the Ethernet/ISO8802-3 [ISO8802-3] convention. Specifically, it states:

The following multicast address MUST be used to address the set of all CM MAC sublayers; for example, when transmitting Allocation Map PDUs:
01-E0-2F-00-00-01

So, to answer your question, yes, it will be sent with all UCDs.

Regards,
-Brady

By: Sathish Kumar

Sathish Kumar — Thu, 12 Nov 2009 04:49:13 +0000

Hi Brady,

Yet another informative post. Simply these posts are real stepping stones to understand the base of working of Cable modem.

Enormous amount of info is present in UCD.
This PDU DA 01E02F000001, you mentioned this a Multicast address.

Whether this address is unique for each UCD, that will be sent for CM’s ?

Thanks,
Sathish.

By: Priya

Priya — Wed, 14 Oct 2009 02:23:14 +0000

Hi Brady,

I got really confused at one point. UCD contains the US frequency to be used for transmitting from CM right. But if there are 8 US for 1 DS and 8 UCDs are sent,Then what will each UCD contain and which US frquency will be chosen??

I know This is a silly question but I’m helpless.

Could you please help me??

Thanks,
Priya

By: Carol

Carol — Tue, 03 Mar 2009 20:31:11 +0000

Hmmm, when I cut and paste the preamble into the text editor, I see 80 words before the start of the line ending in “33 F7″, but I’m sure there’s something I’m misunderstanding that makes you come up with 78.

Anyway, thanks for your speedy reply and this great blog!

Oh, and you might want to wait till I note that 160Ksym/sec would be correct and 1.6Msym/sec would be wrong before you recruit me!

By: Brady

Brady — Tue, 03 Mar 2009 20:08:04 +0000

Carol,

I really should recruit you to help me on these blogs – you are reading them very thoroughly! You are correct on the symbol rate, it is 160 ksym/sec, the lowest rate DOCSIS supports. 1.6 Msym/sec is not even a supported rate, so I completely dropped the ball there. Supported DOCSIS rates are 160, 320, 640, 1280, 2560 and 5120 ksym/sec. Yes, I also made a typo – I’ll fix these in a minute and no one will know aside from these comments.

Regarding the preamble offset, I have double checked this and believe that it is correct. 624/8 bits = 78 bytes. Counting in from the beginning of the preamble 78 bytes (two hex characters) leaves you with 14 “F3″ bytes left in the preamble before the first “33″ byte. This matches exactly with the Request preamble shown. Maybe I’m not seeing what you are seeing.

-Brady

By: Carol

Carol — Tue, 03 Mar 2009 18:03:59 +0000

Hi Brady,

I’m wondering if there is a typo or two in this post.

First, I think the symbol rate should be 1600000 symbols/sec, if it is 1.6M as you state in the text.

Second, I think the Preamble Offset for the Request IUC should be 640 instead of 624, or am I misunderstanding how that’s derived? If I did understand it as an index into the Preamble Pattern sent by the CMTS, then I’m assuming that only the the length and offset are really sent by the modem as that would be enough to derive the preamble itself.

Sorry if the second question is very elementary, but I’m brand new to docsis.

Thanks! Carol

PS: I think the second paragraph of the explanation should read “is the PDU DA” instead of “in the PDU DA”.

By: Brady

Brady — Tue, 03 Mar 2009 13:19:01 +0000

Hi Ishwar,
If you are talking about prorogation delay, well an electromagnetic signal propagates through coax cable at about 1.17 nanoseconds per foot. But I suspect that you were referring to a T1 timeout as defined in Annex B of the DOCSIS 2.0 specification. A T1 Timeout occurs when a cable modem fails to receive a UCD after five (5) consecutive UCD broadcast intervals. Since the UCD broadcast interval is two (2) seconds, a T1 timeout should occur at 2 * 5 = 10 seconds. T1 timeouts are actually pretty rare, but if you have them you probably have some pretty substantial downstream impairments or faulty equipment.

My next blog due out this afternoon will talk a lot more about T3 And T4 timeouts, which are much more common.

-Brady

By: Ishwar

Ishwar — Tue, 03 Mar 2009 10:32:06 +0000

for the guys who are new to DOCSIS,
ICU means “Interval Usage Code” the name it self explains all

Thanks Brady for the blog, This is to early to ask but I cant hold myself I have to get the answer for this question
What about the delay involved in the transmition of these msgs like if CMTS sends UCD at time interval t1 the CM will receive this at time interval t2 ?

By: Brady

Brady — Sat, 28 Feb 2009 19:59:58 +0000

Thanks for the feedback Brian, it is much appreciated. -Brady