CAS - MF Interview question

jrambar

Hi All,

I went through an intensive interview this past Thursday on SIP and PBX to IAD integration. One of the questions that I failed on was:

You are trying to interface a Cisco 2821 with NM-HDV2 to a PBX via T1. There is a red alarm on both ends. You test both ends with a loopback and it comes up fine. The cable tested fine and has been verified to be a T1 cross cable. Changed out the cable, same thing. Put on a monitoring tool and nothing tells what the issue is. From a troubleshooting standpoint, what are things that should be checked considering all that has been done above?

Anyone have any ideas?

I answered all questions correctly but this one. He said he was pleased with my answers and gave me a homework assignment to work on:

- Research CAS - MF and how it applies to a customer PBX
- Research interdigit timers in CAS MF

He will give me a week to research this.

I have always dealt with CCS not much of CAS so this is why this is my weak point. I know MF stands for Multi Frequency, can someone give me a straight forward definition on how DTMF differs from MF?

dtlokee

Sounds like they recently had a problem with this and thought it would be a good interview question

I am not sure a problem with DTMF/MF and inter-digit delay would yeild an alarm condition but would cause calls to fail. DTMF and MF use different frequencies to represent values (siginaling)

http://cisco.com/univercd/cc/td/doc/product/tel_pswt/vco_prod/std_prg/stpra.htm

jrambar

I'm sure the interview question is not related to the "homework" he gave me I know what DTMF is but not sure what MF is or how it sounds.

Back to the interview question, I know it's vague, but do you have a list of what could cause an alarm to come up considering all that has been done per the description? In the event he asks me again I want to be prepared I know doing a debug would help me find the answer, but what would cause a red alarm on both ends?

mikej412

T1 Alarm Troubleshooting

jrambar

Thanks Mike! That helped alot!

Last question - What would be the difference between MF and DTMF? I did multiple searches and cannot get a straight forward answer or definition??

dtlokee

Aas far as I know MF is a really outdated method of signaling on trunks only and was replaced by DTMF for trunk signaling and for the end stations to dial (vs the old pulse methods). I haven't run into it but I guess if you had a really old PBX you would need to set the signaling method on the voice port to MF instead of DTMF (default). Still I will say this won't cause an alarm on the interface, only line cding and framing would cause the alarm he described.

I am not sure but I think MF had something to do with SS5 but that is just purely a random thought I think I read once and could be completely wrong, but it may give you something more to research (or possibly a complete waste of time!)

mikej412

dtlokee wrote:

MF had something to do with SS5

Right, the old stuff. Now we have DTMF and SS7.

I think MF sends 10 digits and 5 auxiliary signals and was used for trunk connections in the old days -- then DTMF took over the phone system, all the way out to the key pads on touchtone phones.

So I'm thinking the interdigit thing may be related to MF being "fixed length" -- and this is where I'm crawling out on a limb. Now a days you can keep punching digits until you match a dial plan, so the interdigit timeout default may be too long when using DTMF and an "unlimited length dial plan" and people may give up after waiting 10 seconds and think nothing is happening. I'd guess MF sends the first 10 digits and doesn't care about the rest (which might get sent down the line to the far end device). So with MF, it may also wait for 10 digits before sending... ??

As far as CAS MF -- DTMF would be the default, so if you want to use MF you'd use the tone command to set it.

Hope this helps..... you might have to post your homework report here when you're done.

Oh, and if the interview didn't scare you from wanting the job -- Good Luck!

Signaling System No. 7 (SS7/C7): Protocol, Architecture, and Services may have some useful information, since it starts off with the history of signaling. It is available of Safari (and book or PDF from CiscoPress.com ).

jrambar

Thanks guys! You all led me in the right direction of what I need to know ( I didn't know how to word it or where to begin looking ). Now I find an abundance of information:

http://en.wikipedia.org/wiki/Signaling_System_No._5

http://en.wikipedia.org/wiki/Multi-frequency

http://en.wikipedia.org/wiki/Two-out-of-five_code

I guess these are the answers to my first question. Interdigit timers are next, all I know so far is an interdigit timer resets itself after each number is pressed. I think Mike is correct that it may have something to do with it's fixed 10 digit length.

mikej412

Something else to pay attention to while researching..... I remember that MF uses pairs for signaling -- so I'm trying to remember if that's the beep-beep, beep-bop, bop-beep, bebop-shabop, beep-bebop, etc.... pairs. It may send the 2 distinct tones for each piece of information.

DTMF uses a combination of low and high frequencies -- simultaneously. The spacing of frequencies is such that the when you combine the high and low frequencies, it will never match any of the individual high/low frequencies.

I'm thinking if the timing intervals were the same, MF could take twice as long as DTMF to send the same information......

But the more I think about, I'm not so sure that any of this stuff in the CCVP got down to the detail of your interview homework.

jrambar

No wonder he gave me a week to research, it's hard to find anything on MF anymore. I got a wave file of DTMF (standard punching of numbers, etc) and I THINK i heard MF before, I'm not sure, it would be nice to hear it to fill in the grey area's.
I guess what they are trying to start up is integrating CAS or CCS to existing customer PBX's (old or new). I assume there are some customers who have the old legacy PBX's out there and they are trying to bring some VOIP solution to it. Wish me luck! I need this job!

jrambar

Well what do you know, I found some. Can someone verify if these are actually MF tones? They do not sound much different.

http://freesound.iua.upf.edu/tagsViewSingle.php?id=8814