Re: Clear channel question? -- A much more confusing explanation.....

[Peace, Jeremiah J (Jeremiah)]

Conceptually ESF or SF looks like this:

The ESF or SF "Frame" is 193 bits, with (8bits*24channels)192 bits of DS0
channels
and 1 frame bit.
       [ 1bit frame(for esf/sf)][8bit DS0-1][8bit DS0-2].....[8bit DS0-24]

Keep in mind that a DS1/T1 is synchronies, and as such there is no
start/stop bits.
So, the clock uses the Framing bit and a specific repeating pattern 
(001011 in the case of ESF) to keep it's clock in sync, and to make sure
it is looking at the correct 193bit frame.  It turns out that there are so
many 
bits available, ((193bit frame)/(T1 rate of 1.544Mbps) = 8Kbps Frame bits)),
that you
can provide much more than just clock sync.  So, AT&T updated supper frame, 
to ESF(extended super frame) to include a CRC check, Error count, And
maintenance 
channel all in this 8kbps frame/sync channel.  

Hope this helps.

Oh, one last thing. If you purchase a Clear channel/common channel/ or
Robbed bit
DS1/T1 from your provider you will still need choose framing and Line
coding.

The choices for framing are: D4(or SF), & ESF. The best choice being ESF.
now line coding choices are AMI/ZCS or B8ZS (chose B8ZS for ISDN-PRI or
Data!)

These have to do with how the Physical T1 places bits on the wire. AMI
stands
for Alternate Mark Inversion with Zero Code Substitution, and B8ZS stands
for
Bypolar 8bits with Zero Substitution.  Basically, most digital signals are
DCvlts to
begin with, DC is extremely susceptible to noise. So what happens is you
must 
alternate the way you express a Bit onto the wire. So that the first "1" bit
is +15vlts(for
instance) with the second "1"bit is -15vlts, and every "0"bit is 0vlts
relative to carrier.

Now, in a synchronies environment two many 0's in a row, will cause the
clock to "Slip",
and so something must be done. AMI/ZCS, just places a 1 in there somewhere.
This 
could cause CRC  failures in TCP/IP as data is "changed" in-transit.  B8ZS
on the 
other hand changes the way that the Bits are expressed, replacing 8 0's in a
row with
a +15 1bit, -15 1bit, a 0vlt 0bit, -15 1bit, and +15 1bit. Causing a
"Bipolar error" that
the end point recognizes and re-places with 8 0's. There is of-course more
to it than
that but that is the general concept.

Hope this helps.

--Jeremiah Peace
--Systems Engineer
--Lucent Technologies
--A+, MCSE,CCNA,LCTE VoIP,Definity G3.

At 08:16 AM 7/26/00, Kent wrote:
>Priscilla,
>
>
>As you mentioned that the robbed bits in the super
>frames are only for voice signals, does this mean, if
>I am only transfering data, I should tell me carrier
>not use the robbed bits?

Yes.

>Still a bit confused about
>the clear channels, does this mean, there is not bits
>robbed in this channel?

Yes

>are they still using ESF?

Sort of. I think technically the term "ESF" defines more than just the 
robbed bits, but in real-world practice ESF tends to refer to a channel 
where bits are being robbed. You'd have to read the small print in the 
documentation of the device you're configuring and/or talk to your carrier 
to be sure about how the term is used.

>if
>it is esf, by default every 6, 12, 18 and 24 bit are
>robbed, if these bit are part of 64k and they are
>robbed, how can we still have 64k?


You don't get 64 Kbps if you're configured for robbed-bit signalling.

If you're only sending data, and no voice, tell your carrier not to rob 
bits. Ask for a "clear channel" or "transparent channel." In this case, 
each DS0 is a full 64 Kbps. The least significant bit of each DS0 in frames 
6, 12, 18, and 24 is no longer robbed.

You will still use the framing bit that is specified by either the ESF or 
SF format, however. But this framing bit is the 193rd bit of each frame and 
is not part of any of the DS0s, so there's no robbing going on.

ESF says to look at the 193rd bit in frames 4, 8, 12, 16 and 20 and 24 for 
the framing pattern of 001011. This same bit is also used for diagnostics 
and a block check field. The diagnostic function looks at the 193rd bit in 
every other frame, and the block check field looks at the 193rd bit in the 
2nd, 6th, 10th, 14th, 18th, and 22nd fields.

It's all very ugly and hard to explain without a white board and some 
actual product or carrier service documentation to look at. ;-)

Priscilla

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