Hey Ole,
It really is 8000 samples. The number 8000 comes from Nyquist's
theorem which in a nutshell states that in order to properly represent
an analog signal in digital form we must sample twice the frequency
range. Since an analog signal is represented from 0-4Khz we must sample
an analog signal (2*4000) times a second in order to transport it with
minimal loss. Thus we get 8000 samples a second. Each sample is encoded
with 8 bits to represent an analog signal at anyone time, thus we get:
8 bits/samples * 8000 samples/sec = 64,000 bits/sec (look familiar?)
Now, if we look at a single frame of a channelized T1 we can see that
we have twenty-four 8-bit timeslots plus a single bit for framing or:
Data Channels (23B+1D)
24 channels * 8 bits/sample * 8000 samples/second = 1,536,000bps
Framing Bit
1 bit/sample * 8000 samples/second = 8000 bps
So, we can see that for data we have 1,536,000 bps and 8,000 bps for framing,
which equals 1,544,000 bps. This is why people think of a T1 as 1.544Mb
but it's true throughput is only 1.536Mb.
So, what is the framing bit used for? Framing allows the CSU/DSU to find
the timeslots in the channelized T1. Remember, that 8000 T1 frames are
flying out of the circuit every second, and the CSU/DSU must know how
to find the T1 frames in this circuit (don't ask me why, but I can't stop
thinking of Lucille Ball in the bon/bon episode :-)
In order to maintain sync, the CSU/DSU must grab every 193rd bit off of
the line, string them together and see if it forms the "pattern" it is looking
for. The type of framing used describes the pattern to look for.
When using Superframing twelve of these bits are strung together and must
form the following pattern of "100011011100". If it doesn't form this
pattern, the CSU/DSU is said to slip and it must then resync its
timing to the signal.
Extended Superframing is an extension of Superframing and instead of
trying to collect twelve of these framing bits, it collects 24 of these
bits for syncing to the signal.
So, what about robbed bit signaling (RBS)? RBS is not a function of
what type of framing you are using. RBS also known as Channel Associated
Signaling (CAS) is a technique in which supervisory and signaling information
is sent by removing the 8th bit of the 6th and 12th Superframe or the
8th bit of the 6th, 12th, 18th and 24th bit of the extended superframe.
The overall affect for those using Channel Associated Signaling is that you
don't need a D channel, however all your B channels use 56kbps.
You can find more information on the following topics in McGraw Hill BCRAN:
http://www.bookpool.com/.x/p9zwbh4yt6/sm/0072124806
Channelized T1/E1 frame format: 208-213
Framing and linecoding: 226-228
Robbed bit signaling/Channel Associated Signaling: 230-231
The purpose of this book was to go beyond the Ciscopress material, which
focuses on the BCRAN exam material only and add information that seems
to have been ignored by Ciscopress's BCRAN. You will note that in this
book, when
a topic is not covered on the BCRAN exam, there is a note indicating such,
so that you don't review material not required for the exam....ok shameless
plug over... :-)
Adam Quiggle
At 05:31 PM 7/11/00, Ole Drews Jensen wrote:
>Hi Subbi,
>
>You are dragging me out in orbit now, but I'll try to hang on.
>
>To get back to the ISDN PRI...
>
>The 8000 samples, are you sure that it's not 8 k-samples (8*1024) = 8192
>samples???
>
>That would make sense since the 192 bits (24*8) multiplied with 8192 equals
>1572864 which again equals 1536 kbps (1572864/1024). That brings us back to
>24 channels where the last one is only used for signaling - not data.
>
>Ole
>
>~~~~~~~~~~~~~~~~~~~~~~~~~~
> Ole Drews Jensen
> Systems Network Manager
> CCNA, MCSE, MCP+I
> RWR Enterprises, Inc.
> [EMAIL PROTECTED]
>~~~~~~~~~~~~~~~~~~~~~~~~~~
>
>-----Original Message-----
>From: Subramanian Nallasivam [mailto:[EMAIL PROTECTED]]
>Sent: Tuesday, July 11, 2000 4:14 PM
>To: Ole Drews Jensen
>Cc: [EMAIL PROTECTED]
>Subject: Re: Some ISDN PRI questions
>
>
>Hi Ole,
>
> Let me take the case of T1. In T1 all the 24 channel are used for
>sending data. There will be one framing bit which constitiute to 193 bits.
>So
>193 * 8000 = 1544 kbps. And one bit from every sixth frame in case of
>superframe(which consists of 12 T1/DS1 frames) will be robbed for
>signalling.
>The robbed bit will be the least significant bit and this concept is known
>as
>robbed-bit signalling. If I am wrong then please correct me.
>
>Thanks,
>-Subbi.
>
>
>Ole Drews Jensen wrote:
>
> > Hi Subbi,
> >
> > That would still only leave 184 * 8000 = 1472000 bps or (1472000/1024)
> > 1437.5 kbps for data.
> >
> > Ole
> >
> > ~~~~~~~~~~~~~~~~~~~~~~~~~~
> > Ole Drews Jensen
> > Systems Network Manager
> > CCNA, MCSE, MCP+I
> > RWR Enterprises, Inc.
> > [EMAIL PROTECTED]
> > ~~~~~~~~~~~~~~~~~~~~~~~~~~
> >
> > -----Original Message-----
> > From: Subramanian Nallasivam [mailto:[EMAIL PROTECTED]]
> > Sent: Tuesday, July 11, 2000 3:54 PM
> > To: Ole Drews Jensen
> > Cc: [EMAIL PROTECTED]
> > Subject: Re: Some ISDN PRI questions
> >
> > Hi,
> >
> > I am new to ISDN concepts, but here they are....
> >
> > We have 23 DSO for carrying data. Each channel carries 8 bit word or
> > something. So
> >
> > 23 * 8 = 184
> >
> > One channel is used for carrying signal . So the number comes out to be
> >
> > 184 + 8 = 192
> >
> > Since 8000 samples are sampled per second , the calculation comes to be
> >
> > 192 * 8000 = 1536 Kbps.
> >
> > T1 uses in-band signalling ( uses same data channel for signalling also)
> > where
> > as ISDN PRI uses out-band signalling (uses seperate channels for signals).
> >
> > I am new to ISDN cocepts. If I am wrong please correct me.
> >
> > Thanks,
> > -Subbi.
> >
> > Ole Drews Jensen wrote:
> >
> > > The ISDN BRI has two 64 kbps B channels and one 16 kbps D channel. That
> > > gives us a maximum of 128 kbps bandwidth for data which makes sense.
> > >
> > > The ISDN PRI has twenty three 64 kbps B channels and one 64 kbps D
> > channel.
> > > That should give us a maximum of 1472 kbps bandwidth for data, and not
> > 1536
> > > kbps as normally specified unless it is possible to use the D channel
>for
> > > data too ???
> > >
> > > The ISDN PRI in Europe has thirty 64 kbps B channels and one 64 kbps D
> > > channel. That should give us a maximum of 1920 kbps for data, and not
>2048
> > > kbps as normally specified even if the D channel is used for data too.
> > >
> > > Are these numbers just used because it's easier to remember the 1536 and
> > > 2048 values, or is there a hidden factor here???
> > >
> > > A last question: I sometimes hear people refer to an ISDN PRI as a T1.
>The
> > > T1 has the same amount of channels and bandwidth, so the question is -
>are
> > > there really a big difference between these two specifications, or is it
> > > more or less the same???
> > >
> > > Some or all of these questions might be answered later on in my book,
>but
> > I
> > > can't wait any longer - I want to know NOW :-)
> > >
> > > Thanks for any comments on this.
> > >
> > > Ole
> > >
> > > ~~~~~~~~~~~~~~~~~~~~~~~~~~
> > > Ole Drews Jensen
> > > Systems Network Manager
> > > CCNA, MCSE, MCP+I
> > > RWR Enterprises, Inc.
> > > [EMAIL PROTECTED]
> > > ~~~~~~~~~~~~~~~~~~~~~~~~~~
> > >
> > > ___________________________________
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> > > Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
>
>
>___________________________________
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**************************************************
Adam Quiggle
Senior Network Engineer
MCI Worldcom/BP Amoco
CCNA, MCNE, MCSE
[EMAIL PROTECTED]
**************************************************
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