Clear Channel on a T1

Thu Mar 22 19:08:55 UTC 2001

> 
> Here's a few more details -
> We provision T1.
> 
> Salesdroid asks "You wanna give us $$$ for clear channel?"
> 
> Us: "..... ?"
> 
> Salesdroid: "It's so you can get 64kbit rather than the normal 56k"
> 
> Us: "We thought all DS-0s were 64kbit?"
> 
> Me: "I know! I'll ask NANOG!"

When you're ordering a full DS-1, it almost never makes a difference,
because telco just passes everything, including the framing, without
even looking at the individual channels.

When you're asking Telco's to do DACSing on the circuti, it matters. 
The relevant difference is what happens with a DS0 is cross-connected
by the Telco from one DS-1 to another.  There are two options:

(1) Just move the bits:  Pull 8 bits at a time from timeslot X the
incoming DS-1, slap them out on timeslot Y in the outgoing DS-1.  This
is what you want if you are doing data.  This is not what you want if
you are doing voice with channel-associated signalling.

(2) Just move the bits, *except* that it also receives the signalling
bits from the incoming DS-1 and regenerates them on the outgoing DS-1. 
This will run data (unless you're doing Nx56), because it will
rearrange the LSB.  But it is necessary for voice using
channel-assocaited signalling.

The above explanation doesn't help unless you know what channel
associated signalling is.  I'll briefly describe it since it's probably
somewhat off charter:

The bits in a T1 are sent as frames, consisting of one framing bit
followed by 8 bits from DS0#1, followed by 8 bits from DS0#2, and so
on.

The frames are then organized into groups of 12 frames (Super Frame) or
24 frames Extended Super Frame).  The LSB of each byte is used for
signalling purposes in Frames 6 and 12 (or 6/12/18/24).  Thus, if
you're cross-connecting individual DS0's, it's possible that when
you're clocking in Frame #12 on the incoming DS-1, you'll be clocking
out frame, say, #3 on the outgoing DS1.  In that case, you would lose
the signalling, because the signalling bit on Frame 12 would go out on
Frame 3 (where it's no longer a signalling bit); similarly, the
essentially random LSB that comes in on Frame 3 would go out on Frame
6, so you'd effectively have a random signalling bit.

So if you want the signalling to be kept right, you don't order clear
channel, and then the DACS (Cross-connect system) regenerated the
signalling on the outgoing DS-1, based on what it is receiving.  If you
aren't using signalling (for example, if you're doingg data), then you
order clear channel.

If you're ordering a Full T1, it raraly matters, because when they
corss-connect everything at the T1 level, they generally just copy the
framing bits through with everything else, so Frame 1 is going out when
Frame 1 is coming in.  

(Note: replace DS-1 with T1 in the above if it helps you read it
better.)

(e-Mail privately if you want more details.)

     -- Brett