Ultrix V4.2 sources et al...

From: Fred N. van Kempen <waltje_at_pdp11.nl>
Date: Wed Mar 10 03:35:06 2004

On Tue, 9 Mar 2004, Michael Sokolov wrote:

> > Qbus VAX, have connectors for a T1 line on them? Not being very
These are multi-line connections, which basically means that on the
outside, you connect the CSU/DSU of the telco T1 line to the card.
Within the card, that T1 (1,544Mbps total, in 24 channels of 64Kbps)
connection gets split into these 24 separate channels- either as an
ISDN PRI line (1xD and 23xB) or as a multiplexed digital line
carrying 24 individual phone line channels. This basically saves
you and the telco from having to find 24 "good" copper pairs.

The card then handles the "lines" as 24 phone lines, for voice, fax,
whatever (in the multiplex case; its a lil different in the ISDN

> In telephony applications a T1 is split into 24 channels each carrying a phone
> call. When, however, someone speaks about T1 as a dedicated line for Inet, a
> device called a CSU/DSU takes the entire T1 payload (all 24 channels together)
> and turns it into an EIA-422 or V.35 synchronous serial stream that a router
> then has to deal with.
No. A T1 *is* a digital peer-to-peer connection between the CO (telco)
and your location. The line runs at 1,544Mbps, and is terminated by a
CSU/DSU at both ends. This is just a digital bit stream.

Depending on HOW the line is used, things change. In the U.S., the most
common use was to either multiplex 24 digitized (using straight PCM)
analog phone lines into a single copper line.. easier for transportation,
saved on copper lines (read: cost and copperline real estate on the
poles)and it was more immune to all sorts of interference. In this
scenario, the CO simply takes 24 analog lines, PCM's these into 24
digital bit streams (at 56 or 64Kbps) and TDM's these onto the T1
bit stream as defined above. No magic, just TDM timeslots. On the
far (customer) end, the CSU output was connected to a channel bank,
which un-TDM's the channels, and converts them back to analog ports,
so you end up with 24 RJ11 jacks again. This was used with older
modem banks (Jay probably remembers the stacks of those we used
to have ;-) and older PBX'es.

Also, if the customer equipment could deal with multiplexed T1 by
itself, you could skip the channel bank, and plug in the output of
the CSU straight into the device, and it would do the de-muxing itself.
Many recent versions of these even had the CSU built in, so you could
just simply plug in the RJ12 from the telco :) We ISP's *loved* this,
as you got one wire from the wall punch panel into the modem bank, with
not much left to break...

For example, I still have Cisco 3640's running with such a setup: they
get fed the R12 from the wall, demodulate the signal ("CSU"), un-TDM
it, and chop it into 24 digital phone channels. These then get fed
to digital modem boards, and the router software does teh rest. Nice
and clean, no analog stuff anymore, pure digital connection.

For ISDN connections (which werent used much in the U.S.), things are
similar: the line is the same, as is the CSU. The DSU is, too, because
U.S. ISDN PRI service (well, most of em *grin*) are magically defined
as having 23 data channels ("lines") and one signalling channels, for
a total of 24 again. So.. after we feed the signal through the CSU
and DSU, we again get 24 digital bitstreams. Channels 0 to 22 are
the B (data) channels, and channel 23 is the signalling (D) channel.

The rest is software. On most T1 controllers, you can indeed tell it
to use "digital multiplexed" or "isdn pri" mode of operation :)

So... this is "channelized service", since the stuff in the CO does
something channels. If you want to just take the raw pipe (the
digital bit stream at 1,544Mbps) you get *transparent* service, which
is simple: the DSU at the CO defined two channels - channel 0 for
monitoring, at 16Kbps, and channel 1 for data at 1,536Mbps. Some
ISP's didnt do the monitoring channel, which gave you an extra
16Kbps... these were called "managed" and "unmanaged" lines.

With transparent service, you just get the bit pipe. This obviously
is used by the ISP's to deliver bandwidth to customers for their
data connections, and what is seen by most as ".. a T1".

Oh. yeah, and then there is Fractional T1. You guessed it.. rather
than allocating a full pipe, the DSU's get programmed to only allocate
as many channels as are needed/requested by the customer. If I wanted
a 256Kbps line, they would program the DSU to put me on time slots A,
B, C and D (4x64). These were usually larger DSU's, which could handle
up to 8 serial links to be multiplexed over the T1 sigal. Later, most
of this was done within the routers.

The above goes for E1's too, which are 32-channel connections used
outside of the U.S. These run at 2,048Mbps, and have the same options
as the T1's above. E1's are usually based on fiber carriers, though,
and not the (largely analog in nature) T1/T3 carriers.


Fred N. van Kempen, DEC (Digital Equipment Corporation) Collector/Archivist
Visit the VAXlab Project at                         http://VAXlab.pdp11.nl/
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Email: waltje_at_pdp11.nl     BUSSUM, THE NETHERLANDS / Mountain View, CA, USA
Received on Wed Mar 10 2004 - 03:35:06 GMT

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