In article , Clive Coleman
writes
Years ago the base band from your phone was multiplexed in a group
using FDM. Then these were multiplexed into larger groupings called
supergroups. Is the system still the same?

No.

The line card in the concentrator will digitise the phone signal (8000
samples per second using 8 bit encoding (mu-law IIRC)). All traffic
within the network uses these digitised streams. This is the source of
the 64k theoretical limit for modems.

An E1 is 2048000 bits per second. It's split into 64000 frames per
second, each 32 bits long. Bit 0 is used for clocking, bit 16 for
signalling, and the other 30 bits are the 30 voice channels on the
carrier. E1s are then multiplexed up into larger groups, but again at
the octet level.

The basic switching device is a piece of RAM. Say you've got 32 E1s
coming in (so 960 voice channels plus 64 signalling bits) on a link.
Switching basically involves re-ordering these bits between the input
and output. You read the 1024 bits in order into a RAM, then read them
out in the appropriate order.

[There are two 64000 bps data streams, one for each direction of the
call. They get handled separately though, of course, in
synchronisation.]

Note that ISDN simply involves doing the digitising at the customer
premises (or using data directly). An ISDN-2 multiplexes two channels, a
16000 bps D channel, and some framing bits into a (IIRC) 160kbs signal.
The line card then splits it back out.

Another question, having ADSL coming down the same line as my normal
telephone, I gather it would need to be on a carrier of some sort or
other, DSB,VSB,SSB with or without the possibility of suppressed
carrier etc. anyone out there able to give me a clue? And what would
the frequencies of the carrier be?

Beyond my knowledge base, I'm afraid. I know the spectrum is divided up
in the order phone, spacer, uplink, downlink, and there are complex
power limit curves for the different types of ADSL equipment.

--
Clive D.W. Feather | Home:
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