telecom@ucbvax.ARPA (03/16/85)
From: Jon Solomon (the Moderator) <Telecom-Request@BBNCCA>
TELECOM Digest Fri, 15 Mar 85 13:35:01 EST Volume 4 : Issue 170
Today's Topics:
900 Service
no long distance service
RE: T1, etc.
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Date: Thu 14 Mar 85 21:36:05-EST
From: S.PAE%MIT-EECS@MIT-MC.ARPA
Subject: 900 Service
To: TELECOM@BBNCCA.ARPA
Does anyone know what the pricing structure is for 900 service? What sort
of calling volume do you have to have before you would clear a profit?
Any information or pointers to information would be appreciated.
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Date: Thu 14 Mar 85 19:49:42-PST
From: David Roode <ROODE@SRI-NIC.ARPA>
Subject: no long distance service
To: abc@BRL-TGR.ARPA, telecom@BBNCCA.ARPA
One standby way of restricting long distance access would be to
install a semi-public coin telephone. This is the kind of coin
telephone that incurs a monthly fee (greater than the usual phone
line), but no long distance calls are possible on such a phone.
Recently, a problem happened with a local jail. It seems
the inmates were using many purloined calling card numbers
and Sprint, MCI, etc. codes. The sheriff's solution:
He is going to ask the telephone company to install
rotary dial phones. Currently they have something that
looks like a Charge-a-call on a metal device that rolls
from cell to cell, with a ruggedized conduit being uncoiled
to carry the wire along with the phone. The inmates are allowed
local calls on this phone, and so can apparently get
to Sprint.
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Date: 15 Mar 1985 09:44 PST
From: Art Berggreen <ART@ACC>
Subject: RE: T1, etc.
To: TELECOM@BBNCCA
This is a summary of information which I have obtained from a short
investigation of T1 and related services.
All T1 services are based on "T1 carrier" circuits. These circuits
employ twisted pair cables to transmit serial bit streams at 1.544 MB/sec.
(or 2.048 MB/sec in Europe). Due to losses in the twisted pair cable,
repeaters are required every mile or so to recover and regenerate the
data. The data bits are sent using a technique known as Alternate Mark
Inversion (AMI). In AMI, a pulse is sent for every "one" bit and no pulse
for every "zero" bit. In order to avoid capacitive DC line charging, every
pulse is generated in the opposite polarity from the previous pulse.
Receive timing information must be derived from the incoming pulse stream.
In order to insure that receive timing remains synchronized with the bit
stream, there must be a minimum number of pulses over a given time. This is
usually addressed by allowing no more than 15 consecutive zeros between
one bits. Other techniques exist to overcome the consecutive zeros problem.
These techniques (B8ZS and HDB3) detect sequences of consecutive zeros
and send pulses which violate the alternating pattern of pulses. The
receiving end recognizes these specific violations and converts them back
to zero bits.
Usually imposed on the basic bit stream is a data framing pattern. This
framing is used to subdivide the data stream into 24 Time Division
Multiplexing (TDM) slots (32 slots in Europe) which carry independent
voice or data traffic. In North America, each frame consists of a framing
bit followed by eight bits for each of the 24 TDM channels for a total of
193 bits per frame. This works out to an overall bit rate for each
subchannel of 64KB/sec. Frames themselves are grouped into "superframes"
of 12 frames or "extended superframes" of 24 frames. The framing bit
follows a pattern which is used to locate boundaries of superframes.
In North America, one bit is "robbed" in each of the subchannels every
sixth frame to carry circuit signalling information. This only leaves
7 bits in each subchannel that can carry data without being corrupted,
limiting data transmission to 56KB/sec (7/8 of 64KB/sec). In Europe,
the first subchannel carries framing information, and the sixteenth
subchannel carries signalling information. Therefore all 8 subchannel
bits can carry data, resulting in all 64KB/sec available for data
transmission. The subchannels can also carry Asynchronous data streams
by sampling the asynch stream at 64KB/sec and regenerating the
asynch stream at the remote end to within a 64KB/sec resolution.
Due to sampling resolution, asynchronous data can only be supported
up to 19.2KB/sec. The basic 1.544 MB/sec service is referred to
as "DS1" and the 64KB/sec subchannel service is referred to as "DS0".
Several companies sell TDM multiplexers which use standard framing on
T1 circuits to provide up to 24 communication ports with standard
RS-232C interfaces. Most of these will run up to 56KB/sec synchronous
or 19.2KB/sec asynchronous. Some of them can combine the bandwith
of several channels and provide higher speed ports than 56KB.
T1C is similar to T1 but conveys 48 subchannels in 385 bit frames by
running at 3.080 MB/sec.
In order to migrate toward ISDN capabilities, a new framing and signalling
standard is evolving called Digital Multiplexed Interface (DMI). This
standard will support full 64KB/sec subchannels by reserving the 24th
subchannel for signalling. The interpretation of the data on the signalling
channel is defined by one of four operating modes. Modes 0 and 1 are intended
to be compatible with existing signalling mechanisms. Modes 2 and 3
provide for ISDN capabilities by defining the signalling channel to
carry an HDLC framing sequence. The HDLC messages carry the signalling
information for the other subchannels.
"Art Berggreen"<Art@ACC.ARPA>
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Date: 14 March 1985 23:29-EST
From: Leigh L. Klotz <KLOTZ @ MIT-MC>
To: telecom-request @ BBNCCA
I received a solicitation call from ITT or some subsidiary recently.
They, as usual, wanted to sign me up then and there for long distance
service. When I queried them on rates they responded "up to 30% less
than AT&T." The caller said that she could not provide me with better
rate information than that, and I said I was uninterested. She repeated
that it was always 30% less than AT&T, and said yes when I asked if that
meant their rates were tied to AT&T's rates for each particular call.
She also said that other long-distance companies do not publish their
long-distance rates.
Is all this correct?
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End of TELECOM Digest
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