T1 links

[Art Berggreen]

I've seen some confusion on this newsgroup about "T1" links, so I
thought that I'd post this msg which I posted earlier on TELECOM.

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".

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.

What most users buy from the telco is 56KB service.  Modems (typically
Bell 303 type) carry the user's 56KB data to the central office where
telco TDM equiptment multiplexes the data into a T1 trunk.  The data is
routed to the remote central office where it is demultiplexed and delivered
to the remote user site by modems.

Most of the ARPANET is connected via 56KB service which the government
leases from the telcos.  Some of the higher volume IMP-IMP links run
at 230KB, and some of the transoceanic links (usually via satellite)
run at lower speeds.

    				Art Berggreen<Art at ACC.ARPA>
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