T1: Physical vs Logical Circuit

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A book on WAN technologies divides circuits into physical and logical.
Physical circuits are subdivided into exclusive-use (like a printer
cable or an RS-232 cable to a modem) and shared-use (Ethernet or Token
Ring between a PC and a server).

A logical or virtual circuit is defined as "a communications path that
appears to be a single circuit between the source and destination
nodes.  A virtual circuit is not a physical circuit; it is a logical
conection between devices configured over one or more physical
circuits."  These are subdivided into PVCs and SVCs.  

This all seems a bit arbitrary to me.  After all, an Ethernet is not a
single piece of wire.  Are these commonly accepted definitions?

Is a T1 always a PVC?  Would it be considered a physical circuit if
both end points are connected by the same switching office?

Re: T1: Physical vs Logical Circuit

On Mon, 13 Mar 2006 16:18:12 -0600, Bob Simon wrote:

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A T1 can be a physical circuit, as all were in the beginning. These days
it is often split out of a higher level physical circuit, like a fiber
optic path at one of the OC levels. That would make it virtual, but, since
it is always there, rather than being set up and taken down conversation
by conversation, it is a PVC in that case. It really has nothing to do
with the physical location of the end points.

The terms are in general use. Your proposed definition isn't very precise.

Re: T1: Physical vs Logical Circuit


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I am confused by your comment that all T1s were physical in the
beginning.  As I understand it, any interlata T1 consists of an IXC
plus two local loops.  That's at least three circuits that appear to
be a single circuit to the nodes terminating it.  So it would be
logical, not physical.  No?

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I did not propose a definition, at least not intentionally.  What is
it that you think I proposed?

Re: T1: Physical vs Logical Circuit

Run a 800 metres long 24 AWG, 0.5 mm 2-pair/4-wire twisted pair copper
from the network facility (local exchange or CO or DLC) to the customer
premises. Next, put a G.703 T1/E1 signal on the loop; assume it gets to
the CSU/DSU at the customer. Now, this local loop has only a single
stretch copper - no fibre, no HDSL, no G.Shdsl, no RF - so, is this
T1/E1 circuit physical or logical?


Re: T1: Physical vs Logical Circuit

On Mon, 13 Mar 2006 22:24:46 -0600, Bob Simon wrote:

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T carrier predates IXCs and LATAs. As DL notes, we should really be
talking about DS1 since T1 refers to a specific carrier system, which
carries 24 DS0s over two pairs of wires. This would make the DS1 circuit
physical. I guess you could call the DS0s virtual circuits, but I don't
think the term existed in the early days.

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I phrased it that way because I don't think what you quoted is a standard
way of stating it. Don't worry about it. If you want definitions with some
degree of authority, root around in ITU-T documents. There is probably
something there.

Re: T1: Physical vs Logical Circuit

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That might well be true, but is not significant since not all T1
carrier is interlata, nor does it necessarily have "two local

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It predates those terms, but obviously not the entities the
terms apply to.

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David isn't wrong, but be careful how you interpret what he
said.  Technically the DS1 standard defines an _interface_, not
a circuit and not a carrier system.  A T1 of course is any one
of several specific carrier systems, virtually all of which use
a DS1 interface (but which might have almost anything between
two DS1 interfaces.  (David was, if I remember correctly,
referencing the original T1 Carrier systems from the Bell
System, and in fact many but not all T1 Carrier systems are
compatable at the physical level.  Others are compatable only at
the DS1 interface.)

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A DS1 *interface* is physical, but the _circuit_ (that supplies
the data to it) may or may not be.  An obvious example would be
a Frame Relay or Cell Relay circuit where the customer interface
is a DS1.  Clearly the DS1 is physical, but the overall circuit
may be only logical.

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It existed.  But there is no such thing as a physical DS0.  It
defines a 64Kbps time slice in a DS1 level circuit.

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Very true.  Another interesting source of definitions (maybe
more interesting, simply because it is available on the
Internet, is Federal Standard 1037C,

   http://www.its.bldrdoc.gov/fs-1037 /

For example, it says that DS1 means,

   digital signal 1 (DS1): A digital signaling rate of 1.544
   Mb/s, corresponding to the North American and Japanese T1

How's that for ambiguous?  But that *is* the definition..., of
course a bunch of techies would actually want the
specifications, not the definition!

All terms such as T1, T2, T3, T4 etc are referenced to
"T-Carrier" (I've reformatted this to make it readable),

    T-carrier: The generic designator for any of several
    digitally multiplexed telecommunications carrier

       Note 1: The designators for T-carrier in the North
       American digital hierarchy correspond to the designators
       for the digital signal (DS) level hierarchy. See the
       table on the following page.

       Note 2: T-carrier systems were originally designed to
       transmit digitized voice signals. Current applications
       also include digital data transmission. (188)

       Note 3: If an "F" precedes the "T", a fiber optic cable
       system is indicated at the same rates.

       Note 4: The table below lists the designators and rates
       for current T-Carrier systems.

       Note 5: The North American and Japanese hierarchies are
       based on multiplexing 24 voice-frequency channels and
       multiples thereof, whereas the European hierarchy is
       based on multiplexing 30 voice-ifrequency channels and
       multiples thereof.   See table below.

The table lists First Level (DS1), Intermediate Level (DS1C),
Second Level (DS2), Third Level (DS3), Fourth Level (DS4), and
Fifth Level (DS5), and give rates for North American, Japanese,
and European standards.

(I don't expect the above will answer as many questions as it
generates.  It should, however, make for some interesting
discussion, and I would hope a lot of good understanding can
come from that.)

Floyd L. Davidson            <http://www.apaflo.com/floyd_davidson
Ukpeagvik (Barrow, Alaska)                         floyd@apaflo.com

Re: T1: Physical vs Logical Circuit

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None of the above. A T1 is facility that carries a DS1 signal.  T1
has 2 metallic pairs, repeaters midway on every 6Kft span, and goes
back to 1962..

Most of what users call a "T1" is really a DS1 carried over one pair
using HDSL technology. It's delivered as 2 pair to you, however...

A host is a host from coast to coast.................wb8foz@nrk.com
& no one will talk to a host that's close........[v].(301) 56-LINUX
Unless the host (that isn't close).........................pob 1433
is busy, hung or dead....................................20915-1433

Re: T1: Physical vs Logical Circuit

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Regarding your last paragraph.

Assuming that you are referring to a physical four wire interface when
referring to a T1 that terminates in an end office switch, then the
constituent DS0 time slots, or a combination of DS0's, would represent the
logical channels within a DS1 signal native to the physical T1 interface.

The PVC label would apply in either a Trunk, or Line side end office

A few examples are:

ISDN T1 PRI DS0, H0, H1 channels
DS0 trunks (AKA: Flex-Path in Verizon's former NYNEX territory).
TR08 line side termination when built as an Integrated DLC (AKA: SLC)
Line side DS1 switch termination with Loop or Ground Start A-B signaling.

In each example the subtending circuit(s) will carry a PVC label since an
end to end information carrying connection will not normally exist between
the end user, and the network.


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