T1: Physical vs Logical Circuit

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.

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...

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?

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

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?

VG

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.

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.

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

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

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.)

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.

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

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

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

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 systems.

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.)

Bob Simon wrote in news: snipped-for-privacy@4ax.com:

Bob.

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 termination.

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.

Bill