>Seeing that nobody had really addressed your question yet, I decided to
>take a stab at it.
>
>T1 is a high-speed digital carrier facility developed by AT&T in the 1950s
>to support long-haul pulse-code modulation (PCM) voice transmission. T1
>provides digital voice circuits or "channels." There are 24 channels per
>each T1 line or "trunk."
>
>AT&T describes their Digital Carrier System as a "two-point, dedicated,
>high capacity, digital service provided on terrestrial digital facilities
>capable of transmitting 1.544 Mbps. The interface to the customer can be
>either a T1 carrier or a higher order multiplexed facility."
>
>So, what do you think? To send data on this do we need to encapsulate it?
>You betcha, as Leo on TechTalk would say. That's why we have "encapsulation
>frame-relay" and "encapsulation ppp," among others.
>
>Hope that helps a bit.
>
>Priscilla
Let me come at this a little differently, and get one of my
opportunities to say something is layered -- just not exactly
corresponding to the Seven Deadly Layers.
At the lowest level are the signal (i.e., electrical or optical)
standards that define single bits. These may be associated with a
particular connector, but they also have particular timing
characteristics. DS1 and E1 describe bit streams that run at certain
data rates:
1.544 MBps for DS1
2.032 Mbps for E1.
I distinguish between the DS1 and T1 signals. T1 is a DS1 signal
specifically running over copper pairs. You can have radio or optical
DS1 that is not on wire. DS-x also is a level in a digital
multiplexing hierarchy.
DS-x or E-x streams are isochronous: bits have to fall into time
slots in the stream, but there are no in-stream timing bits (as with
asynchronous signals) or external timing (as with synchronous
signals).
For the next layer up, there are constraints on the bit stream. These
constraints are used to maintain signal quality, for limited control
(e.g., triggering loopbacks), etc. Bit level framing includes
algorithms such as AMI and B8ZS.
The next level up synchronizes multiple stream into frames, so the
frames can be associated with multiplexed subchannels. SF and ESF are
common methods, along with CAS and CCS as means of sending control
information.
On top of the subchannels go the basic data link framing from
protocols such as frame or ATM with AAL. Both of these families go
up yet another layer, with a protocol type field starting the data
field, a field based on 802.2.
It's late. I hope I'm not rambling too much.
>
>
>
>>On Wednesday, February 07, 2001 at 06:36:34 PM, Santosh Koshy wrote:
>>
>> > Pardon my ignorance here....
>> > Is there such a thing as just a "T1 Link"...
>> >
>> > of what I understand T1 / Frac T1 resides in the physical layer.... Dont u
>> > need something at the Data Link Layer (such as Frame Relay) before you can
>> > enable IP or IPX at layer 3....
>> >
>> >
>> > _________________________________
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>> >
>
>
>________________________
>
>Priscilla Oppenheimer
>http://www.priscilla.com
>
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