[Asterisk-Users] Should I choose DSL @ 1.5 or a full T1?

[Rich Adamson]

> You guys have me second guessing my training and experience in this
> area, so;
> 
> 1. If I am wrong I apologize to the group.
> 2. I have been trying for a few minutes to find confirmation either way.
> 
> >From what I know about the modulation techniques used by DSL (DMT, CAP,
> QAM) it is impossible for the transceiver in the device to transmit and
> receive at the same time (unless there is discreet "channels" for each
> path and a very good transceiver).
> 
> Does anyone have any definitive technical resources confirming that any
> form of xDSL technology can transmit and receive at precisely the same
> time (not interleaved).

I'd have to guess that your past training and experience might have been
based on some specific product where your thoughts might be correct.
Or, someone may have been trying to make a point with some specific
product or chip set that essential suggested "at any specific microsecond
in time, the chip set can either send or receive (half duplex)", and
there are some that operate that way. But, the majority of current day
quality dsl products, when viewed over a longer period of time then a
microsecond, operate in full duplex mode.

Some exceptions to that are specific products from Net-2-Net that are
oriented around sdsl, etc.

If you truly attempt to transmit and receive data simultanously through
the majority of current day products, you can prove to yourself which
ones operate in full duplex vs half.

> Can anyone provide a more logical explanation of why the outbound
> latency on every DSL modem tested increases with inbound traffic? Even
> at rates well below the maximum data rate, Not the case on a T1. My
> explanation is that the additional latency is due to packet scheduling
> and queuing mechanisms required by the technology.

Someone else already discussed the effects of TCP acknowledgements
impacting full duplex or throughput.  The same is roughtly true with 
many udp sessions (such as sip and iax since asterisk doesn't send a 
packet until after its received a packet).

FWIW, our local telco is now providing 3.0 meg down and 768 up on the
same cable pair that was previously used for 1.5 down and 768 up. They
were using Cisco's dslams, but not sure if they are still in use or
if they've been swapped out. Anyway, they are deploying those speeds
right now with full duplex. Technology moves on. :)