ADSL is Asymetric AND ALSO half-duplex
SDSL @1.5mbs is Symmetirc but only HALF-duplex
I disagree with that.
ADSL means Asymmetric Digital Subscriber Line
Asymmetric means, that Up- and Downstream rates are different, usually
downstream is (much) faster than upstream. And all ADSL variations I
know of are full-duplex. Full-duplex has nothing to do with asymmetric
speed ratio in any way, it just says that sending and receiving can be
done at the same time.
Just think about how TCP works and the consequences it would have over a
half-duplex link. A packet you send in one direction needs to be
acknowledged from the receiver. As an example, a file transfer is a
series of big packets from the server to the client, this generates a
stream of Ack packets from the client to the server. If the downstream
channel needs to be stopped every time an Ack packet is sent back to the
server you would largely destroy the high downstream speeds by constant
interruption. But that would be the only possibility to send the Ack
packet, as half-duplex can't send and receive at the same time. This
rules this out pretty much.
And SDSL is Symmetric Digital Subscriber Line
The difference between this and ADSL is that the Downstream is the same
as the Upstream speed. Therefore these types of services are most of the
time more expensive, but if you like to run a server or VPN gateway it
helps greatly, as these are situations, where the upstream speed is
required as well as the downstream.
Your comparison between a 1.5mb T1 and a 1.5mb SDSL is wrong as you
describe it.
A "normal" T1 (or whatever speed) leased line is (normally) a point to
point link between two locations, assume you as a customer on one side
and the ISP on the other side. This guarantees you a constant speed
because nothing happens in between.
Any xDSL connection does not work like that. There you have a point to
point connection between your xDSL equipment and the central office,
where you'll find something like a DSLAM. That device has many ports
which has all the connections to many of the subscribers connecting to
the same central office. From there to the ISP you have a backbone
infrastructure, this can be an ATM network, but it could be anything
capable of transferring data. In many cases, this link has to be rented
by the ISP from a Telco. And this link is where overbooking happens. As
an example, if you have a 100 customers connected to a CO each with a
10mb/1mb (down/up) ADSL, the theoretically achievable maximum transfer
rate by the customer is 100*10mb, equal 1gb for the downstream and
100*1mb =100mb for the upstream. But the ISP won't buy 1gb link from the
CO to his location, he will overbook that link with the assumption, that
not all these 100 customers use their downstream/upstream link at the
same time at full speed. If they try to do, and if the overbooking
ration is 2:1, each of them will only get half the "promised" speed.
This is true for ADSL as well as SDSL. And if you don't get a stable
throughput with SDSL at the speed you purchased, this is the overbooking
of the link between CO and ISP.
If you have the chance to compare an SDSL 1.5mb with a T1 1.5mb from the
same ISP 1:1 you can then guess his overbooking ratio. Most if not all
will keep this ratio as a big secret, but it's an industry standard to
do it.
But that said, SDSL service should have a better ratio than ADSL as you
are paying more money. And you can judge the ISP by his service.
If you have an ISP where you've got an 1.5mb SDSL which you only gives
you half that speed I'd consider another ISP! I run SDSL links for
customers where we can easily get 1.2mb up- and downstream at the same time.
This all assumes that the Pipe of the ISP to the Internet at large can
cope with the traffic from his customers, which is not always the case.
Or you test the speed with servers directly connected to that ISPs network.
Oliver
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