[MLB-WIRELESS] ISDN at local call costs

[Jonathan Oxer]

Hi All,

This might be starting to get a bit off-topic, but maybe non-wireless
data-carriage will be of interest to some (when you're working with an
'experimental' network like this, the more background and general
knowledge you have the better). Anyone not interested in ISDN or DOV, or
already an expert in it, hit delete now!

The throughput you get from ISDN channel bonding is the sum of the
channel capacities, ie: 2 x 64kbps B (bearer) channels gives you 128kbps
total symmetrical throughput. ISDN capacity is quoted as an uncompressed
bit rate (ie: true throughput) and can handle many protocols, unlike
modem type connections, which are quoted based on the compression used
in the specified transport protocol.

Normal ISDN channels can be bonded very easily in pretty much all
equipment, and DOV channels can be bonded in most equipment by using
'multilink-PPP' (sometimes labelled MPPP or other things in the
software).

Comparing a 64kbps ISDN channel to a V90 (theoretically 56kbps) modem
connection is not as easy as it sounds, because a V90 connection is
assymetrical with double the incoming data rate to the outgoing data
rate, is based on theoretical optimum conditions and is *after*
compression, while the ISDN's 64kbps is *before* compression. In plain
terms, that means with ISDN you'll get a full 64kbps in both directions,
which with V90 you may get 44kbps for incoming data and 22kbps for
outgoing data, if you're lucky.

In addition, modems squeeze all their signalling / handshaking overhead
into the same data stream, resulting in a further reduced throughput.
ISDN uses a separate 16kbps D (signalling) channel, which is just used
to handle handshaking and connection negotiation. That leaves the B
channels free to just handle data.

Sorry about that digression, the issue at hand was DOV.

I've set up a couple of DOV connections, one from Croydon to Croydon
(from my home to my office, dialling into a Lucent Portmaster 3) and one
from Sassafras to the CBD (friend's house dialling into an ISP). Funnily
enough, the short link from my home had pathetic stability while the
long link from my friend's home was very stable, and this is a point to
watch: Telstra will have care-factor 0 if you have trouble with a DOV
connection. They see people using DOV as exploiting a tariff loophole to
avoid paying the much higher timed or bill-capped plans for a real ISDN
data connection, and their tech support won't care in the slightest if
your connection doesn't work.

Apologies here to Telstra ppl on the list, I'm not having a go at
Telstra, it's just the way things are: Telstra have a specific range of
products, and if you use a product for a purpose they didn't intend and
in fact they have a more expensive product that *is* intended for that
purpose, you can't really expect them to help you when things don't work
as planned.

I say this because for me, they didn't. The line I was using had a very
poor lead-in from the street, with a badly corroded connection giving
noise on the line. The line passed Telstra's tests, but DOV wouldn't
stay up for more than a few hours at a time. When it went down, it would
sometimes be really unstable for a while, resulting in a lot of redials.
With my system redialling up to 150 times / day, at $0.20c odd or
whatever a local call costs, that results in a *huge* phone bill.

Of course I complained to Telstra, but even though they admitted there
was a problem with the DOV connection, they refused to fix the line
because it was being used for a purpose they didn't intend the product
to be used for, and worked OK when used as a pure data line.

End result: I dropped DOV, converted the ISDN line back to PSTN and got
ADSL.

Now, back to general DOV issues. Throughput on a DOV or ISDN connection
is dependant on the standard used, such as whether the equipment is
running ETSI ISDN (the European standard which Australia uses, with
64kbps B channels) or US ISDN (the US standard which uses 56kbps B
channels). In Australia, DOV should theoretically get the full 64kbps /
channel, but in practice you may not: because most of this gear
originates in the US, I've found that some supports pure ISDN at 64kbps
but drops to 56kbps when running DOV because when the equipment is
'Europeanised' by the US manufacturer, they convert the software for
ISDN to the ETSI standard but don't convert the software for DOV.

End result: Remote Access Servers (RAS's) like the Lucent Portmaster 3
which support ISDN at 64kbps but drop to 56kbps for DOV. Note that both
ends must support the same speed: if your CPE (Customer Premises
Equipment in the lingo) DOV card supports 64kbps / channel but your ISP
is running a PM3, you'll never get more than 56kbps / channel out of it
when running in DOV mode.

Not that a clean 56kbps (or 112kbps) of throughput is bad - in fact
compared to a modem, it's pretty amazing. Just something to watch when
setting it up. It worked really well from my friend's place in
Sassafras, at least.

Anyone care to buy a barely-used NetJet ISDN / DOV PCI card? I'm sure
it'd work well for someone else, it's just my line that was the problem.

Jonathan Oxer
Director
Internet Vision Technologies

> uhm youll be able to get full 128k because the b? d? channel handels 
> the signalling youll need possibly a isdn router unless you have kind of
> dial 
> up on your pc, also you can join to regular modems to double your
> connection
> its only going to get you maybe 6-8kbs tops due to the 33.6k modems
> 
> 
> Bede
> 
> danny.koreny at edion.com wrote:
> > 
> > Yep, DoV is cool. Will be using it soon to link mountainwireless to my
> > place..  I dont think i'll be able to link up tecoma to caulfiled with a
> > wireless card :-(
> > 
> > I think you can multilink 2 B channels to get 112kbs. :-)




--
To unsubscribe, send mail to minordomo at melbwireless.dyndns.org with a subject of 'unsubscribe melbwireless'  
Archive at: http://melbwireless.dyndns.org/cgi-bin/minorweb.pl?A=LIST&L=melbwireless
IRC at: au.austnet.org #melb-wireless