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ISDN
FAQ & SOME COMPARISONS WITH ADSL
What
is ISDN?
How is it Better?
What Does This Mean for Me?
So Where to Now?
Technical Explanation.
Multilinking to get
Broadband
What is ADSL?
Comparisons.
Summary.
What
is ISDN?:
ISDN (Integrated Subscriber Digital Network)
is a digital telephone connection over the same copper cables you have for
your current standard analogue connection, or POTS (Plain Old
Telephone System), or in Telstra's jargon PSTN (Public
Switched Telephone Network). There are mainly 2 forms
of ISDN you would come across today, BRA (Basic Rate Access)
and PRA (Primary Rate Access). BRA allows 2
channels of communication over the one pair of wires, (2xB channels @
64kbs and a D channel @ 16kbs, which the system uses for control data, not you). PRA allows up to 30, but we
will only concentrate on the BRA technology, and more on that later.
ISDN technology transmits data synchronously as distinctly
different to analogue technology which transmits data asynchronously.
The reason data can be transmitted synchronously on an ISDN connection, is
because of that small 16k control D channel. That channel transmits
the control bits between "ends" in regard to quality, size and
type of data "packets". Analogue systems do not have that
"control" facility, packets have to travel back and forth bit by
bit, as well as avoiding "collisions". A theoretical rule
is around a 4 times speed improvement synchronous over asynchronous
transmission.
How is it better?:
The analogue signalling of the PSTN network, well over a century old now,
conveyed voice signal (noise), from point to point so a conversation could
be had, anywhere in the world where there is a phone connection, albeit with some smart repeater boosting. The actual fact that this signal
is "noise" made for some interesting results, many of the older
generation may have experienced in the earlier days of telecommunication. Being "noise" made the signal open for
other spurious electronic "noises" to join the
"conversation". Many of you have experienced hearing the
squealing nature of a fax signal, or even a modem, that "noise"
is data. With the modern communications era of
faxing and internet, this "noise" interference on bad lines causes havoc for
good results and data transmission.
With the digital network system, a device at the phone line entry point in
your home or office, converts the analogue signal to digital, a
series of "pings", each having a "high bit" or
"low bit". This makes for a much more "un-corruptible"
signal, given the electronic "noises" on all lines that the
analogue signal competes with. For some 10-20 years, once your
connection got to an exchange, it was converted to digital for
transmission to its destination exchange anyway, then to be converted back again
to analogue for the last leg, from exchange to subscriber. This has
been popularly termed the "last mile".
What does this mean for me:?
As information communications became more popular for business needs as
well as home, as an extension to voice, the inadequacies of the ageing
Telstra network were becoming more apparent. Faxes took forever to
transmit, modems were hopelessly slow to access internet
connections. Modems originally were manufactured to communicate at
2400bps. Most of the lines could handle that, even fence wire,
(about the level of quality of some Telstra country services).
But as technology became more sophisticated for internet use, modems were
developed to 14.4kbps, 28.8kbps, 33.3kbps and 56kbps. That is a
theoretical communications speed expected today by the modern analogue
modem. BUT all this is expected on the ageing and decaying PSTN.
The digital signal development became the saviour of this high speed
"noise" based analogue system. Analogue signal can only
transmit at 56k downstream, and 33k upstream asynchronously (one direction
at once, like 2 way radio), when everything is perfect. If not
perfect it falls back to an acceptable speed to when data can get through
with no corruption,
remembering the return signal is 50% line speed of the received signal. ISDN
signal travels at 64k synchronously (both directions at once), same speed
both ways, and because of the digital nature, there is no fallback in
speed.
So where to now:? Up
to recent times, (2001), ISDN services were still too expensive for most
small businesses and of course more than most households could
justify. Telstra's claim that ISDN was costly to provide allowed them
to get away with some very exorbitant charges, even local calls were time
charged. As most exchanges were not up to handling the switching
needed for ISDN, data had to be trunked to major exchange centres.
The huge upheaval of the telecommunications industry during the 1997-2000
era saw the ACA (Australian Communications Authority) step in and lay down
some conditions for all the emerging telecommunication ventures
starting. One was the national "low rate" internet
connection call number (01983) for equitable access to an internet point
of presence (PoP), and another was to have an affordable digital
connection to almost all of Australia where a standard phone cable was
situated. The service was called "ISDN Home".
Nowadays a single ISDN Home connection, affording up to 128kbs
communication, would be deemed "midband" rather than "broadband".
It was deemed to be only available on residential subscribed services,
call charges were to be in line with standard residential and business
calls, and rental was also to be in line with standard services. Telstra were to be the only provider to install the service, and had a
monopoly on the provision of call services for a period of 3 months
A
more technical explanation: You
may have sussed out by now, even with a few technical limitations, this
tried and proven communications platform was to be a saviour for some bad
bush cables. That's right. With the use of "ISDN
Extenders", signal can travel up to 20km. It is really a Pair
Gain System (PGS). We won't get too technical on PGS, but
Telstra has been using PGS for a decade, to provide extra analogue
services where copper pairs had been damaged, or supply of free pairs for
new services had expired. PGS basically lets from 2 to 16 call
frequencies travel
over the same pair of wires simultaneously without interfering with each
other. The higher end of of the PGS was for exchanges, but many a
cable fault was "fixed" using a 2 port PGS. This system
works great for voice, even acceptable for faxes, but
"bloody" hopeless for data (internet) on an analogue
service. BUT the story with digital signal is very different,
Remember digital signal is not "noise based", it is made up of
high bits and low bits. To achieve the separation of the 2 signals,
different frequencies are used, just like the different frequencies of
your 2 way radios.
As mentioned at the beginning of this article, there are two main levels
of ISDN service, BRA and PRA. The Basic Rate Access is what the ISDN
Home service is. "Pair Gaining" 2 "channels" of
signal on the one pair of cable to provide the 2 numbers allocated. Primary Rate Access provides 30,
but who wants that in a home? So with ISDN Home, the only analogue
signal is from the mouthpiece of your phone to the ISDN controller box
installed in the home at the point of entry into the Telstra service
pair. That is a vast improvement to many Km's of bad cable to an
exchange. The same with your fax and analogue modem. Speeds
vastly improve to nearer the theoretical 56k, and as there is no
electronic noise interference, "dropouts" should be non
existent. Feedback from some badly earthed electric fences would be
the exception, that "hammer" would take down any form of
communication.
How
do we get "Broadband" out of these Channels?:
Where ISDN systems really come to the fore in high speed
communications, is multiple channels can be "multilinked"
together. A Basic Rate Access provides 2 channels of 64k each.
Multilinked, that provides 128k. Multiple services can be
multilinked together to create a very "broadband" of service,
eg: 2 ISDN Home services multilinked together gives a 256kbs
"broadband" service. Larger Corporations and Internet
Service Providers can multilink many channels of a PRA (Primary Rate
Access) service to provide up to a 2mbs huge pipeline of data service
(eg:30x64=1920kbs) Also remember ISDN is synchronous signal, meaning
data travels both ways simultaneously, giving an exponential increase in
CLEAR data transmission, over an analogue service.
What
is ADSL?: ADSL
(Asynchronous Digital Subscriber Line), is
still a digital revolution for phone lines, but does not carry voice as we
know the analogue system. ADSL services "piggyback" on an
analogue service to provide a fast digital signal for internet access and
data transmission. You have to have an analogue PSTN or POTS service for
the ADSL system to be provided. The ADSL signal is transmitted at a
much higher frequency than the analogue signal, therefore being very fast
in comparison, and is Asynchronous. This being data packets
can travel only one way at once, and at a 25% less upstream speed
than downstream. Common speeds provided are 256k download, 64k
upload, up to 1.5mb download 256k upload. All this is contingent to
the main source being able to "pump" the data out at the premium
rate.
All sounds too good to be true? Yeah, BUT BUT BUT. ADSL
signal, with current technologies and cable condition will only travel
reliably 3500m of cable length. Will NOT work (or is incompatible)
with a PGS (pair gain system) and has to have an unbroken point to point
connection. Hence the continuos connection nature this service
provides, not a dialup type service as is analogue or ISDN, as the
connection has to be established and conditioned each time you connect.
Lets look at some technical statistics of the Telstra National
Network. There are just over 4000 exchanges in Australia. Some
3000 odd were built using the PGS for economies and faster provision of
services in remote and developing areas. The larger ones of these
are being converted, economies of scale prevailing. Others are
having the necessary technology installed, but only to provision the
services that are compatible for distance and PGS conversion back to
single gain. So you can see that ADSL will not be an option for
100,000's of country/rural subscribers, even regional centre subscribers,
and even more so, some very large areas of suburbia, where Telstra have
done a cheap job of service provision. Many of us will be
"pushing daisies" before an ADSL type service becomes a reality,
regardless what the politicians purport. Also ADSL is incompatible
with ISDN
So
lets look at some comparisons of service type and costing
Speed:
PSTN: From 7.2kbs (hopeless) to a theoretical 56kbs (in heaven)
ISDN: 2 channels @ 64k each per BRA service, can be multilinked
ADSL: From 256k down/64k up to 1.5m/256k (if you have the $'s)
Distance:
PSTN: Up to 25km as a rule, anywhere a 48volt DC current can get to.
ISDN: Up to 20km, needs a better conditioned line to get 100v DC through.
ADSL: 3.5km cable length, then anywhere an analogue service is, AS LONG AS
THERE
IS NOT A PAIR GAIN SYSTEM BETWEEN YOUR SITE AND THE EXCHANGE.
Service
Prices:
These
are for ONLY the telephone service, not ISP charges
PSTN:
From low $20's to high $30's/mth, depending on service and features
ISDN: $45.50/mth for the ISDN Home service, then same features cost as PSTN
ADSL: No extra charge on your PSTN phone bill, costs charged by ISP
Installation:
PSTN:
$209inc if new service down to $55 if reconnecting a service
ISDN: $325 new service, $190 upgrade from PSTN, or periodic specials as
low as $55.
ADSL: Up to $130 establishment fee, depending on ISP
So
now for a summary of the ISDN Home servcice
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The
service can only be installed by Telstra, even if upgrading a current
service, provided by an alternate carrier.
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If
using periodic special connection/conversion offers by Telstra, a
condition is that you select Telstra as your preferred carrier for ALL
services (rental, local and long distance/international) for up to 2
years, (as at 1-9-04). A change of lessee application (or
WinBack) will need to be performed if you currently use an alternate
carrier.
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If
NOT using special discount offers of connection/conversion, (ie: new
connection @ $325, conversion of PSTN @$190), Telstra only require a
change to rental and local charges, for the minimum contract period,
currently 3 months. A churn of all services to your long
distance carrier can then be performed without penalty charges by
Telstra.
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ISDN
Home call charges are similar to all Homeline connection
charges, AND ALSO AS OF JAN 10 2005, A 24HR 7 DAYS A WEEK STD
CAPPING, @ $2.40 FOR UP TO 3hrs CALL DURATION TO FIXED LINE NUMBERS,
AND UP TO 20min CALL DURATION TO MOBILES PHONES, IS AN AVAILABLE
OPTION. A full list of current call charges is available in the
Telstra SFOA, www.telstra.com.au/sfoa/docs/isdn.doc
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An
ISDN Home connection provides 2 channels of communication over a
single cable pair and allocation of 2 numbers. In the case of a
conversion to ISDN Home, the primary number will be retained and an
auxiliary number allocated for the second channel. If you
already have 2 PSTN sevices, it is in order to cancel one, upgrade the
other, retaining both numbers for the new service.
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The
installed NT1+II unit provides many features, one being calls between
channels at no flagfall or charge, and an ISDN Data call rate of
30c/min or part thereof per channel, to an Internet Service Provider
using an 01983 access number. There is no flagfall connection
cost with these calls, just the timed rate charge on your account.
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For
longer time ISP connection requirements, there is a Telstra ISDN
Internet Plan that can be applied to the ISDN Home connection.
This caps ISDN Data call charges to $16.50/mth (as at 1-9-04).
This roughly equates to 27 odd hours of ISP connection time @ 128k, or
54hrs @ 64k per month. This charge is added to the current
$45.50/mth rental fee.
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Whilst
this page is not intended to campaign for any particular telephone
service provider, there are many who offer alternative charging plans
to Telstra, once the qualifying period is elapsed, for ISDN Home
connections. That is the subscribers choice.
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One
point must be remembered over and over again, ISDN Home and ISDN2 is
the ONLY better terrestrial broadband connection available above PSTN
capabilities, to 100,000's of rural subscribers in Australia for many,
many years to come.
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During
many publicity campaigns, Telstra have NO right to lead subscribers to
believe that alternate carriers cannot maintain services "in the
Bush". This is emphatically pointed out by the TIO (www.tio.com.au),
The telecommunications service obligation compels the actual service
must meet stated guidelines, regardless of provider, outside of City
areas. Alternate providers have as much fault/service directive
to a service as Telstra themselves. This applies to ISDN Home
connections as well.
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During
power outages, ONLY the Tel1 and SBUS ports remain active on the
NT1+II unit. Therefore it is suggested the primary number be
applied, and primary phone be connected to Tel1 in case of emergencies
during the outage.
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