17-05-2013, 04:13 PM
ISDN
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ISDN Overview
Motivation and History
A long time ago, the entire telephone network was analog. This was bad, because as a voice went farther
down the line, and through more switches, the quality became worse and worse as noise crept in. And
there was no way to eliminate the noise, no way to know what the signal was supposed to be. Digital
encoding promised a way to encode the audio such that you'd know what the signal was supposed to be.
As noise crept in, you could eliminate it throught the phone network, assuming it wasn't worse than the
variation between different digital encoding levels.
With the transistor revolution, this theory became possible, and the phone companies began converting
their own networks over to digital. Today, you have to search pretty hard to find a phone company switch
that isn't digital. They call their network the Integrated Digital Network, or IDN.
This solved many of the phone company's problems. However for a variety of reasons, it has been
attractive to make the phone network completely digital, from end to end. For computer users, this is
ideal, because we can eliminate those clumsy modems, and will hopefully benefit from higher speed. For
the phone companies, they can eliminate the last of the noise and loss from the audio data. And for
dreamers, this will enable a wide variety of different services to be delivered to the customer over a
single interface.
What is ISDN?
ISDN stands for Integrated Services Digital Network. It is a design for a completely digital
telephone/telecommunications network. It is designed to carry voice, data, images, video, everything you
could ever need. It is also designed to provide a single interface (in terms of both hardware and
communication protocols) for hooking up your phone, your fax machine, your computer, your
videophone, your video-on-demand system (someday), and your microwave. ISDN is about what the
future phone network, and information superhighway, will look like (or would have looked like).
ISDN was originally envisioned as a very fast service, but this was a long time ago when it was hoped to
have fiber all the way to your house. It turned out that running all that fiber would be too expensive, so
they designed ISDN to run on the copper wiring that you already have. Unfortunately, that slowed things
down considerably - too slow for quality video, for instance.
ISDN has been very slow in coming. The standards organizations have taken their time in coming up
with the standards. In fact, many people consider them to be out of date already. But on the other side of
the coin, the phone companies (especially in the U.S.) have been very slow at desiging products and
services, or marketing them with ISDN in mind.
B-ISDN
That brings is to B-ISDN. B-ISDN is Broadband ISDN. (The older ISDN is often called Narrowband
ISDN.) This is not simply faster ISDN, or ISDN with the copper to your home finally upgraded to fiber.
B-ISDN is a complete redesign. It is still capable of providing all the integrated services (voice, data,
video, etc.) through a single interface just like ISDN was supposed to. But it will do it a lot faster than
ISDN could. Of course, that copper to your house will still have to be replaced with fiber. But B-ISDN is
still in development - it seems to be moving faster than ISDN, but it is still quite a ways off.
Fitting things together
In order to understand what ISDN is, you have to understand a bit about modern telephony. You'll
invariably find lots of buzzwords, or in most cases buzz-acronyms, that seem to overlap in a terribly
complex way. That's because they do overlap considerably. Nevertheless you can generalize about how
certain things fit together. Hopefully I can sum it all up in a few almost correct categories.
There are two parts of a telephone network: the phone company's part, and the customer's part. The
customer's part today is largely just the telephone, some house wiring, and some connectors. The phone
company's part is lots more wire, fiber, switches, computers, and lots of expensive and complicated stuff.
ISDN is concerned (almost) entirely with the customer's part of the network. ISDN gets the data from
you, to the phone company in a standard way. What they do with it in order to get it to its destination is
entirely up to them. This is a very simple, important concept. If you understand this, then when someone
says something like "SONET is the future of the modern telephone network" you'll know that they're
talking (mostly) about what goes on inside the phone company, and between phone companies. They are
probably right, but it is also true that "ISDN is the future of the modern telephone network" especially if
you mean B-ISDN. They're just the future of different parts of the telephone network.
ISDN: The Big Picture
Let me say at this point, that B-ISDN is not ATM, nor vice-versa. ATM is Asynchronous Transfer Mode.
It is a very important part of B-ISDN, arguably the most important part. But it is just one part. (Also note
that ATM was not used at all in original ISDN.
Some people refer to older ISDN as Narrowband ISDN, which is what I will do. This is sometimes
abbreviated as N-ISDN, however National ISDN is also sometimes abbreviated this way and I'd rather
avoid the confusion.
So to reiterate, ISDN and B-ISDN are concerned with what goes on inside of your house, and to some
extent, what goes on between your house and the phone company. It defines the physical network in your
house, what kind of wires can be used, what the phone jacks look like, how fast the data network is, how
the phones or other phone devices communicate with each other, and how the communicate with the
phone company. Again, to confuse things, ISDN defines how your phone communicates with your
neighbors phone and with telephones around the world, but this only covers things like my phone telling
your phone who I am.
All about Narrowband ISDN
Access Interfaces Provided
You might be tempted to call these the "services" provided by the phone company, but you have to be
careful using the word service with ISDN, because it means things like audio, video, etc. - higher level
services. What you can get from the phone company in terms of service are varying data rates, and
various combinations of separate channels for data and signaling. These are access interfaces.
ISDN was designed around the notion of separate channels at 64Kbps. This number springs from the fact
that that is essentially the data rate at which the analog lines are sampled at (8000 samples per second, 8
bits per sample) for the phone company's
IDN. ISDN is essentially combinations of these channels, and also slower 16Kbps channels used only
forsignaling.The 64Kbps channels are called B channels. The 16Kbps channels are called D channels.
The names of the channels allegedly spring from analog circuits being called A-channels (A for analog).
The next type of channel to come along got labeled B, which also happily can stand for binary (some also
say it is the Bearer channel). The D channels were at one time called delta channels, because of their
relationship to the B channels, but that particular greek symble being hard to type, it became D.
There are two main interfaces, Basic Rate, and Primary Rate. The Basic Rate Interface is intended for
home use, and Primary Rate is intended for businesses.
The ISDN Reference Configurations
You can't talk about ISDN without knowing about the reference configurations. This gives you the basic
vocabulary for talking about all of the pieces of ISDN. There are reference configurations for all different
pieces of the ISDN network, and lots of different configurations. The following diagram shows two of
the most commonly referred to configurations. The networks will actually look more complicated than
this; the diagram just serves to apply standard labels to the different parts of the network you'll encounter.
Power
One important issue of ISDN that we aren't used to worrying about is power. Currently the analog phone
system provides it's own power - if the power goes out, your phone still works. However, ISDN requires
more power than the phone company is in the habit of providing. Because of this, each of your ISDN
devices must get it's power some other way. Under normal circumstances, what will happen is that your
NT1 will be plugged in to your house's power. All the ISDN devices in your home will get power from
the NT1. This is one of the reasons that ISDN uses a four wire system for the network - it allows separate
lines for receiving and transmitting and at the same time allows for transmission of power.
Also, those other four unused wires in the 8-pin ISDN jack are specified in the standard to be used for
alternate power supplies. Whether these will actually be used remains to be seen, but it is possible that a
UPS (uninterruptible power supply) could be added to your NT1, and it could use these auxilliary lines to
provide guarenteed power. Note that one of these alternate power supplies is designed to go from the TE
to the NT.