13-08-2012, 12:33 PM
All about Narrowband ISDN
[attachment=31245]
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 for signaling. 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 Achannels
(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.
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.
Your house's network (S/T reference points)
The phone "network" inside of your house will be somewhat more complicated with
ISDN than it is today, in that it will be a true data network. This network is often
called the customer-premises installation or CPI. This network will typically consist
of telephones, computers, fax machines, videophones, and an endless list of pie-inthe-
sky applications, like controlling your thermostat thru ISDN.
Hardware
This is layer 1 (the physical layer) of the S/T bus. This defines the physical network
in your home. The most obvious things this defines, as far as a customer is concerned,
are wiring, connectors, and power, so I'll talk about those first.
ISDN uses a phone jack that looks just like the standard phone jacks in use today,
except that it is a bit wider. Instead of the older 4-pin jacks (which only used two
wires), ISDN uses an 8-pin jack (which only uses four wires). The CPI is based on a
four wire scheme, two wires for transmitting, and two for receiving (which means
you'll probably have to rewire your house). These wires are typically copper wiring
of some sort, and can be longer than most users will ever need.
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.
[attachment=31245]
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 for signaling. 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 Achannels
(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.
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.
Your house's network (S/T reference points)
The phone "network" inside of your house will be somewhat more complicated with
ISDN than it is today, in that it will be a true data network. This network is often
called the customer-premises installation or CPI. This network will typically consist
of telephones, computers, fax machines, videophones, and an endless list of pie-inthe-
sky applications, like controlling your thermostat thru ISDN.
Hardware
This is layer 1 (the physical layer) of the S/T bus. This defines the physical network
in your home. The most obvious things this defines, as far as a customer is concerned,
are wiring, connectors, and power, so I'll talk about those first.
ISDN uses a phone jack that looks just like the standard phone jacks in use today,
except that it is a bit wider. Instead of the older 4-pin jacks (which only used two
wires), ISDN uses an 8-pin jack (which only uses four wires). The CPI is based on a
four wire scheme, two wires for transmitting, and two for receiving (which means
you'll probably have to rewire your house). These wires are typically copper wiring
of some sort, and can be longer than most users will ever need.
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.