Integrated Services Digital Network (ISDN)
refers to a set of communication protocols proposed by telephone companies to
permit telephone networks to carry data, voice, graphics, music, and video. ISDN
was developed to permit faster access over existing telephone systems without
the additional call setup time. It enables the simultaneous transmission of
voice and data signals using end-to-end digital connectivity.
Because ISDN uses existing phone lines, it
requires that the central office be within a certain distance, which limits
service availability
With ISDN, bearer channels
(B channels) carry voice and data signals. These channels occupy a bandwidth of
64 kilobits per second (kbps). Some switches limit B channels to a capacity of
56 kbps. A data channel (D channel) handles signaling at 16 kbps or 64 kbps,
depending on the service type.
ISDN Components
ISDN components include:
- Terminal equipment (TE)
- Terminal adapters (TAs)
- Network-termination devices
ISDN terminals come in two types terminal
equipment (TE) and terminal adapters (TAs). TE refers to end-user devices such
as digital telephones or workstations:
- Native ISDN terminals are referred to as terminal equipment type 1 (TE1). TE1s connect to the ISDN network through a four-wire, twisted-pair digital link.
- Non-ISDN terminals such as DTE that predate the ISDN standards are referred to as terminal equipment type 2 (TE2). TE2s connect to the ISDN network through terminal adapters. The ISDN TA can be either a standalone device or a board inside the TE2.
If the TE2 is implemented as a standalone
device, it connects to the TA through a standard physical layer interface.
Examples include EIA/TIA-232-C, V.24, and V.35. The TA performs the necessary
protocol conversion to allow non-ISDN (TE2) equipment to access the ISDN
network.
Beyond the TE1 and TE2 devices, the next
connection point in the ISDN network is the network termination type 1 (NT1).
At the customer site, the ISDN local loop is
terminated using a network termination type 1 (NT1). These are
network-termination devices that connect the four-wire subscriber wiring to the
conventional two-wire local loop. The NT1's responsibilities include line
performance monitoring, timing, physical signaling protocol conversion, power
transfer, and multiplexing of the B and D channels.
To delineate areas of responsibility within
an ISDN network, ISDN defines a number of reference points within the network.
Reference points define logical interfaces between functional groupings, such
as TAs and NT1s. Reference points provide for a common term usage when
troubleshooting a component of the local loop part of an ISDN network. Vendors
and providers of ISDN equipment use the reference points R, S, T, and U. Table
12-1 illustrates the relationship between devices and ISDN reference points.
ISDN interfaces
NT1
It creates the T interface for premise devices from the U interface. In the original CCITT concept, the NT1 was provided by the Telco as part of the ISDN service.
NT2
This device would do the switching, permitting more than the standard eight devices to
share the T bus by creating perhaps multiple S buses. Therefore, an ISDN (TE) device
can’t really tell if it is connected to an NT1 or NT2.
TE1
The terminal equipment type 1 (TE1) is a standard (there is that word again) ISDN
terminal that is capable of dealing with the B and D channels. In other words, it can
interface with the S/T bus.
TE2
The terminal equipment type 2 (TE2) is a standard device having an RS-232 or V.35
interface. It may be intelligent but doesn’t have an ISDN interface capable of handling the D and B channels.
TA
The terminal adapter (TA) is the semi-intelligent device that lets a TE2 connect to the
S/T ISDN interface. The primary function of the TA is to run the ISDN interface for our
TE2. The functionality varies widely due to the manu-facturers. Some are simple and
support only one TE2; others support two TE2s and an analog telephone.
The U Interface
The U interface is unique to North America and the “open” telephone network
interconnection. This U interface can be either a two-wire or a four-wire connection.
The Physical Interface
Another clever design feature of both the S/T and U interfaces is that they all use the
same RJ-45 type connector.
The S/T and U interfaces carefully select the pin assignments so that accidentally
plugging an S/T connector into a U interface and vice versa doesn’t hurt anything.
Applications of the ISDN Interface
Multiple channels
The plan is to provide access to every
possible home device. The original concept was for up to eight devices.
Telephone
The
obvious starting point is the telephone, which is now a digital telephone.
Instead of
the
telephone conversation being analog from the handset to the central office
where it
becomes
digitized, the conversation can be digitized directly at the source and passed
digitally
all the way through the network to the other end.
Digital Fax
Fax
machines now have to be digital. Therefore, the Group IV fax standard specifies
64
KB/s
fax operation.
Analog Fax
Analog
fax machines use a modem, so it has to plug into the telephone that
would take the analog modem tones and digitize them at 64 KB/s.
Computer/Video Conferencing
Our
computer or video conferencing equipment can use one of the 64 KB/s or bond
both
Bearer
channels together for a 128 KB/s digital channel across the network.
Signaling
The
primary function of the data channel is to provide for signaling, that is, the
setting up
and
tearing down of the switched Bearer channels. At 16 KB/s, the data channel has
more
bandwidth
than is needed for signaling alone. Therefore, when it is not being used for
its
primary
and high priority signaling function, it could be used for other things.
Telemetry
This
feature has never been well defined. The concept is that many household devices
can
be connected to the data channel. This can include an energy management system
that
would let the power company selectively turn off the refrigerator or air
conditioner
for
an hour or so at peak usage time.
Packet Switching
The
16-KB/s data channel has bandwidth to spare. Therefore, the local carrier can
provide
a data service on this excess bandwidth. X.25 is just maturing and is the
logical
packet
switching technology to offer. As it turns out, all the data on the data
channel,
whether
it be signaling data, telemetry data, or X.25 data, are always sent in packets
anyway.
DDR Concepts
Dial-on-Demand Routing (DDR) addresses the
need for intermittent network connections over circuit-switched WANs such as
ISDN. With DDR, all traffic is classified as either interesting or
uninteresting. If traffic is interesting, the packet is passed to the
interface, and the router then connects by dialing to the remote router thus,
the name dial-on-demand routing. DDR is used to cause the router to dial or
receive a dialed connection.
Configuring DDR
It is
important to remember that DDR cannot dial until some traffic is directed out
the dialing interface. For this to occur, two steps are required:
- Define what traffic will trigger the link to dial. This is known as specifying interesting traffic.
- Specify how DDR traffic is routed out the dial interface. This involves configuring a static route so that the router will queue packets to be sent out the dialing interface.
Next, how will the dialing occur, and for
how long? DDR must know what number to call to make the dialed connection and
must know how long the call should remain up. These details are defined with
the following two steps:
- Define a dialer string on the dialing interface. (Dialer maps may be used when dialing to multiple destinations.)
- Define a dialer idle-timeout.
The
dialer string defines the number to call to get to the next-hop router. After a
dialed connection is made, any type of packets can be routed across the link;
however, only interesting traffic will keep the link up. The idle timer counts
the time since the last interesting packet traversed the link. If the time
specified as the idle-timeout expires, meaning that no interesting packets have
traversed the link for the specified amount of time, the connection is brought
down
ISDN
therefore was a great technology-driven service that didn’t really solve a
business
or home need. It is little wonder that ISDN is not widely implemented or used,
but there
are,
as we have noted, some clear exceptions.
The most notable is video
conferencing. Internet
access is also a possibility, but ISDN can’t compete with xDSL technology in performance
for the cost.
Sources:
Broadband Telecommunications Handbook
