What Is Integrated Services
Digital Network (ISDN)?
Integrated Services Digital Network (ISDN) has been under development for a couple of decades
but has been hampered by the lack of applications that can use its speed. It wasn’t until recently
that telecommuting, video conferencing, and
small offices/home offices (SOHOs)
have needed the
capabilities that ISDN offered. Another factor slowing the development of ISDN was that it was
somewhat proprietary in nature. However, this ended when National ISDN-1 became available
in 1992. National ISDN-1 is a standard switch type used by ISDN providers. This standard
enabled vendors to interoperate among devices.
Different service providers adopted different standards, but on a national basis,
so several different ISDN switch types are now “standard.”
Before getting into what ISDN is and does, you first need to understand how our traditional,
or
plain old telephone service (POTS)
, operates. Typically, you pick up the telephone receiver,
you dial the number, and the party answers at the other end. Your voice—which is an analog
wave—is converted into a digital signal through a process called
pulse code modulation (PCM)
.
PCM samples your voice 8,000 times per second and converts the audio level into an 8-bit value.
This 64Kbps channel, or
DS0
, is multiplexed with 23 other channels to form a T-1.
If you do the math, you’ll notice that a T-1 is 1.544Mbps; however, 24 64Kbps is only
1.536Mbps. Where are the other 8Kbps? Before we answer that question, think of the purpose
of a T-1. Each telephone call in the past required two copper wires to carry the voice traffic. A
T-1 was originally designed to carry 24 individual voice calls on the same wire. Each voice call
received its own channel. The underlying technique to carry all 24 channels on the same wire
is called time division multiplexing (TDM). TDM breaks up the circuit into 24 separate channels
and provides a distinct time slot for each.
Now back to the math. Each of the 24 channels is composed of 8 bits, for a total of 192 bits
(8
×
24). According to the Nyquist theorem, we know that we need to sample at 8,000 times per
second to replicate the human voice. Therefore, to produce all 24 channels, the entire 192 bits
must be transmitted 8,000 times each second, for a subtotal of 1,536,000 bits per second, or
1.536Mbps (8,000
×
192).
Specifically, Nyquist states that we should sample at twice the highest data rate
of the sampled signal, and rounding the voice spectrum up to 4Kbps gives us
the 8,000. The 8 bits for each channel comes from the 256 sampling
levels
used
at each sample time.
Now for the missing 8Kbps. A single framing bit is added between each 24-channel frame. Therefore,
an additional 8,000 framing bits are sent each second (remember the sampling rate for human
voice), raising our total to 1,544,000 bits per second, or 1.544Mbps (1,536,000 + 8,000). This
number is the bit rate of the line itself, and the one you commonly see with reference to a T-1 circuit.
Because 8,000 of the bits sent each second are used for framing and not data, however, the maximum
data you could theoretically put on the wire is the smaller number: 1.536Mbps.
ISDN differs from POTS in a couple of ways. First, ISDN data starts off as digital signaling,
so there is no analog-to-digital conversion. Second, call setup and teardown is accomplished
through a dedicated 16Kbps channel also known as a D (data) channel. By using “out of band”
signaling, you have the entire 64Kbps for data. This leaves one or two B (bearer) channels for
your data or voice traffic that does not have an intrusion on the line for clocking or error control.
ISDN then provides unadulterated bandwidth to end users.
ISDN benefits include improved speed over an analog modem, fast call setup (one second or
less, typically), and lower cost than a dedicated point-to-point circuit. DSLs and cable modems
are replacing ISDN in some areas and will continue to do so as they fit the need for high-speed
Internet access to the home. However, ISDN has some advantages over these newer, faster technologies.
Here is a list of the advantages that ISDN can provide:
Ability to dial into many locations simultaneously
High-speed dial-up services for traveling telecommuters
A fault-tolerant link for dedicated lines
Remote SOHO connectivity
Video conferencing