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ISDN is a form of legacy communications that can transmit voice and data signals over the same line, using digital methods of transmission coursed through plain old telephone service (POTS). In the early days of the internet, ISDN lines were very popular with businesses because they were faster than the more basic lines available at that time.

What does ISDN stand for?

ISDN stands for Integrated Services Digital Network. The service was popular with businesses in the 1990s because it allowed fax machines, card readers, modems, and telephones to be connected to one line, via the public switched telephone network (PSTN).

ISDN also offered a faster call setup than other modems, getting up and running in just a few seconds. Today, internet connections are "always on"; but in the early days, copper telephone lines relied on modems that required a long, and error-prone, handshaking process after dialing into the Internet Service Provider.

This speed and convenience made a big difference to business communications when ISDN first came out. The increase in throughput, for example, meant VoIP calling was more practical than it was on slower connections.

What is the difference between ISDN and DSL?

There are a few key differences between ISDN and DSL. For starters, DSL transmits data far faster than an ISDN line can.

That’s because ISDN is a dial-up service that goes through a singular line. DSL connections never need to dial. They are sometimes called “always-on connections.”

Because of that, DSL sends its packets at speeds up to 100 Mbps, while ISDN tops out around 128 Kbps.

While both of these provide the same service, they do so in radically different ways.

History of ISDN

ISDN was born out of necessity. Analog phone networks failed constantly and proved to be unreliable for long-distance connections.

Sometime in the 1960s, the system began to change over to a packet-based, digital switching system. Most notably, Robert Aaron of Bell introduced the T1 system, which allowed a pair of twisted pair lines to carry 1.544 Mbit/s of data over a distance of about one mile. This was used in the Bell network to carry traffic between local switch offices, with 24 voice lines at 64 kbit/s and a separate 8 kbit/s line for signaling commands like connecting or hanging up a call.

The UN-based International Telecommunications Union, or ITU, started recommending ISDN in 1988 as a new system for operating companies to deliver data.

It still took time for communication providers to begin to offer ISDN. This was mainly because both major companies at the time were on separate operating systems. By the 1990s, the National ISDN 1 (labeled N1-2 for short) was created.

While this innovation could improve the quality of communications, an agreed-upon standard still took time to figure out.

Finally, manufactures like Motorola and USRobotics decided to make the transition easier for everyone.

ISDN then launched across the US. It provided consumers with better pricing and higher-bandwidth internet access.

How does ISDN work?

ISDN works by using copper wires to connect to many different channels via dial-up service. The channels can be put into either one of two categories:

  1. B (bearer) channel - this is a channel that transmits voice data, as well as information about the users
  2. D (delta) channels - this is a channel that transmits other types of data, including signaling and packet networking

Together, these two types of channels help manage the information traffic as the “internal” phone system connects to the external plain old telephone service (POTS).

What are the types of ISDNs?

There are two types of ISDN lines:

  • PRI: Primary Rate Interface
  • BRI: Basic Rate Interface

Primary Rate Interface (PRI)

This is the faster of the two variants, but it’s also more expensive. A technology that’s been around since the 1980s, it’s a circuit technology that lets organizations use a single line for up to 23 different transmissions at the same time.

In the PRI setup, you get multiple B channels and D channels which are collectively supported by speeds of up to 2.94 Mbps.

Basic Rate Interface (BRI)

Of the two variants, this is far more affordable. But it’s not called “basic” for nothing—while you can most certainly use it for more than one transmission at a time, it can’t adequately address the needs of larger operations.

The BRI setup also lets you make use of B and D channels. The difference is that this interface is limited to two B channels and one D channel, with a maximum speed of 128 Kbps.

How to set up ISDN

Setting up an ISDN connection involves using a serial port and plugging in the telephone company line.

The process of setting up ISDN involves:

  • Loading the modem driver disk and programming the modem
  • Pointing the modem toward the right phone numbers
  • Setting your connection speeds for each line
  • Directing your modem to dial your ISP (Internet Service Provider) — this phone number should be provided by your ISP
  • If necessary, set your modem for BONDING (the ability to access higher speeds by allowing your modem to dial both phone numbers at once)

Is ISDN still in use today?

Short answer? Sort of.

Long answer -

ISDN is falling out of favor

ISDN may very well still be available in some parts of the world, where getting a broadband connection isn’t really possible just yet. But beyond that, there are few reasons for new businesses, or even established ones, to choose ISDN lines and hardware that’s compatible with it. In fact, it’s barely even ideal for use in the average modern household anymore.

The need for speed

Today, we have much faster connections. ISDN’s speeds of 128kbps to 2.94Mbps may have been blisteringly fast compared to the 56kbps available to home users in the 1990s; but these days, they’re hardly a match for always-on cable and fiber broadband.

The fight for flexibility

In addition, this technology doesn’t give organizations much room for flexibility in terms of expansion, relocation, and the like. This is because ISDN wasn’t able to evolve with the needs of 21st-century businesses, which have come to value agility, mobility, and geographically distributed workforces. Instead, it’s stuck to a model that forces employees to stay put in one location if they want to use the company network.

It’s not worth the investment anymore

Between its inability to catch up with the speed of broadband and its failure to grow with the businesses it used to serve so well, the idea of spending money on the specialized (and pricey!) ISDN technology is becoming harder to justify.

This is why it isn’t really a surprise that ISDN is being phased out in many areas of the globe. British Telecom, for example, announced that they would stop selling new ISDN lines in 2020, with the goal of switching the service off entirely in 2025. In other words, this is now largely recognized as an obsolete technology.

It won’t be long before the world as a whole will stop relying on ISDN completely.

Are there reasons for people to still use ISDN?


Some of the reasons people still choose to use ISDN are:

  • It offers multiple digital services that operate through the same copper wire
  • Digital signals broadcast through telephone lines.
  • ISDN provides a higher data transfer rate.
  • Can connect devices and allow them to operate over a single line. This includes credit card readers, fax machines, and other manifold devices.
  • It is up and running faster than other modems.

Of course, the above are only true if there is limited or no access to DSL and WAN (wide area network) internet.

What does the world after ISDN look like?

Telephony providers are increasingly moving over to using IP calling for all of their connections. Modern PBX systems are designed with this in mind and are generally not expected to work with older lines.

In fact, a growing number of organizations have begun to transition all their communications to the cloud since many solutions of that type make it easier to create customizable virtual work environments.

Cloud-based communications are the future

Cloud computing has made it possible for providers to develop solutions that integrate voice, video, chat, contact center, and enterprise-class API solutions into one global, secure, and reliable platform.

Thanks to technology such as this, co-workers are more connected and productive no matter where they are in the world. Collaboration is easier, and problems are solved more quickly, and possibilities become endless.

So if you want to future-proof your company and avoid wasting money in the long term, you'll need to choose a solution that’s fully compatible with the way people work today, as well as the way they will work tomorrow.

At 8x8, we offer all-in-one business communications options—not just voice calls, but video conferencing, team messaging, and contact center solutions. Whether you're looking to launch a new location or scale your existing communications efforts, we're here to help you. Contact us today to schedule a demo or request a free consultation.