What is ISDN (Integrated Services Digital Network)? - Your ISDN Guide
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What is ISDN?

With so many different methods of bandwidth and digital communication, it’s hard to keep track of each one. For most people, ISDN is probably at the top of that list. So what is ISDN? Simply put, ISDN (short for Integrated Service Digital Network) is defined as an a set of communication standards for the transmission of data, voice, and signaling. It is a circuit-switched telephone network system that transmits packets of data across copper lines. Essentially it was designed to migrate outdated landline technology to digital.

ISDN connections are known for providing better data speeds and higher quality than traditional analogue connections. Faster speeds and better connections allow data to be sent more reliably than the antiquated systems of the past, with little to no distortion, providing a better overall experience.

History of ISDN

ISDN was born out of necessity, when the analogue phone networks of the past failed time over time, and proved to be an unreliable source for long-distance connections. Sometime in the 1960’s the system began it’s conversion to a packet-based, digital switching system that the US voice communications system is built on today.

The UN-based International Telecommunications Union, which is responsible for standardizing international telecommunications, started recommending ISDN in 1988 as a new system for operating companies to deliver data services. Despite these recommendations it still took time for communication providers to begin their implementation of ISDN. This wasmainly because both major companies at the time were on seperate operating systems, which made it hard for them to interoperate.

By the 1990’s, the National ISDN 1 (labeled N1-2 for short) was created, and an effort to begin standardizing the system was started. Despite the knowledge that this could improve the quality of communications, an agreed upon standard still took time to figure out. It was finally adopted some years later when manufactures like Motorola and U.S. Robotics decided to configure standards for equipment that would make the transition easier on everyone. Thus, ISDN was implemented across the US, providing consumers with better pricing and higher-bandwidth internet access.

In recent years, ISDN has been replaced by broadband internet access connections like DSL, WAN and Cable Modems. That said, it is often still used as a backup in the instance that a main line fails.

How ISDN Works

how ISDN works diagramIt’s easy enough to define ISDN, but do you know how it works?

Most people use ISDN for high speed internet when options like DSL or cable modem connections are not available in their area. In general, setting up ISDN is something you’ll want to work in conjunction with your Internet Service Provider (ISP), but many of the steps are self explanatory and can easily be done from your home.

Traditionally, your ISDN will be plugged in through a traditional POTS (Plain Old Telephone Service) line that can access both phone numbers at once. So, you’ll have to make sure you have a working POTS line as well as assigned phone numbers to begin. After that, you’ll follow the steps below to get your voice and data communications up and running.

ISDN Setup

Setting up an ISDN connection involves using a serial port and plugging in the phone line, similar to that of an analog modem. But that’s where the comparisons between the two end. The process of setting up ISDN involves:

  1. Loading the modem driver disk and programming the modem 


    • Pointing the modem towards the right phone numbers
    • Setting your connection speeds for each line
  2. Setting your modem to dial your ISP (Internet Service Provider) — this phone number should be provided by your ISP
  3. If necessary, setting your modem for BONDING (the ability to access higher speeds by allowing your modem to dial both phone numbers at once)ISDN operating speeds

Types of ISDN

There are two different options on the ISDN network — BRI (Basic Rate Interface) and PRI (Primary Rate Interface). The major difference between BRI and PRI is the level of service and reliability that they offer. To sum them up quickly:

  • BRI is the lower tier of service that only provides the basic needs at a lower cost
  • PRI is the main service that provides a better connection, more reliable service, and faster speeds overall

Both PRI and BRI ISDN (Integrated Service Digital Networks) use B channels for transmitting data and D channels for other forms of communication. The difference primarily lies in the number of channels they use to accomplish their transmissions.

For instance, BRI uses only 2 B channels and one D channel and has a maximum speed of 128kbps. PRI, on the other hand, varies in the number of channels it uses given its location and can be adjusted for speeds up to 2.94Mbps. In the US, PRI tends to use 23 B channels and just one D channel, but the amount can be doubled, if necessary, for faster speeds and to provide a back-up D line in case of emergencies.The flexibility with PRI, not to mention the faster network speed, is just one of the reasons why it’s superior to BRI and used by businesses more often. The faster bandwidth speeds and ability to provide a backup connection in case of emergency makes it a clear winner for businesses that rely on their internet connection.

BRI vs PRI diagram in relation to ISDN

Speed: BRI vs PRI

ISDN comes with a set speed for each option. or instance, BRI provides 128 kbit/s over a standard copper wire, broken down into 64kbit/s over the B channel and 16kbit/s over the D channel. Using a PRI can potentially double those speeds.

To breakdown the bandwidth capabilities even further, for BRI there are two separate 64kbit/s channels, but no specification for concentrating them into a 128kbit/s channel.

The two competing standards make use of both B channels to access the 128 Kbps throughput and include:

  • BONDING (Bandwidth On Demand Interoperability Group) — a set of ISDN protocols that allow users to use more than one ISDN B channel. This is also known as multilink, channel aggregation, or load balancing
  • Multilink PPP — a method for splitting, recombining, and sequencing data across multiple links. This is not specific to ISDN and can be applied to other technologies

PRI on the other hand is a T1 circuit that supports 23 B channels at 64kbit/s and 1 D channel at 64kbit/s (as previously mentioned). This gives users the ability to aggregate several B channels to form a larger capacity network.

BRI vs PRI speed visual in relation to ISDN

Benefits of ISDN

So why do people use ISDN? It first started as an alternative to your dial-up connection that provided higher internet speeds comparatively (though at a higher price point).  In order to access the internet with ISDN, users had to connect through a digital modem provided by their service provider.

People still use ISDN for internet access in small, rural areas where broadband internet isn’t an option, but for the most part ISDN for internet access is beginning to be phased out.

In that case, you might be wondering what the advantages of ISDN networking are? Some of the reasons people choose ISDN are:

  • It offers multiple digital channels that operate concurrently through the same copper wire
  • Digital signals broadcast through telephone lines
  • ISDN provides a higher data rate
  • ISDN can connect various devices together 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 in a relatively shorter amount of time than other modems

The cost of ISDN is much higher than some traditional broadband options today, leading people to search for alternatives. One popular alternative is VoIP.

Alternatives to ISDN

What is VoIP? VoIP (aka Voice over Internet Protocol), takes audio signals and turns them into digital data that can be transmitted across the internet. VoIP is beginning to take the place of traditional telephone lines by providing users with a way to make phone calls over the internet without needing access to a physical telephone line.

how VoIP works diagram

There are several different ways users can access the benefits of VoIP, which include ATA (analog telephone adapter), IP Phones, and of course computer-to-computer.

How does ISDN compare with VoIP? Check out the chart below for a streamlined comparison of the two offerings.




Difficult to set-up and easy for the system to fail if a secondary system is not in place

Takes seconds to set-up, low call failures, and can be used as both a primary or secondary solution


Requires external, physical circuit to be installed

Plug into existing internet connection with no external sources needed


Tied to a contract/geographical area code and changes can take a number of weeks to implement

Make changes instantaneously and it is not tied to a geographical location


Expensive to implement and costly to make calls

Low install costs and between 40-90% cheaper to make calls


Outdated with a confirmed end-of-life timescale

Easily customizable, scalable, and cost effective

It’s clear that the VoIP benefits truly outweigh those of ISDN. The flexibility and cost effectiveness alone make it with a second, third, and probably even fourth glance. The above table does a great job at outlining the differences between the two and highlighting the weaknesses an older offering, like that of ISDN.

Shockingly, VoIP users are primarily home and mobile users, but businesses are starting to catch on. Enterprise customers have become the fastest growing group, comprising 30% of the user base of VoIP in just a few years. Why? When you go beyond the promise of a lower cost and flexibility, it’s obvious that VoIP offers a whole lot more than its landline cousin, ISDN.

For a growing number of businesses, one of the most important needs is implementing services that will easily grow and expand as business does. That wasn’t a possibility before, but VoIP is making it a reality. Where it was once clunky, complicated and costly to add lines and take advantage of features, VoIP offers a wide variety of benefits that have businesses scrambling to take advantage. So, what are those benefits?


For businesses, there are few options that compare to the savings of VoIP, which given the situation can average a savings between 50 percent to 75 percent. One huge story of cost saving came from Dell, who used a mobile VoIP workforce and saved a reported $39.5 million. Thanks to it’s cost-savings, approximately 31% of businesses use VoIP in an effort to reduce their expenses and take advantage of the logistical benefits.

In some cases, businesses can take advantage of an auto-attendant feature that potentially allows companies to repurpose their secretary roles, saving an estimated $45,000 a year. Many come built into VoIP where an offering like ISDN would require you to pay extra depending on the features you wanted to add.

If you take a good look at how much landlines cost compared to VoIP, it becomes pretty clear why so many businesses are making the switch. A landline phone system could cost an average of $50 a line, which included only local and domestic calls. A VoIP line? Well, it can save as much as 60%, and it comes with a whole host of other offerings that the ol’ landline didn’t.

cost savings from switching to VoIP


Unlike traditional voice services, you can take your VoIP anywhere as long as you have internet access. It works similarly to your email account, where you simply sign on with your ID and use the network connection as your phone line.

For travelers, this is a game changer, allowing them to make calls from their hotel, or anywhere else for that matter, without worrying about costly long distances charges.

Beyond being able to make a call from anywhere, VoIP gives employees far more options than were ever offered with traditional analogue phones. Unified messaging is a major bonus for VoIP users, a feature which allows remote employees to tap into their business phones through various applications.

A business line gets sent to an employee’s personal phone device, which means that co-workers can get in touch with him or her for quick answers and fast resolutions to problems. It’s an accessibility feature that makes the life of remote workers easier than ever before.


Need to add a line? VoIP makes that easier than ever— this is thanks in large part to the fact that you no longer need to have a physical land line installed. Installing a new VoIP line is basically like getting a new username assigned to you. It can be attached to any device, at any time, and is ready for use in just a few minutes.

This comes into play in a big way for growing companies. The ability to add a new employee line at the click of the button allows for businesses to expand without worrying about tacking on some unexpected additional costs.


With VoIP you control a lot more than you could with traditional landlines. If you’re a business that needs a number for every hub in the country, you can pick and choose what area codes you want to use, making it seem like you have a physical presence even when you don’t.

Most of us won’t even pick-up the phone for an out of area phone number. So, if a business is trying to contact it’s ideal demographic, being able to assign a local phone number where they’re located is a game changer. In the past, businesses were primarily tied to the location they were physically in, making it harder to connect with clients and consumers nationwide.

In addition to choosing your area code, VoIP allows large and small businesses the added benefit of auto-attendant, giving consumers the perception that they’re bigger than they are. A customer might call in and get menu options, but have no idea they’re going to be routed to the same line, and same person, no matter what number they pick. Still, it leaves them feeling like they’re contacting a well organized and professional organization.


Most people are shocked to find that the voice quality on a VoIP line is virtually the same as a landline. As long as users are calling from an area with a reliable internet connection that has good bandwidth, they should experience sound quality that is just as good, if not better, than traditional landlines. Need proof? Just look at Nextiva’s uptime and performance over the last 90 days:

Nextiva's uptime for VoIP

The only issues with VoIP quality is when you run into internet connection issues. Bad bandwidth will equal a bad connection, which means VoIP may now work as well in rural areas that have limited internet connectivity. 


The reason so many businesses are switching to VoIP is largely because of the features it offers. For a significant cost savings, businesses can add:

  • Call forwarding
  • Call waiting
  • Voicemail
  • Caller ID
  • Three-way calling
  • Etc.

Beyond all the call-related features, VoIP allows users to transmit data over the line even while they’re in the middle of a call. Need to send a document to a colleague while talking to them on the phone? Done. Want to send photos to your mother while chatting about your vacation? VoIP can do that as well.

Likely one of the most sought after features for businesses is the video conferencing option. It’s easier than ever to get the entire office on a call, even when you have traveling and remote employees, with the VoIP video feature. Everyone from around the world can virtually sit in the same room, share files, and see each other face-to-face while chatting about the next big company decision. It’s not only the most useful feature, but probably one of the biggest cost-savers that VoIP offers.

VoIP Solutions

Of course, what you can do with VoIP all depends on the cloud network provider you choose and the features they support. With Nextiva, you get the full suite of options at your fingertips.

Even a quick glimpse at what we offer makes it clear we offer the top features modern businesses are most in need of:

  • Anonymous Call Rejection
  • Auto Attendant
  • Professionally Recorded Greeting
  • Busy Lamp Field
  • Selective Caller Acceptance
  • Call Forwarding Busy/No Answer/Not Reachable
  • Call Parking
  • Voicemail to Email Messaging
  • Conference Bridge
  • Call Recording
  • Caller ID
  • Call Waiting
  • Hoteling Host
  • Dial-by-name Directory
  • Attendant Console

Nextiva even provides the option for Nextiva Anywhere, a feature that allows you to connect your calls to your mobile or home phone, making remote employers and employees happier than ever.

Even better? Nextiva Toolbar, which allows users to make and receive calls from their desktop or laptop computers. Access your contacts at the click of a button and change the controls on your call forwarding and voicemail options right through your email client or browser.

Nextiva Toolbar serves as your softphone, otherwise known as a software program for making phone calls through your VoIP service. It’s one of the many reasons that users are so readily switching to VoIP lines. It works just like your mobile or hardline phone, but it allows you to stay connected to your laptop, send data, and keep up with your email at the same time. A softphone, via Nextiva Toolbar, will allow employees to work from one central location that goes everywhere with them— their laptop.

Nextiva app screenshot

With its suite of features, Nextiva provides the kind of cloud communication service that businesses are expecting from their service provider these days.


It’s clear that VoIP is changing the way people experience voice communications. And more than that, it’s giving businesses more flexibility than they’ve ever had before. Adding users through an online portal, connecting cell phones to business lines, and allowing for seamless team collaboration— it’s checks all the boxes. VoIP not only makes life easier, but it reduces the upfront and monthly costs businesses once had to shell out in order to have traditional phone lines.

The antiqued options, like ISDN, no longer give users what they need. Unified communications is the way of the future, so it’s no surprise that VoIP is taking off like a rocket. Are you ready to add it to your business?

Learn more about the benefits of VoIP and Unified Communications by visiting Nextiva.

About the author

Cameron Johnson is a market segment leader at Nextiva. Along with his articles on Nextiva's blog, Cameron has written for a variety of publications including Inc. and Business.com. Cameron was recently recognized as Utah's Marketer of the Year.

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Contact an Amazing Service
Sales Expert
(800) 799-0600

See how Nextiva will transform
the way you communicate.