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Ethernet technology based networks are playing an increasingly important role in industries. Our ETHERLINE® portfolio offers comprehensive solutions for your Ethernet network. You can always rely on our products to avoid downtimes and expensive breakdowns. The systems are made up of durable and robust cables and connection components for passive and active network technology to deliver an effective solution.

Salient Points and Applications of ISDN – Integrated Services Digital Network

ISDN (Integrated Services Digital Network) hasn’t found many takers for connecting to the Internet after the ADSL (broadband) networks became popular. But is connecting to the Internet, the only application of ISDN network? Do read on to find out how ISDN is relevant even in today’s age of high speed networks like Internet Leased Lines, VSAT and 3G – Third Generation Mobile networks!

There are two types of ISDN lines – ISDN BRI and ISDN PRI/E1/T1. This article is about ISDN BRI – Basic Rate Interface supporting two B channels of 64 Kbps each. If you are looking on detailed information about ISDN PRI – Primary Rate Interface / E1/ T1 circuits which can carry 30 channels in a single line, click here.

What is ISDN?

ISDN is the short form of Integrated Services Digital Network which was created as a communication standard for transmission of digital data, video, fax, etc along with voice signals over circuit switched PSTN (Public Switched Telephone Network) networks.

In short, ISDN BRI = 2B + D

Where ‘B’ refers to the 64 bit bearer channel. Each ‘B’ channel corresponds to a 64 Kbps voice channel (8 bits sampled at 8 Khz with G.711 encoding) and ‘D’ refers to the 16 (or 64, rarely) bit signaling channel. So, effectively, one ISDN line has a bandwidth carrying capacity of 128 Kbps and can carry two telephone calls (one in each channel) in the same line.

Advantages of ISDN Lines:

The primary advantage of ISDN line is that it is a Digital Line and hence the voice quality (audio fidelity) is better than analog circuits.

The ISDN line can carry voice (calls), data (Internet), video (video conferencing) and fax information in the same line.

Each ISDN line can provide a bandwidth of 128 Kbps both in the upstream direction as well as the downstream direction – this feature enables it to carry video communications in real time at both ends.

Since ISDN is a circuit switched network, there is little or no data loss while transmitting data (In packet switched networks, there is some data loss along the propagation path and hence information needs to be re-transmitted, sometimes).

Trouble-shooting a Digital Network is easier than trouble-shooting analog circuits. Faults can be rectified faster and the network performance is quite reliable and predictable.

When compared to ADSL, ISDN BRI can travel for longer distances and hence suitable for remote Internet access (in places where ADSL broadband technologies are not available).

With ISDN, the call set and up tear down is faster than analog circuits.

The latency is very less in Digital Circuits and hence it is suitable for transmitting latency sensitive applications like video, voice etc.

ISDN circuits can use a technique called ‘Channel Bonding’ to combine multiple lines and give a higher throughput.

Applications of ISDN:

1. Video Conferencing: ISDN circuits have been very useful for video conferencing. A video conferencing session can be established with a single ISDN line (128 Kbps) and a set-top video conferencing Codec (at both ends). The video conferencing can be done at a much lower bandwidth than a packet switched network (like Internet Leased Lines or high speed broadband) because, ISDN is a circuit switched network and hence there are no packet losses and ISDN can give equal throughput for both upstream and downstream transmissions (which is so critical for sending and receiving real time video in both the ends). Apart from the rental, the payment is only for usage (duration in minutes). So, if your company doesn’t conduct video conferencing sessions frequently, ISDN can still be a good choice! More over, the data network would be free to do what it needs to do – carry data without any congestion and delays. Multiple ISDN (Up to four, in most video conferencing systems) can be terminated simultaneously in video conferencing systems to enable a conference at higher bandwidth (hence higher quality) or conduct a multi-party video conference. Even if you have your entire video conferencing infrastructure on IP, but have to connect one customer on ISDN, you can still do so by using certain devices that convert ISDN to IP and vice versa.

2. Making Couple of Telephone Calls Simultaneously:

With an ISDN line, we can do a number of things simultaneously:

We can dial out two telephone calls (by connecting two analog phones to the adapter)

We can browse the Internet (128 Kbps) and if we get a call during browsing, the call will be connected in one channel (64 Kbps) and the Internet connectivity can continue in the other channel at a lower speed (64 Kbps)

We can send a fax in one channel and simultaneously talk to some one in the other channel (while the fax is being transmitted)

In some countries (like Germany), a lot of ISDN connections are used for basic voice communications (phone calls), just like how analog trunks are being used.

3. ISDN Video Phones: There are certain video phones that connect to the ISDN network and can make a video call (voice and video) to any other similar ISDN phone connected to the ISDN network. The video phones have a bigger screen and built in camera to show the output video as well as capture the video.

4. Broadcasting Industry: The broadcasting industry (especially the radio broadcasting stations) use ISDN network for carrying the audio signals from one station to another as well as from the covering point (sports stadium, etc) to the broadcasting point (radio station). But these days, IP networks are also popular for this application.

5. Digital PBX/ IP PBX: There are ISDN interface cards which allow ISDN lines to be terminated on a corporate PBX (both for Digital – Mixed PBX as well as IP PBX). So, the Digital ISDN lines can be terminated on the PBX and can be used for making outgoing calls as well as receiving incoming calls from any phone in the organization. This can be especially useful for companies that want to use ISDN for video conferencing, as the ISDN lines can be used for normal voice communications (through PBX) when they are not being used for video conferencing.

6. Back-up Internet Connectivity: The ISDN networks can be directly terminated on certain enterprise Routers that provide an automatic fail-over to the ISDN network for accessing Internet when the primary Internet Line (Internet Leased Lines, Broadband etc) is down. Sometimes, ISDN can also be the primary source of Internet connectivity for small companies in remote locations where the ADSL broadband networks are not supported.

Telecom Trunk Lines – Difference between Analog line, PRI/E1/T1 Digital Line & GSM Gateway

In this post, we will see a short overview of the various trunk lines available for connecting your PBX to the external world. We will have a look at analog lines, PRI/E1/T1 Digital lines & GSM Gateway, in particular.  This topic was requested by a reader in the comments section and hence I am writing an article on this topic. If you want to know more about any topic, leave a comment in this blog and I will try to write elaborately on the same.

Why do you need a PBX?

Before we go into the actual post, we need to understand some background about what is a PBX and why we need it. PBX actually refers to Private Branch Exchange. Imagine this situation – You have 100 employees in your organization, and all of them work from their office. Each one of them need to make outgoing calls (to partners, customers, etc) and also need to talk to their colleagues.

If there was no PBX, you have to procure 100 different analog trunk lines (from a Telecom service provider) to give telephone access to all of them. They can call each other, but those calls will also be charged as they are routed from the public telephone exchange. So, obviously this method is inefficient.

So, a small private branch exchange – PBX (Telecom switch) is kept at the organization to achieve two important purposes :

1. To share the incoming (trunk) lines from public telephone exchange to all employees as every employee is not going to be making outgoing calls/ receiving incoming calls all the time. So, instead of buying a trunk line for every employee, you might just buy one line for ten employees (sufficient in most cases) and share them.

2. To make internal calls (based on extension numbers) without any charge. Since the PBX is a mini-switch, it can route calls from one extension to another extension internally.

Analog Trunk Lines:

The phone line (land-line) that you get in your home is an example for an analog trunk line. At homes, one line would suffice. But in organizations, multiple such lines might be required (as the number of telephone users are more). These analog trunk lines carry a single channel of voice call per line (meaning, one person can make or receive a phone call from outside at any given point of time).

For terminating the analog trunk lines in a PBX, special analog trunk cards (12 ports, 24 ports, etc) are used. In an IP PBX, Analog Terminal Adapters (ATA) or PCI based Analog Cards may be used. These devices convert the analog calls into IP so that an IP PBX can understand them and transmit them over the network.

Separate rental / call charges needs to be paid for each analog trunk. Analog trunk lines are sufficient for small organizations which require a few lines.

PRI / E1 / T1 Lines:

For larger organizations that require a lot of trunk lines, procuring additional analog trunks might not be a practical idea as they need to buy separate (perhaps multiple) analog trunk cards to terminate these lines in their PBX (and) they need to pay separate rental for each line where the free call capacity for some individual lines might never be reached!

So, these organizations can procure a digital line called as PRI/ E1 (In Europe/ Asia) or T1 (In America). Each PRI / E1 line can carry up to 30 channels of voice communications simultaneously. They can do this by using multiplexing/ de-multiplexing techniques (at the public telephone exchange end -and- the customer PBX end). A T1 line, similarly can carry up to 24 channels of voice communications simultaneously.

At the customer end, they need to procure a PRI / E1/ T1 Card, to terminate this PRI/ E1/ T1 line in their PBX. Just imagine – One line coming into your organization but thirty people can simultaneously make or receive calls through it! And add to it a single consolidated rental / billing package from the Telecom company. You can read more about PRI lines and its advantages from here.

There is another type of digital line called ISDN line. With ISDN, two simultaneous calls can be made/ received at the same time using a single line.

GSM/ Cellular Gateway:

In some situations, it may not be feasible to lay analog lines (over a hill, rural areas, etc). But those places might be covered by cellular companies/ signal from cell phone towers. In such situations, you can use a GSM / CDMA Cellular gateway device called as Fixed Cellular Terminal and use the Cell phone SIM cards as analog trunks for your PBX. These devices are in fact connected to the analog trunk ports of the PBX.

A Fixed Cellular Terminal is a small device in which you can insert a SIM card and connect the single-pair copper telephone cable coming out of it, directly to a telephone (or) to a PBX. At the other end of the PBX, the extensions (normal phones) are connected. So, when someone makes an outgoing call, the request reaches the PBX and the call is directed to the Cellular Gateway which has the SIM card. Some cellular gateways have multiple SIM cards, enable multiple simultaneous calls to/from outside.

So, even though people might dial a number from their land line extension, the call actually goes through the cellular network using the GSM/ CDMA Cellular Gateway.

VOIP / SIP Trunks:

Of late, VOIP service providers have become very popular. They use the Internet backbone (mostly) to transmit the call as IP packets to the destination. They have gateways (where required) to convert the IP calls to analog calls before passing them on to the local telephone exchanges so that people can reach normal landline/ cell phone numbers using the VOIP Service. You can read more about SIP Trunks from here.

  KraTEL.eu

This article explains what a PRI line is, what are the benefits of having PRI lines for the telephony requirements of an organization, and the dis-advantages of a PRI line. 

What is a PRI line?

There is only one line physically terminating on the customer PBX but still a PRI line can receive/send 30 calls simultaneously! A PRI line is end to end digital circuit.

A PRI (Primary Rate Interface) line is a form of ISDN (Integrated Services Digital Network) line which is a telecommunication standard that enables traditional phone lines to carry voice, data and video traffic, among others.

A PRI circuit consists of two pairs of copper lines terminating on a modem from a service provider premises to the customer premises. It uses multiplexing/de-multiplexing techniques to carry more than one channel in a single circuit. There are two common forms of PRI lines – E1 (which carry 30 channels in the two pairs of copper lines, common in Europe, India) and T1 (which carry 23/24 channels in the two pairs of copper lines, common in United States).

Each channel in a PRI line provides 64 Kbps for data transmission.

A PRI line can connect to both Analog/Mixed EPABX systems and also the newer IP PBX systems. A PRI Card / Interface might be required to terminate the PRI circuit on the PBX.

A PRI line can also be used to connect two PBX systems thereby providing 30 channels between them for interoperability.

Advantages of PRI Lines:

If thirty separate analog trunks are taken instead of one PRI line,

The cost of terminating all the thirty analog trunk lines becomes higher than terminating one PRI line.

There would be thirty rentals to be paid instead of one consolidated lower rental for a PRI line.

Some analog trunks might be used more (uneven distribution of calls) and some lines may not have even crossed the free calls limit.

Terminating 30 analog trunks in a PBX also requires more free slots/cards than the one slot usually occupied by one or even two PRI trunk cards.

Direct Inward Dialing: For each PRI line, the service provider would provide more around 100-500 numbers which can be used by outsiders to call the extension directly, instead of having to go through the PBX Auto-attendant.

Caller ID: Since all the extensions have their own number, this unique number will be displayed in the phones that they are calling to. Some call centre applications are based on the unique caller ID number for differentiation of services.

It is possible to offer both voice and data in the PRI line. Some service providers have dynamic offerings where data is transmitted in all the channels that are free (not occupied by voice) at that given point of time.

Call hunting (Where the call lands in any channel that is free, instead of the called number specifically – For example, if there is one board number but a number of people are calling in at the same time and still a channel is allocated to them .With analog lines, if one number is busy, they need to call in another number manually) is possible by default with a PRI connection, but for the analog trunks this facility needs to be extended by the service provider and enabled on the PBX, involving additional cost at times.

PRI lines can be used for voice connectivity, data connectivity, video conferencing, faxing, and all the above can be done simultaneously too (on different channels).

PRI lines are end-to-end digital lines and hence the clarity is much better than analog trunk lines.

Since they are digital lines, PRI lines are more reliable and trouble shooting is also easier with them. They are mostly on a fiber core ring and hence there is some redundancy.

It is harder to tap into digital lines and listen to the conversations.

There are flexible billing options available with most of the PRI service providers. The billing can be centralized or distributed (department wise, etc).

PRI lines take lesser time to establish calls then analog trunk lines.

Some service providers offer flexible plans where instead of the full 30 channels, they provide and charge for only 20 channels etc. This makes PRI lines more economical for smaller companies.

Dis-advantages of PRI lines:

A PRI line is economical only if the minimum rental charged by the service provider for a PRI line is more than the average value of calls with analog trunk lines every month in an organization. Otherwise, the usage may not even cross the free call value provided by the service provider for a PRI line.

A PRI line is not so economical for long distance/ international calling. An ITSP or SIP trunk service provider who takes the calls over the internet might charge much lesser for international long distance calls.

Inter branch communication between the branches is not free of cost with PRI lines (Some PRI service providers provide this facility, but all your branches may need to have PRI lines from the same service provider and there also might be a minimum revenue commitment for the same). With VOIP systems, inter-branch communication can be done over internet/ leased lines hence reducing the cost drastically.