ISDN & SIP trunk

ISDN vs SIP Trunk

First defined in 1988, the ISDN standard (which stands for Integrated Services Digital Network) was established to carry voice, video, data and other network services on the traditional telephone company network.  ISDN is a set of communication standards for simultaneous digital transmission of voice, video, data, and other network services over the traditional circuits of the public switched telephone network.

After some time SIP trunking came along to challenge the status quo.  Whereas ISDN was designed to enable voice networks to carry data, SIP was designed to enable data networks to carry voice — including advanced communications features ISDN never dreamed of.


This paper uses SIP trunking to describe using the Session Initiation Protocol (SIP) and Real-Time Protocol (RTP) to connect Office Communications Server 2007 R2 directly to an ITSP over an Internet Protocol (IP) connection for the purpose of originating and terminating telephone calls.

What is ISDN?

SDN is a circuit-switched telephone network system, which also provides access to packet switched networks, designed to allow digital transmission of voice and data over ordinary telephone copper wires, resulting in potentially better voice quality than an analog phone can provide. It offers circuit-switched connections (for either voice or data), and packet-switched connections (for data), in increments of 64 kilobit/s. A major market application for ISDN in some countries is Internet access, where ISDN typically provides a maximum of 128 kbit/s in both upstream and downstream directions.


Advantages of ISDN

  • The basic advantage of ISDN is to facilitate the user with multiple digital channels. These channels can operate concurrently through the same one copper wire pair.
  • The digital signals broadcasting transversely the telephone lines.
  • ISDN provides high data rate because of digital scheme which is 56kbps.
  • ISDN network lines are able to switch manifold devices on the single line such as faxes, computers, cash registers credit cards readers, and many other devices. These all devices can work together and directly be connected to a single line.
  • ISDN takes only 2 seconds to launch a connection

ISDN Disadvantages

  • The disadvantage of ISDN lines is that it is very costly than the other typical telephone system
  • ISDN requires specialized digital devices just like Telephone Company.

What is SIP trunking?

SIP is used to create and control the communications sessions that are the basis of VoIP telephony. SIP is also used for instant messaging, presence updates, multimedia, conferencing, and other real-time services that can traverse a SIP trunk.

The “trunk” in SIP trunking is a term from circuit-switched telecommunications. It refers to a dedicated physical line that connects switching equipment.

Why do I need SIP Trunking? What are the benefits?

Deploying SIP trunking can be a big step towards simplifying your organization’s telecommunications and towards preparing for the latest real-time communications enhancements, but the biggest motivation for most organizations is immediate and substantial cost savings.

  • Save money on long distance service Long distance service typically costs significantly less with a SIP trunk connection.
  • Eliminate IP-PSTN gateways (or even your entire PBX) Because SIP trunks connect directly to your ITSP without traversing the publicly switched telephone network, you can get rid of IP-PSTN gateways and their attendant cost and complexity. Some ITSPs will even host a PBX for you, taking over both the PBX hardware and user administration if you choose, with substantial cost savings from reduced complexity, maintenance, and administration.
  • Eliminate a redundant network  Deploying SIP trunking is a logical step towards the goal of having a single, IP-based network, rather than redundant telephone and data networks.
  • Eliminate BRI and PRI subscription fees With a SIP trunk connected directly to an Internet telephony service provider, you can dispense with costly BRIs and PRIs, replacing them with service that can cost significantly less. Furthermore, with SIP trunking you don’t need to buy lines in blocks of 24 or 32. Instead, you can buy the bandwidth you need, in smaller increments, and at better prices.
  • Extend the capabilities of Office Communications Server with new services from ITSPs
    With a SIP trunk in place, you can extend existing capabilities with additional services, such as E9-1-1 emergency calling.

Possible reliability issues

  • ISDN infrastructure has been around for many years and is highly reliable and consistent, whereas SIP trunks require the use of a reliable broadband connection – something that not everyone enjoys.
  • Without access to broadband with a high bandwidth, the quality of the calls made within an SIP trunk network will drop significantly.

Should I replace ISDN with SIP?

ISDN is an acronym of Integrated Services Digital Network, which can be defined as a series of communication standards that allows the simultaneous digital transmission of data, voice, and other services over the public switched telephone network. The introduction of ISDN technology represented a move from analogue connectivity to digital, which offered significant improvements in quality. Although SIP (session initiation protocol) has been around for several years, it is only recently that advances in technology that have now made SIP an viable alternative to ISDN.

The main barrier for any business that wishes to implement SIP into their organisation is whether or not their public branch exchange (PBX) is SIP enabled. If your PBX is SIP enabled, then there are some extremely compelling benefits in switching from ISDN to SIP. If however, your business doesn’t have a SIP based PBX, then SIP should be something that you explore when your PBX is due to be replaced, as all modern PBX equipment is SIP enabled. Some of the advantages that SIP has over ISDN can are explored below:

Lower call costs – SIP trunk providers are usually able to offer lower call costs than ISDN providers, but this will vary on a case by case basis, and the lowest charges are typically offered to customers delivering the highest call volumes.

Free call routing between sites – It may be of particular interest to businesses that operate from multiple sites to know that almost all SIP providers offer all calls made between these site free of charge.

Scalability – SIP Trunks are extremely flexible as they can be scaled up or down to the exact amount of channels that are required by the business. In comparison, provisioning new ISDN lines increase the amount of channels by 10 at a time, which may be a lot more than what is actually needed by a business.

Number portability – One of the most frustrating elements of business telecoms is the fact that if a business is connected to the PSTN via ISDN, then they will be unable to keep the same telephone number if they move outside of their local exchange area. But if the business in question connects to the PSTN via SIP, then the number can continue to be used regardless of the location of the business.

There are also further benefits, such as the ease with which it can be implemented without having to replace all your legacy equipment and factors surrounding scalability and being able to make use of further developments in IP voice technology.

If you want even more information on SIP trunks, ISDN or pricing feel free to contact us at 0844150120 or email

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