ISDN Makes ADSL Attractive

Introduction

The Integrated Services Digital Network (ISDN) is the only fixed-wire end-to-end digital telecommunications network available throughout the world today. It owes its success to its high value for users. Integrated services and multiple user channels in a single line, high quality and sophisticated telephony capabilities; convenience features (“supplementary services”) such as call holding, automatic call-back, calling line identification; fast call set-up; powerful and, most importantly, economical data communication with scalable bandwidth … ISDN users have come to view these and many other features as indispensable. Furthermore, the close correlation between the digital techniques and concepts of ISDN and GSM permits the seamless integration of GSM mobile telephones in existing ISDN infrastructures.

Naturally a completely digital, universally available network has  advantages for the network operator as well. A homogeneous digital infrastructure with defined digital interfaces reduces operating costs and simplifies service. Moreover, ISDN provides an ideal platform for custom service offerings that can distinguish a provider from competitors. In an open telecommunications market, ISDN is even more important than before.

Two quantitatively significant demand trends are observed in the telecoms market today:

        Continued growing demand for ISDN for classic telecommunication and Internet services;

        Budding demand for new IP-based services (Internet, video and multimedia applications)

With regard to the question of which transmission medium the telecommunications market will prefer in the future, a number of broadband alternatives are in competition to serve the latter demand, including fiber optics, TV cable networks, wireless and copper (xDSL).

This white paper describes a way to serve both of the above named demands concurrently over existing copper on the “last mile”: classic ISDN-based provision of telecommunication services in conjunction with broadband ADSL transmission of IP services. These two techniques complement each other ideally in the local loop, carrying an offering that is scalable and transparent for both customers and the network operator.

ADSL Complements ISDN on the “Last Mile”

Telefax, telephony and Internet access, all at the same time, is an everyday demand for corporate users. This calls for integrated communications. Optimum solutions for this important market segment would provide ISDN capabilities in the LAN as well as connect the LAN to the Internet over ADSL. In this case, the company’s entire communications can be carried over a single physical line.

Private users too are increasingly ready for broadband connections to the Internet. The majority of early ADSL adopters will be recruited among the current segment of innovative ISDN users—who naturally will not want to give up their accustomed ISDN convenience.

The following table compares the typical areas of application of ISDN and ADSL.

Internet access for both personal and business use can take place over ADSL or ISDN. Techniques that fully exploit ISDN bandwidth reserves—“Fast Internet over ISDN”—can obtain throughput of up to 250 kbit/s (full duplex).

ADSL’s asymmetric bit rate is not always suitable for LAN-to-LAN connectivity, since this typically involves symmetrical data loads. Scaled ISDN links, in the form of dial-up or leased BRI lines, remain a reliable option for connecting subsidiaries or branch offices to a central LAN.

Because multimedia data plays a subordinate role in remote access to  corporate LANs, ISDN BRI bandwidth will be found sufficient for some years to come, especially with the performance reserves promised by “Fast Internet over ISDN”. ISDN also spares companies the use of VPN technology which would otherwise be necessary.

ISDN and ADSL:  The Technology

A) Frequency Splitting

In most countries ISDN uses a frequency range of 80 kHz. This follows from the 160 kbps (full duplex) bit rate and the 2B1Q line code.

ADSL, like ISDN, specifies the transmission of a bit stream over a twisted copper wire pair. ADSL uses a frequency range of about 1.1 MHz for DMT (discrete multitone).

ITU Recommendation G.992.1 Annex B (ADSL access over ISDN lines) provides for the simultaneous use of ISDN and ADSL.

B) Installation

To prevent interference and to facilitate the mechanics of installation, ISDN and ADSL are split apart both at the subscriber premises and in the central office. This is done by frequency separators called “splitters”. The ISDN-NT is then connected to the subscriber-end splitter. One advantage of the splitter is that it forms a demarcation point between the subscriber installation and the network operator’s plant. The illustration below shows the connection scheme on the customer premises (at right) as well as at the local central office.

Because different modulation techniques are in use and the current standards leave some leeway for varying interpretations, it is not yet possible to combine any ADSL terminal with any ADSL Access Multiplexer (DSLAM). In general, the DSLAM manufacturer must also supply the ADSL controller used at the customer’s site. A new approach to resolve this problem at the customer's premise is based upon modulation adaptive ADSL/ISDN NICs offered by 3rd Party vendors.

C) Services

ISDN specifies the communication protocols up to the network layer (Q.931). This means that ISDN, over and above the pure bit stream, includes information about the type of data and switching. This makes it possible to switch voice and data channels, for example, to the same line number. Because it transports a service indicator and source and destination numbers, ISDN is ideal for computing applications (both data communication and CTI). Calls can be processed by specific applications depending on the service indicator and ISDN numbers. The exact combination of fax, voice and data applications can be further adapted to a variety of requirements by assigning different dial-in numbers.

G.992.1 specifies an ATM interface, but actual ADSL implementations are concentrated in broadband Internet access and the IP services thus available.

D) The Complementary Characteristics of ISDN and ADSL

In contrast to ISDN, ADSL does not yet have a consistently implemented standard end-to-end switching method for services. ADSL is primarily used today as a vehicle for the Internet Protocol. At least two higher-order protocols are in actual use today in the international sphere (ATM and PPP over Ethernet), yet the protocol stacks suggested here vary in details. In contrast to ISDN, ADSL does not provide for circuit-switched terminal-to-terminal  connections. This means subscribers will not make a direct phone call with an “ADSL connection”. Likewise, ADSL will not be used to transfer data directly from one PC to another. To provide these functions, the services concerned must be made available on a packet-switched basis such as IP. Because some of such services over IP do not yet attain the quality of the corresponding circuit-switched methods, and hence are not widely used, the combination of ADSL with ISDN remains ideal.

Advantages of the ISDN –ADSL Combination

          State of the art telecommunication services and Internet access

          Internet (ADSL) added to the interoperability of GSM and ISDN

          High quality of service for telecommunication

          Investment security for both subscribers and network operators (low costs)

          Permits uniform digital design of central office equipment

          Ready for future extensions in communication services (intelligent networks)

AVM Computersysteme Vertriebs GmbH & Co. KG

Berlin, 15 September 2000

Updated version according to contributions of Alcatel, 02/2000

Updated version according to contributions of GIIF Summit 2000 (Berlin)