ISDN implementation: Part 1 -- Understanding the standard

ISDN may be an older WAN technology, but it is still finding new applications. This article therefore offers an introduction to ISDN and how to make it work for your organisation.

ISDN history

ISDN (Integrated Services Digital Network) is a digital telephone standard designed to replace analogue connections by utilizing ordinary copper wires that are used in standard analogue telephone systems. It started as a recommendation within the International Telecommunication Union (ITU)'s Red Book in 1984, although prior to 1992, the ITU was known as the International Telegraph and Telephone Consultative Committee (CCITT). The ITU is responsible for developing recommendations on international standards within the industry.

ISDN was developed to provide digital transmission of both voice and data, resulting in better quality and speeds above those of Public Switched Telephone Network (PSTN) systems.

Getting to know the digital protocol

There are two types of ISDN Channels:

  • The B-Channel "“ This is known as the Bearer ("B") channel, which is a 64 Kbps channel used for voice, video, data or multimedia transfer. These can be aggregated together to give higher bandwidth utilization.
  • The D-Channel "“ This is known as the Delta ("D") channel, which can be either 16 Kbps or 64 Kbps and is used primarily for the signaling between the switching equipment. Some say that this adds to the security of ISDN because the controlling and data channels are separate.

N.B. Digital Signal 0 (DS0) is a basic digital signaling rate of 64K bits, which may be used to describe a single Bearer channel.

BRI (Basic Rate Interface)

BRI can also be known as BA (Basic Access). This operates a single 16 Kbps D channel and two 64 Kbps B channels. Although it isn't usually pointed out, the BRI total speed is 192 Kbps; this is because you have an additional 48 Kbps overhead for framing and synchronization on the D channel (64 * 2) + (16 + 48) = (128 + 64) = 192 Kbps.

PRI (Primary Rate Interface)

PRI can also be known as PA (Primary Access). This can operate in two different modes, depending on geographic location. For European locations, PRI is made up of 30 x 64 Kbps B channels and a single 64 Kbps D channel, giving a total of 2.048 Mbps, which is also known as an E1 line (or DS1). For American and Japanese locations, PRI is made up of 23 x 64 Kbps B channels and a single 64 Kbps D channel, giving a total of 1.544 Mbps, which is also known as a T1 line (or DS1). Framing and Synchronization is at 8 Kbps for T1 or 64 Kbps for E1. T1 PRI is commonly referred to as "23B+D," and E1 PRI is commonly referred to as "30B+D."

N.B. E1 PRI actually has 32 channels, composed of 30 x B Channels, 1 x D Channel and 1 Synchronization Channel.

Digital Signal Levels (DSx)

Digital Signal X is used to describe standard digital transmission rates or levels based on DS0, which is defined as a transmission rate of 64 Kbps. This is the rate for one telephone voice channel. This is based on the ANSI T1.107 guidelines, and the ITU guideline does differ slightly. The following tables show the DS level and the corresponding speed and T/E classification.

T Carrier
Digital Signal Level T Speed T Classification Channels
DS0 64 Kbps N/A 1
DS1 1.544 Mbps T1 24
DS2 6.312 Mbps T2 96
DS3 44.368 Mbps T3 672
E Carrier
Digital Signal Level E Speed E Classification Channels
DS0 64 Kbps N/A 1
DS1 2.048 Mbps E1 32
DS2 8.448 Mbps E2 128
DS3 34.368 Mbps E3 512
DS4 139.264 Mbps E4 2048
DS5 565.148 Mbps E5 8192

As you can see from the tables, the guidelines differ slightly. In fact, depending on what sources you read, these tables may differ slightly.


ISDN will typically use the Point-to-Point (PPP) Tunnelling protocol as its basis for transmitting packets over the ISDN circuit. The IP packets are encapsulated into the PPP packets before the traffic is sent.

PPP provides link-specific control functions via Link Control Protocol (LCP) such as Link Configuration and Link Quality Testing & Address Negotiation. LCP provides more advanced features, such as Multilink, Header Compression, Callback, Scripting, Demand Dialing, Filtering, Tunnelling and Server Routing. There are also authentication mechanisms that can help to ensure that the ISDN connection that is established is from a trusted source. Authentication is optional; it can be performed by the use of PAP, CHAP and EAP. (Although it is not used in ISDN implementations, EAP is a valid authentication method of PPP.)

  • PAP "“ Password Authentication Protocol is not strong, because the password is sent in clear text. PAP occurs during the LCP phase of the PPP connection.
  • CHAP "“ Challenge Handshake Authentication Protocol is much stronger than PAP and is much more widely used. It uses a challenge/response security mechanism that uses a one-way hash function to ensure that the passwords are not sent over the link. The password is hashed and sent over the link. The other side of the link then performs the same hashing function on the password that they have configured; then the link checks to ensure that the two hash values are the same. This can also provide protection against playback.
  • EAP "“ Extensible Authentication Protocol provides the ability to use multiple authentication protocols such as static passwords, CHAP, token cards, biometrics, etc. As you can imagine, since CHAP is available on its own and ISDN cannot really work with token cards or biometrics, it isn't used in ISDN implementations.

About the author:

Chris Partsenidis has over eight years of experience as a senior network engineer, network administrator and network security consultant for companies in Australia, New Zealand and Greece. Chris has a bachelor's degree in electrical technology and holds the following certifications: CNA for Netware 3, 4, 5, CCNA, MCP, DCE (D-link Certified Engineer), LCP (Linux Certified Professional), Network+ and A+.

In addition, Chris is the founder and senior editor of -- a Web site recommended by Cisco Systems in their CCNA Academy program! Today, with over 850,000 page views per month, is amongst the most respected IT security and knowledgebase sites in the world. His site now offers free access to Cisco lab equipment, allowing users to put in practice the theory covered in the ISDN article. is the only site in the world offering such services with a large range of Cisco equipment.

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