WAN Technologies

Introduction To The ISDN Protocol

ISDN History

ISDN (Integrated Services Digital Network) is a digital telephone standard designed to replace analogue connections by utilising ordinary copper wires that are used in standard analogue telephone systems. It started as a recommendation within the ITU's (International Telecommunication Union) Red Book in 1984, although prior to 1992, the ITU was known as the CCITT (International Telegraph and Telephone Consultative Committee). 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 over that of PSTN (Public Switched Telephone Network) systems.

Getting to Know the Digital Protocol

There are two types of IDSN Channels.

• The B-Channel – This is known as the Bearer (“B”) channel which is a 64Kbps channel used for voice, video, data or multimedia transfer. These can be aggregated together to get higher bandwidth utilisation.

• The D-Channel – This is known as the Delta (“D”) channel which can be either 16Kbps or 64Kbps used primarily for the signalling 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 signalling rate of 64Kbits which may be used to describe a single Bearer channel.

BRI (Basic Rate Interface)

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

PRI (Primary Rate Interface)

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

N.B. E1 PRI actually has 32 channels which are comprised of 30 x B Channels, 1 x D Channel and 1 Synchronisation 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 64Kbps. 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 you 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, you can see where the guidelines differ slightly. In fact, depending on what sources you read, these tables may differ slightly.

Point-to-Point Protocol (PPP)

ISDN will typically use the Point-to-Point (PPP) Tunnelling protocol as its basis of 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, 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 which can be performed by the use of PAP, CHAP & EAP (although EAP is not used in ISDN implementations, EAP is however a valid authentication method of PPP).

PAP – Password Authentication Protocol is not strong since 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 then PAP and is much more widely used. It uses a Challenge/Response security mechanism which 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 check 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.

ISDN Layers, Protocols & Components

Introduction

ISDN uses circuit-switching to establish a physical permanent point-to-point connection from the source to the destination. ISDN has standards defined by the ITU that encompass the OSI bottom three layers of which are Physical, Data Link and Network, see Table 1 below.

At the physical layer the ITU has defined the user network interface standard as I.430 for Basic Rate Access and I.431 for Primary Rate Access; please see the ITU-T I.414 “Overview of Recommendations on Layer 1 for ISDN and B-ISDN customer accesses” document on the ITU's website. ANSI has defined the user network interface standard as T1.601. As already stated above, the physical layer uses the normal telephone cabling as its physical cabling structure.

The ISDN B channels will typically utilise a Point-to-Point protocol such as HDLC (High-Level Data Link Control) or PPP frames at Layer 2 however you can sometimes see other encapsulation such as Frame relay. As you would expect, at layer 3 you typically see IP packets. ISDN operates in Full-Duplex which means that traffic can be received and transmitted at the same time.

The ISDN D channel will utilise different signalling protocols at Layer 3 and Layer 2 of the OSI Model. Typically at Layer 2, LAP-D (Link Access Procedure – D Channel) is the Q.921 signalling used and DSS1 (Digital Subscriber Signalling System No.1) is the Q.931 signalling that is used at Layer 3. It is easy to remember which one is used at which layer by simply remembering that the middle number corresponds to the layer it operates at.

Table 1

OSI Layer	B Channel	D Channel
3	IP	DSS1 (Q.931)
2	HDLC/PPP	LAP-D (Q.921)
1	I.430/I.431 or ANSI T1.601

The Different ISDN Components

As part of the ISDN Standards, there are several types devices that are used to connect to the ISDN network which are known as Terminal Equipments (TE) and also Network Termination (NT) equipment. You also have Reference Points which are used to define the connections between the various equipment that is used within the ISDN network.

Terminal Equipment and Network Termination Definitions;

• Terminal Equipment Type 1 (TE1) are devices that can plug directly into an ISDN Network and understands the ISDN standards

• Terminal Equipment Type 2 (TE2) are devices that predate the official ISDN standards and require the use of a terminal adapter (TA) to facilitate plugging into the ISDN Network. These can simply be routers that only have a serial interface on them and not an ISDN WIC. The terminal adapter can plug into the serial interface and allow the router to be used to connect to the ISDN network. Another example would be a Personal Computer (PC).

• Network Termination 1 (NT1) is typically a customer's device that is used to implement the physical layer specification into the ISDN Network (or the NT2 device). This is the U Reference point that connects through to the telco. This operates at Layer 1 of the OSI Model.

• Network Termination 2 (NT2) is typically the telco's device (it's very rare to see this at the customers site) that is used to terminate from the customers NT1 device before traffic hits the ISDN network. This operates at Layer 2 & 3 of the OSI Model and is an intelligent device performing the switching.

• Terminal Adapter (TA) is used to convert TE2 device signalling into signalling that is used by the ISDN switch.

Different ISDN Reference Points

• R – This reference point is used to specify the point between the TE2 device and the TA device.

• S – This reference point is used to specify the point between the customers router and the NT2 device.

• T – This reference point is used to specify the point between the NT1 device and the NT2 device S and T reference points can perform the same functions therefore they are sometimes referred to as an S/T reference point. When we are plugging into the S/T reference point location, the function of the NT2 is redundant since it's built in.

• U – This reference point is used to specify the point between the NT1 device and the telco's termination equipment in the ISDN carrier network, apart from in North America where the NT1 function isn't provided by the carrier network.

Cisco Router Options

With Modular Cisco Routers, they come with slots where you can plug in various cards different types of WAN Interface Cards (WIC). Cisco provide 2 different types of WICs for ISDN support. These different cards provide either a ISDN WIC with the S/T reference points which plug into an NT1 device or an ISDN WIC with a U reference point which has the NT1 built into the WIC itself.

Which WIC is required depends on your location and the telco that provides the ISDN circuit. For example, in North America , they use a two-wire connection which is a WIC card with the U reference point, having the NT1 built into it.

More information regarding the configuration of Cisco routers and ISDN confguration can be found in our Networking section.