Summary

ISDN Development, Components, and Mechanics

Once upon a time, in the early days of the public telephone network, telephone users were began replacing the analog circuits in the core of their networks with packet-based digital signals. Analog circuits still were delivered to the telephone users at the local loop level. ISDN delivers the digital circuits directly to the customers, enabling them to carry a wide variety of traffic over the network. Therefore, ISDN enables users to carry voice, video, data, and other traffic over the existing telephone wiring. When ISDN became an emerging technology, it was envisioned to become the leading worldwide digital network technology, providing access to everyone. Now, with the emergence of xDSL and cable modems, ISDN is losing favor with the home access market. However, ISDN is still a major player in the business market, where it is used to back up primary links and where PRI/E1s are used to carry data and V.90 remote dial-in access. It is also more readily available than DSL and cable, allowing the majority of home users who fail to qualify for cable or DSL to purchase ISDN service.

A standards committee was developed in 1984 to coordinate the ISDN movement, called the International Telephone and Telegraph Consultative Committee (CCITT), now known as the International Telecommunications Union (ITU). The ITU organizes the ISDN protocols according to three general topic areas:

• Protocols that begin with the letter E deal with telephone network standards for ISDN. For example, the E.164 protocol describes international addressing.

• Protocols that begin with the letter I deal with concepts, terminology, and general methods . The I.100 series of protocols deal with general ISDN concepts and the structure of other I-series recommendations. I.200 deals with the service aspects of ISDN, whereas I.300 describes network aspects, and I.400 describes how the User Network Interface (UNI) is provided.

• Protocols beginning with the letter Q cover how switching and signaling should operate . Q.921 describes the ISDN data-link processes of Link Access Protocol (LAPD), which functions like Layer 2 processes in the OSI model and is an encapsulation option on the ISDN D channel. Q.931 specifies Layer 3 functions.

Debugging Q.921 and Q.931 sometimes can be useful for troubleshooting ISDN connectivity problems, and this is discussed in more detail later in the chapter.

The major advantage of ISDN is that it allows multiple digital channels to operate simultaneously over one circuit. Currently, there are three types of ISDN channels:

• B (Bearer) channel ” 64 kbps used for user traffic. Some ISDN switches limit B channels to a capacity of 56 kbps.

• D (Data) channel ” 16 kbps or 64 kbps, depending on the type of ISDN circuit. This channel is used for ISDN signaling, as well as call setup and teardown .

• H channel ” Provides a way to bond multiple B channels. They typically are not used in North America. H channels are implemented as follows :

  - H0 ” 384 kbps (six B channels) used for high-quality audio/high speed digital information
  
  

  - H10 ” 1.472 Mbps (23 B channels) used for teleconferencing/digital information
  
  

  - H11 ” 1.536 Mbps (24 B channels) used for teleconferencing/digital information
  
  

  - H12 ” 1.92 Mbps (30 B channels) used for teleconferencing/digital information
  
  

  - H4 ” 150 Mbps (approximate) used for high-definition TV
  
  

Cisco supports the following two ISDN interface types, which combine the channel types in the preceding list:

• Basic Rate Interface (BRI) ” One 16-kbps D channel + two B channels

• Primary Rate Interface (PRI) ” 1 64-kbps D channel + 23 B channels in North America and Japan, for a total capacity of 1536 kbps. 1 64-kbps D channel + 30 B channels in Europe and other parts of the world, for a total capacity of 1984 kbps .

ISDN Components and Reference Points

To access the ISDN network, customer premises equipment (CPE), which is the local terminating equipment, is used. This equipment performs the functions needed for properly connecting to the ISDN network. The following describes the types of ISDN CPE options:

• TE1 (Terminal Equipment Type 1) ” Designates a device with a native ISDN interface.

• TE2 (Terminal Equipment Type 2) ” Designates a device that does not include a native ISDN interface and requires a terminal adapter (TA) for its ISDN signals.

• NT1 (Network Termination 1) ” Designates a device that converts the BRI signals into a form used by the ISDN digital line, and is the boundary between the carrier's ISDN network and the CPE.

• NT2 (Network Termination 2) ” Designates a device that aggregates and switches all ISDN lines at a customer location. This typically is incorporated into a corporate private branch exchange (PBX).

• TA (Terminal Adapter) ” Used by a TE2 device to convert EIA/TIA-232, V.35, and other signals into BRI signals.

Because CPE can include one or more of these functions, the way in which they connect to the other ISDN devices can vary. Because of this, the ISDN standards refer to these various interfaces as reference points. The reference points simply define the logical points between the previous CPE groups. Figure 7-1 shows the reference points, which are described as follows:

Figure 7-1. ISDN Reference Points

• R ” The connection between a non “ISDN-compatible device (TE2) and a TA

• S ” The connection between the end-user CPE and the NT2

• T ” The connection between the NT2 and the NT1

• U ” The connection between the NT1 and the carrier's ISDN network

In most implementations for BRI users, an NT2 is not used and usually is found only in PBXs. In this case, the CPE-to-carrier interface is referred to as the S/T interface.

Generally, routers come with an integrated S/T ISDN interface, so the reference points would look like they do in Figure 7-2.

Figure 7-2. ISDN Reference Points for a Router with an Integrated S/T ISDN Interface

Additionally, Cisco routers can be purchased with an S/T interface and integrated NT1, so reference points S, T, and U would all be incorporated into the BRI/PRI interface of the Cisco router.

In North America, all ISDN lines are connected to the ISDN switch at the local central office (CO) through a U interface. This U interface requires network termination to convert it into an S/T interface that actually connects to your end equipment. An NT-1 is a separate unit that serves this function and also provides power to your ISDN line. In Europe, the NT-1 is included in telco-provisioned ISDN lines, so all you need is the S/T interface.

When working with Cisco routers, it is important to determine whether the unit has an S/T interface, which will require an external NT-1, or a U interface with a built-in NT-1. Check your hardware manual to determine whether you have the proper equipment.

ISDN Layers

ISDN deals with Layers 1, 2, and 3 of the OSI reference model. ISDN connectivity works only if all three of these layers are operational. Layer 1 is the physical layer, dealing with the physical connections between the router and the ISDN circuit. Layer 2, the data link layer, deals with the Q.921 protocol on the D channel and the encapsulation options for each B channel (HDLC and PPP). Q.921 handles the signaling between the router ISDN interface and the ISDN switch. Layer 3, the network layer, includes the Q.931 protocol over the D channel and network layer protocols (IP, IPX, AppleTalk, and so on) over the B channels. Q.931 handles call setup messages between the calling and called parties. It is important to fully understand the various protocols that operate over the D and B channels when configuring and troubleshooting ISDN.

ISDN Encapsulation Options

ISDN routers can support both PPP and HDLC encapsulations . HDLC is the default encapsulation, but the vast majority of ISDN routers use PPP. The main reason that PPP is chosen over HDLC is because HDLC cannot use both B channels simultaneously. With HDLC, both B channels can be up, but one channel is used to send and the other is used to receive traffic. Each B channel has the capability to provide full duplex (send and receive at the same time), so by using HDLC, you are essentially cutting the possible data rate over the link in half.

PPP was covered in Chapter 4.