Typical Uses of ISDN

Routers frequently use ISDN to create a backup link when their primary leased line or Frame Relay connection is lost. Although the leased line or Frame Relay access link seldom fails, when it does, a remote site might be completely cut off from the rest of the network. Depending on the business goals of the network, long outages might not be acceptable, so ISDN could be used to dial back to the main site.

The ICND exam covers ISDN as well, including the features and configuration used by routers. The scenarios in Figure 15-10 show some of the typical situations in which ISDN can be used, described as follows:

■ Case 1 shows dial-on-demand routing (DDR). Logic is configured in the routers to trigger the dial when the user sends traffic that needs to get to another site.

■ Case 2 shows a typical telecommuting environment.

■ Case 3 shows a typical dial-backup topology. The leased line fails, so an ISDN call is established between the same two routers.

■ Case 4 shows where an ISDN BRI can be used to dial directly to another router to replace a Frame Relay access link or a failed virtual circuit (VC).

Figure 15-10 Typical Occasional Connections Between Routers

Home

"ZL

Office

Leased Line

"ZL

"ZL

ISDN

ISDN

ISDN

ISDN

PRIs allow for larger-scale ISDN because they support far more B channels on a single physical line. Imagine an ISP that supports ISDN, with 1000 customers. If that ISP wanted to support up to 230 concurrent ISDN customers, each using a single B channel, that ISP

would need 10 PRIs (assuming that it was in the United States). Also, each user might want to use both B channels at the same time, doubling the speed to the Internet; to support 2 B channels each for 230 concurrent users, that ISP would need 460 B channels, or the equivalent of 20 PRIs. However, if it just used BRI lines, it would need 230 different physical BRI lines, which probably would be much more expensive, would require more equipment, and would be a cabling hassle.

ISDN supports voice as well as data circuits. ISDN BRI circuits do not support analog voice, but they do support digital voice. You might recall that a single PCM voice call requires 64 kbps and that a single B channel provides 64 kbps. So, ISDN devices, like a terminal adapter, perform the PCM encoding and decoding features and send the voice traffic over a B channel. In fact, most ISDN modems have two RJ-11 ports that can be used to connect a normal analog phone. Figure 15-11 depicts the cabling and some important concepts about how it all works.

Figure 15-11 ISDN Support for Voice

Andy's PC

PSTN

No PCM Needed on Andy's Digital Local Loop

RS-232 Cable

DTMF Tones, Analog Signal

Figure 15-11 ISDN Support for Voice

Andy's PC

No PCM Needed on Andy's Digital Local Loop

RS-232 Cable

DTMF Tones, Analog Signal

Andy's Analog Phone

/ Internal ISDN Card

Helen's Phone

Andy's Analog Phone

/ Internal ISDN Card

Helen's Phone

The analog phone works just like it normally works. You pick it up and punch in some digits, generating DTMF tones. The ISDN TA can't send the tones, so it interprets the tones and generates a signaling message over the D channel. After the telco sets up a circuit over one of the B channels, the TA begins using its PCM codec to convert the incoming analog voice from the phone into PCM digits, sending them across the B channel. In the Figure 15-11 example, the other phone is an analog phone connected to the PSTN at Helen's house. So, the voice switch connected to Helen's phone line converts the incoming digital signal from the back to analog voice using a PCM codec, just like it normally does for a call between two analog phone.

Finally, ISDN supports multiple concurrent data bearer channels. For instance, you can use your PC to dial two different sites at the same time. You can make two calls to the same ISP, increasing the speed. You also can use one B channel for data and make a voice call using the other B channel.

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