Summary

Chapter 8: Fundamental Networks 51communication channels. The B channel is used to carry the information—data, voice, orvideo. The D channel usually is used for controlling and signaling, but it can be used fordata.Here are the types of ISDN:■ BRI: ISDN Basic Rate Interface offers a dedicated 128-Kbps connection using two 64-Kbps B channels. ISDN BRI also uses one 16-Kbps D channel for call setup, control,and teardown.■■PRI: ISDN Primary Rate Interface offers up to 1.544 Mbps over 23 B channels inNorth America and Japan or 2.048 Mbps over 30 B channels in Europe and Australia.ISDN PRI also uses one 64-Kbps D channel for call maintenance.BISDN: Broadband ISDN manages different types of service all at the same time.BISDN is mostly used only in network backbones.Digital Subscriber Line (DSL)DSL is an “always-on” technology. This means that you don’t need to dial up each time toconnect to the Internet. DSL uses the existing copper telephone lines to provide high-speeddigital data communication between end users and telephone companies. Unlike ISDN, inwhich digital data communication replaces analog voice communication, DSL shares thetelephone wire with analog signals.The telephone company limits the bandwidth of the analog voice on the lines. This limitallows the DSL to place digital data on the phone wire in the unused portion of the bandwidth.This sharing of the phone wire allows voice calls to be placed while DSL is connectingto the Internet.You must consider two major points when selecting DSL. DSL has distance limitations.The phone lines used with DSL were designed to carry analog information. Therefore, thelength that the digital signal can be sent is limited, and the signal cannot pass through anyform of multiplexer used with analog phone lines. The other consideration is that the voiceinformation and the data carried by DSL must be separated at the customer site. A devicecalled a splitter separates the connection to the phones and the connection to the local networkdevices.Asymmetric digital subscriber line (ADSL) is currently the most commonly used DSL technology.ADSL has different bandwidth capabilities in each direction. ADSL has a fastdownstream speed—typically 1.5 Mbps. Downstream is the process of transferring datafrom the server to the end user. This is beneficial to users who download large amounts ofdata. The high-speed upload rate of ADSL is slower. ADSL does not perform well whenhosting a web server or FTP server, both of which involve upload-intensive Internetactivities.