Chapter 12 Long-Distance Digital Connection Technologies Pulse Code Modulation DSU/CSU ISDN ADSL Cable Modem

To reproduce human voice up to 4 KHz, Nyquist sampling theorem implies that voice needs to be digitally sampled at 8 KHz ( once every 125 microseconds).

Pulse Code Modulation (PCM) a standard for digital encoding of audio used in the telephone system. PCM takes 8bit samples at 8 Khz (note that bandwidth of digital voice channel is 64 Kbps)

DSU/CSU device which terminates the digital line translates between the digital representation used by phone companies and the digital representation(eg. v.35, RS449, RS232) used by computer industry(figure 12.2)(figure 12.2)

Digital Leased Lines Cost depends on capacity (speed) and distance. Fractional T1 digital circuit – 64kbps, 128kbps, 256kpbs T1 line – Mbps digital circuit capable of handling 24 digital voice circuits Inverse multiplexing (bonding) (figure 12.4)(figure 12.4) – multiple T1 lines for higher bandwidth T3 line(figure 12.3)(figure 12.3) – 45 Mbps digital circuit capable of handling 672 digitized voice circuits.

Fiber Optic Digital Leased Lines OC-1 circuit – 51.8 Mbps capable of handling 810 voice circuits OC-3 circuit – 155 Mbps capable of handling 2430 voice circuits Trunk – a high capacity circuit (eg. T1 trunk, OC-3 trunk) OC-1 SONET frame contains 810 bytes(figure 12.6)(figure 12.6) OC-3(figure 12.5) SONET frame contains 2430 bytes(figure 12.5)

Integrated Services Digital Network (ISDN) Basic Rate Interface(BRI): – 2B channels at 64kbps each and one D channel at 16kbps Primary Rate Interface(PRI): – 24 64kbps channels

Local Loop wiring between subscriber’s residence and nearest Central Office (CO) aka “last mile”

Asymmetric Digital Subscriber Line (ADSL)(figure 12.7) (figure 12.7)(figure 12.7) a local loop technology that provides a higher bit rate downstream to the subscriber than upstream to the provider can run simultaneously over the same wires as the standard phone service is adaptive (overall speed is negotiated between each pair of ADSL modems) uses both frequency division multiplexing(255 carrier frequencies spaced 4.1 KHz apart for downstream data transmission and 31 frequencies for upstream data transmission) and inverse multiplexing overall downstream rate variable up to 6.4 Mbps while upstream rate up to 640 Kbps

Cable Modem uses existing cable tv wiring. Uses broadband signaling (ie. Frequency division multiplexing) to send data. Requires one cable modem at the CATV center one cable modem for each subscriber whose modems use a common carrier frequency to encode data. Each subscriber is assigned an address Users must share overall bandwidth of 10Mbps with other subscribers whose modems use same carrier frequency