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Alternatives to Ordinary Phone Lines & Modems

Alternatives to Ordinary Phone Lines & Modems. Faster Speeds; Called Broadband Services Also Higher Cost Translation devices more expensive than modems Faster (more expensive) transmission line to ISP ISP may charge more because of heavier transmission load

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Alternatives to Ordinary Phone Lines & Modems

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  1. Alternatives to Ordinary Phone Lines & Modems • Faster Speeds; Called Broadband Services • Also Higher Cost • Translation devices more expensive than modems • Faster (more expensive) transmission line to ISP • ISP may charge more because of heavier transmission load • Faster interface than 232 serial port may be needed

  2. Transmission Line Versus ISP • Transmission line may be provided by a transmission carrier, not the ISP • Users have to pay separate fees to carrier and ISP • Some ISPs also provide transmission line • But not all do • Transmission line costs must always be considered PC ISP Transmission Line

  3. Translator Devices • Device versus Line Device Transmission Line Translator Analog Line Digital Line Analog Device Codec Digital Device Modem DSU

  4. ISDN Integrated Services Digital Network Codecs DSUs

  5. ISDN • Get ISDN line from telephone company • Multiplexes (mixes) three channels on one UTP wire pair to the desktop D Channel: control signals B Channel ISDN Modem 2B+D 1101 Multiplexed Onto One Set of Wires B Channel

  6. ISDN • Two B Channels are 64 kbps • Original idea: one for voice, one for data B Channel 1101 B Channel

  7. ISDN • Can now “bond” the two B channels for 128 kbps data if ISP supports it • D Channel is 16 kbps (for supervisory signaling) Bonding Telephone Network B ISDN Modem ISP B 128 kbps

  8. ISDN Costs • Must install an ISDN line • Will cost more than telephone line • Actually, could use your existing phone line • But then would need one B channel for telephony

  9. ISDN Costs • Line is expensive • $60-$80/mo plus installation fee • ISP charge is separate; may charge more for ISDN access • Dial Up: Not always connected • Do not have to pay for full-time use • Need “ISDN modem” (expensive)

  10. ISDN Modem • ISDN “Modem” is a Misnomer • Modem is for for digital device, analog line • ISDN line is digital • ISDN modem really contains • Codec to link analog telephone to digital ISDN line • Data service unit (DSU) to translate between PC digital format and ISDN digital format (voltage levels, timing, etc.)

  11. ISDN Modem • Codec • Translates analog device signal into digital signal for propagation • Constantly samples the intensity of the voice analog signal Sample 0110010 1/8,000 sec

  12. ISDN Modem • Codec • Divides each second into 8,000 sampling periods • Only measures intensity of voice signal in each Sample Intensity Value 0110010 1/8,000 sec Sampling Period

  13. ISDN Modem • Codec • Measures voice intensity as an 8-bit intensity value (0-255) • Overall, sends 8 bits 8,000 times per second (64 kbps) Sample Intensity Value 0110010 1/8,000 sec Sampling Period

  14. ISDN Modem • Codec • This is why telephone channels are 64 kbps • Designed for digitized voice • Often steal 8 kbps for supervisory signaling, so 56 kbps Sample Intensity Value 0110010 1/8,000 sec Sampling Period

  15. DSU (Data Service Unit) • DSU translates between different digital formats • Device and line are both digital, but still must have translator • Different bit rates • Different number of possible states • Different voltage levels for the states • Different ways to represent ones and zeros Digital Line DSU

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