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 Analog LineDigital Line Analog Device Codec Digital Device ModemDSU Translator Transmission Line Device

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 1101 B Channel B Channel 2B+D Multiplexed Onto One Set of Wires ISDN Modem D Channel: control signals

6. ISDN Two B Channels are 64 kbps –Original idea: one for voice, one for data 1101 B Channel 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) Telephone Network Telephone Network ISP ISDN Modem ISDN Modem 128 kbps B B Bonding

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 1/8,000 sec 0110010

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

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 1/8,000 sec Sampling Period Intensity Value 0110010

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 1/8,000 sec Sampling Period Intensity Value 0110010

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 DSU Digital Line