Alternatives to Ordinary Phone Lines

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
2BD
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
ISP
ISDN Modem
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