Modem

1
Modem
2
Modem connection
PC
PC
modem
modem
110011
110011
DCE
DCE
DTE
DTE
3
Modem type

• Asynchronous, Synchronous
• Half-Duplex, Full-Duplex

4
Modulation method

• FSK 1 bit per baud. max speed 2400
  bps
• PSK, DPSK up 3 bit per baud
• QAM combines ASK and PSK, encoding
  4-7 bits per baud
• TCM same as QAM, but adds extra bits
  for error correction

5
Signal Constellation
distance sqrt(C2D2)
point defines a legimate signal change
C
C
angle sin-1
sqrt(C2D2)
D

• distance defines signal? amplitude
• angle defines phase shift

6
Constellation samples
V.22 600 baud 1200 bps PSK
V.22bis 600 baud 2400 bps QAM
V.32 2400 baud 9600 bps TCM
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V.29 Signal Constellation
90

• 8 phases with 4 amplitudes QAM
• 2400 baud, 9600 bps

1010
3sqrt(2)
0010
1000
1011
sqrt(2)
0011
5
0000
3
0
180
0001
0111
1111
1001
0101
0110
0100
1101
1110
1100
270
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Some standards

• V.32bis
• 2-dimensional TCM with 128 pts.
• transmission rate up to 14,400 bps
• V.32ter
• 2-dimensional TCM with 512 points
• Transmisiion rates up to 19,200 bps
• V.34
• 4-dimensional TCM with 768 points
• Transmisiion rates up to 28,800 bps

9
Interface V.S. Operating rate
interface speed
operating rate
modem
PC
10
Error correcting

• LAPD (Link Access Protocol D) based on
  HDLC sychronous protocol
• MNP (Micrcom Networking protocol) trademark
  of Microcom
• TCM modulation with error detecting code
• V.42 CRC error correcting standard

11
MNP protocol

• Microcom Networking Protocol (MNP) Modem
  protocol developed by Micocom, Inc.
• Divided into classes 1-10 MNP modems
  negotiate to operate at the highest supported
  class
• Considered a de facto industry standard

12
V.42 Recommendation

• V.42
• Defines error-checking standard
• uses CRC for each data block
• V.42bis
• Defines modem compression using the Lempel Ziv
  method

13
Fallback
14
Smart modem

• Introduces by Hayes Microcomputer Products
• known as Hayes-comptible modems
• Properties
• standard modem with microprocessor
• automatic dialling
• provides command in ASCII form
• Hayes command
• known as Hayes-comptible modems
• AT DT 9,9985800

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Hayes Response Codes

• 0 OK
• 1 CONNECT
• 2 RING
• 3 NO CARRIER
• 4 ERROR
• 5 CONNECT 1200
• 6 NO DIALTONE
• 7 BUSY
• 8 NO ANSWER
• 10 CONNECT 2400