13 - Winter 2005 ECE ECE 766 Computer Interfacing and Protocols 1 Modems Goal: Carry data over telephone lines Modulator / Demodulator Baud rate / Bit

Winter 2005

ECE

ECE 766Computer Interfacing and Protocols

113 -

ModemsModems

• Goal: Carry data over telephone lines• Modulator / Demodulator• Baud rate / Bit rate / Bandwidth relationship

Amplitude

Frequency300 600 3000 3300

2400 Hz for data

3000 Hz for voice

Utilize this bandwidth asefficiently as possible

Winter 2005

ECE


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Theoretical Bit RatesTheoretical Bit Rates

Modulation Half-Duplex Full-Duplex

ASK 2400 1200

FSK <2400 <1200

2-PSK 2400 1200

4-PSK, 4-QAM 4800 2400

8-PSK, 8-QAM 7200 3600

16-QAM 9600 4800

32-QAM 12000 6000

64-QAM 14400 7200

128-QAM 16800 8400

256-QAM 19200 9600

Winter 2005

ECE


313 -

Bell 103/113Bell 103/113

• Full-duplex on 2-wire lines• Similar to CCITT V.21 specifications• FSK modulation, asynchronous• Data rate: 300bps

Amplitude

Frequency300 2225Mark

33002025Space

1270Mark

1070Space

Upstream Downstream

Winter 2005

ECE


413 -

Bell 202Bell 202

• Half-duplex over 2-wire lines

• FSK modulation, asynchronous

• Data rate: 1200bps– Space at 1200Hz, Mark at 2200Hz

• Uses ASK channel at 387Hz (5bps) for signaling, flow control, error control

• Bit rate can be improved using specially conditioned lines

Winter 2005

ECE


513 -

Bell 201Bell 201

• Half-duplex over 2-wire lines at 1200bps or full-duplex over 4-wire lines at 2400bps

• 4-PSK modulation, synchronous

• 1200 baud in each direction only half of the theoretical limit

Dibits Phase Shift

00 45º

01 135º

11 225º

10 315º

Winter 2005

ECE


613 -

Modem FeaturesModem Features• Internal vs. external modems• Hayes compatible (intelligent) modem (AT command set)• Flow control• Synchronization

– Asynchronous, synchronous, pseudo-synchronous (start and stop bits removed)

• Error control– Retransmissions, forward error control– Modem level vs. file transfer level

• Data compression– Effective data rate– Modem level vs. file transfer level

Winter 2005

ECE


713 -

Modem FeaturesModem Features

• Proprietary Modems– Modulation– Error control– Data compression

• Fallback rates

• Protocol negotiation

• Echo cancellation

• Scrambling

Winter 2005

ECE


813 -

V.22V.22

• 1200 or 600 bps

• Asynchronous or synchronous

• Full duplex, 4-PSKDibits Phase Shift

00 90º

01 0º

11 270º

10 180º

At 1200bps

Winter 2005

ECE


913 -

V.22bisV.22bis

• 2400 bps• Asynchronous or synchronous input• Data synchronized between modems• Full duplex, 16-QAM• Includes 1200 bps at V.22

Dibits Phase Shift

00 90º

01 0º

11 -90º

10 180º

X X X XQuadrantChange

Bits withinQuadrant

Winter 2005

ECE


1013 -

V.32V.32

• 9600 bps• Asynchronous or synchronous input• Data synchronized between modems• Echo cancellation• Full-duplex • 1800 Hz carrier, 2400 baud, 32-QAM• Trellis coding, 32-bit coding space, only 16 used

at a time• V.32bis 14400 bps using 64-QAM

Winter 2005

ECE


1113 -

V.42 and MNP-4 V.42 and MNP-4 Error Correction ProtocolsError Correction Protocols• V.42 CCITT

• MNP – Microcom Network Protocol

• Assemble packets

• Send synchronously without start/stop bits

• BCC sent with packet

• Error recovery through retransmissions

• V.42 uses LAP-M (link access procedure for modems) with MNP-4 alternative

Winter 2005

ECE


1213 -

V.42bis and MNP-5V.42bis and MNP-5Data Compression ProtocolsData Compression Protocols• Maximum compression of V.42: 4:1• Maximum compression of MNP-5: 2:1• Unlike V.42, V.42bis does not have MNP-5 as

alternative However, some modems have both

• MNP-5– Run length encoding– Adaptive frequency encoding by characters

• V.42bis– Lempel-Ziv-Welch compression– Replaces strings with codes

Winter 2005

ECE


1313 -

56K Modems – V.9056K Modems – V.90

• Traditional modems have limitation of 33.6 kbps• ITU-T announced V.90 in 1998• 56 kbps over PSTN, downstream direction• V.90 replaces proprietary 56K standards by

3Com (X2) and Rockwell (K56Flex)• Client side: Analog modem on analog local loop

to PSTN• Server side: Digital modem with digital

connection to PSTN (e.g. ISDN)

Winter 2005

ECE


1413 -


• Quantization during A/D limits rate No user-to-user V.90 connection

• Pulse Code Modulation (PCM)

• PCM sends m bits 2m values

• (Inverse) PCM m=8, baud rate = 8000 Theoretical limit = 8 x 8000 = 64 kbps

V.90 Client

D/A

A/DLocal Office PSTN V.90

Server

Internet

Winter 2005

ECE


1513 -


• 1 bit used by PSTN 7 bits (64k 56k)

• Noise, power limitations further reduces the number of distinct levels

• Typically, rate is ca. 40 kbps

• Uploads still limited to 33.6 kbps (V.34bis)

Documents

13 - Winter 2005 ECE ECE 766 Computer Interfacing and Protocols 1 Modems Goal: Carry data over telephone lines Modulator / Demodulator Baud rate / Bit