Chapter 3Basic Data Communication Technology

• Business Impact of Modem Technology

• Modems

• Data Compression

• Communication Errors

Business Impact of Modem Technology

• Faster

• Efficient

• Reliable

• Secure

Faster

• New improved model

• Digital transmission– Benefits of Digital Transmission

• Better data integrity (detect & correct error)

• Higher capacity cables (fiber-optic)

• Easier integration (voice, data, video, etc.)

• Better security and privacy (encrypt data)

• Lower cost (large-scale integrated circuitry)

Efficient

• Data compression

• Backward compatibility

Reliable

• Error prevention

• Error detection

• Error correction

• Error control standard

• Flow control

• Line-failure backup

Secure

• Access control– Logical: password, callback– Physical

• Encryption

Modems

– Standard– Functions– Interfaces– Handshaking– Types– Selection Criteria

Standards

• V. by ITU-T (International Telecommunications Union-Telephony Sector)– V. bis & V.ter

• MNP standard (Microcom networking protocols)• Modem standard: V.34 (28.8 Kbps)• Data compression standard: V.42bis & MNP 5• Error control standard: V.42 & MNP 4

Modem Functions

• Convert signals

• Reverse channel for signal one another

• Auto dial/auto answer

• Modem diagnostics for accuracy checking

V.34 Technical Innovation

• Multiple baud rates/carrier • Greater baud rate• Auxiliary management channel• Asymmetric transmit/receive speed• Adaptive line probing• Precoding and nonlinear encoding• Fallback/fallforward• Trellis-coded modulation• V.8 training specification

Modem Interfaces

• Between modem and line– Two- to four-wire cable– RJ-11 plug

• Between DTE and modem– RS-232 interface

• Software– Hayes AT command set

Modems - Handshaking

• Exchange signals between modems

• Test the characteristics of circuit and quality

• determine baud rate, modulation technique and error control

Modem Types - I

• Simplex, half-duplex, full-duplex transmission modems

• Asynchronous or synchronous transmission modems

• Acoustically coupled modems (portable)

• Limited distance modems/short haul modems (less than 20 miles)

• Modem eliminators/null modems (cable less than several thousand feet)

Modem Types - II• Facsimile modems

– Error correction and data compression

• Modem for fiber-optics circuits– Digital-electrical to digital-optical

• Cable modems– DTE to cable television system cable– Internet and cable television system problems

• Cellular modems

Modem Types - III

• Wired/wireless modems

• ISDN modems

• CSU/DSU

• Enhanced V.34 modems

• 56-kbps modems

• Digital simultaneous voice data (DSVD)

• Analog simultaneous voice data (ASVD)

• Asymmetric digital subscriber line (ADSL)

Modem Selection Criteria

• Digital or analog signals

• Asynchronous or synchronous

• Speed

• Distance

• Type of line

• Cost

• Use compatible modems for both end

Data Compression/Compaction

• Purpose: to improve transmission efficiency• Types

– Character compression

– Run length coding

– Character stripping

– Combination of the above three

• Consideration– throughput

– Storage and transmission cost

– Hardware cost & software cost

Circuit Errors

• Background noise (electrical phenomenon)• Impulse noise (spike)• White noise• Attenuation• Envelope delay distortion (different propagation

delay)• Phase jitter • Echo• Crosstalk• Dropouts

Error Prevention

• Line conditioning for leased lines reduce– Attenuation

– Envelope delay distortion

– Noise

– Distortion

• Methods– Shielding or insulation

– Improving connection

– Equalizer (delay distortion)

– Echo suppressor

– Electronic versus mechanical equipment

– Repeaters or amplifiers (attenuation)

– Adaptive protocols: adaptive size packet protocol & dynamic speed shift

Error Detection

• Echo checking every character

• Vertical redundancy checking (VRC) or parity checking

• Longitudinal redundancy checking (LRC) or block check character (BCC)

• Checksum

• Cyclic redundancy checking (CRC)– CRC-16 or CRC-32

Parity & Longitudinal Redundancy Checks Example

• Date bits VRC• 1 1 1 1 1 1 0• 0 0 0 0 0 0 0• 1 0 1 0 1 1 0• 0 0 0 1 0 1 0• 0 0 0 0 1 0 1• 0 0 1 0 0 0 1• 0 1 0 1 0 1 1

• 0 0 1 1 1 0 1 LRC

Checksum Example - I

• Add ASCII decimal face value of the 128 characters in the block– 7 5 6 4 3 2 1 0– 128 64 32 16 8 4 2 1– 1 0 0 0 0 0 1 (A)– 64+1=65– 128 A is 128*65=8320

• Divide the number by 255– 8320/255=32 r 160

Checksum Example - I

• Take the remainder and express in binary format– 7 5 6 4 3 2 1 0– 128 64 32 16 8 4 2 1– 1 0 1 0 0 0 0 0

• Transmit 10100000 (checksum)

Error Correction• Equipment and circuit requirement

– Buffer

– Reverse channel or D channel

• Retransmission - automatic repeat request (ARQ)• Methods

– Discrete ARQ - ACK/NAK: long data block & low error rate

– Continuous ARQ - sliding window protocol with block sequence number: long propagation time

– Selective ARQ

– Forward error correction (FEC): simplex transmission• Trillis-Coded modulation

• High cost (extra bits & codes)

Communication Software Analysis

• Easy to use

• Efficient

• Cost-effective

• Compatibility

• Support overall business objectives

File Transfer Protocols - I

• XMODEM (simple, less reliable error checking)

• XMODEM-CRC (more reliable)

• XMODEM-1K (more efficient)

• XMODEM-G (G for streaming protocol)

• YMODEM-CRC-16 (Reliable, multiple files transfer)

• YMODEM-Batch

• ZMODEM (fast, failure recovery)

File Transfer Protocols - II

• X.PC (pack switching network, multiple sessions on one circuit)

• KERMIT (reliable, not very fast, PC & mainframe)

• Sliding window Kermit (fast)

• Point-to-point Protocol (PPP)– PC to a TCP/IP network– Authentication, compression, error correction,

& packet sequencing

File Transfer Protocols - III

• Serial Line Internet Protocol (SLIP)– Dial-up or leased lines– No error correction

Selection Criteria for File Transfer Protocol

• Error checking

• Error Correction

• Block size

• Batch capability

• Multiple platforms

• Auto recovery

Assignment

• Review chapters 1-3

• Read chapter 4

• Prepare exam 1