Chapter 3 Basic 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 bitsVRC LRC

Checksum Example - I Add ASCII decimal face value of the 128 characters in the block – – – (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 – – – Transmit (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