Broadband Transmission Rong Wang CGS3285 Spring2004

RECOMMENDED READING From textbooks: Page 81-91 of Data Communications: From Basics to Broadband, 3rd Edition by William J. Beyda (ISBN: 0-13-096139-6) Chapter 5 of Data Communications and Networking, 3rd Edition, Behrouz A. Forouzan (ISBN: 0-07-251584-8)

BROADBAND and BASEBAND Baseband transmission A single data signal is transmitted directly on a wire e.g., TD and RD signal on RS-232-C Broadband transmission Data to be transmitted are sent using a carrier signal , such as a sine wave. Many different frequency carrier signals can be transmitted simultaneously, more than one signal can be sent on the same wire. (e.g., cable television signals) In data communications, broadband transmission always uses analog signals to send data.

MODULATION and DEMODULATION Modulation - conversion of digital signals to analog Demodulation - conversion of analog signals to digital

MODEMS A device that performs modulation and demodulation is a modulator/demodulator or a modem. Modems are required for any broadband transmission medium Leased line modems Dial up modems Dumb - user dials destination number on telephone Smart - modem capable of dialing and establishing connection Different modems use different modulation techniques

TYPES OF MODULATION

AMPLITUDE MODULATION (Amplitude Shift Keying - ASK) Amplitude of the carrier signal is varied to represent binary 1 or 0 Frequency and phase remain constant Susceptible to variations in transmission quality Full duplex: add second signal on another frequency

FREQUENCY MODULATION (Frequency Shift Keying - FSK) Frequency of carrier signal is varied to represent binary 1 and 0 Amplitude remains constant Less susceptible to variations in transmission quality Full duplex: add two more frequencies

300 BPS, BELL 103/113 USING FSK ORIGINATING END ANSWERING END Transmit 1070 Hz SPACE 2025 Hz SPACE 1270 Hz MARK 2225 Hz MARK Receive 2025 Hz SPACE 1070 Hz SPACE 2225 Hz MARK 1270 Hz MARK

PHASE MODULATION (Phase Shift Keying - PSK) Frequency of carrier signal is varied to represent binary 1 and 0 Less susceptible to variations in transmission quality Uses fewer frequencies than FSK Full duplex: add second frequency

BIT RATE AND BAUD RATE Bit rate Baud rate The number of bits per second. Baud rate The number of signal units per second. Less than or equal to the bit rate.

Example 1 An analog signal carries 4 bits in each signal unit. If 1000 signal units are sent per second, find the baud rate and the bit rate Solution Baud rate = 1000 bauds per second (baud/s) Bit rate = 1000 x 4 = 4000 bps

Example 2 The bit rate of a signal is 3000. If each signal unit carries 6 bits, what is the baud rate? Solution Baud rate = 3000 / 6 = 500 baud/s

DIBIT PHASE SHIFT KEYING USING THE BELL 212A SPECIFICATION 1200 bps , 600 baud (changes per second) Must recognize four different phase changes Each transition / change, represents 2 bits

BELL 212A SPECIFICATIONS Channel Assignment (1200 bps): ORIGINATING END ANSWERING END Transmit Fc=1200 Hz Fc=2400 Hz Receive Fc=2400 Hz Fc=1200 Hz Phase Shift: DIBIT 212A Shift 00 90 degrees 01 0 degrees 10 180 degrees 11 270 degrees Can fall back to 300 bps using different tones

OTHER MODULATION METHODS Quadrature Amplitude Modulation Uses changes in both amplitude and phase shifting Total of 16 possible states Each state represents different four bit sequence Quadbit 600 baud * 4 bits = 2400 bps 2400 baud * 4 bits = 9600 bps High Speed Dial Up Modems (56 Kbps) Requires 1/2 of connection (ISP end) to be digital If not, drops to 28.8 or 33.6

MAXIMUM LINE TRANSMISSION RATES 4000 Hz - highest bandwidth on telco dialup circuit Pulse Code Modulation (chapter 10) Line sampled 8000 times per second (twice the maximum bandwidth) Each sample converted to 8-bits bytes 8 bits * 8000 samples / second = 64,000 bps 1 bit per byte may be “robbed” for signaling over T-carrier systems Can’t be guaranteed a “clear” channel Therefore, for data each sample limited to 7 usable bits 7 bits * 8000 samples / second = 56,000 bps

MAXIMUM MODEM TRANSMISSION RATES In perfect world, modems would same in the exact same way and exact same time as telco equipment Unfortunately, this is not a perfect world To increase throughput (bps) Use highest, most reliable frequency possible Use multi-bit encoding (dibit, quadbit,etc) Use data compression Use error detection Make circuit as digital as possible

DATA COMPRESSION IN MODEMS MNP-5 (Microcom Network Protocol) up to 2:1 in theory but actually less adds overhead even if data not compressible not good on pre-compressed files (e.g., zipped) v.42bis (CCITT/ITU) up to 4:1 in theory but actually less senses compressibility of data and shuts off if data not warranted works well with pre-compressed files (e.g., zipped) studies show 2.3:1 is more accurate estimate v.44 (CCITT/ITU) newest standard developed for internet applications twice as efficient as v.42 with web pages

ERROR CHECKING/CORRECTION MODEMS Data reorganized into small packets called frames Parity bits removed from each byte Checksum added to each frame to detect errors Frames sent in method similar to block, synchronous communication Two popular standards: MNP (Microcom Network Protocol) Levels 1, 2, 3 & 4 Some data compression included in levels 3 & 4 V.42 (CCITT/ITU) Link Access Procedure for Modems, or LAPM

STATE OF THE ART v.92 (introduced in 2000) Preceded by v.90, K56 and X2 modems 56 kbps modem Uses technology similar to v.34 & v.90 Downstream speed appr. 45-53 kbps Adds new features Modem on hold (longer call waiting times) Quick Connect (faster handshaking) PCM Upstream (48,000 bps) Incorporates v.44 data compression

FACSIMILE AND FAX MODEMS Facsimile, or fax, is the electrical transmission of documents over telephone lines. Analog: Group 1 100 lines per inch 3 min/page Group 2 100 lines per inch 6 min/page Digital: Group 3 200 lines per inch <1 min/page 4800bps + Group 4 400 lines per inch <1 min/page up to 64 kbps Facsimiles use data compression to speed document throughput