1. Broadband Transmission
Rong Wang
CGS3285
Spring2004

2. RECOMMENDED READING From textbooks:
Page of Data Communications: From Basics to Broadband, 3rd Edition by William J. Beyda (ISBN: )
Chapter 5 of Data Communications and Networking, 3rd Edition, Behrouz A. Forouzan (ISBN: )

3. 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.

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

5. 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

6. TYPES OF MODULATION

7. 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

8. 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

9. 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

10. 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

11. 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.

12. 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

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

14. 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

15. 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
degrees
01 0 degrees
degrees
degrees
Can fall back to 300 bps using different tones

16. 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

17. 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

18. 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

19. 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

20. 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

21. 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 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

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