1. Analog and Digital Transmission Interfaces and Multiplexing (Physical Layer) Lita Lidyawati 2012

2. Multiplexing Multiplexing (“ muxing”) allows multiple flows to share a channel, within the limits of the overall capacity.

3. Multiplexing (‘cont) Frequency division (FDM) - analogous to radio spectrum within a cable; not a good environment for data due to noise from “baseband loading”. Time division (TDM) - interleaves bits from slower data streams onto a single, faster data stream.

4. Multiplexing (‘cont)

7. Converting Digital Information to Analog Information

8. Modulation Modulation means varying some property of a signal to impress information on the signal

9. Amplitude Modulation Assuming amplitude 1 = binary 0, and amplitude 2 = binary 1, this signal would represent 0011010

10. Phase Modulation + =

11. Quadrature Amplitude Modulation

12. first and second bit taken as a binary number are the multiple o f 90 o third bit indicates the amplitude

13. Quadrature Amplitude Modulation Example Let's encode a big bit stream: 001010100011101000011110 We break it up into 3-bit triads: 001-010-100-011- 101-000-011-110

14. Digital Transmission The foregoing discussion assumes the signal is modulated according to some continuous input that behaves in a way analogous to the information, for example, the output current from a microphone. Such a sample can be represented as binary numbers, or a “digital” signal

15. Encoding a Digital Signal An encoder samples, or measures the amplitude of the incoming analog signal 8,000 times a second The amplitude of each sample is given a pre-established 8- digit binary code, which is determined by the height of the sample. Each 8-digit binary code is transmitted behind the 8-digit binary code of the previously encoded sample in the conversation, creating a signal of 64,000 b/s (8,000 samples a second at 8 bits per sample.

16. Encoding a Digital Signal

17. Multiple Bits per Baud QAM is an example of the way modern modems can pack a lot of information into a sample. Depending on the quality of the analog channel, it is possible to encode several bits into every sample taken form the channel: multiple bits per baud Given n levels of signal that can be discriminated in each sample based on amplitude frequency or phase, the bit rate is:

18. Multiple Bits per Baud where C is the channel capacity as before and b is the signalling rate (also called sampling rate or baud rate) Shannon’s law defines the absolute limit for C

19. Multiple Bits per Baud Sample analog voice signal at the Nyquist rate = 2 fH (twice the highest frequency if fL= 0), or 2 X 4000 Hz = 8000 samples per second Convert each sample to an 8-bit binary number (called quantizing) using Pulse Code Modulation (PCM) Send this digital data as 8 (bit samples) X 8000 (samples per second), or 64,000 bps

20. Digital Transmission of Voice A group of 24 voice channels requires – 24 X 64 kbps = 1,536,000 bps – which can fit on a T1 carrier channel

21. Digital Audio Fidelity 8-bit PCM is very adequate for telephone use but is not “high fidelity” with regard to either noise or bandwidth. When a digitally encoded signal is converted back to analog, there is an added “noise of quantization”: thus for 8-bit coding S/N=216=65,536=48.2dB so the noise will be no better than 48.2 dB below maximum possible signal level

22. Digital Audio Fidelity For CDROM quality fH = 20kHz and fL= 0; and the sample is encoded in 16 bits; thus

23. Digital Pulse Codes Purpose: Make efficient use of available bandwidth while avoiding errors (also may be designed to eliminate DC component, as required by some media) – Non-Return to Zero-L (NRZ-L): straight binary data – Manchester: 01 = 1; 10 = 0 (two baud per bit); guarantees an equal number of ones and zeros; requires 2x bandwidth in medium – Bipolar alternate mark inversion (AMI): a pulse for each one; every pulse changes polarity

24. Digital Pulse Codes

25. Physical Interfaces EIA-232-D (“ RS-232”) – Most common serial interface – If used for asynchronous transmission, the interface can work with as few as five wires. – Many more pins are defined

26. Physical Interfaces EIA-449 (“ RS-449”) – Higher data rate (up to 2 Mbps) – Balanced line capable – Common on 56/64 kbps and T1/E1 links – Variations include RS-422, V.35 – Built-in loopback capability