Analog and Digital Transmission Interfaces and Multiplexing (Physical Layer)

Lita Lidyawati2012

Multiplexing

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

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.

Multiplexing (‘cont)

Multiplexing (‘cont)

Multiplexing (‘cont)

Converting Digital Information to AnalogInformation

Modulation

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

Amplitude Modulation

• Assuming amplitude 1 = binary 0, and amplitude 2 = binary 1, this signal would represent 0011010

Phase Modulation

+

=

Quadrature Amplitude Modulation

Quadrature Amplitude Modulation

• first and second bit taken as a binary number are the multiple o f 90o

• third bit indicates the amplitude

Quadrature Amplitude ModulationExample

• Let's encode a big bit stream: 001010100011101000011110

• We break it up into 3-bit triads: 001-010-100-011-101-000-011-110

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

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.

Encoding a Digital Signal

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:

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

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

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

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

Digital Audio Fidelity

• For CDROM quality fH = 20kHz and fL= 0; and the sample is encoded in 16 bits; thus

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

Digital Pulse Codes

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

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