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Analog and Digital Transmission Interfaces and Multiplexing (Physical Layer). Lita Lidyawati 2012. Multiplexing. Multiplexing (“ muxing ”) allows multiple flows to share a channel , within the limits of the overall capacity. Multiplexing (‘cont). - PowerPoint PPT Presentation
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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