Figure 6-9: The PSTN: Mostly Digital with Analog Local loops Today's Telephone Network: Predominantly Digital Local Local Loop Loo p (Analog) Switch Digital) Rq( Digital) Residential Switch Trunk line Switch PBX Tel telephone Digital) Digital) Digital Digital) (Analog) Today, everything is digital except for the local loop access line and residential telephones The actual local loop line can carry either analog or digital signals but the equipment at both ends is analog 16
6-16 Figure 6-9: The PSTN: Mostly Digital with Analog Local Loops Trunk Line (Digital) Local Loop (Analog) Local Loop (Digital) PBX (Digital) Residential Telephone (Analog) Today's Telephone Network: Predominantly Digital Switch (Digital) Switch (Digital) Switch (Digital) Today, everything is digital except for the local loop access line and residential telephones. The actual local loop line can carry either analog or digital signals, but the equipment at both ends is analog
Figure 6-10: Codec at the End office Switch Digital Analog Signal ADc Signal p ocal Loo Telephone Codec Digital Switch Home DAC End ofiice a codec at the end office translates between residential analog and Pstn digital signaling ADC= analog to digital conversion DAC digital to analog conversion 6-17
6-17 Figure 6-10: Codec at the End Office Switch Analog Signal Local Loop Telephone Home Codec DAC Digital Switch ADC End Office Digital Signal A codec at the end office translates between residential analog and PSTN digital signaling. ADC = analog to digital conversion DAC = digital to analog conversion
Figure 6-11: Frequency Division Multiplexing(FDM) in Microwave Transmission BoX Codec Operation Microwave uses Channel 1/ circuit a radio transmission for pstn trunk lines Channel 2/ Circuit D Channel 3/ Circuit C Channel 4/Unused Channel 5/ Circuit e Each circuit is sent in a separate channel If channel bandwidth is large there will be fewer channels Voice uses 4 kHz-wide channels to allow more channels 6-18
6-18 Figure 6-11: Frequency Division Multiplexing (FDM) in Microwave Transmission Channel 1 / Circuit A Channel 2 / Circuit D Channel 3 / Circuit C Channel 4 / Unused Channel 5 / Circuit E Each circuit is sent in a separate channel. If channel bandwidth is large, there will be fewer channels. Voice uses 4 kHz-wide channels to allow more channels. Microwave uses radio transmission for PSTN trunk lines Box: Codec Operation
Figure 6-12: Analog-to-Digital Conversion(ADC): Bandpass Filtering and Pulse Code Modulation (PCM) BoX. Codec Step 1: Bandpass Filtering Operation Analog Voice Analog electrical Signal Filter at Subscriber End office Switch At the end office the voice signal is bandpass - filtered to limit its bandwidth to 4 Mhz This permits more calls to be multiplexed on trunk lines 6-19
6-19 Figure 6-12: Analog-to-Digital Conversion (ADC): Bandpass Filtering and Pulse Code Modulation (PCM) Subscriber Analog Electrical Signal Analog Voice Signal Filter at End Office Switch Step 1: Bandpass Filtering At the end office, the voice signal is bandpass-filtered to limit its bandwidth to 4 MHz. This permits more calls to be multiplexed on trunk lines Filter at End Office Switch Box: Codec Operation
Figure 6-12: Analog-to-Digital Conversion(ADC): Bandpass Filtering and Pulse Code Modulation (PCM) Box Codec Operation Step 1: Bandpass Filtering Signal Energy Distribution of Human Speech Along the Frequency Spectrum 0 Hz 300Hz 3400Hz(34kHz) 20 kHz Frequency Bandwidth (3. 1 kHz) Actually, to provide a safety margin, the signal is filtered to between about 300 hz and 3. 4 khz instead of from o hz to 4 khz 6-20
6-20 Figure 6-12: Analog-to-Digital Conversion (ADC): Bandpass Filtering and Pulse Code Modulation (PCM) 0 Hz Signal 300 Hz 3,400 Hz (3.4 kHz) 20 kHz Frequency Bandwidth (3.1 kHz) Energy Distribution of Human Speech Along the Frequency Spectrum Step 1: Bandpass Filtering Actually, to provide a safety margin, the signal is filtered to between about 300 Hz and 3.4 kHz instead of from 0 Hz to 4 kHz. Box: Codec Operation