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Q. How speech is transmitted in digital switching environment by using PCM/TDM?
A digital carrier system is a communications system which uses digital pulses instead of analogue signals to encode information. Figureshows the block diagram for a digital carrier system. This digital carrier system T1 employs PCM-encoded samples from 24 voice band channels for transmission over an optical fibre transmission line or single metallic wire pair. Every voice-band channel has a band width of about 300 Hz to 3000 Hz. Again, multiplexer is simply a digital switch with 24 independent inputs and one time division multiplexed output. PCM output signals from 24 voice-band channels are sequentially selected and connected through multiplexer to transmission line.
When a T1 carrier system, D-type (digital) channel banks perform sampling, multiplexing and encoding of 24 voice-band channels, every channel comprise an eight-bit PCM code and is sampled 8000 times a second. Every channel is sampled at the same rate. Figuredemonstrates the channel sampling sequence for a 24-channel T1 digital carrier system. As the Figureillustrates each channel is sampled once each frame though not at the same instance. Every channel's sample is offset from previous channel's sample by 1/24 of the total frame time. Hence one 64-kbps PCM-encoded sample is transmitted for every voice-band channel during each frame (a frame time of 1/8000 = 125 µs). Afterwards, an extra bit (known as the framing bit) is added to every frame. Framing bit occurs once per frame (8000-bps rate) and is recovered in the receiver, where it's used to maintain frame and sample synchronization between TDM receiver andtransmitter.
3. Why the external characteristics of a DC shunt generator is more drooping than that of a separately excited generator?
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