Understanding the network.pdf - Back to Home

sampling rate is 8,000 times per second. For digital audio, the standard is
47KHz.
• Quantizing—The next step is to round off each PAM signal to the closest
defined voltage value. Each defined value equals a specific analog frequency.
Although this rounding does add some noise to the signal, the symbol steps
are close enough that this "quantizing noise" is essentially inaudible.
• Encoding—Each defined voltage signal has a corresponding 8-bit word. The
final step is to convert quantized analog values into their 8-bit binary words
and transmit them.
After the analog signals have been converted into 8-bit words, they can be grouped
together and transmitted using a multiplexing technique. There are two types of
multiplexing techniques used for digital voice and data transmission: time division
multiplexing (TDM) and statistical time division multiplexing (STDM).
TDM and STDM
TDM is used for the majority of time-sensitive voice and data transmission
applications. With FDM, the transmission medium's available bandwidth is divided
into separate passbands, and each discrete circuit is assigned its own passband and
time-slot to carry the transmission signal. The circuits are then combined together
to form a single channel. Instead of each channel getting its own transmission
passband, however, each discrete circuit gets a chance to send a PCM word, and
then the next channel gets a turn. With TDM, each channel gets a turn to transmit
one PCM word during a transmission cycle. If the channel has no actual data to
transmit, bit-stuffing is used to fill the slot, because the transport frame is made up
of data from each of the discrete channels. Therefore, although TDM is great for
transporting data in real time (like voice and video), if there is no data to transmit,
the bandwidth is wasted.
This is where STDM comes into play. With STDM, each discrete circuit is not
assigned its own channel. Instead, STDM looks at which discrete channels actually
have data to transmit and then allocates the available time-slots to those active
channels to transport their data. The major advantage to using STDM over TDM is
that you can "over-subscribe" the transport circuit. Over-subscribing is the practice
of assigning more devices to a transport circuit than the circuit can transmit at one
time. When using TDM, each device is assigned its own channel. If the device has no
data to transmit, stuffing bits are sent in place of data, and this wastes useable
bandwidth. With STDM, a collection of devices can share the bandwidth, the idea
being that, statistically speaking, not all the devices need to transmit data at the
same time. The practice of over-subscription is commonly used in voice
applications.