Introduction to Satellite Communication 3rd Edition

Table
5.3
Comparison
of
Information
Requirements
for
Traditional
Telecommunication
Networks
Type
of
Information
Bandwidth
Variability
Latency
Requirement
or
Service
Requirement
(
kbps
)
of
Data
Rate
(
One
Way
)
Connection
Paradigm
Traditional
Network
Telephone
(POTS)
16
to
64
40%
duty
cycle
for
Between
150
and
Circuit
switched
PSTN
voice
500
ms
Fax
9.6
to
64
Continuous
but
time
Between
150
and
Circuit
switched
PSTN
varying
500
ms
Local
area
network
10
Mbps
to
1
Gbps
Variable
and
bursty
<
1
second,
depending
Packet
switched
Ethernet
(IEEE
802.3)
(LAN)
on
application
Internet
access
14.4
to
2,048
Highly
variable
to
Between
150
ms
and
Circuit
switched
PSTN;
dedicated
bursty
2
seconds,
depending
(virtual
circuit)
bandwidth
T1/E1
on
content
Video
teleconferencing
128
to
2,048
Continuous
but
time
Between
500
ms
and
Circuit
switched
T1/E1,
Internet
varying
1
second
(virtual
circuit)
Data
file
transfers,
Continuous
during
Between
500
ms
and
Any
Internet
SDH
(fiber),
small
64
to
<
15,000
transmission
10
seconds
Gigabit
Ethernet
large
45,000
to
1
Gbps
Up
to
2
hours
Digital
TV
SD
Approximately
2,500
Continuous
but
time
Up
to
10
seconds
for
Dedicated
bandwidth
Satellite
area
coverage
varying
broadcast
or
transmission;
local
radio
narrowcast
broadcast
and
cable
systems
HD
10,000
Continuous
but
time
Up
to
10
seconds
for
Dedicated
bandwidth
Satellite
area
or
spot
beam
varying
broadcast
or
coverage;
local
radio
narrowcast
broadcast
or
cable
systems

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156
Modulation, Multiple Access, and Impairments
5.1.2
Analog-to-Digital Conversion
The process by which a continuous information signal is converted into a stream
of digital data is called A/D conversion. Quite obviously, the reversal of digital
data to analog information is D/A conversion. A/D conversion is necessary to
convert analog signals, which generally are associated with some natural form of
activity or communication (e.g., voice or image), into a bit stream that can be
applied to a digital network. Many user devices such as personal computer audio
processors (‘‘sound cards’’) and video codecs perform this function as a first step
prior to delivery of the information to the Earth station. In applications such as
MSS telephony and DTH reception, the user terminal must perform the appropriate
direction of conversion.
The first step in A/D conversion is to sample the analog signal at equally spaced
points in time. Electronic devices that perform that measurement and conversion
operate at high rates, corresponding to twice the highest baseband frequency. The
voltage range being sampled is divided into narrow bands called quantization levels,
illustrated in Figure 5.2. (This linear quantization scheme is not representative of
real-world A/D systems which use some form of audio compression.) The logic of
the A/D converter puts out a numerical code, typically in the binary system.
The A/D conversion process introduces some inaccuracy into the end-to-end
link. Called quantization error, it is the small amount of information loss due to
the use of steps (as opposed to transferring a continuous range of voltage). The
basic relationship for quantization error, measured by the ratio of signal power to
quantization noise, is: