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5.1.1
Digital Information Sources and Bandwidth Requirements
The range of digital information sources that provide input to satellite communica-
tion networks has grown rapidly over recent years. It begins with text and simple
graphic images and extends through standard definition and high definition televi-
sion, and streaming broadband and interactive multimedia. In this section, we
merely wish to identify the more familiar and promising of what can only be
described as an opening cornucopia.
The basic digital information inputs are classified according to the following
criteria:
Table 5.2
SONET (North America) and SDH (Worldwide) (Entries Are Expressed in Mbps)
Bit Rate
SONET Level
SONET Designation
(
Mbps
)
SDH Level
SDH Designation
STS-1
OC-1
51.840
STS-3
OC-3
155.52
1
STM-1
STS-9
OC-9
466.560
STS-12
OC-12
622.080
4
STM-4
STS-18
OC-18
933.120
STS-24
OC-24
1,244.160
STS-36
OC-36
1,866.240
STS-48
OC-48
2,488.320
16
STM-16
154
Modulation, Multiple Access, and Impairments
•
Bandwidth or data rate requirement at the user interface, usually a peak or
maximum value in kilobits, megabits, or even gigabits per second;
•
Variability of this data rate, that is, whether it is constant, time varying, or
bursty;
•
Acceptable end-to-end delay (latency), which is more important for inter-
active services and less important for one-way information distribution on
a broadcast, multicast or narrowcast basis;
•
Communication connection paradigm, namely, dedicated bandwidth,
circuit-switched or packet-switched.
Table 5.3 presents traditional telecommunication networks and services, evalu-
ated against the parameters listed above. Those four criteria are probably the most
important, but there are many more that can be critical to the success of a particular
network or service. The secondary factors, such as call setup time, ease of use,
availability, and cost per bit or minute of use, are important differentiators in the
competitive commercial marketplace.
The entries in Table 5.3 are suggestive of the options available over traditional
networks that are predominantly terrestrial in nature. Satellite communications
must, of necessity, reflect and accommodate these as they represent the mainstream.
There is a heavy dependence on fiber-optic systems and their inherent large band-
width capacity and high quality of transmission. Accomplishing this over satellite
links is not often practical, as discussed in Chapter 4. However, more and more
data communications are finding their way to wireless networks and these are
subject to many of the transmission issues that we associate with satellites.
In the past, terrestrial networks have employed standard transmission schemes
such as time division multiplexing (TDM) and Asynchronous Transfer Mode
(ATM). These share bandwidth among several different communication channels,
but accomplish this in fundamentally different ways. In TDM, the data rate for
each channel is fixed and dedicated to it; a variation of TDM called statistical
multiplexing allows some variability among the channels as long as the maximum
total is not exceeded (see Figure 3.5). ATM is a packet-switching scheme with a
fixed-length packet called a cell. Each 53-byte cell transfers only 48 bytes of data,
and so it takes many cells to service a particular demand. The switching ‘‘fabric’’
associated with ATM is based on very fast hardware technology and fiber optic
lines, and so the latency associated with this packet approach is quite low.
TDM and ATM are prevalent in a variety of applications, but may be replaced
ultimately by Internet-based protocols and services. This is because the Internet
offers the greatest versatility in serving users, who employ wire cable, fiber optics,
wireless systems, and satellite links. The only data communication scheme that
accommodates all of this so far is the Internet Protocol family (and in particular
TCP/IP). The dominance of TCP/IP results from its widespread use within LANs
and WANs, both in terms of use in the enterprise environment and at home.
Essentially every modern computer from to the PCs to largest server in a server
farm provides the foundation, as do the public networks that interconnect the
computers and applications.
5.1
Digital Baseband Signals and Hierarchies
155
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