Page 38 - Fundamentals of Communications Systems

1.4 Chapter One

1.3 Modern Communication System Engineering
Modern communication systems are very complex systems and no one engi-
neer can be an expert in all the areas of the system. The initial communication
systems were very simple point-to-point communication systems (telegraphy)
or broadcast systems (commercial radio). As these systems were simple, the
engineering expertise could be common. As systems started to get more so-
phisticated (public telephony), a bifurcation of the needed expertise to address
problems became apparent. There was a need to have an engineer who under-
stood the details of the physical channel and how the information was trans-
mitted and decoded. In addition there was a need to have an engineer who
abstracted the problem at a higher level. This “higher level” engineer needed to
think about switching architectures, supporting multiple users, scalability of
networks, fault tolerance, and supporting applications. As the amount of infor-
mation, system design options, and technology to implement these options grew,
further subdisciplines arose within the communications engineering field.
Modern communication systems are typically designed in layers to compart-
mentalize the different expertise and ease the interfacing of these multitude of
expertises. In a modern system, the communication system has a high-level net-
work architecture specification. This high-level architecture is typically broken
down into layers for implementation. The advantage of the layered architecture
in the design process is that in designing a system for a particular layer the
next lower layer can be dealt with as an abstract entity and the higher layer
functions do not impact the design. Another advantage to the layered design
is that components can be reused at each layer. This allows services and sys-
tems to be developed much more quickly in that designs can reuse layers from
previous designs when appropriate. This layered design eliminated monolithic
communication systems and allowed incremental changes much more readily.
An example of this layered architecture is the open systems interconnection
(OSI) model. The OSI model was developed by the International Organization
for Standardization (ISO) and has found significant utilization in practice. The
OSI reference model is shown in Figure 1.1. Each layer of abstraction communi-
cates logically with entities at the same layer but produces this communication
by calling the next lower layer in the stack. Using this model, for instance,
it is possible to develop different applications (e.g., e-mail vs. web browsing)
on the same base architecture (e.g., public phone system) as well as provide a
method to insert new technology at any layer of the stack without impacting the
rest of the system performance (e.g., replace a telephone modem with a cable
modem). This concept of a layered architecture has allowed communications
to take great advantage of prior advances and leap-frog technology along at a
phenomenal pace.
This text is entirely focused on what is known as physical layer commu-
nications. The physical layer of communications refers to the direct transfer
of physical messages (analog waveforms or digital data bits) over a commu-
nications channel. The model for a physical layer communication abstraction
is shown in Figure 1.2. Examples of physical communication channels include