Wireless Network Design: Optimization Models and Solution ...

10 Dinesh Rajan
The goal of this chapter is to introduce the basic components of a wireless communications
system. To understand the functioning of digital communication devices,
we first introduce the layering principle that is commonly used in the design
of these systems.
2.1.1 Layered Architecture
The task of transmitting information from a transmitter device to a receiver device is
a complex process that requires careful design, analysis, and implementation. This
challenging task is usually accomplished with a layered architecture such as the
open systems interconnection (OSI) model proposed by the international standards
organization (ISO), which is shown in Figure 2.1. An OSI model, typically consists
of 7 layers which function together to accomplish the required data transmission
task. The 7 layers and a brief description of their functionality are:
• Physical Layer: This layer performs the basic task of converting digital bits into
signals that may be sent and recovered over the transmission medium.
• Data Link Layer: This layer provides a certain amount of data reliability over a
particular link and also assists in separating the signals from the various users
that are transmitted over the same channel.
• Network Layer: The network layer provides functionality such as routing to enable
data packets to be efficiently sent over multiple hops in a network.
• Transport Layer: This layer provides a mechanism to ensure end-to-end reliability
of data. The transport layer is also tasked with creating congestion control
mechanisms that reduces packet losses in a network occurring due to buffer overflows.
• Session Layer: The session layer provides services such as directory assistance
to enable efficient and reliable data delivery.
• Presentation Layer: This layer typically provides data encryption and compression
functionality.
• Application Layer: This layer includes all other tasks involved in the data transmission
process as well as preparing the data for user or machine consumption.
The advantages of a layered architecture is that the design and implementation of
each layer is simplified and can be done independent of the implementation of the
other layers, as long as the interfaces with the layer above and below it are standardized.
This freedom in implementation allows many of these layers to be reused in
many different systems. For instance, the transmission control protocol (TCP) functionality
(which forms the backbone for the Internet) could be the same irrespective
of what physical layer transmission scheme is used. Also, changes in the implementation
of one layer are transparent to the other layers. One of the disadvantages
of a layered architecture is that the resulting end-to-end system performance may
not be optimized. For instance, the use of TCP functionality which was originally