07-12-2012, 12:34 PM
Communication Networks
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Introduction
A computer network is the infrastructure that allows two or more computers (called
hosts) to communicate with each other. The network achieves this by providing a set
of rules for communication, called protocols, which should be observed by all
participating hosts. The need for a protocol should be obvious: it allows different
computers from different vendors and with different operating characteristics to
‘speak the same language’.
This chapter introduces the fundamental concepts of computer networks. We
will first look at constituent network components and various network types, and
then describe a reference model for network protocol architectures which we will
expand upon throughout the rest of this book. We will also discuss the role of
international standards and major standards organizations.
Network Components
Figure 1.1 shows an abstract view of a network and its hosts. The network is made
up of two types of components: nodes and communication lines. The nodes
typically handle the network protocols and provide switching capabilities. A node is
usually itself a computer (general or special) which runs specific network software.
The communication lines may take many different shapes and forms, even in the
same network. Examples include: copper wire cables, optical fiber, radio channels,
and telephone lines.
A host is connected to the network by a separate communication line which
connects it to one of the nodes. In most cases, more than one host may be
connected to the same node. From a host’s point of view, the entire network may be
viewed as a black box, to which many other hosts are connected. Each host has a
unique address allocated to it by the network. For a host to communicate with
another host, it needs to know the latter’s address. All communication between hosts
passes through the nodes, which in turn determine how to route messages across the
network, from one point to another.
The Physical Layer
The physical layer is concerned with the transmission of raw data bits over
communication lines. Physical layer standards and protocols are concerned with
issues such as the following:
· How a physical circuit is established between communicating devices.
· How the circuit is terminated when no longer needed.
· The physical form (e.g., voltages, frequencies, timing) in which data bits (binary
values 0 and 1) are represented.
· Whether transmission of data can take place in one or both directions over the
same physical connection.
· Characteristics of the physical media that carry the signals (e.g., copper wire,
optical fiber, radio waves).
· Characteristics of the connectors used for connecting the physical media.
· How data from a number of sources should be multiplexed before transmission
and demultiplexed upon arrival, and the type of multiplexing technique to be
used.
· The type of modulation to be used for transmitting digital data over analog
transmission lines.
The physical layer accounts for much of the tangible components of a network,
including cables, satellites, earth stations, repeaters, multiplexers, concentrators, and
modems. Physical layer protocols and standards are of mechanical, electrical,
functional, and procedural nature.
The Transport Layer
The aim of the transport layer is to isolate the upper three layers from the network,
so that any changes to the network equipment technology will be confined to the
lower three layers (i.e., at the node level). Transport layer protocols are concerned
with the following issues:
· Establishment and termination of host-to-host connections.
· Efficient and cost-effective delivery of data across the network from one host to
another.
· Multiplexing of data, if necessary, to improve use of network bandwidth, and
demultiplexing at the other end.
· Splitting of data across multiple network connections, if necessary, to improve
throughput, and recombining at the other end.