Data Communication & Computer Networks
Solution Assignment 1
Q1: Explain the major classification of Computer Networks?
Ans: In its simplest form, data communication takes place between two devices that are
directly connected by some form of point-to-point transmission medium. Often, however,
it is impractical for two devices to be directly, point-to-point connected. This is so for one
(or both) of the following contingencies: The devices are very far apart. It would be
inordinately expensive, for example, to string a dedicated link between two devices
thousands of miles apart. There is a set of devices, each of which may require a link to
many of the others at various times. Examples are all of the telephones in the world and
all of the terminals and computers owned by a single organization. Except for the case of
a very few devices, it is impractical to provide a dedicated wire between each pair of
devices. The solution to this problem is to attach each device to a communications
network. The two major categories into which communications networks are traditionally
classified: wide-area networks (WANs) and local-area networks (LANs). The
distinction between the two, both in terms of technology and application, has become
somewhat blurred in recent years, but it remains a useful way of organizing the
Wide-area networks have been traditionally been considered to be those that cover a large
geographical area, require the crossing of public right-of-ways, and rely at least in part on
circuits provided by a common carrier. Typically, a WAN consists of a number of
interconnected switching nodes. A transmission from any one device is routed through
these internal nodes to the specified destination device. These nodes (including the
boundary nodes) are not concerned with the content of the data; rather, their purpose is to
provide a switching facility that will move the data from node to node until they reach
their destination. Traditionally, WANs have been implemented using one of two
technologies: circuit switching and packet switching. More recently, frame relay and
ATM networks have assumed major roles.
In a circuit-switched network, a dedicated communications path is established between
two stations through the nodes of the network. That path is a connected sequence of
physical links between nodes. On each link, a logical channel is dedicated to the
connection. Data generated by the source station are transmitted along the dedicated path
as rapidly as possible. At each node, incoming data are routed or switched to the
appropriate outgoing channel without delay. The most common example of circuit
switching is the telephone network.
A quite different approach is used in a packet-switched network. In this case, it is not
necessary to dedicate transmission capacity along a path through the network. Rather,
data are sent out in a sequence of small chunks, called packets. Each packet is passed
through the network from node to node along some path leading from source to
destination. At each node, the entire packet is received, stored briefly, and then
transmitted to the next node. Packet-switched networks are commonly used for terminal-
to-computer and computer-to-computer communications.
Packet switching was developed at a time when digital long-distance transmission
facilities exhibited a relatively high error rate compared to today's facilities. As a result,
there is a considerable amount of overhead built into packet-switched schemes to
compensate for errors. The overhead includes additional bits added to each packet to
introduce redundancy and additional processing at the end stations and the intermediate
switching nodes to detect and recover from errors. With modern high-speed
telecommunications systems, this overhead is unnecessary and counterproductive. It is
unnecessary because the rate of errors has been dramatically lowered and any remaining
errors can easily be caught in the end systems by logic that operates above the level of the
packet-switching logic; it is counterproductive because the overhead involved soaks up a
significant fraction of the high capacity provided by the network. Frame relay was
developed to take advantage of these high data rates and low error rates. Whereas the
original packet-switching networks were designed with a data rate to the end user of
about 64 kbps, frame relay networks are designed to operate efficiently at user data rates
of up to 2 Mbps. The key to achieving these high data rates is to strip out most of the
overhead involved with error control.
Asynchronous transfer mode (ATM), sometimes referred to as cell relay, is a culmination
of all of the developments in circuit switching and packet switching over the past 25
years. ATM can be viewed as an evolution from frame relay. The most obvious
difference between frame relay and ATM is that frame relay uses variable-length packets,
called frames, and ATM uses fixed-length packets, called cells. As with frame relay,
ATM provides little overhead for error control, depending on the inherent reliability of
the transmission system and on higher layers of logic in the end systems to catch and
correct errors. By using a fixed-packet length, the processing overhead is reduced even
further for ATM compared to frame relay. The result is that ATM is designed to work in
the range of 10s and 100s of Mbps, compared to the 2-Mbps target of frame relay. ATM
can also be viewed as an evolution from circuit switching. With circuit switching, only
fixed-data-rate circuits are available to the end system. ATM allows the definition of
multiple virtual channels with data rates that are dynamically defined at the time the
virtual channel is created. By using full, fixed-size cells, ATM is so efficient that it can
offer a constant-data-rate channel even though it is using a packet-switching technique.
Thus, ATM extends circuit switching to allow multiple channels with the data rate on
each channel dynamically set on demand.
Local Area Networks
As with wide-area networks, a local-area network is a communications network that
interconnects a variety of devices and provides a means for information exchange among
those devices. There are several key distinctions between LANs and WANs:
1. The scope of the LAN is small, typically a single building or a cluster of buildings.
This difference in geographic scope leads to different technical solutions, as we shall see.
2. It is usually the case that the LAN is owned by the same organization that owns the
attached devices. For WANs, this is less often the case, or at least a significant fraction of
the network assets are not owned. This has two implications. First, care must be taken in
the choice of LAN, as there may be a substantial capital investment (compared to dial-up
or leased charges for wide area networks) for both purchase and maintenance. Second,
the network management responsibility for a local network falls solely on the user.
3. The internal data rates of LANs are typically much greater than those of wide area
networks. Traditionally, LANs make use of a broadcast network approach rather than a
switching approach. With a broadcast communication network, there are no intermediate
switching nodes. At each station, there is a transmitter-receive that communicates over a
medium shared by other stations. A transmission from any one station is broadcast to and
received by all other stations. A simple example of this is a CB radio system, in which all
users tuned to the same channel may communicate. More recently, examples of switched
LANs have appeared. The two most prominent examples are ATM LANs, which simply
use an ATM network in a local area, and Fiber Channel. We will examine these LANs, as
well as the more common broadcast LAN’s.
Q 2: Explain structure and Function of OSI layers?
Ans: The OSI, or Open System Interconnection, model defines a networking framework
for implementing protocols in seven layers. Control is passed from one layer to the next,
starting at the application layer in one station, and proceeding to the bottom layer, over
the channel to the next station and back up the hierarchy.
Application (Layer 7):-
This layer supports application and end-user processes. Communication partners are
identified, quality of service is identified, user authentication and privacy are considered,
and any constraints on data syntax are identified. Everything at this layer is application-
specific. This layer provides application services for file transfers, e-mail, and other
network software services. Telnet and FTP are applications that exist entirely in the
application level. Tiered application architectures are part of this layer.
Presentation (Layer 6):-
This layer provides independence from differences in data representation (e.g.,
encryption) by translating from application to network format, and vice versa. The
presentation layer works to transform data into the form that the application layer can
accept. This layer formats and encrypts data to be sent across a network, providing
freedom from compatibility problems. It is sometimes called the syntax layer.
Session (Layer 5):-
This layer establishes, manages and terminates connections between applications. The
session layer sets up, coordinates, and terminates conversations, exchanges, and
dialogues between the applications at each end. It deals with session and connection
Transport (Layer 4):-
This layer provides transparent transfer of data between end systems, or hosts, and is
responsible for end-to-end error recovery and flow control. It ensures complete data
Network (Layer 3):-
This layer provides switching and routing technologies, creating logical paths, known as
virtual circuits, for transmitting data from node to node. Routing and forwarding are
functions of this layer, as well as addressing, internetworking, error handling, congestion
control and packet sequencing.
Data Link (Layer 2):-
At this layer, data packets are encoded and decoded into bits. It furnishes transmission
protocol knowledge and management and handles errors in the physical layer, flow
control and frame synchronization. The data link layer is divided into two sublayers: The
Media Access Control (MAC) layer and the Logical Link Control (LLC) layer. The MAC
sub layer controls how a computer on the network gains access to the data and permission
to transmit it. The LLC layer controls frame synchronization, flow control and error
Physical (Layer 1):-
This layer conveys the bit stream - electrical impulse, light or radio signal -- through the
network at the electrical and mechanical level. It provides the hardware means of sending
and receiving data on a carrier, including defining cables, cards and physical aspects. Fast
Ethernet, RS232, and ATM are protocols with physical layer components.