DATA COMMUNICATION BASICS

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Oct 27, 2013 (3 years and 7 months ago)

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DATA COMMUNICATION BASICS


This tutorial describes introductory concepts in data communications.



What a
protocol

is




What a
packet
is




What a
gateway

( or
router
) and
routing

are




The concept of a layered protocol architecture




Layer services




Data
encapsulation




Data fragmentation and reassembly




Why standards are needed




The concept of an open system




Overview of the OSI 7
-
layer model and the TCP/IP model


What Is a Communication Protocol?


Communication
protocols

define the manner in which peer processes communicate
between computer hardware devices. The protocols give the rules for such things as
the passing of messages, the exact formats of the messages and how to handle error
conditions.


If two computers are c
ommunicating and they both follow the protocol(s) properly,
the exchange is successful, regardless of what types of machines they are and what
operating systems are running on the machines. As long as the machines have
software that can manage the protocol
, communication is possible.

Essentially, therefore, a computer protocol is a set of rules that coordinates the
exchange of information.


What Is a Packet?


The term packet is used often in data communications, sometimes incorrectly.

To transfer data eff
ectively, it is usually better to transfer uniform chunks of data than
to send characters singly or in widely varying sized groups. Usually these chunks of
data have some information ahead of them ( called the header ) and sometimes an
indicator at the end

(called the trailer). These chunks of data are loosely called
packets. In some data communications systems, "packets" refer to the units of data
passed between two specific layers in a protocol hierarchy e.g. the Data Link Layer
and the Network Layer of t
he OSI 7 layer model.


The amount of data in a packet and the composition of the header or trailer may vary
depending on the communications protocol as well as some system parameters, but
the concept of a packet always refers to the entire chunk of data (i
ncluding header and
trailer).

What Is a Host?


A host typically refers to a computer providing an information or communications
service.

What Are a Gateway, a Router, and Routing?


A
gateway

or
router
is a computer that interconnects two or more networks

and passes
packets from one to the other.

The gateway has an interface to each of the networks to which it is connected.
Generally, congestion on connected networks is avoided by keeping local traffic
confined to the network on which it originates,
except when the packets are destined
for another network. The gateway has the responsibility of acting as the switch that
allows such packets to go from one network to another.


The process by which the paths that packets travel across a network or interne
twork
are chosen is known as
routing
.


Protocol Layering


A wide range of problems may arise in packet
-
based data communication. These
include the following.




Host failure: A host or gateway may fail due to a hardware or software crash.



Link failure: A tr
ansmission link may be damaged or disconnected.



Network congestion: Networks have finite capacity which cannot be exceeded.



Packet delay or loss: Packets are sometimes lost during transmission or may
experience excessive delays.



Data corruption: Transmi
ssion errors can corrupt the data being transmitted.



Data duplication or packets out
-
of
-
sequence: Where more than one route exists
in a network connection, it is possible for transmitted packets to arrive out of
sequence. Also, packets that traverse a ‘lo
ng route’ may be retransmitted,
having been presumed lost. This results in the presence of duplicate frames if
the original and retransmitted packets arrive at the destination.


It is impractical to write a single protocol to handle all of these issues. Th
e early
methods were often based on a single protocol with many interacting components.
The resulting software was difficult to test, debug and modify. Complex data
communication schemes therefore require a family of co
-
operative protocols to handle
all tr
ansmission tasks ( Ref. 1 ).

The modules of protocol software on each machine can be organised into what may be
thought of as layers. Each layer handles a communication sub
-
problem. This is
illustrated in Figure 1.


.


Layer Services


Connection
-
Oriented

and Connectionless Services


The services that layers provide to the layers above them may be classified into two
types : connection
-
oriented and connectionless. In a connection
-
oriented service, the
sender of data first establishes a logical connection w
ith the ( prospective ) receiver of
the data, uses the connection ( sends the data ), and then terminates the connection (
Ref. 2). During the establishment of the connection, a fixed route that all packets will
take is defined, and information necessary t
o match packets to their session and
defined route is stored in memory tables in the gateways ( Ref. 3 ). Connection
-
oriented protocols provide in
-
sequence delivery; that is, the service user receives
packets in the order they were sent. Packet re
-
sequenci
ng may have to be
implemented internal to the protocol in order to achieve this, but the service user only
receives correctly sequenced packets ( Ref. 4 ).


In a connectionless ( or datagram ) service there is no initial end
-
to
-
end setup for a
session; eac
h packet is independently routed to its destination. When a packet is
ready, the host computer sends it on to a gateway. The gateway examines the
destination address of the packet and passes the packet along to another gateway,
chosen by a route
-
finding al
gorithm. A packet may go through many gateways in its
travels from one host computer to another. The fact that routes are dynamically
chosen means that it is possible for different packets from a single session to take
different routes to the destination (

Ref. 3 ). It is also possible for packets, having
traversed different routes to the destination, to arrive out of order and be passed on as
such to the layer above.


Thus, connectionless networks economise on gateway memory and connection set
-
up
time, whi
le connection
-
oriented networks economise on routing decisions (which
have to be redone for every packet in a connectionless network) ( Ref. 3 ).


Reliable and Unreliable Services


Services can also be classified according to the ‘quality of service’ that
they provide
to the layers above. There are two types of service quality : reliable and unreliable. A
reliable service is one that endeavours never to lose data during a transfer and to
provide the data error
-
free to the service user. In such a scheme the
receiver is
required to acknowledge the receipt of each item of data, to ensure that no data is lost
in transit. In addition to this, the receiver checks each data item received for errors,
informing the source if an error is detected and that another copy

of the affected data
should be sent.


The acknowledgement process required for reliable service introduces delays and
overhead. There are some cases when it is more important for the service to be free of
delays than for it to be one hundred percent relia
ble. In such situations an unreliable
service may be used. An unreliable service is implemented by omitting the
requirement for acknowledgements for the data received. Error checking may be done
by the receiver on each block of data, and when one is detect
ed ( even when it is only
a single unknown bit ) the complete data block discarded. When an unreliable service
is implemented in a given layer, reliability is typically implemented by some higher
layer.


Data Encapsulation, Segmentation ( Fragmentation ) a
nd Reassembly


Encapsulation is the technique used by layered protocols in which a protocol accepts a
message from a protocol above it and places it in the data portion of the lower level
protocol’s message unit ( Ref. 1 ). As data move down the layers, ad
ditional
information will be appended to it, and it may be
segmented

into smaller pieces. The
encapsulation process is illustrated for the
TCP/IP
model in Figure 2.



Say an application process on one host has some data it wishes to send. It passes the
data to the transport layer, which ( possibly ) modifies the data then attaches the
transport header, T, to it and passes the resulting item to the Internet layer. The
In
ternet layer treats the entire message received as data, may make some
modifications, and attaches its own header, I, to the message. This message is then
passed to the network access layer and the process is repeated. Here, the message
from the Internet l
ayer is enclosed in a physical network frame, the header from this
layer being represented as N. The data are then transmitted over the medium. Note
that, in each layer (below the application layer), depending on the protocol, a trailer
may be appended to
the message in addition to the header as is mentioned here.


On the receiving end the various headers are successively stripped off as the message
moves up the layers until it reaches the receiving process. Data are
reassembled

( i.e.
the segmentation sub
-
process is reversed ) if they had been fragmented during their
downward path in the transmitting system. At each layer the data at the receiving end
( after reassembly, if performed ) match data at the sending end ( before segmentation,
if performed ) ( Re
f. 4 ).


Data segmentation and reassembly are illustrated in Figure 3.



Why Are Data Communication Standards Needed?


Standards prevent a situation arising where two seemingly compatible systems really
are not. Data communication standards are created to ensure that individual products (
possibly from two independent sources ) designed to perform data communications
tasks

will actually be able to work together to perform these tasks. Once the standards
are adhered to, the respective designers should not need to collaborate in order to
achieve compatibility.


What Is an Open System?


Many definitions for the term
open syste
m
exist. Perhaps the best definition that can
be offered here is that an
open system
is one for which the architecture is not a secret.
The description of the architecture has been published or is readily available to
anyone who wants to build products for

a hardware or software platform. This
definition of an open system may be applied to both hardware and software.


Open systems are in contrast to
vendor
-
owned

or
proprietary

systems. In such
systems the vendor developes an architecture which he keeps a se
cret from the general
population. Consequently, only persons the vendor authorizes can build products that
will work with the vendor's products. Years ago, open systems were virtually
nonexistent. It was customary for hardware manufacturers to have product

lines which
bound clients to that manufacturer for all software and hardware needs. Some
companies took advantage of the captive market, charging outrageous prices or
forcing unwanted configurations on their customers.


The equipment from one manufacturer

that adheres to an open system standard can be
used interchangeably with the equipment from any other manufacturer that adheres to
that particular standard. This is so regardless of the other specifics of the vendors'
respective equipment ( e.g. operating

system, if this is not itself a part of the protocol
). The customer therefore has choices.


Established Data Communication Standards


Some vendors have established data communication models based on the layered
approach previously described. The vendors
base their systems or products ( such as
network operating systems ) on their own model. An example of this is Novell’s
IPX/SPX protocol architecture, upon which Novell Netware, a popular network
operating system, is based. As stated before, in such cases,

to develop applications for
a particular vendor’s products, one must adhere to that vendor’s model. This tends to
lead to considerable complication when products or systems from a number of
different vendors are to be integrated in some way. To address th
is problem, two
groups concerned with the interconnection of independent systems, the ISO and
DARPA, have defined commonly
-
used models for systems interconnection.


The International Standards Organization ( ISO ) developed a model for the
connection of op
en systems ( systems that are open for communication with other
systems ) known as the Open Systems Interconnection ( OSI ) reference model. The
OSI model has seven layers : Physical, Data Link, Network, Transport, Session,
Presentation, Application. The O
SI model is not a network architecture in that it does
not specify the actual protocols to be used in each layer ( Ref. 2 ). Rather, the model
clearly defines the functions to be performed by each layer and the boundaries
between each layer. The OSI model
is illustrated in Figure 4.



Prior to, and concurrently with the development of the ISO’s OSI model, the US
Department of Defense Advanced Research Projects Agency (DARPA) was heavily
involved in internetworking activity. It was such activity that result
ed in the creation
of the Internet. As a result of this activity, DARPA created a four layered model of
data communication known as the TCP/IP suite, which has become the de facto
standard for interconnection, despite the ‘official’ status of the OSI model
. The
TCP/IP model is illustrated in Figure 5.



The TCP/IP and OSI standards are the two major open system, vendor
-
independent,
standards in use ( Ref. 5 ).


REFERENCES


1) COMER, Douglas,
Internetworking with TCP/IP: Principles, Protocols and Architectu
re
,
Prentice
-
Hall, Englewood Cliffs, New Jersey, USA, 1988.

2) TANENBAUM, Andrew S.,
Computer Networks ( 3rd Ed. )
, Prentice
-
Hall, Englewood Cliffs,
New Jersey, USA, 1996.

3) MACKIE
-
MASON, Jeffrey K., VARIAN Hal R.,
Economic FAQs About the Internet
,
University of Michigan, Ann Arbor, MI 48109
-
1220, USA, June 1995.

4) McCONNELL, John,
Internetworking Computer Systems : Interconnecting Networks and
Systems
, Prentice
-
Hall, Englewood Cliffs, New Jersey, USA, 1988.

5) HALSALL, Fred,
Data Communications
, Computer Networks and Open Systems (3rd Ed.)
,
Addison
-
Wesley Publishers Limited, 1992.

Authored by
Cameron Stroke

October 20, 2011