Download Network Applications and Applications Protocols

Essential terms and concepts
Network Applications and Applications Protocols




Most computer networks support similar network-related applications like email.
The function of these applications across different networks is similar; the manner
in which they are implemented is protocol dependant.
An application program provides the users with an interface to interact with the
application, and it contains a related application protocol that defines the manner
in which an application communicates over the network.
Although nodes have to use the same application protocol, users are not restricted
in their use of application programs.
Computer Communications and Networking Models
Decentralized systems




Users maintain their own systems and there is no electronic exchange of resources
or information among these separate systems
Provides user with computing independence – it moves control closer to the end
user
Can result in data redundancy
Can be expensive to implement in terms of increased costs for hardware,
maintenance, and support
Centralized systems





A single computer houses all the data of an organization and users have access to
the data via terminals
All systems operations are under the auspices and control of a single department
Data redundancy and inconsistency are reduced or eliminated
Promote and ease data sharing among users
Are not as reliable as decentralized systems because of the presence of a single
point of failure
Distributed systems




Consists if independent computers interconnected to one another
Resources are made available to the user in a transparent manner; from the user’s
perspective it appears as if it was a single system
The key to the inherent transparency of a distributed system is specially designed
software generically called Network Operating System (NOS), which is usually
independent of a computer’s native operating system. A NOS can also be built
into the native operating system.
A distributed system can be thought of as an hybrid of decentralized and
centralized systems.
Client server




Can be thought of as dividing a network transaction in two parts: The client side
(or front end) provides the user with an interface for requesting services from the
network, and the server side (or back end) is responsible for accepting user
requests for services and providing these services transparent to the user.
Both terms – client and server – can be applied to either application programs or
actual computing devices
In TCP/IP, server processes on some systems such as UNIX are commonly
referred to as daemons and are designated by the letter d at the end of a program’s
name.
o HTTPD: Server side, Web server process
o HTTP: Client side, Web application process, (Browser)
A typical client / server interaction works as follows:
o A server process is started on a host, notifies the host that it is ready to
accept client requests and then waits for a client to request a specific
network application service
o Independent of the server process, a client process is started, usually by a
user through an application program. A request for service is sent by the
client process to the host that is providing the requested service and the
server program running on the host respond to the request
o When the server process has fully honoured the client’s request, the server
returns to a “wait” state and waits for another client request from the same
or another client.
Peer-to-Peer



Each networked host runs both the client and server parts of an application. This
is accomplished by installing the same NOS on all hosts within the network,
enabling them to provide resources and services to all other networked hosts.
Can be less reliable than Client / Server based networks
Unlike Peer-to-Peer networking, in the client / server model a network service can
only be provided if a server program responsible for servicing a request is running
on a particular host. Example: Dynamic Host Configuration Protocol (DHCP)
responsible to provide IP addresses automatically.
Web-Based



Is still based on the Client / Server model, it deserves a separate recognition
because of it’s potential for reshaping the manner in which resources are
provisioned to the end user
Generally involve the use of specially designed network-based devices,
commonly called network appliances, netappliances, information appliances, and
Internet appliances.
Network appliances usually rely on application service providers (ASP) to furnish
users with computing resources via the Internet.
Servant




Enables local and remote systems to be both client and server, similar to the Peerto-Peer model.
Napster is a good example of this model; all the hosts running Napster are
concurrently servers and clients.
Unlike the Peer-to-Peer model, which is one-to-one, or the client server model,
which is one-to-many, the servant model is many-to-many.
This concept is expected to be expanded into general file-sharing model that will
enable all interconnected hosts to exchange any type of file residing on their hard
drive.
Communication Service Methods and Data Transmission Modes
Serial and Parallel communications




Serial communication is data transmission in which the bits representing a
character of data are transmitted in sequence, one bit a time, over a single
communication channel.
Parallel communication refers to simultaneous transmission, each on a separate
channel, of all the bits representing a character.
Parallel communication requires a relatively complex communication link, which
is achieved through the use of multiwire copper cables. The longer the link, the
worse the degradation of the electrical signal.
In most networking applications, parallel communication is limited to peripherals
directly connected to a system and for communication between systems that are
relatively close to each other.
Synchronous and Asynchronous communications




Synchronous communication implies that each node monitors communication
between two nodes. If data is to be transmitted or received, then the nodes are
aware of this transmission almost immediately and prepare for the exchange
based on ordered data rates and sizes. It is also tied to the clocking inherent to the
link
Asynchronous communication is achieved by encapsulating the data by special
start and stop bits. A direct consequence of the inclusion of these start-stop bits
in the bit stream is that data can be transferred at any time by the sending node
without the receiving node having any advance notification of the transfer. The
receiving node does not necessarily know when the data string is being sent or the
length of the message.
Isochronous communication involves establishing specific bandwidth and data
rate requirements that a session will need so data flow between source and
destination is continual and uninterrupted. This is critical for delivery of
applications that require a Constant Bit Rate (CBR) of information to be sent and
delivered over the communication interface.
Most terminals, dialup modems and local links are asynchronous by nature.

Synchronous communication tends to be more expensive because of the need for
sophisticated clocking mechanisms in the hardware. However, Synchronous
communication can eliminate up to 20% of the associated overhead inherent to
asynchronous communication. This would allow greater data throughput and
better error detection. Synchronous communication is typically seen in higher
speed connections.
Simplex and Duplex communications



In simplex communication mode data may flow only in one direction; one device
assumes the role of sender and the other one assumes the role of receiver. These
roles may not be reversed.
In half-duplex transmission, data may travel in either direction, but only one unit
can send at any one time. While one is in send mode the other is in receive mode.
A full-duplex transmission involves a link that allows simultaneous sending and
receiving of data in both directions. It involves two separate but parallel
transmissions occurring simultaneously.
Analog and Digital Communications
In any computer communication system, data are transmitted across a medium from
sender to receiver in the form of electrical signal.
Analog communication




Refers to any physical device or signal that can continuously vary in
strength or quantity, for example voltage in a circuit.
Signals flow across a wire in the form of electromagnetic waves. When
viewed by an oscilloscope, these signals appear as continuous waves
called sinusoidal waves.
Sinusoidal wave have three attributes:
o Amplitude: Level of voltage on a wire
o Frequency: Number of oscillations, or cycles, of a wave in a
specified length of time
o Phase: The point to which a wave has advanced within it’s cycle
A frequency rate of one cycle per second is defined as one Hertz (Hz).
Thus, Hertz is a measure of frequency in cycles per second
Digital Communications

Refers to any physical device or signal that is coded in the binary form


A binary code is a system that uses two symbols zero (0) and one (1) to
represent data. A single 1 or 0 is referred to as binary digit, commonly
called a bit.
Signal is discrete, that is there is no in between. The signal consists of
only 2 states: electrical current applied or no current at all. On = 1, Off = 0
Speed and Capacity of a communication Channel


From a most general perspective, “speed” implies data rate, that is, how fast
data can be transmitted from one node to another.
Capacity implies the amount of data that can be carried by a communication
channel.
Bandwidth and Baud rate

In analog communications, bandwidth refers to the total capacity of a
communication channel.
o It is the difference between the highest and lowest frequencies
capable of being carried over the channel.

In Digital communications, bandwidth refers to the data rate
o It is the amount of data tat can be transferred over a
communication medium in a given period.
o Data is measured in bits per seconds (“bps”) and can vary
considerably from one type of channel to another.
Data rate (measured in bits per seconds) should not be confused with baud rate. A
baud is a measure of signalling speed. It is the number of discrete changes in a
single period of signal. Although baud represents a measurement of data
transmission speed, it does not correspond to the number of bits transmitted per
second.
Throughput
There is a difference between the maximum theoretical capacity of a
communications channel and the actual data transmission rate. Extraneous factors
such as node processing capability, input/output processor speed, operating
system overhead , communications software overhead, and amount of traffic on
the network at a given time all serve to reduce the actual data rate.
Noise
In the context of computer communications and networking, noise is any
undesirable, extraneous signal in a transmission medium.


Ambient noise: Also called thermal noise, is always present and is
primarily generated by transmission equipment (transmitters,
receivers, repeaters). Can also be induced by external sources such
as fluorescent light transformers, electrical facilities, and heat.
Impulse noise: Consists of intermittent signals induced by external
sources such as lightning, switching equipment and heavy
electrically operated machinery. It increases or decreases a circuit
signal level.
Whichever the type or source, noise degrades the quality of and performance of a
communications channel and is one of the most common causes of transmission
errors in computer networks. Although noise is always present, much of it can be
avoided through proper cable installation.
Multiplexing and Switching


Multiplexing is a technique used to place multiple signals on a single
communication channel.
Switching is a process that involves establishing an appropriate path, which data
message will follow as it travels throughout a network en route between a sending
source and a destination node.
Multiplexing


In it’s simplest form, it involves combining data from several relatively
low-speed input channels and transmitting these data across a single highspeed circuit.
Many different transmissions are possible using a single medium. (One
channel for voice, one channel for data, one for video) Each of these
separate, independent transmissions can occur simultaneously.
Multiplexing strategies:


Frequency division multiplexing (FDM): Partitions the available
transmission bandwidth in narrower bands, each being a separate channel.
Each subfrequency is customized to the bandwidth of data it must carry.
FDM transmissions are parallel by nature.
Time division multiplexing (TDM): Enables more than one signal to be
transmitted over the same channel but at different time interval. TDM




assigns to each node connected to a channel an identification number and
a small amount of time in which to transmit. TDM based transmissions are
serially sequenced.
Statistical multiplexing: Allocates part of a channel’s capacity only to
nodes that have to transmit. This permits a greater number of devices to be
connected because not all devices need to transmit at the same time.
Demand access multiplexing (DAM): Involves the creating and
management of a pool of frequencies. Pairs of communications
frequencies are assigned to a requesting station – one pair for
transmission, one pair for reception. When one or the other of the stations
are finished communicating the frequencies are deallocated and returned
to the frequency pool.
Wavelength division multiplexing (WDM): Used with fiber-optic cables.
The light sources, which are of different wavelength, are combined by a
WDM multiplexer and transmitted over a single line.
Inverse multiplexing: The reverse of multiplexing, Instead of
partitioning a single communication medium into several channels, an
inverse multiplexer combines several ‘smaller” channels into a high-speed
circuit. Also called line aggregation.
Switching



Circuit switched: Dedicated physical circuit is established between source
and destination nodes before any data transmission can take place. Remains in
place for the duration of a transmission. Reserved exclusively for that senderreceiver pair. (Ex: Telephone communication)
Packet switched: Instead of using a dedicated physical circuit, nodes share a
communication channel via a virtual circuit.
o A virtual circuit is a non-dedicated connection through a shared
medium that gives the user the appearance of a dedicated, direct
connection. Created by multiplexing a physical link so that the
physical link can be shared by multiple network programs or data
transmission.
o In a packet switched network, messages are partitioned into packets,
which may contain only a few hundreds bytes of data, accompanied by
addressing information and sequence numbers.
o Packet switched networks can employ 2 methods to deliver packets:
Virtual circuit or datagram service. In a virtual circuit all packets are
transported along the same virtual path as if it were a dedicated circuit,
and store and forward method is employed: All packets are stored and
then forwarded. In Datagram switching packets are transmitted
independently of each other, packets can travel along different paths
and can arrive out of order.
In a circuit switched network only the first data message carries the
destination address, which is needed to initially set up the link.
Circuit-Switching vs Packet Switching
Circuit Switched
Packet Switched
Bandwidth is allocated in advance and
is garanteed for the entire
transmission
Bandwidth is acquired and
released dynamically on an asnedded basis
Once circuit is established, the full
capacity of the circuit is available for
use, and the capacity of the circuit will
never be reduced due to other
network activity
Several communications can
occur concurently between nodes
using the virtual links over the
same physical channel
Circuit costs are independent of the
amount of data being transmitted and
hence any unused bandwidth is
wasted
As packet-switched networks
become overloaded with more
traffic, delays and congestion are
introduced
Packet-switched networks are
more cost effective and offer
better performance than circuitswitched networks.
Network Architecture and the OSI Reference Model
Network architecture is a formal, logical structure that defines how network devices and
software interact and function. It defines communications protocols, message formats,
and standards required for interoperability.
OSI Model




The Open Systems Interconnections (OSI) formally defines and
codifies the concept of layered network architecture.
Each layer is independent and isolated from other layers
Each layer is responsible for performing specific set of functions and
for providing specific set of services. Specific protocols define both
the service and the manner in which these services are provided.
Seven layers
o Application
o Presentation
o Session
o Transport
o Network
o Data link
o Physical

Each layer consists of two parts: a service definition, which defines the
type of services a layer provides, and a protocol specification that
details the rules governing the implementation of a particular service.
Lower layers provide services to upper layers.
OSI Services types
The OSI layers provide Connection-Oriented or Connectionless services. Some
layers may also provide multiplexing. Services are available at service access
points (SAP), with each SAP having a corresponding address.


Connection-Oriented: Implies that prior to the transfer of data a physical
(and virtual) is established. The link remains for the duration of
transmission, when transmission is terminated then the link is removed. (
wasted bandwidth, high potential for hung network, and guaranteed
sequential arrival of packet at destination.)
Connectionless: No physical link is established prior to data transmission.
Message partitioned into packets and routed through the network. Each
packet is independent of the others, must carry a destination address and
can arrive out of sequence. Can be Reliable or unreliable.
o Reliable: Requires an acknowledgement of receipt of data.
Acknowledged datagram service
o Unreliable: Does not require acknowledgment of receipt of data.
Datagram service.
<-----------Hardware--------->
<-------------------Software------------------>
Summary of OSI Layers and Functions
Aplication (7) Provides user application services and
procedures
Application or ServiceOriented Layers
Presentation (6) Structures data in a mutually agreed
format; concerned with issues such as how to code and
format data; includes data encryption.
Session (5) Controls process communications;
responsible for segmenting, buffering, and
synchronization
Transport (4) Provides end-to-end control; responsible Delivery and
for partitioning and reassembling messages
verification services
Network (3) Provides routing services for transferring
Communication or
data across the network; performs network management, Network Oriented
packet formatting
Layers
Data Link (2) Organizes data into frames; provides flow
initialization, flow control, link termination, and error
control
Physical (1) Transfers bits across link, defines physical
characteristics of media