What is a comm. Network?
A comm. Network is a set of equipment & facilities that provides a service: the transfer of information between entities/users located at various geographical points. Examples:
Telephone networks, Computer networks , television broadcast networks, cellular telephone networks & the internet
Two unique capabilities that form the basis for many existing & unlimited number of future network based services:
Extreme high speeds allows users to gather large volume of
information nearly instantaneously
With the aid of computers, user can take action at a
Examine networks from the point of view
of the services
Radio & Television broadcasting (the most
common comm. Service)
The role of the user is passive
Relatively high audio & video quality is
expected
A significant amount of delay (order of sec.
or more) can be tolerated even in ‘live’ broadcasts
From the point of view of the services they provide to the user (Cont.)
Telephone service ( most common RT service) The service is connection oriented
RT requirement that delays cannot be greater than
250 ms
The service must be reliable
The delivered voice signal must be intelligible Not only to recognize the speaker but also to
discern subtleties in intonation, mood and so on
High degree of availability
Cellular telephone service
Offer mobility to users within a regional
area covered by an interconnected array of smaller geographical areas called cells
Design compromises resulting lower voice
quality, lower availability and greater exposure to eavesdropping
Handing off mechanism should be provided
Should support roaming service while
Electronic mail
User provides a text message and an address
to a mail application
MA interacts with a local MS which transmits
to a DS across a computer network
Destination user retrieves message using a
MA
It is not a RT service, so fairly large delays
can be tolerated
Not necessarily connection oriented service
Does require reliability
World Wide Web (WWW)
WWW application consists of a framework
for accessing information from computers connected to the internet
WWW is accessed through a browser
program
Allows user to access other documents by
clicking hyper links
Does not require RT response but excessive
Video on demand
It is not RT and can tolerate delay as long
as the responsiveness expected is not affected
Frame jitter or delays between consecutive
frames should not vary too much
The service requires relatively high audio &
video quality
Security and privacy are a concern in the
From the above discussion, we saw that
different user applications impose different requirements on the services provided by the network in terms of:
Transfer delay
Reliability of service
Accuracy of transmission
Volume of information that can be
transferred
Cost & convenience
Examine network from the point of
view of the network designer
The task of the designer is to develop an
overall network design that meets the user requirements in a cost-effective manner.
The following essential functions must be
provided by any network:
• Basic user service-the primary services that
the network provides to its users
• Switching approach- the means of
Examine network from the point of
view of the network designer
• Terminal- the end system that connects
to the network
• Information representation-the format of
the information handled by the network
• Transmission system- the means for
transmitting information across a physical medium
• Addressing- the means for identifying
Examine network from the point of
view of the network designer
• Routing- the means for determining the path
across the network
• Multiplexing- the means for connecting multiple
information flows into shared connection lines
Several approaches can provide these essential network functions. Let’s see the telegraph
network, the TN and the internet as examples of how these approaches have been applied in
Key factors in comm. Network evolution
Four factors:
Technology- the technology must be
available to implement the service in a cost-effective manner
Regulation- government regulations
must permit such a service to be offered
Market- the market for the service must
exist
Standards- a set of technical
Role of technology
Capabilities of various technologies
improved dramatically over the past two centuries along with the
reductions in cost
Fundamental physical considerations
place limits on what technology can ultimately achieve
As no signal can propagate faster than
Role of technology
The capabilities of a given technolgy
Role of technology
Role of technology
Other key technologies like signal
processing technology & digital computer technology augmented in the development of comm. Networks
Computer memory capacity & computer
processing play key role in the operation of network switches & the implementation of network protocols
Every 18 -24 months, the same dollar buys
Role of regulation
Traditional POTS have been government
regulated
First move by FCC in 1968
The break up of AT&T in 1984 gives an
impetus
At last, the telecomm act of 1996
A global trend towards deregulation
Role of the market
Ultimately determined by a customer’s
willingness to pay, which depends on the cost , usefulness & appeal
Usefulness depends on there being a
critical mass of subscribers
Cost reduces due to economies of scale
Early 1970s, picture phone service failed
due to market limitation
Late 1970s, Cellular radio telephony is
Role of the standards
Standards are basically agreements, with
industry-wide, national & possibly
international scope that allow equipment manufactured by different vendors to be interoperable
De jure standards result from a consultative
process- like some telephone standards developed by ITU
Some standards are set by
Role of the standards
De facto standards arise from the
dominance of certain products in a market- like Microsoft's windows OS or Intel’s
microprocessors
The existence of standards enables smaller
companies to enter large markets, thus
results in increased rate of innovation and evolution of both the technology & the
Who’s who in the telecomm world
ITU has three main sectors
Radio Comm sector (ITU-R)
Telecomm standardization sector (ITU-T) (former
CCITT)
Development sector (ITU-D)
ITU has five classes of members
Administrations ( national PTTs)
Recognized private operators (AT&T, MCI, BT)
Scientific & industrial organizations
Regional telecomm organizations (ETSI)
Other interested organizations (banking & airline
networks)
