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INTRODUCTION
A Communications Model
• Source
– generates data to be transmitted
• Transmitter
– Converts data into transmittable signals
• Transmission System
– Carries data
• Receiver
– Converts received signal into data
• Destination
– Takes incoming data
Simplified Communications
Model - Diagram
Simplified Data
Communications Model
Networking
• Point to point communication not
usually practical
– Devices are too far apart
– Large set of devices would need
impractical number of connections
• Solution is a communications
network
Simplified Network Model
Wide Area Networks
• Large geographical area
• Crossing public rights of way
• Rely in part on common carrier
circuits
• Alternative technologies
– Circuit switching
– Packet switching
– Frame relay
– Asynchronous Transfer Mode (ATM)
Network Models
Multi-layer Network Models
• The process of transferring a message
between sender and receiver is more
easily implemented by breaking it down
into simpler components.
• Instead of a single layer, a group of
layers are used, dividing up the tasks
required for network communications.
• The two most important such network
models are the OSI and Internet
models.
The OSI Reference Model
• Stands for Open Systems
Interconnection
• Created by the International
Standards Organization (ISO) as a
framework for computer network
standards
• Released in 1984, the model has 7
layers
The OSI 7-layer Model
Application: provides a set of utilities used by
application programs
Presentation: formats data for presentation to the
user, provides data interfaces, data compression and
translation between different data formats
Session: responsible for initiating, maintaining and
terminating each logical session between sender and
receiver
Transport: deals with end-to-end issues such as
segmenting the message for network transport, and
maintaining the logical connections between sender
and receiver
Network: responsible for making routing decisions
Data Link: deals with message delineation, error
control and network medium access control
Physical: defines how individual bits are formatted
to be transmitted through the network
The OSI Environment
OSI as Framework for
Standardization
Layer Specific Standards
Elements of Standardization
• Protocol specification
– Operates between the same layer on two
systems
– May involve different operating system
– Protocol specification must be precise
• Format of data units
• Semantics of all fields
• allowable sequence of PCUs
• Service definition
– Functional description of what is provided
• Addressing
– Referenced by SAPs
The Internet (TCP/IP) Protocol
• Stands for Transmission Control
Protocol/ Internet Protocol. Used on the
Internet.
• TCP/IP’s 5 layer suite was developed to
solve to the problem of internetworking
• Network layers can also be placed in
three groups:
– application layer (includes the application
layer),
– internetwork layer (includes the transport
and network layers)
– hardware layer (includes the data link and
physical layers).
The Internet’s 5-Layer Model
Application: used by application program
Transport: responsible for establishing
end-to-end connections, translates domain
names into numeric addresses and
segments messages
Network*: responsible for end-to-end
addressing and routing, determines
destination address if unknown
Data Link*: deals with message
delineation, error control & network access
Physical*: defines how information will be
transmitted through the network
*same as corresponding layer in OSI model
Network Models
OSI v TCP/IP
Message Transmission Using
Layers
• Network model layers use protocols, i.e., sets
of rules to define how to communicate at each
layer and how to interface with adjacent
layers.
• Generally, messages travel down all network
layers.
• When a message is sent to the next layer, that
layer places it in an envelope and adds
addressing information related to that layer.
• At the receiving end, messages travels up
through the network layers, each layer
removing the envelopes added when the
message was sent.
Fig. 1-4 Message transmission
using layers
NETWORK STANDARDS
The Importance of
Standards
• Standards are necessary in almost every
business and public service entity.
• The primary reason for standards is to
ensure that hardware and software
produced by different vendors can work
together.
• The use of standards makes it much
easier to develop software and hardware
that link different networks because
software and hardware can be developed
one layer at a time.
The Standards Making
Process
Two types of standards:
– Formal standards are developed by an
official industry or government body.
– Defacto standards emerge in the
marketplace and supported by several
vendors, but have no official standing.
The Standards Making
Process
Formal standardization process has three
stages
1. Specification stage: developing a
nomenclature and identifying the problems to
be addressed.
2. Identification of choices stage: those
working on the standard identify the various
solutions and choose the optimum solution
from among the alternatives.
3. Acceptance, the most difficult stage:
defining the solution and getting recognized
industry leaders to agree on a single, uniform
solution
Telecommunications
Standards Organizations
• International Organization for
Standards (ISO)
– Member of the ITU, makes technical
recommendations about data
communications interfaces.
Telecommunications
Standards Organizations
• International Telecommunications
Union - Telecommunication
Standardization Sector (ITU-TSS)
– Technical standard setting organization
of the UN ITU. Formerly called the
Consultative Committee on International
Telegraph and Telephone (CCITT)
– Comprised of representatives of over
150 Postal Telephone and Telegraphs
(PTTs), like AT&T, RBOCs, or common
carriers.
TC Standards Organizations
– American National Standards Institute (ANSI)
– Institute of Electrical and Electronics Engineers
(IEEE)
– Electronic Industries Association (EIA)
– National Institute of Standards and Technology
(NIST)
– National Exchange Carriers Association (NECA)
– Corporation for Open Systems (COS)
– Electronic Data Interchange -(EDI) of
Electronic Data Interchange for Administration
Commerce and Transport (EDIFACT).