a distributed-queue dual-bus network (DQDB) is a distributed multi-access network that (a) supports integrated communications using a dual bus and distributed queuing, (b) provides access to local or metropolitan area networks
(Routing is the process of selecting paths in a network along which to send network traffic.)
OSI and TCP MODELS of Networking
The Theoretical Network
• Networks are complicated structures with
many interrelated parts.
• To identify the various parts that makeup
networks and how they work together , it is
useful to have a network model.
• International standard organization (ISO)
established a committee in 1977 to develop a
reference model for understanding computer
• Open Systems Interconnection (OSI) reference
model is the result of this effort.
• In 1984, the Open Systems Interconnection
(OSI) reference model was approved as an
international standard for communications
• Communication Architecture provides Strategy
for connecting host computers and other
necessary elements for data communication
• OSI model is a layer architecture design and It
is easy to debug network applications in a
layered architecture network.
• Term “open” denotes the ability to connect
any two systems which conform(obey the rules) to
the reference model and associated standards.
• The OSI model describes how information or
data makes its way from application
programmes (such as spreadsheets) through a
network medium (such as wire) to another
application programme located on another
• The OSI reference model divides the path of
moving information between computers over a
network medium into SEVEN smaller and more
manageable functions .
• This separation into smaller more manageable
functions is known as layering.
7 Layers of OSI Reference Model
Provides services that directly
support user applications
Translates data formats and adds
Establishing and maintaining
Adds identifiers to processes and
deals with error-handling information
Handles internetwork sequencing
,addressing and routing
Adds error checking information and
organizes bits into frames
Transmits and receives bits over
• Responsible for sending bits from one
computer to another.
• Deals with physical connection to the
network and with transmission and reception
mechanical, electrical, functional and
levels, timing of voltage changes, maximum
connectors, and other similar attributes are
defined by physical layer specifications.
Data link Layer
• Data link layer attempts to provide reliable
communication over the physical layer
• Create and detect frame boundaries.
acknowledgement and retransmission scheme.
• Implement flow control.
• Supports points-to-point communication.
• Supports simplex, half-duplex or full-duplex
• Implements routing of frames (packets) through
• Defines the best possible path the packet should
take from the source to the destination. Handles
overcrowding in the network.
• Defines logical addressing so that any endpoint
can be identified
• translates logical network
physical machine addresses.
• Purpose of this layer is to provide a reliable
mechanism for the exchange of data between
two processes in different computers.
• Ensures that the data units are delivered error
• Ensures that
• Ensures that there is no loss or duplication of
• Session layer provides mechanism for controlling
the dialogue between the two end systems. It
defines how to start, control and end
• a logical connection to be established on an enduser’s request.
• Any necessary log-on or password validation is
also handled by this layer.
• Session layer is also responsible for terminating
• also provide check-pointing mechanism
• data can be
from the last
• Presentation layer defines the format in which
the data is to be exchanged between the two
• Also handles data compression and data
encryption ,data translation, interpretation of
• Application layer interacts with application
programs and is the highest level of OSI model.
• such as file transfer, electronic mail, remote
• When a programmer writes an application
program that uses network services ,application
program will access this layer.
Communication between Layers
• Moving from the top, down - messages get larger and larger !
! A message is passed down, and the lower layer adds a
header to it. This is called encapsulation.
• Moving from the bottom, up - messages get smaller and smaller
! ! A message is first stripped of it's header, and then the inner
contents (the "data" portion) is passed up. This is
layer 0 or 8
y they exist
the User, as
down to it
IEEE 802 categories
• The institute of IEEE in February 1980 formed a
project called project 802 to help define certain
LAN standards, though “802” is sometimes
associated with the date the first meeting was
held — February 1980.
• IEEE 802 refers to a family of IEEE standards
dealing with local area networks and metropolitan
• IEEE 802 splits the OSI Data Link Layer into two sub-layers
named Logical Link Control (LLC) and Media Access Control
(MAC), so that the layers can be listed like this:
1. Data link layer
i. LLC Sub layer
ii. MAC Sub layer
2. Physical layer
• The IEEE 802 family of standards is maintained by the IEEE
802 LAN/MAN Standards Committee (LMSC). The most
widely used standards are for the Ethernet family, Token
Ring, Wireless LAN, Bridging and Virtual Bridged LANs. An
individual Working Group provides the focus for each area.
Some Working groups
Bridging (networking) and
Defines the MAC layer for a
Broadband LAN using
Fiber Optic TAG
• As network management is easier due to the
• Each layer has its own a set of rules and
requirements called protocol.
• The protocol defines the format of the data being
exchanged, and the control and timing for the
handshake between layers.
• Protocols that work together to provide a layer or
layers of OSI model are protocol stacks or suit.
• The protocol stack is an implementation of
a computer networking protocol suite. The terms are
often used interchangeably. Strictly notice, the suite
is the definition of the protocols, and the stack is the
software implementation of them.
• Protocol stacks are often divided into three major sections: media,
transport, and applications.
• A particular operating system or platform will often have two welldefined software interfaces: one between the media and transport
layers, and one between the transport layers and applications.
a) The media-to-transport interface defines how transport protocol
software makes use of particular media and hardware types ("card
For example, this interface level would define how
TCP/IP transport software would talk to Ethernet hardware.
b) The application-to-transport interface defines how application
programs make use of the transport layers.
For example, this interface level would define how a web
browser program would talk to TCP/IP transport software.
Internet protocol suite TCP/IP
• The Internet protocol suite is the networking
model and a set of communications protocols
used for the Internet and similar networks. It is
commonly known as TCP/IP.
• The Internet protocol suite and the layered
protocol stack design were in use before the OSI
model was established.
• It is one of the commonly used networks today.
The layers of
suite near the
closer to the
hile those near
the bottom are
closer to the
of the data.
It was developed by Department of defend’s Advanced projects
Research Agency DARPA, an agency of the United States
Department of Defense under its project on network
interconnection , started in 1969.
• TCP/IP provides end-to-end connectivity specifying how data
should be formatted, addressed, transmitted, routed and received
at the destination.
• It has four layers,
The link layer
The internet layer (IP)
The transport layer
The application layer
• Application layer (process-to-process):
This is the scope within which
applications create user data and
communicate this data to other processes
or applications on another or the same
host. This is where the higher level
protocols such as HTTP (hypertext
transfer protocol), etc. operate.
• Transport layer (host-to-host):
• This is where flow-control, errorcorrection, and connection protocols
exist, such as TCP.
• This layer deals with opening and
maintaining connections between
• The internet layer has the task of
exchanging data grams across network
• It defines and establishes the Internet.
This layer defines the addressing and
routing structures used for the TCP/IP
• Link / Network access layer:
• Despite the different semantics of layering in
TCP/IP and OSI, the link layer is often described
as a combination of the data link layer (layer 2)
and the physical layer (layer 1) in the (OSI)
• Management of reliable data delivery,
• access to physical networks
• Binary data transmission
As data is being sent from one computer, it will pass from the top
layer to the bottom. On the receiving end, the data will then be
rebuilt from the bottom layer to the top.
Each layer a packet of information travels through adds
what is called a header. Think of it in terms of a Russian
• When the packet is being rebuilt on the receiving
end, each header is unpackaged the same way.
• Since each layer of the TCP/IP model does a unique
task separate of the other layers, we refer to the data
package at each layer with different names. For
instance, the data package at the Application Layer is
called a message, while the same data package at the
Internet Layer is called a datagram.
• Once the data packet leaves link layer it is converted
into a bit stream of electrical pulses, commonly
referred to as 1’s and 0’s.
• all transport layer in TCP
implementations must choose
whether or how to provide
reliability. UDP(User Datagram
Protocol) provides data
integrity/errorless data via a
checksum but does not
guarantee delivery; TCP
Protocol) provides both data
integrity and delivery
guarantee by retransmitting
until the receiver acknowledges
the reception of the packet.
Connectionless vs. connection
• These are the two ways in which
communications between computers can
• Former assumes that all data will get
through ,do not check delivery &
• Faster working
• Connectionless Network Service is an OSI
Network Layer service.
Connectionless vs. connection
• Later presume that some data will be lost
or disordered .
• Assures data delivery in sequential order
by retaining data and negotiating for
Which is better?
Historically, a few companies tried to create protocol
stacks based on ISO/OSI, mostly throughout the 1980s
(Unger man-Bass, even the early versions of Microsoft
However, no implementation ever seen was "complete"
in that it covered all 7 layers. The ISO/OSI model is
simply more theoretical than practical.
On the other hand, the TCP/IP model is easy to
implement and there are numerous implementations of
Either way, during the mid 1980s, it became clear that
the stack of TCP/IP protocols that was built according
to the TCP/IP model were winning in the market.
Whatever "OSI inspired" products there were,
disappeared from the market.