159.334 Computer Networks
Introduction to the OSI Model and the
Physical Layer
Professor Richard Harris
School of Engineering and Advanced
Technology (SEAT)
Learning Objectives
159.334 Computer Networks
You will be able to:
Describe the concept of Layering as it applies to
communication protocols
Identify and describe the basic components in the OSI
Reference Model such as:
the N+1, N and N-1 Layers,
Layer Entities,
the Service Access Point (SAP),
the role of primitives,
the difference between Inter-Layer and peer communication,
the basic functions of the various Layers of the OSIRM
Identify and describe the layered components of a TCP/IP
network
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References
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Forouzan, “Data Communications and Networking”, 4th
Edition
Tanenbaum, “Computer Networks”, 4th Edition
Cisco CCNA1 Module 10 - part 1
Stallings, William 2000 ‘Data and Computer
Communications’, Prentice Hall, Sixth Edition
Russell, Travis 1997 ‘Telecommunications Protocols’,
McGraw Hill
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Introducing Protocols
(Protocols in a Data Network)
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D
D
X.25
A
A
Network
Access
Internal
Protocols
Network
Access
High Layer (Level) User Protocols
(End to End)
Network Access
Protocols
Highly
standardise
d
Describe
how to use
a network
Specified as
if each user
is the only
network
user.
Internal
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Protocols
Introducing Protocols
(A Protocol Model for a Data Network)
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The Packet Switching Network of the Previous Slide can
be viewed in terms of a Layered Model
A
D
Network
Higher Layer (Level) Protocols
Higher
Layers
X.25 (L3)
X.25 (L3)
LAPB
X.21
LAPB
X.21
x
y
z
x
y
z
X.25 (L3)
X.25 (L3)
LAPB
X.21
LAPB
X.21
Node D
Node A
X.25 Network Access
Layers 1,2 & 3
Higher
Layers
Internal
Protocols
X.25 Network Access
Layers 1,2 & 3
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Introducing Protocols
(A Definition)
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According to the “Webster's New World Dictionary ”
A protocol is .. "a set of rules governing the
communications and the transfer of data between
machines, as in computer systems".
Other dictionaries will talk about the “rules of
interaction” between organisations, countries, people
or machines. The point here is that for two entities to interact /
communicate / co-operate there must be a set of rules
that describe how the communication is to take place
(including definitions of the semantics and syntaxes
involved)
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Introducing Protocols
(A Definition - 1)
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To “handle” complex protocols
designers “compartmentalised”
communication protocols into
Layers
Communications and data
transfer between machines takes
place at many "layers".
Each layer provides a Service
(via InterLayer Communication)
to it’s higher Layer through the
SAP
And communicates with its peer
in the remote system
Layer N+1
Layer N
Layer N-1
L1 SAP
Layer 1
Physical Medium
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Introducing Protocols
(A Definition - 2)
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These layers can be
implemented as software,
hardware or a combination of
both
InterLayer Communications can
be anything from software
messages to the raising or
lowering of voltage levels
Peer Communication is always
in terms of abstract Protocol
Data Units (PDUs).
The only actual communication
is on the Physical Medium
Layer N+1
Layer N
Layer N-1
L1 SAP
Layer 1
Physical Medium
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Introducing Protocols
(Entities and Primitives)
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Layer N+1
Layer N
Entities
in each
Layer
(N+1) Entity
Response
Indication
Confirm
Request
Primitives
at the
Layer
Boundary
(N+1) Entity
Layer N+1
Protocol
(N) Entity
Layer N
Protocol
(N) Entity
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Introducing Protocols
(Goals of the OSIRM)
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The Open Systems
Interconnection Reference
Model (OSIRM) effort strove to
define the functions and
protocols necessary for any
computer system to connect to
any other computer system,
regardless of the manufacturer.
In 1977, the International
Organisation for
Standardisation (ISO) started
work on the OSIRM. The Basic
reference model was
documented in 1983 as ISO
standard 7498.
OSI has standardized
many of these protocols,
but only a few are in
widespread use.
The layering concept,
however, has been
widely adopted by every
major computer and
communications
standards body and
most proprietary
implementations as well.
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Introducing Protocols
(OSIRM and the Future)
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Whilst ISO defined protocols for the OSIRM may not be
in wide use. Communication product vendors relate their
commercial offerings to the OSIRM.
That is…… Protocol X is a Transport Layer Protocol, or
Protocol Y is a Session Layer Protocol
It is, and will continue to grow to be the framework for
modelling of communication systems
While “Internet” protocols (i.e. those developed by the
IETF) will be commercially dominant.
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OSI Layered Model
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Communication between layers
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Overview of the Layers
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Application
To translate, encrypt and compress data
Presentation
Session
To provide reliable process to process
message delivery and recovery
To establish, manage and terminate
sessions
Transport
Network
To organise bits into frames; to provide
hop-to-hop delivery
To allow access to network resources
To move packets from source to
destination; to provide internetworking
Data Link
Physical
To transmit bits over a medium; to
provide mechanical and electrical
specifications
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Introducing Protocols
(The Basic OSI Reference Model - 1)
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Relating certain
network
equipment
classes to the
OSIRM
7. Application
6. Presentation
5. Session
S/W
Layers
Comms
Subnetwork
H/W
Layers
Gateway
4. Transport
3. Network
Router
2. Data Link
Bridge
1. Physical
Repeater
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Introducing Protocols
(The Basic Reference Model - 2)
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7. Application
6. Presentation
5. Session
4. Transport
3. Network
Defines network segmentation and network address
scheme. Connectivity over multiple network segments
Cornerstone on which all upper layers are based.
Units is “Packets”
Creates packet headers & checksum trailers. Packages
datagrams into frames. Detects errors. Regulates data
flow. Maps hardware addresses. (FDDI, Ethernet, T1)
Units is “Frames”
2. Data Link
1. Physical
Defines physical and electrical specifications for
transmission. Defines connector types and pin-outs,
voltage and current. (e.g., RS-232, V.35)
Units is “Bits”
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Introducing Protocols
(The Basic Reference Model - 3)
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7. Application
6. Presentation
5. Session
4. Transport
3. Network
Connection negotiation, establishes and maintains
connection, and synchronises dialog. (RPC, Nethios,
service 'ports')
Unit is “Message”
Assures end-to-end reliability. Translates & manages
message communication through subnetwork. Insure data
integrity and deals with packet sequencing. (e.g. TCP)
Unit is “Segment”
2. Data Link
1. Physical
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Introducing Protocols
(The Basic Reference Model - 4)
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7. Application
6. Presentation
Manages program requests that require access to services
provided by a remote system.(FFP, NFS, MHS, Netware
requester)
Unit is “Message”
5. Session
4. Transport
3. Network
Translates data format of sender to data format of receiver.
Also performs encryption. Provides data compression,
translation and encryption.
Unit is “Message”
2. Data Link
1. Physical
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Introducing Protocols
(End and Intermediate Systems - 1 )
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Various Levels
of Peer Communication
7. Application
7. Application
6. Presentation
6. Presentation
5. Session
5. Session
4. Transport
4. Transport
3. Network
3. Network
3. Network
2. Data Link
2. Data Link
2. Data Link
1. Physical
1. Physical
1. Physical
User A
End System
Intermediate Node
User B
End System
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Introducing Protocols
(End and Intermediate Systems - 2 )
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Each of the seven layers represents one or more
protocols that define the functional operation of
communications between user and network
elements.
All protocol communications between layers are
"peer-to-peer" - depicted as horizontal arrows
between the layers. Standards span all seven
layers of the model.
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Introducing Protocols
(Following Interlayer & Peer Communication)
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Actual
Communications
Path
Various Levels
of Peer
Communication
7
7
6
6
5
5
4
4
3
3
3
2
2
2
1
1
1
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Connection-oriented and
connectionless services
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The service each layer provides to the layer above can
be classified into two different types,
Connection oriented service requires a pair of processes
in the two computers to establish a connection before
sending data.
In connectionless service, data packets are independent
of each other and they may not arrive at the destination
in the proper order.
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Connection-oriented service over a
single link
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In connection-oriented service, a logical connection is
established by having both sides initialize variables and
counters needed to keep track of which frames have
been received and which ones have not
In connectionless service, this connection does not
need to be established, therefore data frames can get
lost, corrupted, duplicated, and out of order.
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Connection-oriented service over
multiple links
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In connection-oriented service, a path from the source to
the destination must be established before any data
packets can be sent. This connection is called a VC
(virtual circuit).
In connectionless service, no advance setup is needed.
In this context, the packets are called datagrams. The
datagrams carries the full destination address, and each
is routed through the system independent of all the
others.
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Application layer
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Provides the user interface, application programs,
access to distributed information services
ex.
File Transfer Protocol (FTP), E-Mail
Other important tasks
Identification and checking the availability of the called party
Agreement on encryption mechanisms and responsibility for
error recovery
Procedures for initiation and release of a dialog
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Presentation Layer
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Concerned with the representation of syntax of data
during transfer of information between two application
processes
Performs the necessary conversion if two application
processes use different syntaxes
ex. Two people who speak different languages communicating
with each other. The PL does the job of the interpreter. The PL at
each end translate the message to English before handing it to
SL
Performs encryption using a secured key if requested by
the AL
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Session Layer
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Sets up and clear the dialog between two PL protocol
entities
In the case of half duplex (two way alternate) mode of
dialog, it controls the exchange of data (dialog units)
Token management- in case both sides are not
supposed to attempt the same operation at the same
time, users are issued with tokens. Only the user who
has the token at a given time can perform the critical
operation
Can establish synchronization points during lengthy
transactions as requested by the user via SL
Reports of non-recoverable data to higher layers.
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Transport Layer
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Provides higher layers with a message transfer facility
which is efficient, reliable and cost-effective and
independent of the network layer
Can offer various classes of service because of the
varied quality of service (QOS) provided by different
networks - ranging from basic functions needed for
connection establishment and data transfer to full error
control and flow control
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Network Layer
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Concerns with routing messages from the source to the
destination through the network. This may require
making multiple hops at various nodes in the network
The main tasks of the NL are
Providing services to the TL, such as connection oriented or
connection less, explicit confirmation of delivery and priority
schemes
Routing of packets
Congestion control
compatibility issues associated with internetworking
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Data Link Layer
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DLL converts a raw transmission media (coaxial cable,
satellite channel) into a line that appears free of
transmission errors to the network layer and it concerns
about the link established between two ends of a single
transmission facility, i.e. no nodes or switching centres
in the middle
The main tasks are
Services provided to the NL,such as unacknowledged
connectionless service, acknowledged connectionless service
and connection oriented service
framing
error control
flow control
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Physical Layer
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The physical layer concerns with issues related to the
utilisation of a given physical medium (copper wire,
optical fiber, free space) for digital data transmission
The main tasks of the physical layer are
defining voltage levels of various signals and bit rates
analogue to digital and digital to analogue conversion
error control coding
multiplexing schemes
modulation and demodulation
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Some Revision on TCP/IP Networks
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Nodes in a TCP/IP network are modelled using a 5 layer
model.
They can also be modelled using the OSI 7 layer model..
It is just that the TCP/IP community use a 5 layer model
Note that in the past they have used a three layered
model…. So their model complexity is growing
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A TCP/IP Node and a Rough
Equivalence to OSI Layering
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An element of the
communications
stack
Transport Control Protocol
User
Processes
User Datagram
Protocol
- Unreliable Datagram
service but supports
the ability to find
multiple
Applications on the
target machine
5,6,7 Applications
UDP
TCP
4
3
2
1
IP
NDP
- Provision of a reliable
connection oriented
service
InterNet Protocol Layer
- Routing on IP Addresses
- Provision of an
unreliable Datagram
service over multiple
networks
Network Dependent Protocols
- Those protocols
necessary to connect to
a network
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A TCP/IP Network Model
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The Internetwork
User
Processes
- A collection of networks
that provide an
unreliable Datagram
service for the transport
of UDP or TCP messages
User
Processes
Applications
Applications
UDP
UDP
TCP
IP
NDP
IP
Network 1
NDP
TCP
IP
Network 2
NDP
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A TCP/IP Network Model
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User
Processes
User
Processes
Applications
Applications
UDP
UDP
TCP
IP
NDP
IP
FR or ATM
NDP
TCP
IP
FR or ATM
NDP
The Subnetwork
- A collection of networks that can carry an IP
Datagram (e.g. Frame Relay or ATM)
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Addresses in TCP/IP
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Relationship of layers and addresses
in TCP/IP
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Physical addresses
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