Chapter 1
Read (again) chapter 1
Why use networks?
Resource sharing
Reliability
Cost
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Network Hardware:
Type of Transmission
Broadcast
Multicasting
Point-to-point
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Network Hardware:
Metropolitan Area Networks (MANs)
On the level of a city
There are no switching elements
Example: Distributed Queue Dual Bus
(DQDB) standardized under 802.6
Source
Hosts
Source
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Network Hardware:
Wide Area Networks (WANs)
Large Geographical area
Switching elements (p.s. nodes, I.S, or
Data switching exchanges, router)
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Network Hardware: Topologies
Main characteristics?
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Network Hardware:
Wireless Networks
Wireless does not mean mobile, and
mobile does not mean wireless
Wavelan, Cellular Digital Packet Data
(CDPD)
802.11
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Network Hardware:
The OSI Reference Model
What is it?
Who created it?
Why was it created?
What is it used for, today?
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Network Software: Protocol Hierarchy or Layered Design
Host 1
Layer 3 protocol
Layer 3
Host 2
Layer 3
Layer 2/3 Interface
Layer 2 protocol
Layer 2
Layer 2
Layer 1/2 Interface
Layer 1 protocol
Layer 1
Layer 1
Physical Medium
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OSI - The Model
A layer model
Each layer performs a subset of the required
communication functions
Each layer relies on the next lower layer to
perform more primitive functions
Each layer provides services to the next
higher layer
Changes in one layer should not require
changes in other layers
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OSI as Framework for
Standardization
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Network Hardware: The OSI Reference Model
Host A
Application
Interface
Presentation
Interface
Session
Interface
Transport
Interface
Network
Interface
Data link
Interface
Physical
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Application Protocol
Presentation Protocol
Session Protocol
Transport Protocol
Network Protocol
Data link Protocol
Physical Protocol
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Host B
Application
Presentation
Session
Transport
Network
Data link
Physical
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Network Hardware: The OSI Reference Model
Host A
Application
Interface
Application Protocol (End to End layer)
Presentation Protocol (End to End layer)
Presentation
Interface
Presentation
Session Protocol (End to End layer)
Session
Interface
Transport
Session
Transport Protocol (End to End layer)
Interface
Network Protocol
Network
Interface
Data link Protocol
Data link
Interface
Physical
Host B
Application
Physical Protocol
Routing the packets over the net(s)
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Transport
Network
Data link
Physical
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The OSI Reference Model...
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The OSI Reference Model...
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The Physical Layer
Transports raw bits, Defines the
physical (electrical and mechanical)
specifications
Characteristics of transmission medium
Signal levels
Data rates
etc.
Example: RS232, V24, V35, X.21,
ISDN...
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The Data Link Layer
Has 2 main functions


Error detection and correction
MAC (Medium Access Control)
Examples:SLIP, PPP, Ethernet, HDLC,
SDLC
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The Network Layer
Has 3 main functions

addressing

routing

some congestion control
Examples: IP
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The Transport Layer
It is an end-to-end layer, dealing with

flow control

congestion control

may offer end-2-end reliability
Examples: TCP, UDP.
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The Session Layer
- Used for :
• session management
• Synchronization
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The Presentation Layer
Concerned with

“compatibility” (encoding)

encryption (security purposes)
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The Application Layer
It is communication software directly
accessible to the user
Examples: FTP, Telnet, Network Virtual
monitor, HTTP….
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Summary on OSI layers...
The International Organization for Standardization (known as ISO, an abbreviation taken from its name in French)
has devised a model for the design of communication protocols (known as the open systems interconnection (OSI)
model). In this model each communicating entity has seven layers of protocol. The bottom layer (1) is known as the
physical layer, and it essentially corresponds to the wire. The next layer (2) is known as the data link layer. It
provides the means for putting data on the wire (and for taking it off). An example is ethernet. The next layer (3) is
known as the network layer. Its primary responsibility is to see that the data travels to the intended destination
(perhaps via a number of intermediate points). The next layer is known as the transport layer. Its job is to see to it
that the data, which is transferred between machines by the network layer, reaches the desired party at the
destination machine. The notion of a “connection” is maintained by this layer. The next layer is known as the
session layer. This layer doesn’t really do very much (if anything); it is responsible for maintaining the notion of a
“session.” Sessions might be in one-to-one correspondence with transport connections; there might be two
successive sessions on the same connection; or one session might span multiple connections (e.g., the first
connection was terminated due to a communication failure, the session continues as soon as
communication is reestablished). The next layer is known as the presentation layer. Its job is to deal with the fact
that different machines have different representations for data (i.e. it must somehow translate between data
representations) and to deal with such concerns as compression or encryption
of data. Finally, the application layer is where all other software resides. However, it has been discovered that there
is “system software” that logically fits above the presentation layer. The only place to put it is in the application
layer, so “real” application software sits on top of the application layer.
The bottom three layers are sometimes known as the communications subnet. If our data must pass through a
number of machines on their way to the destination, each intermediate machine has an implementation of these
lower layers to forward the data on.
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TCP/IP Protocol Suite
Dominant commercial protocol
architecture
Specified and extensively used before
OSI
Developed by research funded US
Department of Defense
Used by the Internet
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Telnet
TCP UDP
Telnet
TCP UDP
IP
Ethernet
Protocols
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IP
Ethernet
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Application
Transport
Internet
Services
Physical link
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OSI versus TCP/IP
Application
Transport
Network
Physical link
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TCP/IP Protocol Architecture(1)
Application Layer

Communication between processes or applications
End to end or transport layer (TCP/UDP/…)

End to end transfer of data

May include reliability mechanism (TCP)

Hides detail of underlying network
Internet Layer (IP)

Routing of data
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TCP/IP Protocol Architecture(2)
Network Layer

Logical interface between end system and
network
Physical Layer


Transmission medium
Signal rate and encoding
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PDUs in TCP/IP
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Some Protocols in TCP/IP Suite
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Network Standardization
What is it ?
Why ?
How is it done ?
Who does it?
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Big Standards Organization
International standards



ITU (International Telecom. Union) old CCITT
ISO
IEEE
Internet Standards


IAB
IRTF, IETF
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Conclusion
You must be pretty familiar with:




the layered view of a communication
system
know the function of each layer
the mapping of communication protocols to
the layers
chapter 1 in textbook
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