BCIS 4630 Fundamentals of IT Security
NETWORKING (1)
Dr. Andy Wu
OSI Model
• Network communications protocols are developed using the
Open System Interconnection (OSI) model.
• An ISO standard for worldwide communications.
• The OSI model defines a framework for implementing protocols
in seven distinct layers.
• Each layer makes use of the services of the layer below it to
provide a set of specific services to the layer above it.
• Adjacent layers communicate with each other through a welldefined interface, so that in principle, we could change one
layer or even completely rewrite it without affecting the others.
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OSI Layers
Layer
Function
Examples
Application
User interface
Program level communication
HTTP, SMTP
Presentation
Presentation format of data
Data conversion if needed
ASCII, JPEG, MP3,
Encryption
Session
Establish and maintain communication channels
OS, Application access
scheduling
Transport
Reliable or unreliable end-to-end communication
Identifies upper layer processes that will receive data
TCP, UDP, SPX
Network
Logical addressing and routing
Delivering packets from source to destination
IP, IPX
Data Link
Transforming physical layer into a reliable link and
presenting it to upper layers
Includes MAC (802.3) and LLC (802.2) sublayers
Ethernet
Physical
Transmission of bit stream over physical medium
Signaling
EIA/TIA 568B
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OSI Mnemonic
Away
Application
Pizza
Presentation
Sausage
Session
Throw
Transport
Not
Network
Do
Data Link
Please
Physical
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Encapsulation
• When data is passed down from higher
layers to lower ones, each layer creates its
header and places the data given to it by
the next-higher layer behind its own
header, thereby encapsulating the higher
layer’s data.
• The Data Link layer also creates a trailer.
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Encapsulation – Layer 4
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Encapsulation – Layer 3
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Encapsulation – Layer 2
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Network Communication
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Decomposition – Layer 2
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Decomposition – Layer 3
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Decomposition – Layer 4
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Protocol Data Unit
• A term that describes a set of bytes that includes the layer’s
header and trailer (if any) and all data encapsulated.
• From Layer X’s perspective, the higher-layer headers and the
user data form one large data field.
Layer(s)
PDU
Application, Presentation,
Session
Data
Transport
TCP Segment
UDP Datagram
Network
IP Packet
Data Link
Frame
Physical
Bit
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