Comptia Network +: Dissecting The Osi Model
Comptia Network +: Dissecting The Osi Model
Comptia Network +: Dissecting The Osi Model
Chapter 2
Dissecting the OSI Model
Objectives
What is the purpose of a Network model?
What are the layers of the OSI model?
What are the characteristics of each layer of the
OSI model?
How does the TCP/IP stack compare to the OSI
model?
What are the well-known TCP and/or UDP port
numbers for a given collection of common
applications
The Purpose of Reference Models
• It breaks network communication into smaller, simpler parts that
are easier to develop.
• It facilitates standardization of network components to allow
multiple-vendor development and support.
• It allows different types of network hardware and software to
communicate with each other.
• It prevents changes in one layer from affecting the other layers
so that they can develop more quickly.
• It breaks network communication into smaller parts to make
learning it easier to understand.
Warriors of the Net
The OSI seven-layer model
7 Application
6 Presentation
5 Session
4 Transport
3 Network
2 Data Link
1 Physical
Presentation
Session
Transport Segments
Network Packets
Physical Bits
1
2
3
4
7
Physical Layer
Application
Presentation
Session
Transport
AAAA
B_B_ ABCA_CABCA_C
CCCC
Statistical Time-Division
Multiplexing (StatTDM)
• Busy channels get more time on the wire
AAAA
B_B_ ABCACABCAC
CCCC
Frequency Division Multiplexing
(FDM)
• Example: signals sent with different colors
through the same fiber optic cable
AAAA
AAAA
B_B_ B_B_
CCCC
CCCC
Layer 1 Devices
• Cables
• Wireless access points
• Hubs
– Because they don’t pay any attention to
addresses, they just deliver signals to every
connected device like a crossover cable
Data Link Layer
Application
Presentation
Session
• Physical Addressing
Transport • Logical topology
MAC
• Method of transmitting
Network on the media
Data Link
Presentation
Session
• Logical addressing
Transport • Switching
• Route discovery and
Network selection
• Connection services
Data Link • Bandwidth usage
• Multiplexing strategy
Physical
Presentation
Session
• TCP/UDP
Transport • Windowing
• Buffering
Network
Data Link
Physical
Window Size 2
Segment 2
Segment 3
Sender Receiver
Ack 4
Window Size 4
Segment 4
Segment 5
Segment 6
Segment 7
Ack 8
Network
Data Link
Physical
Session
Transport
Network
Data Link
Physical
Session
Transport
Network
Data Link
Physical
Application Layer
• Closest to the user
• Hands data to an application in the format
it expects, with no addresses or other
transmission artifacts
• Examples: a downloaded file, an email
message
The TCP/IP Stack
Application
Transport
Internet
Network Interface
The TCP/IP and OSI Models Compared
OSI Stack TCP/IP Stack
Application
Presentation Application
Session
Transport Transport
Network Internet
Data Link
Network Interface
Physical
• No handshake, acknowledgements,
sequencing, or flow control
Common Ports
Link Ch 2d for flash cards
Port Types
Port numbers are assigned in various ways, based
on three ranges:
System Ports (0-1023), System Ports are assigned by IETF process for
standards-track protocols, as per RFC6335. Also known as well-
known-ports
User Ports (1024-49151) ,User Ports are assigned by IANA using the
"Expert Review" process, as per RFC6335