OSI model Essay

Submitted By mikeyklein
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Pages: 4

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 video

\Quick Summary of Layers 1-4

OSI Layers in Wireshark

1. Physical Layer
Wiring Standards
Asynchronous and Synchronous Communications
Synchronizing Bits
Two devices must agree on when one bit stops and another bit starts
Uses start and stop bits
Internal clocks are synchronized at each end of the cable
Bandwidth Usage
Multiple channels share the same medium
Ex: cable TV uses frequency division multiplexing (each channel uses a different frequency range)
The whole medium is used for one transmission
Example: Ethernet
Time-Division Multiplexing (TDM)
Each channel gets the same amount of time on the wire
Statistical Time-Division Multiplexing (StatTDM)
Busy channels get more time on the wire
Frequency Division Multiplexing (FDM)
Example: signals sent with different colors through the same fiber optic cable
Layer 1 Devices
Wireless access points
Because they don’t pay any attention to addresses, they just deliver signals to every connected device like a crossover cable

2. Data Link Layer

MAC Addresses
Physical Address
Built into the network interface

Connection Services
Flow control
Prevents sender from sending data faster than the client can accept it
Error control
When a frame is received, a checksum is used to detect errors
Usually a Cyclic Redundancy Check (CRC)
If the receiver's checksum does not match the sender's checksum, the frame is discarded and resent
Layer 2 Devices
Network Interface Cards (NICs)
3. Network Layer
IP Address
Logical address
Changes when the device is moved
Packet switching
Data is broken into packets
Many packets travel along network connections like cars on a freeway
Circuit switching
A physical line is dedicated to each connection
Ex: old copper landline phone systems
Message switching
Store-and-forward, like email
Layer 3 Devices
Multilayer Switches
4. Transport Layer
Transmission Control Protocol (TCP)
Connection-oriented and reliable
Handshake makes sure both ends are ready
Segments are acknowledged and resent if necessary
User Datagram Protocol (UDP)
Connectionless and unreliable
No handshake
Best-effort delivery, no acknowledgements
ICMP (Internet Control Message Protocol)
At layer 4
Used by ping and traceroute, and to indicate errors such as dropped packets
5. Session Layer
Example of a Session
User logs in with a username & password
All data now has a special significance until that user logs off, or the session times out, or is terminated some other way
Layer 6 Protocol H.323 (voice or video)
NetBIOS (file sharing)

6. Presentation Layer

7. Application Layer
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
The TCP/IP and OSI Models