OBJ 1.

1
OSI model – OBJ 1.1
OSI developed in 1977 by international organization for
standardization
OSI model = OSI Stack
OSI model is reference model when you troubleshoot your
network
There is 7 layers in OSI model : Please Do Not Throw
Sausage Pizza Away
1- Physical
2- Data link
3- Network
4- Transport
5- Session
6- Presentation
7- Application
Data name changes while moving through the models : Do
Some People Fear Birthdays:
At layer 7-6-5 it called Data
At layer 4 it called Segments
At layer 3 it called Packets
At layer 2 it called Frames
At layer 1 it called Bits
 Layer 1 (Physical)
Where transmission of bits across the network occurs and
includes physical and electrical network characteristics
Transition modulation: if it changes during the clock cycle then
a 1 is represented otherwise a 0 is represented.
Cables is also part of the physical layer
Asynchronous uses start and stop bits to indicate when
transmissions occur from the sender to the receiver
Synchronous uses a reference clock to coordinate the
transmissions by both sender and receiver.
Broadband divides bandwidth into separate channels
Baseband uses all available frequencies on a medium (cable) to
transmit data
Multiplexing involves optimizing a limited amount of resources
for more efficient utilization
Multiplexing enables simultaneous usage of a baseband
connection by multiple users.
Examples of physical layer: wired
 Fiber optic cable
 ethernet cable
 coaxial cable
Examples of wireless:
 Bluetooth
 Wi-Fi
 Near field communication (NFC)
Examples of infrastructure devices:
 Hubs , Access points and Media converters.
Layer 1 devices are essentially repeaters, passing along whatever
is received
 Layer 2 (Data Link)
Packages data into frames and transmits those frames on the
network
Media access control (MAC): physical addressing system of a
device which operates on logical topology.
The first 6 digits of mac address is vendor code
The last 6 digits of mac address is unique value
Logical link control (LLC): provides connection services
and allows acknowledgement of receipt of messages.
LLC is the most basic form of flow control
Logical link control provides basic error control functions by
using checksum
Communication across layer 2 can be synchronized
according to three different schemes:
Isochronous: network devices use a common reference
clock source and create time slots for transmission.
synchronous: network devices agree on clocking method to
indicate beginning and end of frames and can use control
characters.
Asynchronous: network devices reference internal clocks
and use start and stop bits for synchronization.
Layer 2 devices:
 Network interface cards
 Bridges
 Switches
 Layer 3 (Network)
There is 3 main ways to forward or route the data:
 Packet switching the most common in our houses
 Circuit switching
 Message switching
packet switching is data is divided into packets and then
forwarded
circuit switching is dedicated communication link is established
between two devices
message switching is data is divided into messages which may
be stored and then forwarded
Route discovery and selection manually configured as a static
route or dynamically through a routing protocol
Connection services augment layer 2 connection services to
improve reliability. More flow control and packet reordering
Packet reordering ensures that all data reaches the receiver
correctly at the end destination
Internet control message protocol (ICMP) sends error
messages and operational information to an IP destination.
Devices of layer 3 :
 Routers
 Multi-layer switch
IPv4
IPv6
 Layer 4 (Transport)
Transport layer dividing line between the upper layers and the
lower layers of OSI model
TCP, UDP protocols
Transmission control protocol (TCP): connection-oriented
protocol that is reliable way to transport segments across the
network
User datagram protocol (UDP) connectionless protocol that is
an unreliable way to transport segments across the network.
TCP = segments, UDP = Datagram
TCP UDP
Connection-oriented Connectionless
Reliable Unreliable
Segment retransmission and No windowing or
flow control through retransmission
windowing
Segment sequencing No sequencing
Acknowledges segments No acknowledges

Use TCP when ensuring someone receives something.

Like audio or video streaming, UDP does not require obtaining
every single piece of the file.
Windowing is going to allow the clients to adjust the amount of
data in each segment. Windowing sends information while
minimizing retransmission and maximizing throughput.
Buffering occurs when devices allocate memory to store
segments if bandwidth is not readily available.
Examples of layer 4:
 TCP And UDP
 WAN accelerators
 Load balancers and firewalls
 Layer 5 (Session)
keeps conversations separate to prevent intermingling of data.
1- Set up session is checking of user credentials and
assigning numbers to sessions to help identify them.
2- Maintain session is where data transfers back and forth
across the network.
 Transfer data.
 Reestablish connection.
 Acknowledge receipt
3- Tear down session is ending of a session after the transfer
is done or when the other party disconnects.
Tearing down a session can be mutually or after the
communication has finished.
Layer 5 examples:
H.323 used to set up, maintain and tear down voice and video
connections.
Real-time transport protocol (RTP) used for streaming
audio/video (phone call or face time session).
netBios is a method used to share files over the network.

Layer 6 (presentation)
Formats the data to be exchanged and secures that data with
proper encryption.
Data formatting or encryption = Layer 6
Data formatting is formatting by the computer to have
compatibility between different devices
American standard code for information (ASCII) text-based
language to use.
GIF
JPG
PNG
ASCII ensures the data is readable by the received system,
provides proper data structures and negotiates data transfer
syntax for application layer 7.
Encryption used to scramble the data in transit to keep it
secure from prying eyes and provide data confidentiality.
Layer 6 examples:
 Scripting languages – html, xmp and java script.
 Standard text – ASCII
 Pictures – GIF, PNG
 Movies files – MP4 - MOV
 Encryption algorithms - TLS

Layer 7 (Application)
 Provides application-level services where users communicate
with the computer.
File transfer – network transfer.
Application services unite communications components
from more than one network application.
Service advertisements send out announcements to other
devices on the network to state the services they offer.
Example of layer 7:
 Email applications
 Web browsing
 Domain name service
 File transfer protocol
 Remote access.

Encapsulation and Decapsulation

the process of putting headers and trailers around some
data.
Protocol data unit (PDU) a single unit information transmitted
in a computer network.
SYN (synchronization) used to synchronize connection during
the three-way handshake.
ACK (Acknowledgement) used during the three-way
handshake, but also used to acknowledge the successful
receipt of packets.
FIN (Finished) used to tear down the virtual connections
created using the three-way handshake and SYN flag.
RST (Reset) used when a client or server receives packet
that was bit expecting during the current connection.
PSH (Push) used to ensure data is given priority and is
processed at the sending or receiving ends.
URG (Urgent) like PSH and identifies incoming data as
urgent.
MAC Address is a physical address that is used to identify a
network card on a local area network.
EtherType used to indicate which protocol is encapsulated
in the payload of a frame.
A frame being sent at layer 2 will also contain a payload.
42 bytes using VLANs and 46 no VLANs
Wireshark is packet analyzer that can pull apart some
network traffic and show different layers of the OSI model