OSI Model

Question 1

OSI mOdel Overview

Answer 1

Developed in 1977 by International Organization for Standardization (ISO)
▪ Called the OSI model or OSI stack
▪ Consists of 7 layers
▪ Useful in troubleshooting networks
▪ Serves as a reference model in networks

Question 2

Purposes of Reference Model

Answer 2

Categorize functions of the network into particular layer(s)
▪ Compare technologies across different manufacturers
▪ By understanding its functions, you can understand how best
to communicate with that device

Question 3

OSI Model Layers

Answer 3

7. Application
8. Presentation
9. Session
10. Transport
11. Network
12. Data Link
13. Physical

Question 4

Data Types in OSI Model

Answer 4

7. Application —————- Data
8. Presentation—————-Data
9. Session————————Data
10. Transport———Segments
11. Network————–Packets
12. Data Link—————Frames
13. Physical—————Bits

Question 5

Layer 1 (Physical)

Answer 5

Transmission of bits across the network
▪ Physical and electrical characteristics
▪ Characteristics:
● How bits are represented on the medium
● Wiring standards for connectors and jacks
● Physical topology
● Synchronizing bits
● Bandwidth usage
● Multiplexing strategy

Question 6

Layer 1 Examples

Answer 6

Cables
● Ethernet
● Fiber optic
▪ Radio frequencies
● Wi-Fi
● Bluetooth
▪ Infrastructure devices
● Hubs
● Wireless Access Points
● Media Converters

Question 7

Layer 2 (Data link)

Answer 7

▪ Packages data into frames and transmitting those frames on the network,
performing error detection/correction, and uniquely identifying network
devices with an address (MAC), and flow control
● MAC
● Physical addressing
● Logical topology
● Method of Transmission
● Link Layer Control (LLC)
o Connection services
o Synchronizing transmissions

Question 8

Layer 2 Examples

Answer 8

Network Interface Cards (NIC)
▪ Bridges
▪ Switches

Question 9

Layer 3 (Network)

Answer 9

Forwards traffic (routing) with logical address
● Example: IP Address (IPv4 or IPv6)

Question 10

Logical Link Control (LLC)

Answer 10

Provides connection services
▪ Acknowledgement of receipt of a message
▪ Flow control
● Limits amount of data sender can send at one time to keep
receiver from becoming overwhelmed
▪ Error control
● Allows receiver to let sender know when an expected data frame
wasn’t received or was corrupted by using a checksum

Question 11

Logical Address

Answer 11

Numerous routed protocols were used for logical addressing over the
years:
● AppleTalk
● Internetwork Packet Exchange (IPX)
● Internet Protocol (IP)
▪ Only Internet Protocol (IP) remains dominant
● IP v4
● IP v6

Question 12

How should data be forwarded or routed? Layer 3

Answer 12

Packet switching (known as routing)
● Data is divided into packets and forwarded
▪ Circuit switching
● Dedicated communication link is established between two devices
▪ Message switching
● Data is divided into messages, similar to packet sw

Question 13

ICMP (Layer 3)

Answer 13

Used to send error messages and operational information about an IP
destination
▪ Not regularly used by end-user applications
▪ Used in troubleshooting (ping and traceroute)

Question 14

Layer 3 Examples

Answer 14

▪ Routers
▪ Multilayer switches
▪ IPv4 protocol
▪ IPv6 protocol
▪ Internet Control Message Protocol (ICMP)

Question 15

Layer 4 (Transport Layer)

Answer 15

▪ Dividing line between upper and lower layers of the OSI model
▪ Data is sent as segments
▪ TCP/UDP
▪ Windowing
▪ Buffering

Question 16

TCP (LAYER 4)

Answer 16

▪ Connection-oriented protocol
▪ Reliable transport of segments
● If segment is dropped, protocol detects it and resends segment
▪ Acknowledgements received for successful communications
▪ Used for all network data that needs to be assured to get to its
destination

Question 17

Layer 4 Examples

Answer 17

▪ TCP
▪ UDP
▪ WAN Accelerators
▪ Load Balancers
▪ Firewalls

Question 18

Layer 5( Session)

Answer 18

▪ Think of a session as a conversation that must be kept separate from
others to prevent intermingling of the data
▪ Setting up sessions
▪ Maintaining sessions

Question 19

Layer 5 Examples

Answer 19

H.323
● Used to setup, maintain, and tear down a voice/video connection
▪ NetBIOS
● Used by computers to share files over a network

Question 20

Layer 6 (Presentation)

Answer 20

▪ Responsible for formatting the data exchanged and securing that data
with proper encryption
▪ Functions
▪ Data formatting
▪ Encryption

Question 21

Layer 6 Examples

Answer 21

HTML, XML, PHP, JavaScript, …
▪ ASCII, EBCDIC, UNICODE, …
▪ GIF, JPG, TIF, SVG, PNG, …
▪ MPG, MOV, …
▪ TLS, SSL, …

Question 22

Layer 7 (Application)

Answer 22

Provides application-level services
● Not Microsoft Word or Notepad
▪ Layer where the users communicate with the computer
▪ Functions:
● Application services
● Service advertisement

Question 23

Layer 7 Examples

Answer 23

E-mail (POP3, IMAP, SMTP)
▪ Web Browsing (HTTP, HTTPS)
▪ Domain Name Service (DNS)
▪ File Transfer Protocol (FTP, FTPS)
▪ Remote Access (TELNET, SSH)
▪ Simple Network Management Protocol (SNMP)

Question 24

Encapsulation

Answer 24

The process of putting headers (and sometimes trailers) around some data

Question 25

Decapsulation

Answer 25

o Action of removing the encapsulation that was applied
o If we move down the OSI layers from 7 to 1, we encapsulate data
o If we move upward from layers 1 to 7, we decapsulate data
o A protocol data unit is a single unit of information transmitted within a
computer network
▪ Layer 1 - bits
▪ Layer 2 - frames
▪ Layer 3 - packets
▪ Layer 4 - segments if TCP or datagrams if UDP

Question 26

SYN (Synchronization flag)

Answer 26

▪ The most well-known flag in TCP communications because it is used to
synchronize the connection during the three-way handshake

Question 27

ACK (acknowledge flag)

Answer 27

Used during the three-way handshake, but it is also used to acknowledge
the successful receipt of packets

Question 28

FIN (Finished flag)

Answer 28

▪ Used to tear down the virtual connections created using the three-way
handshake and the SYN flag
▪ The FIN flag always appears when the last packets are exchanged
between a client and server and the host is ready to shutdown the
connection

Question 29

RST (reset flag)

Answer 29

Used when a client or server receives a packet that it was not expecting
during the current connection

Question 30

PSH (push flag)

Answer 30

▪ Used to ensure that the data is given priority and is processed at the
sending or receiving ends

Question 31

URG (urgent flag)

Answer 31

▪ It is like the Push flag and identifies incoming data as “urgent”
▪ The main difference is PSH is used by a sender to indicate data with a
higher priority level where URG is sent to tell the recipient to process it
immediately and ignore anything else in queue
● Source and Destination ports
o are just like the ones used in UDP, they dictate where the
data is coming from and where it is going to
● Length
o Used to indicate how many bytes the UDP packet is,
including its header and its data
● Checksum
o Not a mandatory field, but it can be used to provide some
validation that the UDP data being sent was received with
some level of integrity

Question 32

EtherType field

Answer 32

Used to indicate which protocol is encapsulated in the payload of the
frame
▪ As data moves from layer 7 to layer 1, that data is encapsulated
● At layer 4, we add our source and destination ports
● At layer 3, we add our source and destination IP addresses
● At layer 2, we add our source and destination MAC addresses
▪ Once we get to layer 1, we are simply transmitting our layer 2 frames as a
series of 1’s and 0’s over the medium
▪ Once that host is found, it will keep decapsulating the information all the
way up to layer 7, where its application can read and understand the
underlying data