Lecture 2

Question 1

How many layers does the The Open Systems Interconnection (OSI) model have and what are they

Answer 1

1. Application
2. Presentation
3. Session
4. Transport
5. Network
6. Data link
7. Physical

Question 2

The OSI model: peer communication

Answer 2

each layer between 2 computers behave as if they are directly communicating with each other

Data passes from top (application layer) to bottom to the physical layer and back up on
the pair computer

Question 3

What does each layer in the OSI model do

Answer 3

has its own function and interacts with the layer below it

similarly each layer provides service to the layer above it.

Question 4

Data from the application layer is split into what

Answer 4

Protocol Data Units (PDUs)

Question 5

As data makes its way down the OSI model, each layer below adds its “_____”. Similar to putting
a letter into an envelop.

Answer 5

header (encapsulation)

Question 6

When data gets to the destination host’s physical layer, as it passes up the stack, and the
header is removed (_____).

Answer 6

decapsulation

Question 7

The OSI Model Application layer (L7)

Answer 7

Application Layer (L7)

• Provides interface for user applications to access the network
• User applications include web browsers, email clients, networked games etc.

Common protocols at L7 include:
• Hyper Text Transfer Protocol (HTTP), port#: 80 or 8080
• Hyper Text Transfer Protocol Secure (HTTPS), port#: 443
• File Transfer Protocol (FTP), port #: 20 or 21
• Domain Name Service (DNS), port#: 53
• Simple Mail Transfer Protocol (SMTP), port#: 25

Question 8

The OSI Model Presentation Layer (L6)

Answer 8

• Ensures that data can be read by the receiving system.
• Handles data formatting, e.g., displaying graphics embedded in web pages
• Translation, e.g., translating special characters between MACs and Windows or Linux
• Handles Encryption and decryption.

Common protocols at L6 include:
- SSL or TSL security
- JPEG, PNG image formatting
- MP4, MP3, AVI, multimedia formats

Question 9

The OSI Model Session Layer (L5)

Answer 9

• Enables two devices hold on-going communications called “sessions”.
• This allows the 2 devices exchange data seamlessly for as long as the session lasts.
• Activities includes user login/logoff and authentication used in online shopping or banking applications.
• Handles synchronization and ordering of packet received, such as in streaming videos.

Question 10

The OSI Model Transport layer (L4)

Answer 10

• Manages data transfer between applications across a network.
• Ensures reliable delivery, error corrections.
• TCP and UDP are common protocols here
• TCP: reliable data delivery using sequencing, acknowledgements, flow control, etc.
• UDP: best effort and does not usually guarantee delivery of PDUs

Question 11

The OSI Model Network Layer (L3)

Answer 11

• Handles logical addressing
  -Translates logical address to physical address.
• Routes data from the source to destination node across network(s).
• Also manages traffic management including flow control and access control.
• Devices include: layer 3 switches, routers
• Protocols include:
  • Internet Protocol (IP) address
  • Internet Control Message Protocol (ICMP)
  • RIP, BGP, OSPF etc.

Question 12

The OSI Model Data Layer (L2)

Answer 12

• Media Access Control Layer
• Works with frames
• Error checking using different models including Cyclic Redundancy Checks (CRC) control.
• Detects and manages collisions on network lines.
• Devices include layer 2 switches and bridges.
• Protocols include:
  • MAC address
  • Spanning Tree Protocol (STP).
  • Ethernet, ATM etc.

Question 13

The OSI Model Physical Layer (L1)

Answer 13

• Converts data bits into electrical signals and vice versa.
• Deals with data encoding.
  -Handles physical media for data transmission including Ethernet cables, fiber optics, coaxial etc.
• Hardware connection
• Devices include:
  • Hubs and repeaters
  • NIC
  • Cables

Question 14

The TCP/IP Model Application layer (L1):

Answer 14

• Combines the Application, presentation and session layers of the OSI model into one.
• Interfaces with the end user
• Creates data which is sent to the transport layer.

Question 15

The TCP/IP Model Transport Layer (L2):

Answer 15

• Similar function as the transport layer of the OSI model.
• TCP and UDP are the most common protocols.
• Creates a SEGMENT by adding a TCP header to data received from the application layer or
• Creates a DATAGRAM by adding a UDP header to data received from the application layer.

Question 16

The TCP/IP Model Internetwork or Internet Layer (L3)

Answer 16

• Responsible for logical addressing and routing.
• Adds an IP header to segments or datagrams received from the transport layer above it to
  create a PACKET.

Question 17

The TCP/IP Model Network Link Layer (L1):

Answer 17

• Combines the physical and data link layers of the OSI model.
• Adds frame header to the packet received from the Internetwork layer above it to create a
  FRAME.
• Frames are then sent to the network media as
  bits.
• The reverse is carried out at the receiver’s end, where the headers are stripped off (decapsulated) at each layer, on its way up to the application layer.