Chapter 1

Question 1

What are the OSI 7 layers?

Answer 1

Please Do Not Tell Salespeople Anything

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

Question 2

What does OSI stand for?

Answer 2

Open Standards Interconnection

Question 3

What does ISO stand for?

Answer 3

International Organisation for Standardisation

Question 4

What is a Protocol?

Answer 4

Set of rules that computer devices follow to establish and maintain communications

Question 5

What is Modulation

Answer 5

Convert logical data bits into physical signals

Question 6

What is Demodulation

Answer 6

Convert signals into logical data bits

Question 7

Describe the 1. Physical Layer

Answer 7

Defines physical aspects of signal transmission and receipt across media. E.g Wireless needs RF, copper patch lead needs voltage signal, fiber needs light signal etc.

Question 8

Describe the 2. Data Link layer

Answer 8

Frame delivery and error detection.

Three main functions, Media Access Control / Link Layer Addressing / Error Detection

Question 9

Describe the 3. Network Layer

Answer 9

Establish communications across multiple LANs or WANs using best path available.

Specific path can change each time depending on multiple factors.

Achieved using two techniques.
Logical Addressing - Unique source and destination (IP addresses)
Path Discovery and Selection - Algorithms and protocols find all possible paths and select the best one.

Three main responsibilities of transport layer
Segmentation
De-segmentation
Error Control

Question 10

Describe the 4. Transport Layer

Answer 10

Control link reliability of the given link using segmentation and error control.

Uses protocols like TCP and UDP

TCP - Allows transport layer keep track of messages and re-transmission those that fail.

UDP - Stateless or connection-less. Transport layer does not keep track of the messages.

Question 11

Describe the function of Media Access Control

Answer 11

Controls how to access media

Controls devices to ensure only 1 device transmits at one time (e.g one person talks at a time).

Carrier sense is used to sense state of the carrier or media. If a transmission is detected then station must wait its turn.

Question 12

Describe Link Layer Addressing

Answer 12

Each station identified with MAC address. Message sent out is with header information of intended recipient. All stations get message but will ignore.

Question 13

Describe Error Detection

Answer 13

On receiver side, Layer 2 helps detect errors that can occur during the Layer 1 transmission. Helps protect against corrupted or incomplete messages.

Carried by adding a checksum to data in the trailer. Called Cyclic Redundancy Check (CRC)

Question 14

Describe Layer 4 - Segmentation

Answer 14

Senders TCP or UDP proecess accepts files from application and divides to smaller pieces called segments. Each piece is passed down to the lower layers and transmitted.

Question 15

Describe Layer 4 - De-Segmentation

Answer 15

Receiver acceptes each segment and puts them back together in correct order. Processed by application.

Question 16

Describe Layer 4 - Error Control

Answer 16

Verification of information to avoid errors that could occur at layers 1-3

Question 17

Describe Layer 5 - Session Layer

Answer 17

Setup, Maintenance and tear down of session between devices.

The conversation between two computer devices.

Question 18

Describe Layer 6 - Presentation Layer

Answer 18

Transforms data into the format that the application accepts.

Typical Processes are:
Compression/Decompression
Encryption/Decryption
Code Translation

E.g. Application sends message is clear text, presentation layer encrypts message to keep data safe. On other end the receiver decrypts using key.

Question 19

Describe Layer 7 - Application Layer

Answer 19

OSI layer that interacts directly with the user via an application.

This layer determines the identity ad availability of the communication partners.
Provides network serivces to user applications, such as FTP/email/video conference

Examples of app layer are
HTTP (TCP for transport)
HTTPS (TLS for presention and TCP for transport)
FTP (TCP)
DNS (uses UDP)
TFTP (UDP)

Question 20

What is Layer Header and Encapsulation?

Answer 20

Header used to establish commication and control information.

Header generated on specific layer by sender can only be read at the same level of the reciever side.

Encapsulation - Process where OSI layer adds header. Always done sender side
Decapsulation - Process to read and interpret the header information. Always done by receiver side.

Question 21

What is Protocal Data Units (PDUs)

Answer 21

Structure that considers the header and payload or data for each layer.

There is no PDU at Layer 1

Three key tersmf related to PDUs
Segment - Encapsulation done in Layer 4 (PDU4), TCP or UDP segment
Packet - Encapsulation done at Layer 3 (PDU3)
Frame - encapsulation done at Layer 2 (PDU2), ethernet frames/wifi frames.

Question 22

What is different about the PDU at Layer 2

Answer 22

Has a trailer appended to the end of the data.

Used to detec errors during transmission. The CRC or FCS (frame check sequence)