Chapter 1 Flashcards
What are the OSI 7 layers?
Please Do Not Tell Salespeople Anything
- Application
- Presentation
- Session
- Transport
- Network
- Data Link
- Physical
What does OSI stand for?
Open Standards Interconnection
What does ISO stand for?
International Organisation for Standardisation
What is a Protocol?
Set of rules that computer devices follow to establish and maintain communications
What is Modulation
Convert logical data bits into physical signals
What is Demodulation
Convert signals into logical data bits
Describe the 1. Physical Layer
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.
Describe the 2. Data Link layer
Frame delivery and error detection.
Three main functions, Media Access Control / Link Layer Addressing / Error Detection
Describe the 3. Network Layer
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
Describe the 4. Transport Layer
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.
Describe the function of Media Access Control
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.
Describe Link Layer Addressing
Each station identified with MAC address. Message sent out is with header information of intended recipient. All stations get message but will ignore.
Describe Error Detection
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)
Describe Layer 4 - Segmentation
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.
Describe Layer 4 - De-Segmentation
Receiver acceptes each segment and puts them back together in correct order. Processed by application.
Describe Layer 4 - Error Control
Verification of information to avoid errors that could occur at layers 1-3
Describe Layer 5 - Session Layer
Setup, Maintenance and tear down of session between devices.
The conversation between two computer devices.
Describe Layer 6 - Presentation Layer
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.
Describe Layer 7 - Application Layer
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)
What is Layer Header and Encapsulation?
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.
What is Protocal Data Units (PDUs)
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.
What is different about the PDU at Layer 2
Has a trailer appended to the end of the data.
Used to detec errors during transmission. The CRC or FCS (frame check sequence)
