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Network+ (N10-009) > OSI Model > Flashcards

OSI Model Flashcards

1.1

1
Q

Open Systems Interconnect Model (OSI)

A

■ Developed in 1977 by the International Organization for Standardization
■ OSI is a reference model
● Used to categorize the functions of a network
● Useful for troubleshooting

○ Networks today operate under the TCP/IP mode
○ Layers

■ Physical - Layer 1
■ Data Link - Layer 2
■ Network - Layer 3
■ Transport - Layer 4
■ Session - Layer 5
■ Presentation - Layer 6
■ Application - Layer 7
○ Networks are designed to make data flow across networks

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2
Q

Physical Layer

A

(BITS)

■ First layer of the OSI model where transmission of bits across the network
occurs and includes physical and electrical network characteristics
■ Data type occurs as bits
● Binary bits represented as a series of 1s and 0s

○ Transition Modulation

■ Switching between levels to represent 1 or 0
● Copper Wire (Cat5/Cat6) – Uses voltage (0V for 0, +5V/-5V for 1)
● Fiber Optic Cable – Uses light (on for 1, off for 0)

○ Connector Standards

■ RJ-45 Connector – Used in CAT5/CAT6 cables

■ Wiring Standards
● TIA/EIA-568A
● TIA/EIA-568B
■ Crossover cables – TIA/EIA-568A on one end, and TIA/EIA-568B on the
other end
■ Straight-through cables – TIA/EIA-568B on both ends

○ Physical Topology

■ Different physical network layouts
● Bus
● Ring
● Star
● Hub-and-Spoke
● Full Mesh
● Partial Mesh
■ Based on how cables are physically connected

○ Synchronization

■ Asynchronous Communication
● Start and stop bits for out-of-sync data transmission
■ Synchronous Communication
● Real-time communication using a common time source

○ Bandwidth Utilization

■ Broadband
● Divides bandwidth into separate channels (e.g., cable TV)

■ Baseband
● Uses all frequency of the cable all the time (e.g., telephone)

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3
Q

Layer 1 Devices

A

■ Simply repeat whatever they receive
■ No logic or decision-making at Layer 1

■ Cables – media
● Fiber optic
● Ethernet
● Coaxial
■ Wireless Media
● Bluetooth
● Wi-Fi
● Near field communication

■ Infrastructure Devices
● Hubs
● Access points
● Media converters

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4
Q

Multiplexing

A

● Allows multiple people to use a baseband connection at the same
time

■ Time Division Multiplexing (TDM)
● Allocates dedicated time slots

■ Statistical Time Division Multiplexing (StatTDM)
● Dynamically allocates time slots based on when people need it

■ Frequency Division Multiplexing (FDM)
● Divides the medium into channels

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5
Q

Layer 2

A

Data Link

■ Responsible for packaging bits from Layer 1 into frames and transmitting
them across the network
■ Performs error detection and correction, identifies devices using MAC
addresses, and provides flow control

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6
Q

MAC Address (Media Access Control Address)

A

■ A means for identifying a device physically and allowing it to operate on a
logical topology
■ A unique 48-bit physical addressing system is assigned to every network
interface card (NIC) produced
● Written in hexadecimal numbers
● First 24 bits – identify the manufacturer
● Remaining 24 bits – identify the specific device
■ Crucial for logical topology – identifying devices on the network

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7
Q

Logical Link Control (LLC)

A

■ Provides connection services and acknowledges message receipt,
ensuring controlled data flow
■ Most basic form of flow control
● Limits data sent by a sender and prevents receiver overwhelm
■ Uses a checksum to detect corrupted data frames

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8
Q

■ Isochronous Mode

A

● Common reference clock
● Time slots for transmissions
● Less overhead

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9
Q

Synchronous Method

A

● Devices use the same clock, with beginning and ending frames,
and control characters for synchronization

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10
Q

Asynchronous (layer 2)

A

● Devices reference own clock cycles
● No strict control over communication timing

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11
Q

Layer 2 Devices

A

■ Network Interface Cards (NICs)
■ Bridges
■ Switches
● Intelligent use of logic to learn and send data to specific devices
based on MAC addresses
○ Switch Operation
■ Switches use CAM tables with MAC addresses to identify physical ports
connected to devices
■ Enables selective data transmission to specific areas in the network.

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12
Q

Layer 3

A

Network Layer
■ Concerned with routing and forwarding traffic using logical addresses.

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13
Q

Logical Addressing

A

■ IP variants – common logical addressing schemes
● IPv4 – written in dotted octet notation which are four sets of
numbers separated by dots (e.g., 172.16.254.1)
● IPv6
■ Other protocols – these were replaced by IP (Internet Protocol)
● AppleTalk
● IPX (Internetwork Packet Exchange)

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14
Q

Packet Switching (Routing)

A

● Data is divided into packets and then forwarded
● Most commonly used method

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15
Q

Circuit Switching

A

● A dedicated communication link is established between two
devices

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16
Q

Message Switching

A

● Data is divided into messages which may be stored and then forwarded.

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17
Q

Route Discovery and Selection

A

■ Routers maintain routing tables for determining the best path

● Dynamic protocols (e.g., RIP , OSPF) enable routers to share and
update route information

■ Routing protocols help decide how data is going to flow across the
network and how the routers are going to communicate that information

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18
Q

Connection Services at Layer 3

A

■ Augments Layer 2 services

■ Involves flow control
● Prevents sender from overwhelming the receiver

■ Packet reordering
● Ensures data packets arrive and are reassembled in the correct
order

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19
Q

Internet Control Message Protocol (ICMP)

A

■ Used for sending error messages and operational information to an IP
destination

■ PING
● Most commonly used ICMP
● Helps troubleshoot network issues by testing connectivity and
response times

■ Traceroute
● Traces the route of a packet through the network

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20
Q

Layer 4

A

Dividing line between the upper layers and the lower layers of the OSI
model

■ Upper Layers
● Transport
● Session
● Presentation
● Application

21
Q

Segments

A

■ Data Type in Transport Layer

22
Q

TCP (Transmission Control Protocol)

A

Segment – data type for TCP
● Connection-oriented protocol that is a reliable way to transport
segments across the network
● With acknowledgement
● Uses Three-Way Handshake
○ SYN – synchronization
○ SYN-ACK – synchronization - acknowledgement
○ ACK – acknowledgement
● Windowing for flow control
● Used for all network data that needs to be assured to get to its
final destination

● Reliable
○ Uses Three-way Handshake
● Connection-oriented
● Segment retransmission and flow control through windowing
● Sequencing
● Acknowledgment of segments

23
Q

■ UDP (User Datagram Protocol)

A

Datagram – data type for UDP
● A connectionless protocol that is an unreliable way to transport
segments (datagram)
● Used for audio and visual streaming
● No three-way handshake and less overhead
● No acknowledgment or retransmission

● Unreliable
○ No Three-way Handshake
● Connectionless
● No retransmission and no windowing
● No sequencing
● No acknowledgment of datagrams

24
Q

Windowing

A

■ Allows clients to adjust the amount of data in each segments during
transmission
■ Optimize throughput and bandwidth
■ Open or close window based on retransmissions

25
Buffering
■ Occurs when devices allocate memory to store segments if bandwidth is not readily available Buffer ● Temporary storage for segments ■ Prevents overflow by clearing segments
26
Layer 4 Devices
Layer 4 Devices ■ Protocols ● TCP and UDP ■ Devices ● WAN accelerators ● Load balancers and firewalls
27
Layer 5
Session Layer ■ Manages sessions, ensuring separate conversations to prevent data intermingling
28
Setting Up Session
■ Checking of user credentials and assigning numbers to sessions to help identify
29
Maintaining Session
■ Continuous data transfer between parties ■ If connection breaks, it will require re-establishment ■ Includes acknowledgement of data
30
Tearing Down a Session
■ Ending a session once communication goals are achieved ■ Mutual agreement or one party disconnects
31
Layer 5 Devices and Protocols
○ Layer 5 Devices and Protocols ■ H.323 ● Used for setting up, maintaining, and tearing down voice and video connections ● Operates over the real-time transport protocol (RTP) - Example is FaceTime. ■ NetBIOS ● Utilized by computers for file sharing over a network ● Commonly associated with Windows file sharing
32
Layer 6
Presentation layer ■ Responsible for formatting data for exchange and securing it through encryption
33
Data Formatting
■ Formatting data by a computer to have compatibility between different devices ■ Formats ● American Standard Code for Information Interchange (ASCII) ○ Text-based language to use ○ Ensures data is readable by receiving system ○ Provides proper data structures ○ Negotiates data transfer syntax for the Application Layer (Layer 7) ● GIFs – motion pictures ● JPEG – photographs ● PNG – Internet images ■ Formats enable compatibility between different devices
34
Encryption
■ Used to scramble data in transit to keep it secure and provide data confidentiality ■ Transport Layer Security (TLS) ● Ensures secure data transfer ● Creates an encrypted tunnel, protecting sensitive information
35
Scripting languages in Layer 6
■ Control how ASCII text is displayed on the screen ● HTML ● XML ● PHP ● JavaScript
36
Encryption Algorithms
■ Scrambles data to provide confidentiality and security during transit and storage ● TLS ● SSL (Secure Sockets Layer) ■ Focus on Security
37
○ Application Layer (Layer 7)
■ Provides application-level services where users communicate with the computer ■ Focus on lower-level applications ● File transfer ● Network transfer
38
Layer 7 Protocols
■ Email Applications ● POP3 ● IMAP ● SMTP ■ Web Browsing ● HTTP ● HTTPS ■ Domain Name Service (DNS) ■ File Transfer Protocols ● FTP ● FTPS ● SFTP ■ Remote Access ● Telnet ● SSH ● SNMP
39
Service Advertisement
■ Applications send announcements to other devices on the network ■ Devices advertise the services they offer ● Printers and file servers managed by Active Directory ● Self-advertising devices like wireless printers
40
Application Services
■ Unites components for more than one network application ● File transfer ● File sharing ● Email ○ Low-level protocols ■ POP3 (Post Office Protocol 3) ■ IMAP (Internet Message Access Protocol) ■ SMTP ( Simple Mail Transfer Protocol) ● Remote access ● Network management ● Client-server processes
41
Encapsulation & Decapsulation
○ Encapsulation ■ Process of putting headers and sometimes trailers around data ○ Decapsulation ■ Removing the applied encapsulation to access the original data ○ OSI Model Layers ■ Moving down from Layer 7 to 1 – encapsulation ■ Moving up from Layer 1 to 7 – decapsulation
42
Protocol Data Units (PDUs) in OSI Model
■ A single unit of information transmitted in a computer network ● Terminology used for each layer is written as L(layer number) PDU ○ Example – L7 PDU for Layer 7 ■ There are special names for the PDUs for layers 1, 2, 3, and 4 ● Layer 1 – Bits ● Layer 2 – Frames ● Layer 3 – Packets ● Layer 4 – Segments (TCP) or Datagrams (UDP)
43
TCP Header (Layer 4)
■ 10 mandatory fields, totalling 20 bytes of information ● Source port ● Destination port ● Sequence number ● Acknowledgment numbers ● TCP data offset ● Reserved data – always set to zero ● Control flags ○ SYN – synchronize connection in three-way handshake ○ ACK – acknowledgment of the successful receipt of data ○ FIN (Finished) – tears down connections created by three-way handshake ○ RST (Reset) – used when an unexpected packet is received ○ PSH (Push) – ensures data is given priority ○ URG (Urgent) – identifies incoming data as urgent ● Window size ● TCP checksum ● Urgent pointer ● mTCP – optional
44
UDP Header (Layer 4)
■ 8-byte header ● Source port ● Destination port ● Length – indicates the total packet bytes ● Checksum – not mandatory
45
IP Header (Layer 3)
■ Contains various fields ● Version ● Length of IP header ● Type of service ● Total length of packet and header ● Identifier ● Flags ● Fragmented offset ● Time to live ● Protocol ● Header checksum ● Source IP Address ● Destination IP Address ● Options and Padding
46
Ethernet Header (Layer 2)
■ Features a few things ● Destination MAC Address ● Source MAC address ○ MAC Address ■ Physical address that is used to identify a network card on a local area network ■ Processed by switches
47
EtherType field
○ Used to indicate which protocol is encapsulated in the payload of a frame ■ IPv4 or IPv6
48
● Payload
A frame being sent at Layer 2 will also contain a payload ○ Data that being sent across the network ○ 42 bytes – using VLANs ○ 46 bytes – no VLANs ○ Maximum Transmission Unit (MTU) ■ Maximum size for payload ● 1500 bytes for Ethernet ● Jumbo Frames ○ Frames larger than 1500 bytes ○ Require reconfiguring MTU
49
Data Transmission Travel
■ Encapsulation of data and adding header at each layer ● Layer 4 – source/destination ports ● Layer 3 – source/destination IP addresses ● Layer 2 – soure/destination MAC addresses ● Layer 1 – data transmitted as 1s and 0s ■ Decapsulation at each intermediate device until the final host is reached ■ Final host decapsulates to Layer 7 for application understanding

Network+ (N10-009) (6 decks)

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