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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
Q

Buffering

A

■ 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
Q

Layer 4 Devices

A

Layer 4 Devices
■ Protocols
● TCP and UDP
■ Devices
● WAN accelerators
● Load balancers and firewalls

27
Q

Layer 5

A

Session Layer
■ Manages sessions, ensuring separate conversations to prevent data
intermingling

28
Q

Setting Up Session

A

■ Checking of user credentials and assigning numbers to sessions to help
identify

29
Q

Maintaining Session

A

■ Continuous data transfer between parties
■ If connection breaks, it will require re-establishment
■ Includes acknowledgement of data

30
Q

Tearing Down a Session

A

■ Ending a session once communication goals are achieved
■ Mutual agreement or one party disconnects

31
Q

Layer 5 Devices and Protocols

A

○ 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
Q

Layer 6

A

Presentation layer
■ Responsible for formatting data for exchange and securing it through
encryption

33
Q

Data Formatting

A

■ 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
Q

Encryption

A

■ 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
Q

Scripting languages in Layer 6

A

■ Control how ASCII text is displayed on the screen
● HTML
● XML
● PHP
● JavaScript

36
Q

Encryption Algorithms

A

■ Scrambles data to provide confidentiality and security during transit and
storage
● TLS
● SSL (Secure Sockets Layer)
■ Focus on Security

37
Q

○ Application Layer (Layer 7)

A

■ Provides application-level services where users communicate with the
computer
■ Focus on lower-level applications
● File transfer
● Network transfer

38
Q

Layer 7 Protocols

A

■ 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
Q

Service Advertisement

A

■ 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
Q

Application Services

A

■ 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
Q

Encapsulation & Decapsulation

A

○ 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
Q

Protocol Data Units (PDUs) in OSI Model

A

■ 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
Q

TCP Header (Layer 4)

A

■ 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
Q

UDP Header (Layer 4)

A

■ 8-byte header
● Source port
● Destination port
● Length – indicates the total packet bytes
● Checksum – not mandatory

45
Q

IP Header (Layer 3)

A

■ 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
Q

Ethernet Header (Layer 2)

A

■ 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
Q

EtherType field

A

○ Used to indicate which protocol is encapsulated in the
payload of a frame
■ IPv4 or IPv6

48
Q

● Payload

A

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
Q

Data Transmission Travel

A

■ 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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