Section 3: OSI Model
Decapsulation - Layer 7 Application : OSI Model
Action of removing the encapsulation that was applied. If we move upward from layers 1 to 7, we decapsulate data A protocol data unit is a single unit of information transmitted within a computer network ▪ Layer 1 - bits ▪ Layer 2 - frames ▪ Layer 3 - packets ▪ Layer 4 - segments if TCP or datagrams if UDP
Packet Reordering (Connection Services) : Layer 3: OSI Model
Allows packets to be sent over multiple links and across multiple routes for faster service
Logical Link Control (LLC) - Error Control : Layer 2 Data Link - OSI Model
Allows receiver to let sender know when an expected data frame wasn't received or was corrupted by using a checksum
Windowing - Layer 4 Transport : OSI Model
Allows the clients to adjust the amount of data sent in each segment. Continually adjusts to send more or less data per segment transmitted ● Adjusts lower as number of retransmissions occur ● Adjusts upwards as retransmissions are eliminated
Source MAC Address - Layer 7 Application : OSI Model
Allows the source to find the destination by using this type of addressing
Application services - Layer 7 (Application) : OSI model
Application services unite communicating components from more than one network application Examples: ● File transfers and file sharing ● E-mail ● Remote access ● Network management activities ● Client/server processes
Layer 5 - Session / Setting Up a Session : OSI Model
Check user credentials Assign numbers to session to identify them Negotiate services needed for session Negotiate who begins sending data
transmission control protocol (TCP) - Layer 4 Transport : OSI Model
Connection-oriented protocol that's a Reliable transport of segments ● If segment is dropped, protocol detects it and resends segment. Acknowledgements received for successful communications Used for all network data that needs to be assured to get to its destination
user datagram protocol (UDP) - layer 4 transport : OSI Model
Connectionless protocol that's Unreliable transport of segments across the network ● If dropped, sender is unaware ▪ No retransmission ▪ Good for audio/video streaming ▪ Lower overhead for increased performance
Message Switching (routing) - Layer 3: OSI Model
Data is divided into messages, similar to packet switching, except these messages may be stored then forwarded
Packet Switching (routing) - Layer 3: OSI Model
Data is divided into packets and forwarded Ex - its going to keep switching the packet from place to place until it gets to its final destination. Most networks use this. (home network, small office, and the internet)
Which of the following applies to data as it travels from Layer 1 to Layer 7 of the OSI model? - OSI Model Quiz
De-encapsulation - Data encapsulation and de-encapsulation in a computer network is a necessary process. De-encapsulation in networking is performed at the receiver side or destination side as data moves from layer 1 to layer 7 of the OSI model. As information travels up the layers of the OSI layer, information added from the sender's encapsulation process is removed layer by layer.
Circuit Switching (routing) - Layer 3: OSI Model
Dedicated communication link is established between two devices
Buffering - Layer 4 Transport : OSI Model
Devices, such as routers, allocate memory to store segments if bandwidth isn't readily available ▪ When available, it transmits the contents of the buffer ▪ If the buffer overflows, segments will be dropped
(Broadband) : Layer 1 Physical - OSI Model
Divides bandwidth into separate channels ● Example: Cable TV
Layer 4 Transport : OSI Model
Dividing line between upper and lower layers of the OSI model ▪ Data is sent as segments ▪ TCP/UDP ▪ Windowing ▪ Buffering
Layer 5 - Session / Tear Down a Session : OSI Model
Due to mutual agreement ● After the transfer is done Due to other party disconnecting
Layer 7 Application Devices : OSI Model
E-mail (POP3, IMAP, SMTP) Web Browsing (HTTP, HTTPS) Domain Name Service (DNS) File Transfer Protocol (FTP, FTPS) Remote Access (TELNET, SSH) Simple Network Management Protocol (SNMP)
Time-Division Multiplexing (TDM) -- Limited Network: Layer 1 Physical - OSI Model
Each session takes turns, using time slots, to share the medium between all users
(bits represented on the medium) : Layer 1 Physical - OSI Model
Electrical voltage (copper wiring) or light (fiber optics) represent 1's and 0's (bits) Current State ● If 0 volts, then 0 is represented ● If +/- 5 volts, then 1 is represented Transition Modulation - If it changed during the clock cycle, then a 1 is represented, otherwise, a 0
Examples at Layer 1 - Cables : OSI Model
Ethernet Fiber optic
Data Formatting - Layer 6 Presentation : OSI Model
Formats data for proper compatibility between devices ● ASCII ● GIF ● JPG ▪ Ensures data is readable by receiving system ▪ Provides proper data structures ▪ Negotiates data transfer syntax for the Application Layer (Layer 7)
Network Layer 3: OSI model
Forwards traffic (routing) with logical address - Example: IP Address (IPv4 or IPv6) -- Ex - 172.16.254.1 (IPv4) Logical addressing Switching - which is called routing (which is how we transfer things at the network layer, layer 3) Route discovery and selection Connection services Bandwidth usage Multiplexing strategy
Layer 5 Session Device examples : OSI Model
H.323 - Used to setup, maintain, and tear down a voice/video connection. Operates over the RTP (Real time protocol) / Ex - Skype, FaceTime, YouTube NetBIOS - Used by computers to share files over a network
Layer 6 Presentation Device Examples : OSI Model
HTML, XML, PHP, JavaScript, (scripting) ASCII, EBCDIC, UNICODE, (standard text) GIF, JPG, TIF, SVG, PNG (pictures) MPG, MOV, (movie files) TLS, SSL (encryption algorithms)
Which type of network device operates at layer 1 of the OSI model and requires connected devices to operate at half-duplex using CSMA/CD? - OSI Model Quiz
HUB - A hub is a network hardware device for connecting multiple Ethernet devices together and making them act as a single network segment. A hub operates at the physical layer (Layer 1) of the OSI model. All devices connected by a hub are in a single collision domain and a single broadcast domain, therefore they must use half-duplex for communication and CSMA/CD.
Examples at Layer 1 - Infrastructure Devices : OSI Model
Hubs Wireless Access Points Media Converters
Decapsulation - URG (or urgent) flag / Layer 7 Application : OSI Model
It is like the Push flag and identifies incoming data as "urgent" The main difference is PSH is used by a sender to indicate data with a higher priority level where URG is sent to tell the recipient to process it immediately and ignore anything else in queue
According to the OSI model, at which of the following layers is data encapsulated into a frame? - OSI Model Quiz
Layer 2 - The data layer, or layer 2, is the second layer of the seven-layer OSI model. The data link layer encapsulates data into frames for delivery between nodes on the same network. Data is transmitted at Layers 5, 6, and 7 of the OSI model. At Layer 4, the data is encapsulated into segments. At layer 3, the segments are encapsulated into packets. At layer 2, the packets are encapsulated into frames. At layer 1, the frames are encapsulated into bits
Media Access Control (MAC) - Logical Topology : Layer 2 Data Link - OSI Model
Layer 2 devices view networks logically - Ring, bus, star, mesh, hub-and-spoke
Connection Services - Layer 3: OSI Model
Layer 3 augment Layer 2 to improve reliability
What layer of the OSI model is responsible for data encryption and character set conversion, such as ASCII to UTF-8? - OSI Model Quiz
Layer 6 - The presentation layer (layer 6) establishes the way in which information is presented, typically for display or printing. Data encryption and character set conversion (such as ASCII to EBCDIC) are usually associated with this layer. The presentation layer translates information in a way that the application layer understands. This layer also translates information from the application layer to the session layer.
Logical Link Control (LLC) - Flow Control : Layer 2 Data Link - OSI Model
Limits amount of data sender can send at one time to keep receiver from becoming overwhelmed
Frequency Division Multiplexing (FDM) -- Limited Network) : Layer 1 Physical - OSI Model
Medium is divided into various channels based on frequencies and each session is transmitted over a different channel example - Broadband
Statistical Time-Division Multiplexing (StatTDM) -- Limited Network) : Layer 1 Physical - OSI Model
More efficient version of TDM, it dynamically allocates time slots on an as-needed basis instead of statically assigning
Examples at Layer 2 : Data Link - OSI Model
Network Interface Cards (NIC) Bridges Switches - can use logic to learn which physical ports are attached to which devices based on their MAC addresses. They can send data to specific devices in the network, allowing us to pick up and choose different lines of communication to go to different areas
Synchronous : Layer 2 Data Link - OSI Model
Network devices agree on clocking method to indicate beginning and end of frames Uses control characters or separate timing channel
Asynchronous : Layer 2 Data Link - OSI Model
Network devices reference their own internal clocks and use start/stop bits
Isochronous : Layer 2 Data Link - OSI Model
Network devices use a common reference clock source and create time slots for transmission Less overhead than synchronous or asynchronous
Decapsulation - URG (or urgent) flag -- Checksum / Layer 7 Presentation : OSI Model
Not a mandatory field, but it can be used to provide some validation that the UDP data being sent was received with some level of integrity
Layer 2 - Data Link : OSI Model
Packages data into frames and transmitting those frames on the network, performing error detection/correction, and uniquely identifying network devices with an address (MAC), and flow control ● MAC ● Physical addressing ● Logical topology ● Method of Transmission ● Link Layer Control (LLC) - Connection services - Synchronizing transmissions
Flow Control (Connection Services) : Layer 3: OSI Model
Prevents sender from sending data faster than receiver can get it
Layer 7 Application : OSI Model
Provides application-level services and where the users communicate with the computer ● Not Microsoft Word or Notepad Functions: ● Application services ● Service advertisement
Logical Link Control (LLC) : Layer 2 Data Link - OSI Model
Provides connection services and Acknowledgement of receipt of a message
Layer 6 Presentation : OSI Model
Responsible for formatting the data exchanged and securing that data with proper encryption ▪ Functions ▪ Data formatting ▪ Encryption
Layer 3 Network Device Examples : OSI Model
Routers - Looks like a die icon here ( a circle with four arrows, this is a depiction of what a router looks like in a logical diagram) Multilayer switches - Works like a regular switch and a router combined. Has the features of a layer 2 switch and a layer 3 router in the single device, which is why it's considered a layer 3 device. IPv4 protocol IPv6 protocol Internet Control Message Protocol (ICMP)
Route Discovery and Selection - Layer 3: OSI Model
Routers maintain a routing table to understand how to forward a packet based on destination IP address Manually configured as a static route or dynamically through a routing protocol ● RIP ● OSPF ● EIGRP
Service Advertisement - Layer 7 (Application) : OSI Model
Some applications send out announcements to to other devices on the network to state services they offer on the network Some centrally register with the Active Directory server instead - Example: Printers, File servers
Which network device operates at layer 2? - OSI Model Quiz
Switch - A basic switch operates at layer 2 of the OSI model. For the exam, unless they mention a "multilayer switch" or "layer 3 switch", always assume they are referencing a basic layer 2 switch. A router is a layer 3 device. A repeater is a layer 1 device. A firewall will operate layers 3 through 7, depending on the type of firewall.
Layer 4 Transport Device Examples : OSI Model
TCP UDP WAN Accelerators -Where we try to add compression to our IP packets, and then we send over those segments over through using WAN accelerators to get them through our network faster. Load Balancers Firewalls - Blocking and allowing different protocols and ports to go through them
Decapsulation - SYN (or synchronization) flag / Layer 7 Presentation : OSI Model
The most well-known flag in TCP communications because it is used to synchronize the connection during the three-way handshake
Encapsulation - Layer 7 Presentation : OSI Model
The process of putting headers (and sometimes trailers) around some data. If we move down the OSI layers from 7 to 1, we encapsulate data
Layer 5 - Session : OSI Model
Think of a session as a conversation that must be kept separate from others to prevent intermingling of the data ▪ Setting up sessions ▪ Maintaining sessions ▪ Tearing down sessions
Data Types in the OSI Model
Top Data -- Layers 5 - 7 : Application Layer (7), Presentation (6), Session (5) Segments -- Layer 4: Transport Packets -- Layer 3: Network Frames -- Layer 2: Data Link Bits -- Layer 1: Physical Don't Some People Fear Birthdays Bottom
OSI Model Layers : (OSI Model)
Top Layer 7: Application Layer 6: Presentation Layer 5: Session Layer 4: Transport Layer 3: Network Layer 2: Data Link Layer 1: Physical Please Do Not Throw Sausage Pizza Away Bottom
Layer 5 - Session / Maintaining a Session : OSI Model
Transfer the data Reestablish a disconnected session Acknowledging receipt of data
Layer 1 - Physical: OSI Model
Transmission of bits across the network occurs and includes Physical and electrical network characteristics This is will tell us whether we're using fiber or copper cables, whether Cat5 or Cat6, or if we're using radio frequency in the case of Wi-Fi. Characteristics: ● How bits are represented on the medium ● Wiring standards for connectors and jacks ● Physical topology ● Synchronizing bits ● Bandwidth usage ● Multiplexing strategy
Decapsulation - ACK (or acknowledgement) flag / Layer 7 Presentation : OSI Model
Used during the three-way handshake, but it is also used to acknowledge the successful receipt of packets
Decapsulation - PSH (or PUSH) flag / Layer 7 Presentation : OSI Model
Used to ensure that the data is given priority and is processed at the sending or receiving ends
Decapsulation - URG (or urgent) flag -- Length / Layer 7 Presentation : OSI Model
Used to indicate how many bytes the UDP packet is, including its header and its data
EtherType field - Layer 7 Application : OSI Model
Used to indicate which protocol is encapsulated in the payload of the frame As data moves from layer 7 to layer 1, that data is encapsulated ● At layer 4, we add our source and destination ports ● At layer 3, we add our source and destination IP addresses ● At layer 2, we add our source and destination MAC addresses Once we get to layer 1, we are simply transmitting our layer 2 frames as a series of 1's and 0's over the medium Once that host is found, it will keep decapsulating the information all the way up to layer 7, where its application can read and understand the underlying data
encryption - Layer 6 Presentation : OSI Model
Used to scramble the data in transit to keep it secure from prying eyes and Provides confidentiality of data ▪ Example: TLS to secure data between your PC and website
Internet Control Message Protocol (ICMP) - Layer 3: OSI Model
Used to send error messages and operational information about an IP destination Not regularly used by end-user applications Used in troubleshooting (ping and traceroute)
Decapsulation - FIN (or finished) packet / Layer 7 Presentation : OSI Model
Used to tear down the virtual connections created using the three-way handshake and the SYN flag The FIN flag always appears when the last packets are exchanged between a client and server and the host is ready to shutdown the connection
Decapsulation - RST (or reset) flag / Layer 7 Presentation : OSI Model
Used when a client or server receives a packet that it was not expecting during the current connection
Media Access Control (MAC) - Physical Addressing : Layer 2 Data Link - OSI Model
Uses 48-bit address assigned to a network interface card (NIC) by manufacturer Ex - D2:51:F1:3A:34:65 First 24-bits is the vendor code - D2:51:F1 Second 24-bits is a unique value - 3A:34:65
(Synchronous) : Layer 1 Physical - OSI Model
Uses a reference clock to coordinate the transmissions by both sender and receiver
(Baseband) : Layer 1 Physical - OSI Model
Uses all available frequency on a medium (cable) to transmit data and uses a reference clock to coordinate the transmissions by both sender and receiver ● Example: Ethernet
(Asynchronous) : Layer 1 Physical - OSI Model
Uses start bits and stop bits to indicate when transmissions occur from sender to receiver
Examples at Layer 1 - Radio Frequencies : OSI Model
Wi-Fi Bluetooth
Decapsulation - URG (or urgent) flag -- Source & Destination Ports / Layer 7 Application : OSI Model
are just like the ones used in UDP, they dictate where the data is coming from and where it is going to (layer4)
(How are the cables connected) : Layer 1 Physical - OSI Model
devices view networks from a physical topology perspective ▪ Includes: ● Bus ● Ring ● Star ● Hub-and-Spoke ● Full Mesh ● Partial Mesh
(TIA/EIA-568-B Cable) : Layer 1 Physical - OSI Model
is standard wiring for RJ-45 cables and ports
Destination MAC Address - Layer 7 Application OSI Model
physical address that is used to identify a network card on the local area network
traits of transmission control protocol (TCP) - Layer 4 Transport : OSI Model
reliable connection-oriented segement retransmission and flow control through windowing segment sequencing acknowledge segments
(Straight-thru cables) : Layer 1 Physical - OSI Model
typically use T-568B on both ends, but could use T-568A on both
traits of user datagram protocol (UDP) - layer 4 transport : OSI Model
unreliable connectionless no windowing or retransmission no sequencing no acknowledgement
(Crossover cables) : Layer 1 Physical - OSI Model
use T-568A and T-568B
