OSI Model Flashcards
OSI Model/Stack (7 Layers)
1) Physical (Bits)
2) Data Link (Frames)
3) Network (Packets)
4) Transport (Segments)
5) Session (Data)
6) Presentation (Data)
7) Application (Data)
OSI Model Data Types
Bits (Physical) Frames (Data Link) Packets (Network) Segments (Transport) Data (Session, Presentation, Application)
Layer 1 (Physical): Functions
Transmission of bits across the network How bits are represented Physical Topology/Wiring Standards Synchronizing bits Bandwidth usage Multiplexing strategy
Layer 1 (Physical): How Bits are Represented
Electric voltage (copper) or light (fiber)
Current State:
0 volts = 0
+/- 5 volts = 1
Transition Modulation: If it changed during the clock cycle, then a 1 is represented (otherwise 0)
Layer 1 (Physical): Cable Connections
Layer 1 devices view networks from a physical topology perspective
Bus, Ring, Star, Hub & Spoke, Full Mesh, Partial Mesh
Layer 1 (Physical): Communication Synchronization
Asynchronous: Uses start bits & stop bits to indicate when transmissions occur from sender to receiver.
Synchronous: Uses a reference clock to coordinate the transmissions by both sender & receiver
Layer 1 (Physical): Bandwidth Utilization (B vs. B)
Broadband: Divides bandwidth into separate channels (Cable TV)
Baseband: Uses all available frequency on a medium (cable) to transmit data & uses a reference clock to coordinate the transmissions by sender & receiver
Layer 1 (Physical): TDM
Time-Division Multiplexing:
Each session takes turns, using time slots, to share the medium between all users
Layer 1 (Physical): StatTDM
Statistical Time-Division Multiplexing:
More efficient version of TDM, it dynamically allocates time slots on an as-needed basis instead of statically assigning
Layer 1 (Physical): FDM
Frequency-Division Multiplexing:
Medium is divided into various channels based on frequencies and each session is transmitted over a different channel (broadband)
Layer 1 (Physical): Examples
Cables (Ethernet, Fiber Optic) Radio Frequencies (Wi-Fi, Bluetooth) Infrastructure Devices (Hubs, WAPs, Media Converters)
Layer 2 (Data Link): Functions
Packages data into frames & transmitting those frames on the network, performing error detection/correction, and uniquely identifying network devices with a MAC address, and flow control.
(Physical addressing, logical topology, method of transmission)
Layer 2 (Data Link): LLC
Logical Link Control:
Provides connection services
Acknowledgement of receipt of a message
Flow Control: Limits amount of data sender can send at one time to keep receiver from becoming overwhelmed
Error Control: Allows receiver to let sender know when an expected data frame wasn’t received or was corrupted by using a checksum
Layer 2 (Data Link): Communication Synchronization
Isochronous: Network devices use a common reference clock source & create times slots for transmission (less overhead than synchronous or asynchronous)
Synchronous: Network devices agree on clocking method to indicate beginning & end of frames (uses control characters or separate timing channel)
Asynchronous: Network devices reference their own internal clocks & use start/stop bits
Layer 2 (Data Link): Examples
NICs
Bridges
Switches
Layer 3 (Network): Functions
Forwards traffic (routing) with logical address (IPv4/IPv6) Logical addressing Switching Route discovery & selection Connection services Bandwidth usage Multiplexing strategy
Layer 3 (Network): Logical Address
Numerous routed protocols were used for logical addressing over the years:
AppleTalk, Internetwork Packet Exchange (IPX), Internet Protocol (IP)
Only IP remains dominant (IPv4, IPv6)
Layer 3 (Network): Data Routing/Forwarding (3 Switching Types)
Packet Switching: Data is divided into packets & forwarded.
Looks for most efficient route available. Once data reaches destination, packets are reassembled.
Circuit Switching: Dedicated communication link is established between two devices (sender/receiver). (Not ideal for data, best for voice)
Line remains idle between transmission spurts, wasting bandwidth unless using voice.
Message Switching: Data is divided into messages, similar to packet switching, except these messages may be stored, then forwarded.
Layer 3 (Network): Connection Services
Flow Control: Prevents sender from sending data faster than receiver can get it.
Packet Reordering: Allows packets to be sent over multiple links & across multiple routes for faster service.
Layer 3 (Network): ICMP
Internet Control Message Protocol:
Used to send error messages & operational info about an IP destination.
Not regularly used by end-user apps
Used in troubleshooting (ping/traceroute)
Layer 3 (Network): Examples
Routers, multilayer switches, IPv4/IPv6, ICMP
`Layer 4 (Transport): Functions
Dividing line between upper/lower layers of the OSI model Data is sent as segments TCP/UDP Windowing Buffering
Layer 4 (Transport): TCP
Transmission Control Protocol:
Connection-oriented protocol
Reliable transport of segments (if dropped, it’s detected and will get resent)
Acknowledgements received for successful communications
Segment sequencing
Used for all network data that needs to be assured to get to its destination
Layer 4 (Transport): UDP
User Datagram Protocol: Connectionless protocol Unreliable transport of segments (unaware of drops) No retransmission, no segment sequencing Good for audio/video streaming Lower overhead for increased performance
Layer 4 (Transport): Windowing
Allows clients to adjust the amount of data sent in each segment
Continually adjusts to send more/less data per segment transmitted
Adjusts lower as number of retransmissions occur
Adjusts upwards as retransmissions are eliminated
Layer 4 (Transport): Buffering
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 get dropped
Layer 4 (Transport): Examples
TCP UDP WAN Accelerators Load Balancers Firewalls
Layer 5 (Session): Functions
Think of a session as a conversation that must be kept separate from others to prevent intermingling of the data
Setting up, maintaining, and tearing down sessions
Layer 5 (Session): Setting up a Session
Check user credentials
Assign numbers to session to identify them
Negotiate services needed for session
Negotiate who begins sending data
Layer 5 (Session): Maintaining a Session
Transfer the data
Reestablish a disconnected session
Acknowledging receipt of data
Layer 5 (Session): Tearing Down a Session
Due to mutual agreement (after transfer)
Due to other party disconnecting
Layer 5 (Session): Examples
H.323 - Used to setup, maintain, & tear down a voice/video connection
NetBIOS - Used by computers to share files over a network
Layer 6 (Presentation): Functions
Responsible for formatting the data exchanged & securing that data with proper encryption
Data formatting
Encryption
Layer 6 (Presentation): Data Formatting
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 6 (Presentation): Encryption
Used to scramble the data in transit to keep it secure from prying eyes
Provides confidentiality of data
Example: TLS to secure data between device/website
Layer 6 (Presentation): Examples
HTML, XML, PHP, Javascript ASCII, EBCDIC, UNICODE GIF, JPG, TIF, SVG, PNG MPG, MOV TLS, SSL
Layer 7 (Application): Functions
Provides application level services (not Word or Notepad)
Layer where the users communicate with the computer
Application services, Service advertisement
Layer 7 (Application): Application Services
Application services unite communicating components from more than one network application.
(File transfers, email, remote access, network management activities, client/server processes)
Layer 7 (Application): Service Advertisement
Some applications send out announcements
States the services they offer on the network
Some centrally register with the AD server instead
(Printers, file servers)
Layer 7 (Application): Examples
Email (POP3, IMAP, SMTP) Web Browsing (HTTP, HTTPS) DNS FTP/FTPS TELNET/SSH SNMP
Encapsulation
The process of putting headers (and sometimes trailers) around some data.
Moving down the OSI layers from 7 to 1.
Decapsulation
The action of removing the encapsulation that was applied.
Moving up the OSI layers from 1 to 7.
PDU
Protocol Data Unit:
A single unit of info transmitted within a computer network.
Layer 1 = Bits
Layer 2 = Frames
Layer 3 = Packets
Layer 4 = Segments (TCP), or Datagrams (UDP)
SYN Flag
Synchronization Flag:
Used to synchronize the connection during the three-way handshake.
ACK Flag
Acknowledgement Flag:
Used during the three-way handshake, but is also used to acknowledge the receipt of packets.
FIN Packet
Finished Packet:
Used to tear down the virtual connections created using the three-way handshake and the SYN flag.
Always appears when the last packets are exchanged between a client/server & host is ready to shutdown the connection.
RST Flag
Reset Flag:
Used when a client or server receives a packet that it was not expecting during the current connection.
PSH Flag
Push Flag:
Used to ensure that the data is given priority and is processed at the sending or receiving ends.
URG Flag
Urgent Flag:
Like the Push flag, identifies incoming data as urgent.
PSH is used by sender to indicate data with a high priority.
URG is sent to tell the recipient to process immediately & ignore anything else in the queue.
EtherType Field
Used to indicate which protocol is encapsulated in the payload of the frame.
Layer 4 = Source/destination ports
Layer 3 = Source/destination IPs
Layer 2 = Source/destination MACs
