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
