OSI Model (1.1 & 5.3) Flashcards
OSI Model Overview & Objective
- OSI Model (Open Systems Interconnection)
- Purpose of Reference Model
- OSI Model Layers
- Data Types in the OSI Model
OSI Model (1977 ISO / 7 layers / troubleshooting / reference model)
- Developed in 1977 by International Organization for Standardization (ISO)
- Called the OSI model or OSI stack
- Consists of 7 layers
- Useful in troubleshooting networks
- Serves as a reference model in networks
Purpose of Reference Model (categorized in layers / comparing tech / learning and understanding to communicate)
▪ Categorize functions of the network into particular layer(s)
▪ Compare technologies across different manufacturers
▪ By understanding its functions, you can understand how best to communicate with that device
OSI Model Layers (PDNTSPA)
7: Application
6: Presentation
5: Session
4: Transport
3: Network
2: Data Link
1: Physical
(Please / Do / Not / Throw / Sausage / Pizza / Away!)
Data Types in the OSI Model (DSPFB?)
7 / 6 / 5 = Data 4 = Segments 3 = Packets 2 = Frames 1 = Bits (Don't / Some / People / Fear / Birthdays?)
Layer 1 (Physical) ( Transmission / Physical and Electrical / bits)
Physical Layer (Layer 1)
▪ Transmission of bits across the network
▪ Physical and electrical characteristics
▪ Characteristics:
● How bits are represented on the medium
● Wiring standards for connectors and jacks
● Physical topology
● Synchronizing bits
● Bandwidth usage
● Multiplexing strategy
How are bits represented on the medium? ( 1’s and 0’s / 0 = 0, +/- 5 = 1 / clock cycle)
▪ 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
How are the cables wired? (RJ45 standard = TIA/EIA-568-B / crossover = T568A or B / straight-thru = 68B on both ends or 68A)
▪ TIA/EIA-568-B is standard wiring for RJ-45 cables and ports
▪ Crossover cables use T-568A and T-568B
▪ Straight-thru cables typically use T-568B on both ends, but could use T-568A on both
How are the cables connected? (Layer 1 = B R S H-S FM PM)
▪ Layer 1 devices view networks from a physical topology perspective ▪ Includes: ● Bus ● Ring ● Star ● Hub-and-Spoke ● Full Mesh ● Partial Mesh
How is communication synchronized? (Asyn = Start and Stop to indicate (When) / Syn = Reference Clock to coordinate (Lead) )
▪ Asynchronous
● Uses start bits and stop bits to indicate when transmissions occur from sender to receiver
▪ Synchronous
● Uses a reference clock to coordinate the transmissions by both sender and receiver
How is bandwidth utilized? (Broadband = Channel / Baseband = Ethernet)
▪ Broadband ● Divides bandwidth into separate channels ● Example: o Cable TV ▪ Baseband ● 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: o Ethernet
How can we get more out of a limited network? (TDM = take turns / STDM = needed basis / FDM = combined transmission on single line/channel)
▪ Time-Division Multiplexing (TDM)
● Each session takes turns, using time slots, to share the medium between all users
▪ Statistical Time-Division Multiplexing (StatTDM)
● More efficient version of TDM, it dynamically allocates time slots on an as-needed basis instead of statically assigning
▪ Frequency-Division Multiplexing (FDM)
● Medium is divided into various channels based on frequencies and each session is transmitted over a different channel
o Broadband
Examples at Layer 1 (Cables / Radio Frequencies / Infrastructure)
▪ Cables ● Ethernet ● Fiber optic ▪ Radio frequencies ● Wi-Fi ● Bluetooth ▪ Infrastructure devices ● Hubs ● Wireless Access Points ● Media Converters
Layer 2 (Data Link) (Data into frames / correction / identify / flow) (MAC, addressing, logical top, transmission, LLC)
Data Link Layer (Layer 2) ▪ 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) o Connection services o Synchronizing transmissions
Media Access Control (MAC) (Physical Addressing / Logical Topology / Transmission)
▪ Physical addressing
● Uses 48-bit address assigned to a network interface card (NIC) by manufacturer
● First 24-bits is the vendor code
● Second 24-bits is a unique value
▪ Logical topology
● Layer 2 devices view networks logically
● Ring, bus, star, mesh, hub-and-spoke, …
▪ Method of transmission
● Many devices are interconnected
● Determines whose turn it is to transmit to prevent interference with other devices
Logical Link Control (LLC) (Connection services / acknowledgement of receipt / flow control / error 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
How is communication synchronized? (Iso = ref clock for time slots / Syn = clocking for beginning to end / Asyn = internal clocks for start/stop bits)
▪ Isochronous
● Network devices use a common reference clock source and create time slots for transmission
● Less overhead than synchronous or asynchronous
▪ Synchronous
● Network devices agree on clocking method to indicate beginning and end of frames
● Uses control characters or separate timing channel
▪ Asynchronous
● Network devices reference their own internal clocks and use start/stop bits
Examples at Layer 2 (NICs, Bridges, Switches)
▪ Network Interface Cards (NIC)
▪ Bridges
▪ Switches
Layer 3 (Network) (Forwards traffic / IPv4 or IPv6)
Network Layer (Layer 3) ▪ Forwards traffic (routing) with logical address ● Example: IP Address (IPv4 or IPv6) ▪ Logical addressing ▪ Switching ▪ Route discovery and selection ▪ Connection services ▪ Bandwidth usage ▪ Multiplexing strategy
