Section 3: OSI Model Flashcards
OSI Model
Application (7)
Presentation (6)
Session (5)
Transport (4)
Network (3)
Data Link (2)
Physical (1)
data on each OSI layer:
layers 5, 6, 7: data
layer 4: segements
layer 3: packets
layer 2: frames
layer 1: bits
Physical Layer
transmission of data (bits) and electrical signals across the network
Connector Standards
RJ-45 - Used in Cat5/Cat6 cables
Wiring Standards
TIA/EIA-568A
TIA/EIA-568B
Crossover cables
TIA/EIA-568A on one end, TIA/EIA-568B on the other
Straight-through cables
TIA/EIA-568B on both ends
Asynchronous Communication
out of sync data transmission
includes a start and stop bit, like leaving a voicemail
synchronous Communication
real-time communication, uses a common time source or clock
Broadband
divides bandwidth into separate channels
e.g. cable TV carrying 200 channels
Baseband
Uses all available frequencies on a cable to transmit data.
e.g. a telephone, one call at a time, ethernet
Multiplexing
allows multiple usage of a baseband connection by multiple users
layer 1 devices
fiber optic cable, ethernet, coaxial, bluetooth, wi-fi, NFC
hubs, access points, media converters
devices are essentially repeaters
Data Link Layer
package data/bits into frames to transmit
controls information flow
Media Access Control (MAC)
physical addressing system using a unique 48-bit physical address in hexidec assigned to every network interface card (NIC)
D2:51:F1:3A:34:64
D2:51:F1 = vendor
3A:34:64 = unique
Logical Link Control (LLC)
provides connection services and acknowledgement of messages sent correctly
e.g. a device can request for less information or a resend
provides error control functions using checksums
Isochronous method
devices use a common reference clock and create time slots for transmission
Synchronous method
devices agree on a clocking method, has beginning and ending frames
asynchronous
network devices reference internal clocks to start and stop bits
layer 2 devices
network interface cards, bridges/switches
switches
smarter devices that send data to specific devices based on the MAC address
uses CAM tables with MAC addresses
Network Layer
concerned with routing/switching, logical addressing, route discovery
Internet Protocol (IP)
IPv4 or IPv6
Packet switching
data is divided into packets, then forwarded
most networks use this
e.g. putting a letter in an envelope and sending it off
Circuit switching
dedicated communication link established between two devices
e.g. a phone call makes a temporary connection
message switching
data is divided into messages which may be stored and then forwarded
e.g. mail arriving at the destination and waiting until its forwarded
route discovery and selection
manually configured as a static route or dynamically (RIP, OSPF, EIGRP) both using a routing table
decide how data is going to get there
Connection services
provides additional reliability for layer 2 connection such as flow control and packet reordering
packet reordering
ensures all the data that is split up reaches the destination
Internet Control Message Protocol (ICMP)
Sends error messages and information to an IP destination
e.g. ping, tracert
layer 3 devices
routers/multi layer switches
Transport Layer
division between the upper and lower layers of the OSI model
datatype: segments
Transmission Control Protocol (TCP)
data is referenced as segments
connection-full protocol; reliable
Three-way handshake
Client sends SYN packet
Server sends SYN-ACK
Client sends ACK, then their data
e.g. certified priority mail
User Datagram Protocol (UDP)
data is referenced as datagram
connectionless; “unreliable” things may get lost
Windowing
allow clients to minmax adjust the amount of data in each segment to maximize throughput
e.g. if you are getting too many retransmissions, so you need to slow down
Buffering
devices allocate memory to store segments if it’s receiving too much data to go through too little bandwidth
layer 4 devices
TCP and UDP protocols
WAN accelerators (compresses IP packets to go faster)
load balancers and firewalls (filtering ports/protocols)
Session Layer
keeping conversations seperated to prevent intermingling of data
set up session
checks credentials then assigns random numbers to the session to identify them
maintain session
where data transfers back and forth across the network
any breaks will be reestablished and acknowledgement of data as well
tear down session
ending of a session after the transfer is done or a disconnection
H.323
used to setup, maintain, teardown voice and video communications
operated over Real-Time Transport Protocol (RTP)
NetBIOS
used to share files over a network
Presentation Layer
Data Formatting and Encryption
data formatting
data is formatted to have compatibility between different devices
e.g. ASCII = text, GIF, JPG, PNG
ensures data is readable, provides redundant data structures, negotiates data transfer syntax for layer 7
encrpytion
scrambles data in transit for security
e.g. (Transport Layer Security) TLS encryption
layer 6 “devices”
scripting languages, html, php
standard text
Pictures
Video files
Encryption algorithms (TLS, SSL, etc.)
Application Layer
Where users communicate with the computer, for the computer to be transported
e.g. file transfer, network transfer
application services
e.g. file transfer, file sharing, email protocol , remote access
service advertisement
sending out announcements to other devices to state a service they offer
e.g. printers
layer 7 “things”
Email applications
(POP3, IMAP, SMTP)
Web browsing (HTTP, HTTPS)
DNS, FTP, RDP, SSH, SNMP, etc.
Encapsulation
putting headers and trailers around data
moving down the OSI model, encapsulation happens
e.g. encapsulating a letter in an envelope
Decapsulation
moving up the OSI model, decapsulation happens
e.g. removing an envelope to see the letter
Protocol Data Unit (PDU)
A single unit of information transmitted in a network
called L(layer number) PDU (L7 PDU)
data names: bits, frames, packets, etc.
how encapsulation works
each layer adds their header/trailer with metadata to the “data”, and passes it down
at layer 4, adds source and dest ports
at layer 3, adds source and dest IP address
at layer 2, adds source and dest mac address
at layer 1, transmit layer 2 frames as 0s and 1s
TCP header
20 bytes of information in the Header:
source/dest port
sequence/acknowledgement number
control flags
TCP control flags
SYN: Synchronization of connection
ACK: used to acknowledge successful transport of packets
FIN (Finished): tears down the virtual connection
RST (Reset): an unknown packet is received to reject
PSH (Push): data is given priority to be processed
URG (Urgent): identifies data as urgent, processed first and immediately
UDP Header
8 bytes, Source/Dest Port, Length, Checksum (optional)
IP header
version, length, service, total length, identifier, flags, checksum, source/dest IP, etc.
Ethernet Header
Source/Dest MAC address
EtherType (which IP protocol is encapsulated)
VLAN tag (optional)
Frame payloads
ethernet: 42 byte min, 46 byte no VLANs
MTU
Default frames: 1500 bytes
Default Jumbo Frames: 9000
