The OSI Model N10-009

Slide 1 of 10  |  N10-009 Obj 1.1  |  OSI Model
The OSI Model:
7 Layers of Communication
A systematic framework to understand and troubleshoot network communication.
A user reports they can't access a website. Is it a DNS issue (Layer 7)? A routing problem (Layer 3)? A bad cable (Layer 1)? Understanding the OSI model gives you a systematic framework to isolate the problem layer by layer.
10 Slides N10-009 Obj 1.1 OSI Model Concepts + Context
Slide 2 of 10  |  Layer 7
Layer 7: Application
The layer closest to the end user — where applications meet the network.
Description
Provides network services directly to user applications. Users interact with the network through email clients, web browsers, and other software at this layer. It interfaces between the user and the network stack below.
Key Functions
Provides network services to applications.
Handles high-level protocols.
Interfaces between user and network.
Manages resource sharing.
Common Protocols
HTTP / HTTPS80 / 443
FTP20 / 21
SMTP25 / 587
DNS53
SSH22
Telnet23
SNMP161 / 162
Network Devices
Firewalls (L7 / Next-Gen)
Load Balancers
Proxy Servers
Exam tip: If a user can ping an IP but not browse a website, the issue is likely at Layer 7 (DNS, HTTP).
Slide 3 of 10  |  Layer 6
Layer 6: Presentation
Data translation, formatting, encryption, and compression.
Description
Acts as a translator between the network and the application. Handles data formatting, encryption, and compression to ensure data sent by one system can be read by another.
Key Functions
Data translation and formatting.
Encryption and decryption.
Data compression.
Character encoding (ASCII, Unicode).
Common Protocols
SSL / TLS443
JPEGdata format
GIFdata format
MPEGdata format
ASCIIencoding
EBCDICencoding
Network Devices
Encryption devices
Gateway (protocol conversion)
Exam tip: SSL/TLS operates at the Presentation layer. If you see encryption/decryption questions, think Layer 6.
Slide 4 of 10  |  Layer 5
Layer 5: Session
Establishes, manages, and terminates connections between applications.
Description
The Session layer manages dialogs between systems. It establishes sessions, maintains them during data transfer, and tears them down cleanly. Handles synchronization checkpoints for long transfers.
Key Functions
Session establishment.
Session maintenance.
Session termination.
Synchronization and checkpointing.
Common Protocols
NetBIOS137-139
RPC135
PPTP1723
SAPsession mgmt
SDPsession description
Network Devices
Gateways
Exam tip: The Session layer manages dialogs. NetBIOS and RPC are the exam favorites here.
Slide 5 of 10  |  Layer 4
Layer 4: Transport
End-to-end delivery, segmentation, error recovery, and flow control.
Description
Ensures complete data transfer between hosts. Provides error recovery, flow control, and can guarantee delivery (TCP) or provide fast, connectionless service (UDP). Handles port addressing to identify upper-layer applications.
Key Functions
Segmentation and reassembly.
Error detection and recovery.
Flow control.
Port addressing.
Common Protocols
TCP0-65535reliable
UDP0-65535fast
SCTPmulti-stream
DCCPcongestion ctrl
Network Devices
Firewalls (L4)
Load Balancers
Exam tip: TCP = reliable (three-way handshake, port numbers). UDP = fast (no overhead, no guarantee). Know the difference.
Slide 6 of 10  |  Layer 3
Layer 3: Network
Logical addressing, routing, and path selection across networks.
Description
Handles logical addressing (IP addresses) and determines the best path for data to travel across networks. Routing decisions are made here. Packets are the Protocol Data Unit (PDU) at this layer.
Key Functions
Logical addressing (IP).
Routing and path selection.
Packet forwarding.
Fragmentation.
Common Protocols
IP (IPv4 / IPv6)proto 4/41
ICMPproto 1
OSPFproto 89
BGP179
RIP520
ARPL2/L3 bridge
Network Devices
Routers
Layer 3 Switches
Firewalls
Exam tip: If a packet can't reach a remote network, the problem is likely Layer 3 -- check IP addressing and routing tables.
Slide 7 of 10  |  Layer 2
Layer 2: Data Link
Physical addressing (MAC), framing, and media access control.
Description
Provides node-to-node data transfer. Handles physical addressing (MAC addresses) and packages raw bits into frames. Manages access to the physical medium and performs error detection via CRC.
Key Functions
Physical addressing (MAC).
Framing.
Error detection (CRC).
Media access control.
Common Protocols
Ethernet802.3
Wi-Fi802.11a/b/g/n/ac/ax
PPPpoint-to-point
HDLChigh-level data link
Frame Relaylegacy WAN
Network Devices
Switches
Bridges
NICs
Wireless Access Points (WAPs)
Exam tip: MAC addresses, switches, and VLANs all live at Layer 2. ARP resolves IP to MAC (bridges L3 and L2).
Slide 8 of 10  |  Layer 1
Layer 1: Physical
Raw bit transmission: cables, signals, electrical specifications.
Description
Deals with the actual physical connection between devices. Defines electrical, mechanical, and procedural specifications for transmitting raw bits over the physical medium. No framing or addressing -- just ones and zeros.
Key Functions
Bit transmission.
Physical topology.
Encoding and signaling.
Physical medium specifications.
Common Protocols
Ethernet (physical)Cat5e/6/6a
USB2.0/3.0/C
Bluetooth2.4 GHz
DSLcopper telephone
ISDNlegacy digital
Network Devices
Hubs
Repeaters
Cables
Connectors
NICs
Exam tip: If LEDs are off, cables are damaged, or the NIC is dead -- it's a Layer 1 problem. Always check physical first.
Slide 9 of 10  |  Memory Aids
OSI Memory Aids
Two mnemonics and a quick-reference layer stack.
Top-Down (Layer 7 to 1)
All   People   Seem   To   Need   Data   Processing
Application · Presentation · Session · Transport · Network · Data Link · Physical
Bottom-Up (Layer 1 to 7)
Please   Do   Not   Throw   Sausage   Pizza   Away
Physical · Data Link · Network · Transport · Session · Presentation · Application
Quick Reference Stack
7
Application HTTP/HTTPS, FTP, SMTP, DNS, SSH, SNMP
6
Presentation SSL/TLS, JPEG, GIF, MPEG, ASCII, EBCDIC
5
Session NetBIOS, RPC, PPTP, SAP, SDP
4
Transport TCP, UDP, SCTP, DCCP
3
Network IP (IPv4/IPv6), ICMP, OSPF, BGP, RIP, ARP
2
Data Link Ethernet, Wi-Fi (802.11), PPP, HDLC, Frame Relay
1
Physical Ethernet (physical), USB, Bluetooth, DSL, ISDN
Slide 10 of 10  |  N10-009 Obj 1.1
The OSI Model -- Key Takeaways
7 Facts to Carry Out of This Presentation
L7 Application -- where users meet the network. HTTP, FTP, SMTP, DNS, and SSH all live here. Firewalls can inspect at this layer.
L6 Presentation -- translates, encrypts, and compresses data. SSL/TLS is the key exam protocol at this layer.
L5 Session -- manages dialogs between applications. NetBIOS and RPC are the most-tested protocols here.
L4 Transport -- TCP for reliability (handshake, guaranteed delivery), UDP for speed (connectionless, no overhead). Port numbers live here.
L3 Network -- IP addressing and routing. Routers and Layer 3 switches operate here. Can't reach a remote subnet? Start at Layer 3.
L2 Data Link -- MAC addresses, switches, frames, and VLANs. ARP bridges Layer 3 logical addressing to Layer 2 physical addressing.
L1 Physical -- bits on the wire. Dead LEDs, bad cables, failed NICs. Always check physical layer first in any troubleshooting scenario.