1. Click to edit Master subtitle style
Chapter 2: OSI Specifications
Click to edit Master subtitle style

2. Chapter 2 Objectives
The Following CompTIA Network+ Exam Objectives Are Covered in This Chapter:
5.0 Industry standards, practices, and network theory
5.1 Analyze a scenario and determine the corresponding OSI layer
• Layer 1 – Physical
• Layer 2 – Data link
• Layer 3 – Network
• Layer 4 – Transport
• Layer 5 – Session
• Layer 6 – Presentation
• Layer 7 – Application 2

3. Chapter 2 Objectives
The Following CompTIA Network+ Exam Objectives Are Covered in This Chapter:
5.2 Explain the basics of network theory and concepts
• Encapsulation/de-encapsulation
• Modulation techniques
o Multiplexing
o De-multiplexing
o Analog and digital techniques
o TDM 3

4. Internetworking Models
In the late 1970s, the Open Systems Interconnection (OSI) reference model was created by the International Organization for Standardization (ISO) to break through this barrier.
The OSI model was meant to help vendors create interoperable network devices and software in the form of protocols so that different vendor networks could work with each other.
The OSI model is the primary architectural model for networks. It describes how data and network information are communicated from an application on one computer through the network media to an application on another computer. The OSI reference model breaks this approach into layers.
Layered architecture – understand computer processes and see what must be accomplished on any one layer and how 4

5. Advantages of Reference Models
Advantages of using the OSI layered model include, but are not limited to, the following:
It divides the network communication process into smaller and simpler components, thus aiding component development, design, and troubleshooting.
It allows multiple-vendor development through standardization of network components.
It encourages industry standardization by defining what functions occur at each layer of the model.
It allows various types of network hardware and software to communicate.
It prevents changes in one layer from affecting other layers, so it doesn’t hamper development and makes application programming easier. 5

6. The OSI Model The OSI model has seven layers: Application (Layer 7)
Presentation (Layer 6)
Session (Layer 5)
Transport (Layer 4)
Network (Layer 3)
Data Link (Layer 2)
Physical (Layer 1)
Its not a physical model, it’s a conceptual and comprehensive fluid set of guidelines 6

7. OSI Layer Functions Application
File, print, message, database, and application services
Presentation
Data encryption, compression, and translation services
Session
Dialog control
Transport
End-to-end connection
Network
Routing
Data Link
Framing
Physical
Physical topology
Divided into two groups:
top 3 rules of how applications within hosts communicate with each other and end users
bottom 4 how data is transmitted from end to end 7

8. The Upper Layers Application Provides a user interface Presentation
Presents data
Handles processing such as encryption
Session
Keeps different applications’ data separate
Transport
Network
Data Link
Physical
Application:
users communicate and interact with the computer
application processes, interfaces, APIs -> through OS
Presentation:
data translation and code formatting
Adapts data to standard format before transmission
data compression, decompression, encryption, decryption
Session
setting up , managing, and tearing down session between presentation layer entities
coordinates communication between systems
organizes their communication through: simplex, half-duplex, full-duplex
keeps app data separate from other apps data 8

9. The Lower Layers Application Presentation Session Transport
Provides reliable or unreliable delivery
Performs error correction before retransmit
Network
Provides logical addressing which routers use for path determination
Data Link
Combines packets into bytes and bytes into frames
Provides access to media using MAC address
Performs error detection not correction
Physical
Moves bits between devices
Specifies voltage, wire speed, and pin-out of cables
Transport,
segments and reassembles data into data streams
end-to-end data transport services
establish logical connections between sending and destination hosts
multiplexing
establish virtual connections
tear down virtual connections
TCP/UDP 9

10. Reliability
Reliable data transport employs a connection-oriented communications session between systems, and the protocols involved ensure that the following will be achieved:
The segments delivered are acknowledged back to the sender upon their reception.
Any segments not acknowledged are retransmitted.
Segments are sequenced back into their proper order upon arrival at their destination.
A manageable data flow is maintained in order to avoid congestion, overloading, and data loss. 10

11. A Connection Oriented Session
Sender
Receiver
SYN
SYN/ACK
ACK
How does the Transport layer employ connection oriented sessions:
Senders TCP process contacts destination TCP process to establish connection -> this results in a virtual circuit
within the handshake, TCP processes agree on amount of information will be sent
The virtual circuit setup is called overhead
Connection Established
Data transfer
(Send bytes of Segments) 11

12. Flow Control 12 Sender Receiver Transmit Buffer full No ready – STOP!
Segments processed
GO!
If a bunch of computers are simultaneously sending datagrams back and forth -> congestion -> bottleneck
prevents sending host from overflowing the buffer in receiving host -> loss of data
delivered segments are acknowledged back to sender upon receipt
if not acknowledged, retransmitted by sender
segments are sequenced into proper order on arrival destination
manageable data flow maintained to avoid congestion, overload, data loss
Transmit 12

13. Connection Oriented Session
A service is considered connection-oriented if:
Virtual circuit is setup (three-way handshake).
Uses sequencing.
Uses acknowledgments.
Uses flow control. 13

14. Windowing Flow Control
Sender
Receiver
Window size of 1
Send 1
Receive 1
Ack 1
Send 2
Receive 2
Ack 2
Slow if: sender waited for ACK after transmission of each segment.
Sender allowed to send without receiving ACK is known as Windowing
used to control the amount of outstanding, unacknowledged data segments
Window size of 3
Send 1
Send 2
Send 3
Ack 4
Send 4 14

15. Acknowledgements 1 2 3 4 5 6 1 2 3 4 5 6 15 Sender Receiver Send 1
Reliable data deliver guarantees that data wont be duplicated or lost
Positive Acknowledgments with Retransmission
requires receiving machine to communicate with sender when data received
Remember how during the handshake, they share information about size of segment?
Send 4
Send 5
Connection lost!
Send 6
Ack 5
Send 5
Ack 7 15

16. The Lower Layers Application Presentation Session Transport
Provides reliable or unreliable delivery
Performs error correction before retransmit
Network
Provides logical addressing which routers use for path determination
Data Link
Combines packets into bytes and bytes into frames
Provides access to media using MAC address
Performs error detection not correction
Physical
Moves bits between devices
Specifies voltage, wire speed, and pin-out of cables
Network
logical device addressing
transports traffic between devices that aren’t locally attached
Routers work at Layer 3
Two types of packets in the network layer:
Data packets: transport user data through internetwork, called routed protocols ie: IP and IPv6
Route-Update packets: to update neighboring routers about networks connected to all routers, called routing protocols ie: RIP, RIPv2, EIGRP, OSPF. Used to help build and maintain the routing table 16

17. Routing at Layer 3 Routing table Routing table 17 3.0 1.0 3.1 1.1 2.1
2.2
1.3
3.3 E0 E0 S0 S0
3.2
1.2
Routing table
NET
INT
Metric 1 E0 2 S0 3
Routing table
NET
INT
Metric 1 S0 2 3 E0
Routing table:
network addresses – protocol specific
Interface – exit interface
metric – distance to the remote network. Different ways of computing this distance: hop count, (bandwidth, delay etc. 17

18. Routers at Layer 3 18 Internet FastEthernet0/0 Serial0 WAN Services
Each router interface is a broadcast domain. Routers break up broadcast domains by default and provide WAN services
Each interface represents a separate network – each assigned a unique network identification number, each host on the network will use the same network number
Key points:
do not forward broadcast or multicast packets
use logical address to determine next-hop router to forward packet
can use access-lists created by admins to control security
can provide layer 2 bridge functions and can route through the same interface
layer 3 devices provide connections between VLANS
provide QOS for specific types of network traffic 18

19. The Lower Layers Application Presentation Session Transport
Provides reliable or unreliable delivery
Performs error correction before retransmit
Network
Provides logical addressing which routers use for path determination
Data Link
Combines packets into bytes and bytes into frames
Provides access to media using MAC address
Performs error detection not correction
Physical
Moves bits between devices
Specifies voltage, wire speed, and pin-out of cables
Data link is the physical transmission of data, handles:
error notification
network topology
flow control
Uses:
hardware addressing (MAC) media access control
formats messages into pieces called data frames 19

20. Data Link Layer (Layer 2)
Logical Link Control (LLC)
Media Access Control (MAC)
802.11
802.3
802.2
MAC – how packets are placed on the media.
physical addressing
logical topology – signal path through a physical topology
line discipline
error notification
ordered delivery of frames
optional flow control
LLC- identifies network layer protocols and encapsulates them
flow control
sequencing of control bits
802 project 20

21. The Lower Layers Application Presentation Session Transport
Provides reliable or unreliable delivery
Performs error correction before retransmit
Network
Provides logical addressing which routers use for path determination
Data Link
Combines packets into bytes and bytes into frames
Provides access to media using MAC address
Performs error detection not correction
Physical
Moves bits between devices
Specifies voltage, wire speed, and pin-out of cables
Physical layer:
send and receives bits (1, 0)
specifies the electrical, mechanical, procedural, and functional requirements for activating, maintaining, and deactivating a physical link
Identify the interface between data terminal equipment DTE -> data communication equipment DCE
DTE is the end of the users device
DCE is the end of the providers side of communication,
usually located at the customer premice (modem or channel service unit/data service unit (CSU/DSU) 21

22. Data Encapsulation PDU Application Presentation Upper layer data
Session
TCP Header
Upper layer data
Segment
Transport
IP Header
Segment
Packet
Network
LLC Header
Packet
FCS
Encapsulation: wrapped with protocol information at each layer
Protocol Data Unit (PDU) – hold the control information attached to the data
User information -> Data -> segments, reliable connection setup for transmission -> packets, logical address to be routed -> frames, hardware address for unique identify -> bits, digital encoding and cocking scheme
Frame
Data Link
MAC Header
Packet
FCS
Bits
Physical 22

23. Modulation Techniques
Modulation is the process of varying one or more properties of a waveform called a carrier signal
In current networks, modulation takes digital or analog signals and puts in in another signal that can be physically transmitted
Modems – perform both modulation and demodulation operations
Analog and Digital modulation use Frequency-Division multiplexing. Several low-pass signals transferred simultaneously over same shared medium
Ethernet uses digital baseband modulation or line coding to transfer digital bit stream
Time-division multiplexing (TDM) method of transmitting and receiving many independent signals over common signal path through synchronize devices 23

24. Summary Summary Exam Essentials Section Written Labs Review Questions24