1. CS 447 Networks and Data Communication
Network Protocols and OSI 7-layer Reference Model
Department of Computer Science
Southern Illinois University Edwardsville
Fall, 2015
Dr. Hiroshi Fujinoki

2. CS 447 Networks and Data Communication
Organization of a Computer Connected to a Computer Network
Computer A
Computer B
Application
Program
Network
Protocol
Wire (Computer Network)
Network Protocol/001

3. CS 447 Networks and Data Communication
Protocol and Interface
Computer A
Computer B
Application
Program
Protocol
Protocol
Wire (Computer Network)
Network Protocol/003

4. CS 447 Networks and Data Communication
Definitions
A network protocol is:
 a set of rules
 that governs communication between two hosts
 implemented as software
Example
- TCP
- NetBIOS (Windows)
- IP
- etc.
- IPX/SPX (Novel Netware)
Network Protocol/002

5. CS 447 Networks and Data Communication
Protocol and Interface
Interface: a set of rules that governs communication
between two directly facing protocol layers
Computer A
Computer B
Application
Program
Protocol
TCP IP
MAC
TCP IP
MAC
Layered structure
Interface
Wire (Computer Network)
Protocol
Stack
Network Protocol/003

6. CS 447 Networks and Data Communication
Summary
A network protocol is a set of rules that govern communication between
two host computers (in the same level)
An interface is a set of rules that govern communication between
two directly facing protocol layers (within a host computer)
Network Protocol/004

7. CS 447 Networks and Data Communication
“End-to-end” and “point-to-point”
Host A
Host B
Point-to-point
NIC
Intermediate Routers
End-to-end
OSI_SevenLayer/001

8. CS 447 Networks and Data Communication
OSI Seven Layer Model
Application Program
Purposes
Layer 7
Layer 6
Layer 5
Layer 4
Layer 3
Layer 2
Layer 1
Application Layer
Interface to user programs
Presentation Layer
Data filters (compression, encryption)
Session Layer
Manage end-to-end connection
Transport Layer
Provide reliable end-to-end connection
Network Layer
Manage point-to-point connection
Datalink Layer
Provide reliable point-to-point connection
Physical Layer
Dealing with electric signals
signals
NIC
Wire (Network Cable)
OSI_SevenLayer/002

9. CS 447 Networks and Data Communication
TCP/IP Protocol Suite:
Application Layer
NFS
(Network File Server)
XDR
(External Data Rep.)
RPC
(Remote Proc. Call)
Network Hardware
TCP
UDP IP
ARP
RARP
Telnet, FTP, SMTP, rlogin,
DNS, HTTP, rcp, finger, etc. 
Presentation Layer
Session Layer
Transport Layer 
Network Layer 
Datalink Layer 
Physical Layer
OSI_SevenLayer/003

10. CS 447 Networks and Data Communication
Layer 1: Physical Layer
Major Function: Dealing with electric signals
Example:
Manchester Signal Encoding
NRZI Signal Encoding
Bipolar-AMI Signal Encoding
Define how the signals are represented
Interpret electric signals, represent them as 1’s or 0’s
OSI_SevenLayer/004

11. CS 447 Networks and Data Communication
Layer 2: Data Link Layer
Major Functions:
Try to detect and correct signal errors, if any
Forward the received signals to the network layer
If uncorrectable errors, gives an error warning to the network layer
Provide Media Access Control (MAC)
- For “shared” network, control who can use the network first
Examples of Shared Network:
 Star (=hub)
 Ring
 Bus
OSI_SevenLayer/005

12. CS 447 Networks and Data Communication
Layer 3: Network Layer
Major Function:
Maintain an point-to-point connection
- Keep track of the sequence number
- Establish a connection
- Continue to check the status of a connection
- Delete a connection when it is done
OSI_SevenLayer/006

13. CS 447 Networks and Data Communication
Layer 4: Transport Layer
Major Functions:
Try to detect and correct packet errors (error control) for E2E
Perform flow control
- If the receiver wants to slow down, reduce transmission rate
- If the network is congested, reduce the transmission rate
Layer 5: Session Layer
Major Functions:
Establish/Manage/Delete (E2E) connections
Full-Duplex/Half-Duplex Control
QoS (specify maximum tolerable delay)
OSI_SevenLayer/007

14. CS 447 Networks and Data Communication
Layer 6: Presentation Layer
Major Functions: Encryption, compression, etc
Raw Data
Encryption
Presentation Layer
Compressed Data
Raw Data
Compression
Presentation Layer
Encrypted Data
Layer 7: Application Layer
Major Function: Interface to your application programs
Example: Network API (Application Program Interface)
OSI_SevenLayer/008

15. CS 447 Networks and Data Communication
Full-Duplex and Half-Duplex
Full-duplex: a connection two host can talk simultaneously
Half-duplex: a connection one of the two hosts can talk at a time
Full-Duplex Circuit
Transmitting
Circuit
Receiving
NIC at host A
NIC at host B
Multiplexer
OSI_SevenLayer/009

16. CS 447 Networks and Data Communication
Half-Duplex Circuit
NIC at host B
Transmitting
Circuit
Transmitting
Circuit
Receiving
Receiving
Circuit
Switch
NIC at host A
Half-duplex: One-way communication at a time
OSI_SevenLayer/009