1. Communication Networks NETW 501
Lecture 2
Network Architecture: OSI Reference Model
Course Instructor: Dr.-Ing. Maggie Mashaly
C3.220

2. Layers and Protocols Fundamental idea:
A piece of software/hardware provides a service to its users but keeps the details of its internal state and algorithms hidden from them
Sounds familiar??
YES!
Used all the time in computer science & programming
E.g: Abstract data types, Data Encapsulation & Object-oriented programming
The same approach is implemented in networks
To reduce their design complexity, networks are organized as a stack of layers, each one built upon the one below it

3. Layers and Protocols Function of each layer is to:
Offer certain services to the higher layers
Make use of services provided by underlying layers
For machines to communicate:
Layer n on one machine carries a conversation with layer n on another machine
Rules and conventions of this conversation are known as Layer n Protocol
Protocol: is an agreement between communicating parties on how communication is to proceed

4. Multilayer Communication
1. Guy 1 speaks only Urdu and English
2. Guy 2 speaks only Chinese and French
Help of translators is needed!
3. Guy 1 passes message to translator
7. Translator translates message, forwards it to Guy 2
Translators agree on common language: Dutch
4. Translator passes message to secretary for transmission
6. Secretary passes message to translator
5. Message sent through fax

5. The OSI Reference Model
How communication occurs?
In reality, no data are directly transferred from layer n on one machine to layer n on another machine
Instead, each layer passes data and control information to the layer immediately below it, until the lowest layer is reached
Lowest layer is the Physical Layer, through which communication actually occurs

6. Physical Layer Function: Transfer of bits across communication link
Transmission Medium
Twisted-Pair cable
Coaxial Cable
Optical Fiber
Radio
Satellite
Infrared
Aspects of a communications link
Mechanical: cable, plugs, pins, ...
Electrical/optical: modulation, signal strength, voltage levels, bit times, …
Functional/procedural: how to activate, maintain, and deactivate physical links…

7. Data Link Layer Data Link Layer Function: Two Sub-Layers
Divides data into Frames
Responsible for detecting /correcting transmission errors
Two Sub-Layers
Medium Access Control (MAC)
Logical Link Control (LLC)
LLC
MAC
Network
Physical
Data Link Layer

8. 1. Medium Access Control (MAC)
What problems does MAC sub-layer handle?
Contention
Two or more nodes want to transmit over the same medium at the same time
Medium Access Control
Defines the rules for accessing the medium
Common MAC Schemes
Token Passing
Random Access
Reservation
Carrier Sense Multiple Access (with Collision Detection) CSMA/CD

9. 2. Logical Link Control (LLC)
What problems does LLC sub-layer handle?
Detection of bit errors; Retransmission of frames
Correction of bit errors
Flow Control
Simplest Error Detection: Parity Checks
Even Parity : Sum of 1s is even
Odd Parity : Sum of 1s is odd
Single Error Detected
Ask for Retransmission
Double Error Goes undetected
LLC
LLC
LLC
LLC
Even Parity Assumed
Even Parity Assumed

10. Network Layer Functions of Network Layer: Routing Addressing
Transfers packets across multiple links and/or multiple networks
Nodes jointly execute routing algorithm to determine paths across the network
Addressing
Each node in the network must have its own unique address (IP address)
Must scale to large networks
Congestion Control
To deal with traffic surges

11. Routing Example D B F A E C 5 7 4 9 3 5 7 6 6 Route Cost ABF 13
AB DF 16
ABEF 12
ACF
ACEF 17

12. Issues in Routing Requirements for efficient routing algorithms:
Computation of routes
- What are possible routes from source to destination
Cope with dynamic changes in costs
- Links might fail or their cost might increase/decrease
Avoid congestion
- Since a good route may attract a lot of traffic and overload the computers on it
Avoid route oscillations
- To guarantee that all data is sent on one route

13. Transport Layer Functions of Transport Layer:
Provide a reliable and efficient network connection
Allowing the upper three layers to operate independent of underlying networks
Deals with
Packet loss
Packet Delay
Packet reordering
Performs flow control and buffering
Port numbers enable multiplexing Message segmentation and reassembly
Connection setup, maintenance, and release

14. Session Layer Functions of Session Layer:
Establishes and maintains a session between two end-users
Dialog management
Full duplex
Half Duplex
In case of connection disruption during large file transfers. It enables the communication to resume from synchronization points
What is the difference between the session layer and transport layer?
The session layer establishes the connection in conjunction with the application
Example: A video session might have multiple transport connections for
Video
Audio
Data (e.g., captions, messages)

15. Presentation Layer
Different computers have different ways of representing the same information
The presentation layer establishes a common ground of communication
Functions of Presentation Layer:
Data Compression
Decreasing as much as possible the amount of bits used to represent information
Security
If an un-authorized person intercepts the message, it is unintelligible

16. Application Layer Function of Application Layer:
Provides services that are frequently required by applications
Example application layer residents
File Transfer Protocol (FTP)
Hypertext Transfer Protocol (HTTP)
Telnet

17. Headers & Trailers
Data
Application
Presentation
Session
Transport
Network
Data Link
Physical
Application
Presentation
Session
Transport
Network
Data Link
Physical AH
Data PH AH
Data SH PH AH
Data TH SH PH AH
Data NH TH SH PH AH
Data DH NH TH SH PH AH
Data
CRC
Bit stream
Each protocol uses a header that carries addresses, sequence numbers, flag bits, length indicators, etc…
CRC check bits may be appended for error detection

18. Peer-to-Peer Communications
Layer n+1 entity
Layer n+1 entity
n-SDU
n-SDU
n-SAP
n-SAP
n-PDU
n-SDU H
Layer n entity
Layer n entity H
n-SDU
n-PDU
SDU: Service Data Unit
PDU: Protocol Data Unit
SAP: Service Access Point

19. Peer-to-Peer Communications: Protocol
Layer n in one machine interacts with layer n in another machine to provide a service to layer n+1
The entities comprising the corresponding layers on different machines are called peer processes.
The machines use a set of rules and conventions called the layer-n protocol.
Layer-n peer processes communicate by exchanging Protocol Data Units (PDUs)

20. Peer-to-Peer Communications: Protocol
Communication between peer processes is virtual and actually indirect
Layer n+1 transfers information by invoking the services provided by layer n
Services are available at Service Access Points (SAP’s)
Each layer passes data & control information to the layer below it until the physical layer is reached and transfer occurs
The data passed to the layer below is called a Service Data Unit (SDU)
SDU’s are encapsulated in PDU’s

21. Connection-Oriented & Connectionless Services
Established in three- phases:
Connection setup between two SAPs to initialize state information
SDU transfer
Connection release
E.g. TCP, ATM
Connectionless
Immediate SDU transfer
No connection setup
E.g. UDP, IP
Layered services need not be of same type!
e.g.: TCP operates over IP, IP operates over ATM

22. Segmentation & Reassembly
A layer may impose a limit on the size of a data block that it can transfer for implementation or other reasons
Thus a layer-n SDU may be too large to be handled as a single unit by layer-(n-1)
Sender side: SDU is segmented into multiple PDUs
Receiver side: multiple PDUs are re- assembled into one SDU
Segmentation
n-SDU
n-PDU
n-PDU
n-PDU
Re-assembly
n-SDU
n-PDU
n-PDU
n-PDU

23. Multiplexing Sharing of layer n service by multiple layer n+1 users
Layer n+1 entity
Layer n+1 entity
Layer n+1 entity
Layer n+1 entity
n-SAP
n-SAP
n-SAP
n-SAP
n-PDU
n-SDU H
Layer n entity
Layer n entity H
n-SDU
n-PDU
Sharing of layer n service by multiple layer n+1 users
Multiplexing tag or ID required in each PDU to determine which user an SDU belongs to

24. References NETW 501 Lectures slides by Assoc. Prof. Tallal El-Shabrawy
“Communication Networks 2nd Edition”, A. Leon-Garcia and I. Widjaja, McGraw Hill, 2013
“Computer Networks 4th Edition”, A. S. Tanenbaum, Pearson International