1. Chapter 2 (handout 1– only sections 2.1, 2.2 and 2.3)
Dr. Clincy
Professor of CS
1 of 10
Tough Day: (1) Multiple lectures, (2) Exam covering Ch 4 and 5.1 and 5.2, (3) Lab after exam
Dr. Clincy
Lecture

2. OSI Reference Model
The data communications part of the course will be structured around the OSI reference model
We will focus on the bottom layer – similar to computer architecture (physical)
The remaining layers will be covered in CS3550 (new course) and CS4500 (advanced course in TCP/IP)
Dr. Clincy
Lecture

3. OSI Reference Model Why Study OSI?
An excellent model for conceptualizing and understanding data communications
More granularity in functionality - more functional delineation
Key points:
Modular
Hierarchical (chain of command, pecking order)
Boundaries between layers (called interfaces)
NOTE: the protocols or functionality with in the layer could change however, the interface remains the same – this facilitates the flexibility
Dr. Clincy
Lecture

4. Advantages of Layering
Easier application development
Network can change without all programs being modified
Breaks complex tasks into subtasks
Each layer handles a specific subset of tasks
Communication occurs
between different layers on the same node or stack (INTERFACES)
between similar layers on different nodes or stacks (PEER-TO-PEER PROCESSES)
Dr. Clincy
Lecture

5. OSI’s Layered Approach Example
Network A
Network B
Actual commands invoked, presentation
Top Layer
Some Intermediate Layer
Bottom Layer
Top Layer
Some Intermediate Layer
Bottom Layer
Facilitate the actual communications
Network interfaces, raw bits
How does peer-to-peer communication work ?
Dr. Clincy
Lecture

6. OSI Application Open Systems Interconnection Presentation Session
Transport
Network
Data Link
Physical
Open Systems Interconnection
Developed by ISO
(International Organization for Standardization)
Contains seven layers
Dr. Clincy
Lecture

7. OSI’s Layered Approach
between different layers on the same node or stack (INTERFACE)
between similar layers on different nodes or stacks (PEER-TO-PEER PROCESSES)
Dr. Clincy
Lecture

8. OSI Reference Model ? Bottom 3 layers Top 4 layers
Responsible for getting data or info to destination
Routing and switching occurs
Define the electrical and physical standards
Performs bit ordering, transmission of the bits, and error detecting and correcting
Top 4 layers
Creation and interpretation of the data
Protocols for data formatting, message syntax, dialogue management, message sequences and info presentation are used
Dr. Clincy
Lecture

9. OSI Physical Layer Responsible for transmission of bits
Always implemented through hardware
Encompasses mechanical, electrical, and functional interfaces
Encoding and Decoding issues: how 0’s and 1’s are converted to signals
Signal translation (ie. electrical to optical)
Signal Multiplexing and Demultiplexing
Signal Modulation and Demodulation
Transport medium: Coaxial, Twisted Pair, Optical, etc..
Transmission Rate/Data Rate – how fast to send bits
Transmission mode: transmission direction (simplex, duplex)
Physical Topology: network layout
Dr. Clincy
Lecture

10. OSI Data Link Layer
Responsible for error-free, reliable transmission of data
Framing, Flow control, Error control (detection/correction), Access Methods
Makes use of physical address because with in the same network
Network Layer
Data Link Layer
Physical Layer
Actually sends the packets (groups of frames) from node to node using a routing algorithm
Takes raw data (bits) and transform them into frames, error control, etc.
Transmit and receive the raw data (bits)
Dr. Clincy
Lecture

11. OSI Network Layer Responsible for routing of messages through networks
Concerned with type of switching used (circuit v. packet)
Handles routing among different networks
NOTE: with in the same network, only the DATA LINK layer is needed – amongst multiple networks, the NETWORK LAYER is needed
No need for routing with in the same network (LAN)
Routing across “internetworks”
Makes use of logical address vs physical address because not with in the same network
Dr. Clincy
Lecture

12. OSI Network Layer
Dr. Clincy
Lecture

13. Chapter 2 (handout 1– only sections 2.1, 2.2 and 2.3)
Dr. Clincy
Professor of CS
2 of 10
Dr. Clincy
Lecture

14. Recap - OSI Already covered bottom 3 layers Application Presentation
Session
Transport
Network
Data Link
Physical
Dr. Clincy
Lecture

15. OSI Upper Layers Application Presentation Session Transport
Peer-to-Peer Processes …..
End-to-End nodes only
Dr. Clincy
Lecture

16. OSI Transport Layer Isolates messages from lower and upper layers
Breaks down message size (segmentation) (down) and performs re-assembly (up)
Monitors quality of communications channel (oversee all hops)
Selects most efficient communication service necessary for a given transmission (could change over hops)
Flow and Error control for Source and Sink
Dr. Clincy
Lecture

17. OSI Session Layer
Establishes logical connections between systems (up/down)
Manages log-ons, password exchange, log-offs (up/down)
Terminates connection at end of session (up/down)
Dr. Clincy
Lecture

18. OSI Presentation Layer
Provides format and code conversion services
Examples
File conversion from ASCII to EBDIC
Invoking character sequences to generate bold, italics, etc on a printer
The source and sink could operate using different encoding schemes – the presentation layer makes the translations
Security
Compression
Dr. Clincy
Lecture

19. OSI Application Layer
Provides access to network for end-user (end-user being a human being or software application)
User’s capabilities are determined by what items are available on this layer (ie. remote log-in, file transfer, service, directory service, etc.)
Dr. Clincy
Lecture

20. Recap: What happens at the Intermediate Nodes ?Rx Tx 7
Intermediate Nodes 3 1 1 B C Q T A Z
Dr. Clincy
Lecture

21. COMPLEXITY TO CONSIDER
Any particular node in an internetwork can be functioning as follows simultaneously:
Tx to other internetwork nodes
Rx from other internetwork nodes
Intermediate node to some other internetwork nodes
Dr. Clincy
Lecture

22. OSI in Action: Outgoing File Transfer
The File Transfer Program issues a command to the Application Layer
Application passes it to Presentation, which may reformat, encrypt, encode, compress, passes to Session (adds overhead)
Session requests a connection, passes to Transport (adds overhead)
Transport breaks file into chunks, adds error-checking and flow-control info, process-to-process, passes to Network (adds overhead)
Network selects the data’s route (internetworking), passes to Data Link (adds overhead)
Data Link adds error-control and flow-control info, passes to Physical (adds overhead)
Physical translates bits to signal and transmits the signal, which includes information added by each layer
Dr. Clincy
Lecture

23. OSI in Action: Incoming File Transfer
Physical receives signal and translates to bits, passes to Data Link
Data Link checks for errors and performs flow control on bits, formulates bits into some formation (frames), passes to Network
Network verifies routing (if intermediate node, determines next hop), passes to Transport
Transport checks for errors and performs flow control on the chunks, reassembles the chunks, passes to Session
Session determines if transfer is complete, may end session, passes to Presentation
Presentation may reformat, perform conversions, decode, decrypt, decompress, pass to Application layer
Application presents results to user (e.g. updates FTP program display)
Dr. Clincy
Lecture

24. US Postal System Analogy
Illustrate how the US Postal System is very similar to how networking works
Will help students better understand (versus memorize) data comm and networking
Upper Layers – creating and interpreting the signal, data or info
Lower Layers – getting the signal from one place to the next

25. Dr. Clincy Professor of CS
Chapter 2 and 3 Handout #2
Dr. Clincy
Professor of CS
3 of 10
Dr. Clincy
Lecture 2

26. Data Vs Signal
Fully explain the difference between signal and data before getting into the details
Dr. Clincy
Lecture 2

27. Data vs Signals
We probably have a good idea about digital and analog signals
What about analog and digital data ??
Analog data examples
Voice
Images
Digital data examples
Text
Digitized voice or images
Dr. Clincy
Lecture 2

28. Electromagnetic Signals - Time
Electromagnetic signal can be expressed as a function of time or frequency
Function of time
Analog (varies smoothly over time)
Digital (constant level over time, followed by a change to another level)
ie. sound
ie. bits
Dr. Clincy
Lecture 2

29. Periodic Signal Characteristics
If the signal’s pattern repeats over and over, we called these signals Periodic Signals
Periodic Signals can be either Analog or Digital
Dr. Clincy
Lecture 2

30. Analog Periodic Signal Case
Amplitude (A): signal value, measured in volts
Frequency (f): repetition rate, cycles per second or Hertz
Period (T): amount of time it takes for one repetition, T=1/f
Phase (f): relative position in time, measured in degrees
General sine wave is written as
S(t) = A sin(2pft + f)
Dr. Clincy
Lecture 2

31. Varying S(t) = A sin(2pft + f)
Dr. Clincy
Lecture 2
Note: 45 degrees because p is 180 degrees

32. What is Wavelength ?
The distance an electromagnetic wave can travel in the amount of time it takes to oscillate through a complete cycle
Wavelength (w) = signal velocity x period or propagation speed x period
Recall: period = 1 / frequency
Another perspective of Wavelength: how far did this signal travel AS the signal goes through a FULL cycle ?
Dr. Clincy
Lecture 2

33. Electromagnetic Signals
Electromagnetic signal can be expressed as a function of time or frequency Function of frequency (more important)
Frequency-Domain Plot – peak amplitudes with respect to frequency
Time-Domain Plot – amplitude changes with respect to time
Different signals
Dr. Clincy
Lecture 2

34. Electromagnetic Signals - Frequency
Electromagnetic signal can be expressed as a function of time or frequency
Function of frequency (more important)
Spectrum (range of frequencies)
Bandwidth (width of the spectrum)
When we talk about spectrum, we mean
the range of frequencies the
electromagnetic signal takes on
In the example, the signal has a
Frequency range of f to 3f
Therefore, a electromagnetic signal can
be a collection (addition) of periodic analog
Signals (Composite Signal)
Dr. Clincy
Lecture 2

35. Composite Periodic Signal
According to FOURIER ANALYSIS, a composite signal is a combination of sine waves with different amplitudes, frequencies and phases.
Could converged to a square wave
3rd harmonic
9th harmonic
Dr. Clincy
Lecture 2

36. Electromagnetic Spectrum for Transmission Media
Tell them how to study this chart
Dr. Clincy
Lecture 2

37. Digital Signaling represented by square waves or pulses
Refers to transmission of electromagnetic pulses that represents 1’s and 0’s
1 cycle
amplitude (volts)
time
(sec)
frequency (hertz)
= cycles per second
Dr. Clincy
Lecture 2

38. Digital Signal Rate Each bit’s signal has a certain duration
Example, given a data rate of 50 kbps (or 50,000 bps)
Each would have a 0.02 microseconds duration
Duration (or bit length) = 1/50000 = sec = .02 msec
Dr. Clincy
Lecture 2

39. Digital Signal # bits per level = log2 of (#oflevels)
Sending 1 bit per level
Sending 2 bits per level
How many levels needed to send 5 bits at a time ????
# bits per level = log2 of (#oflevels)
Dr. Clincy
Lecture 2