1. 1-1
Introduction and Review
MET CS-635 Unit 1

2. 1-2
The Network Pipe
• Most basic model of the network
– Phone call
– Tin cans and string
– Two computers connected to one another
• Goal : Move data from one end to other

3. 1-3
Class Exercise
The Living Network Exercise #1
Two individuals are issued cards and are
challenged to communicate the contents of
the cards to each other using only verbal
commands. Participants sit with their backs to
each other. Objective is to observe the
process that takes place, and draw
conclusions about the considerations that
must be taken into account in any networking
situation.

4. 1-4
Standards
• Stations sharing the network media must use
a common set of agreed upon rules to
cooperate
– Protocols
– Standards
• Sources of LAN standards
– IEEE
– IETF
– ATM Forum

5. 1-5
LAN vs. WAN
• Local Area Network
– Privately owned and operated
– All data belongs to your company
– Limited geographic extent
– High Speed
– Building or Campus

6. 1-6
WAN
• Wide Area Network
– You pay somebody to move your data
– You share capacity with other companies
– Wide geographic extent
– Global
• Phone Company
• Leased Lines
• Interconnect LANs

7. 1-7
Basic Network Requirements
• Reliability
• Get data across network securely and error free
• Know when an error has occurred, and handle it
– Corrupted data
– Cable break
• Speed
• Get data across network as fast as possible
• Scalability
• Be able to grow the network
• Be able to migrate to new designs and protocols

8. 1-8
Requirements Continued
• Efficiency
• Divide network capacity among multiple users in an
equitable manner
• MANY approaches to this very fundamental problem
• All users are equal, but some are more equal than
others...
• Cost Effectiveness
• Meet all requirements as inexpensively as possible

9. 1-9
Additional Requirements
• From Interconnections by Perlman
• Scope
• Network should solve as general a problem as possible
• Autoconfigurability
• Plug and play networks
• Auto assignment of addressing
• Auto discovery of topological information

10. 1-10
Data Sources
• Digital Data
– Files, GIF Images, Web Pages, etc.
– Data must still be properly framed for transmission
over the LAN
• Analog Data
– Audio, Video
– Data must be converted first using a codec and
then framed for transmission over the LAN

11. 1-11
Data on LANs
• The LANs we will study all use digital
signaling and digital transmission
• All of the data is converted to 1’s and 0’s by
the time it gets to the network
• The network just moves the 1’s and 0’s

12. 1-12
Signal Encoding
• Moving the 1’s and 0’s
– Need to be able to move digital data over analog
media like copper wire and fiber optic cable
– In fiber, we have presence or absence of light
– In copper, we have a range of voltage levels
• We’ll consider NRZ and Biphase encoding
– NRZ Non-Return to Zero

13. 1-13
NRZ Encoding
• Differential Encoding
– Uses positive and negative voltages as opposed
to absolute voltage levels
– Can reverse wires transmitting differential signal
and it still works
• Easily implemented
• Problems with long strings of 1’s and 0’s
– Clock synchronization gets lost
– So how come FDDI and 100BASE-T use this?

14. 1-14
NRZ-L
• Non-Return to Zero Level
– Constant negative voltage represents 1
– Constant positive voltage represents 0
1 1 0 1 1 0 1 0
0
-V
+V
t

15. 1-15
NRZI
• Non-Return to Zero, Invert on Ones
– Transition up or down at start of bit time means 1
– No Transition means 0
• Users: FDDI and 100BASE-T
1 1 0 1 1 0 1 0
0
-V
+V
t

16. 1-16
Biphase Encoding
• Solve the NRZ synchronization problem by
providing a predictable transition during each
clock phase
• The additional clock transitions double the
bandwidth of the signal
– This puts higher demands on the cabling used
• You can still use differential encoding

17. 1-17
Manchester Encoding
• Transition in middle of the phase
– Low to High is 1, High to Low is 0
– Always a mid-phase transition, so you never lose
the clock
– Users: 10BASE-T and 10BASE-2
1 1 0 1 1 0 1 0
0
-V
+V
t

18. 1-18
Differential Manchester
• Transition in middle of phase again
– Transition just provides clocking
– Transition at start of phase is 0
– No Transition at start of phase is 1
1 1 0 1 1 0 1 0
0
-V
+V
t

19. 1-19
Recovering the Clock
• LANs are, for the most part, asynchronous
networks
• Clocks are locally generated and recovered
• Need a data encoding technique that allows
the clock to be recovered
• Problems with synchronous networks and
clock skew

20. 1-20
Nyquist and PCM
• PCM = Pulse Coded Modulation
– Sample signal at regular intervals
– Send the numbers over the network
• Nyquist’s result:
– max data rate = 2H log2 V bits/second
– H is the bandwidth of the signal in Hertz
– V is the number of sampling levels used
• This fundamental limit discovered in 19241924
drives all digital communication today

21. 1-21
Multiplexing
• Sharing the communications channel
– Need to share with LANs and WANs
– Cost effective use of the capacity
• TDM
– A time slice just for you
– Most LANs
– Phone trunk lines
• FDM
– A frequency just for you
– CATV

22. 1-22
Switching
• Process of establishing a path through the
network
• Need a way to get from here to there
• Many ways to accomplish this
• We will examine:
– Circuit Switching
– Packet Switching
– Cell Switching

23. 1-23
Circuit Switching
• Connection Oriented
• Phone call
• Dedicated circuit established
– Fixed path through network
– Predictable performance and delay
– Wasted bandwidth when quiet
• Call setup and teardown

24. 1-24
Packet Switching
• Connectionless
• Most LANs
• No dedicated circuit established
– Random path through network
– No predictable performance or delay
– Bandwidth only used when needed
• Store and forward network

25. 1-25
Cell Switching
• Hybrid technique used in ATM networks
• Makes efficient use of media like packet
switched network
• Has predictable characteristics of circuit
switched network

26. 1-26
Metrics
• We will use the following performance metrics
throughout the course
– Bandwidth
– A range of frequencies in Hertz
– For signals: Delta between the maximum and minimum
frequency components
– For media: Maximum frequency component that may be
carried by the media
– Data Rate
– Speed at which data is communicated in bits per second

27. 1-27
Metrics Continued
– Capacity
– Maximum Data Rate sustainable by a channel allowing for
Gaussian (thermal) noise
– Shannon’s Equation: C = H log2 (1 + S/N)
– Latency
– Delay in seconds from the time a transmission is initiated
until it is received
– Throughput
– Also called effective data rate
– Rate in bits per second or bytes per second in which actual
application data is moved, in its entirety, across a medium
– Allows for latency and protocol overhead

28. 1-28
OSI Reference Model
Physical
Data Link
Network
Transport
Session
Presentation
Application
1
2
3
Most of our time
Rest of our time
As needed
Course Emphasis OSI Stack
Layer

29. 1-29
Using the stack to communicate
• Systems that wish to communicate use
stacks that implement the same standards
• Different vendors may produce different
implementation but implementations must
both be compliant
• Each layer in a station must interoperate with
corresponding layer on the remote station
• Need interoperability testing
– Note: This is how the trade show “Interop” started

30. 1-30
Interconnecting two hosts
Physical
Data Link
Network
Transport
1
2
3
Physical
Data Link
Network
Transport
1
2
3
Host A Host B

31. 1-31
Making the connection
• Corresponding layers in each system
establish a logical connection
• Data is sent from the application ‘down’ the
stack
• Each layer encapsulates data from next
higher layer
• Encapsulations stripped on way ‘up’ the
receiving stack

32. 1-32
Layer Encapsulation
PHY
DLL
Net
Trans
PHY
DLL
Net
Trans
data
datadata
This is what
goes on the
wire