1. ECE 4450:427/527 - Computer Networks Spring 2017
Dr. Nghi Tran
Department of Electrical & Computer Engineering
Lecture 2: Overview of Computer Network
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

2. Network Definition
Definition: A system that carries a commodity between 2 or more entities via connections
Entities: People, Computers, Cities etc.
Connections: Relationships, Wires, Road etc.
Examples of Network:
Transportation
Computer
Many others
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

3. Computer Network
Definition: A system that carries information between 2 or more entities, in the form of electric signals
Commodity?
Entities?
Connections?
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

4. Example: Transportation Vs Computer Network
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

5. Common Networks Ethernet (LAN)
WiFi, WiMAX, 3G/4G: Mobile wireless network
The Internet:
Internet: Internetwork - interconnecting computer networks with special gateways or routers
Largest communication network adopting IP technology
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

6. How Large is the Internet?
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

7. How Popular is the Internet?
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

8. Traffic of the Internet
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

9. Users of the Internet Dr. Nghi Tran (ECE-University of Akron)

10. Internet: Further Trend
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

11. Internet: Challenges
Internet has grown rapidly and evolved to unprecedented size
With a significant increase in the number of users, providers, and services, the Internet of the future is facing many problems:
How to manage/design a very large system with limited resources
Yet still provide a guaranteed performance
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

12. Objectives Not too ambitious in this course!!!
We take things step by step to help you uunderstand communication network design principles
And to pave an initial road for you to:
Design/operate Comm. Network
Design client/server software
Design equipments
Doing research
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

13. Basic Elements of Network
Divided into two main categories:
Nodes:
Links:
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

14. What is Network Design
A task to make connections among nodes via links so that packages can be exchanged:
Timely, reliably
Using as low resources as possible
etc
To this end, a huge of work required: We need to define 1) network connectivity; 2) network architecture; 3) protocols; 4) applications, 5) interfaces, 6) policies, usages.
As said, we shall take the issues step by step.
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

15. Main Factors in Network Design
What are the most important factors that drive Network Design?
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

16. Applications
Most people know about the Internet (a computer network) through applications
World Wide Web
Online Social Network
Streaming Audio Video
File Sharing
Instant Messaging …
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

17. Example of Application
A multimedia application including video-conferencing
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

18. Network Performance Metrics
When designing a network, we also need some tools to evaluate the design:
Network Performance Metrics
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

19. Network Design
A task to make connections among nodes via links so that packages can be exchanged:
Timely, reliably
Using as low resources as possible
etc
To this end, a huge of work required: We need to define 1) network connectivity; 2) network architecture; 3) protocols; 4) applications, 5) interfaces, 6) policies, usages.
As said, we shall take the issues step by step.
We now start with some Network Connectivity
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

20. We have the following three main categories
Network Connectivity
We have the following three main categories
Directly-connected Network
Circuit-switched Network
Package-switched Network
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

21. Directly-connected Network
Nodes are directly connected
(a) Point-to-Point: Each node is directly connected to all others via a link
(b) Multiple-Access: All nodes share the same physical medium
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

22. Given N nodes, how many connections do we need?
Disadvantages
What are the main disadvantages of a network in which all nodes are directly connected?
Given N nodes, how many connections do we need?
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

23. Switched Network
Fortunately, connection between 2 nodes does not necessary imply a physical connection: connection is achieved among a set of cooperating nodes
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

24. Interconnection of Networks
Two nodes can also be connected by using a set of networks (or clouds): Form an internetwork or internet
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

25. Switched Network Circuit-Switched Packet-Switched
A dedicated circuit is established between two communicating nodes
Example: ?
Packet-Switched
Data is split into blocks called packets or messages.
Store-and-forward strategy: Switches to store and forward packets
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

26. Circuit-Switched Network
Designed in 1878: reserve a dedicated channel for entire communication
No need for a destination address since a path is already established
Once communication is complete, connection is ended and links are released
How many switches do we need?
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

27. Circuit-Switched: Pros and Con?
Interesting note: Circuit switching currently makes a comeback in optical networking
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

28. Packet-Switched Network
Data is divided into packets
Each packet can be delivered independently over the network
Each packet contains identification info (source/destination address seq. number)
Store-and-forward: Key strategy
Each node receive complete package
Store in memory
Forward package to the next node
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

29. Advantages of Packet-Switched
Efficiency in using resources, which is a key requirement of computer network
Flexible multiplexing methods
What is multiplexing?
Service
More connections of lesser quality
No blocking of users
Adaptation
Can adapt to network congestion and failures
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

30. Disadvantages of Packet-Switched
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

31. Multiplexing
Fundamental concept: resources are shared among user, e.g., data of users be multiplexed over links that make up network
Two common methods: Time and Frequency Division Multiplexing
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

32. Statistical Multiplexing
Shortcomings of TDM/FDM:
If no data to send: Link is idle – wasteful
Maximum number of flows is fixed and known ahead of time – not practical to add additional quanta/frequencies
Statistical multiplexing: Division of the communication medium into channels of variable bandwidth: Resource allocated on demand rather than pre-allocated
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

33. Comparison Packet switching allows more users to use network! …..
Example:
1 Mb/s link
each user:
100 kb/s when “active”
active 10% of time
circuit-switching:
10 users
packet switching:
with 35 users, probability > 10 active at same time is less than .0004 N
users
…..
1 Mbps link
Q: how did we get value ?
Q: what happens if > 35 users ?
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

34. More Detail Packet switching allows more users to use network!
Assume there are total N users
The probability for each user to be active is p
Now we need to calculate two probabilities:
What is the probability that we exactly n active users
What is the probability that we have more than n active users
Let start with some simple cases: N=3 and n=1
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

35. Comparison Packet switching allows more users to use network! …..
Example:
1 Mb/s link
each user:
100 kb/s when “active”
active 10% of time
circuit-switching:
10 users
packet switching:
with 35 users, probability > 10 active at same time is less than .0004 N
users
…..
1 Mbps link
More connections of lesser quality
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

36. Recap We have taken an overview about Network
We also define CONNECTIVITY in a Network:
Packet switching with statistical multiplexing
Next step: We shall look at NETWORK ARCHITECTURE
Layering
Protocols
Internet Architecture
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527