1. Computer Communication & Networks
Lecture # 01
Introduction
Nadeem Majeed Choudhary

2. Grading Policy Final Exam: 40% Mid term Exam 20% Assignments 05%
Quizzes: 10%
Labs 25%

3. Quizzes may be announced or unannounced.
Exams are closed-book and extremely time limited.
Exams consist of design questions, numerical, maybe true-false and short answer questions.

4. Reading Text book: Reference books:
Data Communications and Networking,
B.A. Forouzan, McGraw-Hill,
(Latest Available Edition)
ISBN
Reference books:
Computer Networking, a top-down approach featuring the Internet,
J.K.Kurose, K.W.Ross,Addison-Wesley, 2005, ISBN
Computer Networks, A Systems Approach
L. Peterson & Davie
Data and Computer Communication by William Stallings Prentice Hall.

5. Data Communications
The term telecommunication means communication at a distance. The word data refers to information presented in whatever form is agreed upon by the parties creating and using the data. Data communications are the exchange of data between two devices via some form of transmission medium such as a wire cable.

6. Fundamental Characteristics
The effectiveness of a data communication system depend on four fundamental characteristics:
Delivery
Accuracy
Timelines
Jitter

7. Five Components of Data Communication
Message
Sender
Receiver
Medium
Protocol

8. Direction of data flow
Simplex
Half Duplex
Full Duplex

9. Network design
Before looking inside a computer network, first agree on what a computer network is

10. Computer network ? Specialized to handle: Keystrokes Voice Video
Set of serial lines to attach terminals to mainframe ?
Telephone network carrying voice traffic ?
Cable network to disseminate video signals ?

11. What distinguishes a Computer network ?
Generality
Built from general purpose programmable hardware
Supports wide range of applications
Not optimized for special purpose application like making phone calls or delivering television signals

12. Information, Computers, Networks
Information: anything that is represented in bits
Form (can be represented as bits) vs
Substance (cannot be represented as bits)
Properties:
Infinitely replicable
Computers can “manipulate” information
Networks create “access” to information

13. Networks Potential of networking: Network provides “connectivity”
move bits everywhere, cheaply, and with desired performance characteristics
Network provides “connectivity”

14. What is “Connectivity” ?
Direct or indirect access to every other node in the network
Connectivity is the magic needed to communicate if you do not have a direct pt-pt physical link.
Tradeoff: Performance characteristics worse than true physical link!

15. Building Blocks Nodes: PC, special-purpose hardware…
hosts
switches
Links: coax cable, optical fiber…
point-to-point
multiple access …

16. Why not connect each node with every other node ?
Number of computers that can be connected becomes very limited
Number of wires coming out of each node becomes unmanageable
Amount of physical hardware/devices required becomes very expensive
Solution: indirect connectivity using intermediate data forwarding nodes

17. A Network A network can be defined recursively as
two or more nodes connected by a physical link Or
two or more networks connected by one or more nodes

18. Switched Networks A network can be defined recursively as...
two or more nodes connected by a link
white nodes (switches) implement the network
colored nodes (hosts) use the network

19. Switched Networks A network can be defined recursively as...
two or more networks connected by one or more nodes: internetworks
white nodes (router or gateway) interconnects the networks
a cloud denotes “any type of independent network”

20. Switching Strategies Packet switching: store-and-forward messages
Circuit switching: carry bit streams
establishes a dedicated circuit
links reserved for use by communication channel
send/receive bit stream at constant rate
example: original telephone network
Packet switching: store-and-forward messages
operates on discrete blocks of data
utilizes resources according to traffic demand
send/receive messages at variable rate
example: Internet

21. What next ? Hosts are directly or indirectly connected to each other
Can we now provide host-host connectivity ?
Nodes must be able to say which host it wants to communicate with

22. Addressing and Routing
Address: byte-string that identifies a node
usually unique
Routing: forwarding decisions
process of determining how to forward messages to the destination node based on its address
Types of addresses
unicast: node-specific
broadcast: all nodes on the network
multicast: some subset of nodes on the network

23. Wrap-up A network can be constructed from nesting of networks
An address is required for each node that is reachable on the network
Address is used to route messages toward appropriate destination

24. What next ? Hosts know how to reach other hosts on the network
How should a node use the network for its communication ?
All pairs of hosts should have the ability to exchange messages: cost-effective resource sharing for efficiency

25. Multiplexing Physical links and nodes are shared among users
(synchronous) Time-Division Multiplexing (TDM)
Frequency-Division Multiplexing (FDM) L1 L2 L3 R1 R2 R3
Switch 1
Switch 2
Multiple flows on a single link
Do you see any problem with TDM / FDM ?

26. What Goes Wrong in the Network?
Reliability at stake
Bit-level errors (electrical interference)
Packet-level errors (congestion)
distinction between lost and late packet
Link and node failures
distinction between broken and flaky link
distinction between failed and slow node

27. What Goes Undesirable in the Network?
Required performance at stake
Messages are delayed
Messages are delivered out-of-order
Third parties eavesdrop
The challenge is to fill the gap between application expectations and hardware capabilities

28. Networks: key issues Network criteria Performance Reliability Security
Throughput
Delay
Reliability
Data transmitted are identical to data received.
Measured by the frequency of failure
The time it takes a link to recover from a failure
Security
Protecting data from unauthorized access

29. Terminology
The throughput or bandwidth of a channel is the number of bits it can transfer per second
The latency or delay of a channel is the time that elapses between sending information and the earliest possible reception of it

30. Network topologies
Topology defines the way hosts are connected to the network

31. Network topology issues
a goal of any topology
high throughput (bandwidth)
low latency

32. Bandwidth and Latency Bandwidth Latency
1. telecommunications: range of radio frequencies: a range of radio frequencies used in radio or telecommunications transmission and reception
2. computing: communications capacity: the capacity of a communications channel, for example, a connection to the Internet, often measured in bits per second
3. a data transmission rate; the maximum amount of information (bits/second) that can be transmitted along a channel
Latency
A synonym for delay, is an expression of how much time it takes for transmission from one designated point to another

33. Categories of Topology

34. Mostly used network topologies
bus
mesh
ring
star

35. A hybrid topology: a star backbone with three bus networks

36. Hierarchical organization of the Internet

37. LAN, WAN & MAN
Network in small geographical Area (Room, Building or a Campus) is called LAN (Local Area Network)
Network in a City is call MAN (Metropolitan Area Network)
Network spread geographically (Country or across Globe) is called WAN (Wide Area Network)

38. Layering & Protocol Stacks

39. What’s a protocol? human protocols: “what’s the time?”
“I have a question”
introductions
… specific msgs sent
… specific actions taken when msgs received, or other events
network protocols:
machines rather than humans
all communication activity in Internet governed by protocols

40. Protocol
protocols define format, order of msgs sent and received among network entities, and actions taken on msg transmission, receipt
a human protocol and a computer network protocol:
time Hi
TCP connection
req. Hi
TCP connection
reply.
Got the
time?
Get
2:00
<file>

41. Standard
Essential in creating and maintaining an open and competitive market for equipment manufacturers
Guaranteeing national & international interoperability of data & telecommunication technology & process.

42. Layered Tasks
An example from the everyday life
Hierarchy?
Services

43. Why layered communication?
To reduce complexity of communication task by splitting it into several layered small tasks
Functionality of the layers can be changed as long as the service provided to the layer above stays unchanged
makes easier maintenance & updating
Each layer has its own task
Each layer has its own protocol

44. Reference Models
OSI reference model
TCP/IP

45. OSI Reference model Open System Interconnection 7 layers
Crate a layer when different abstraction is needed
Each layer performs a well define function
Functions of the layers chosen taking internationally standardized protocols
Number of layers – large enough to avoid complexity

46. Seven layers of the OSI model

47. Exchange using OSI Model

48. Issues, to be resolved by the layers
Larger bandwidth at lower cost
Error correction
Flow control
Addressing
Multiplexing
Naming
Congestion control
Mobility
Routing
Fragmentation
Security
....

49. Applications E-mail Searchable Data (Web Sites) E-Commerce News Groups
Internet Telephony (VoIP)
Video Conferencing
Chat Groups
Instant Messengers
Internet Radio

50. Research areas in Networking
Routing
Security
Ad-hoc networks
Wireless networks
Protocols
Quality of Service …

51. Readings Chapter 2 (B.A Forouzan) Chapter 1: 1.1, 1.2
Computer Networks, A Systems Approach
L. Peterson & Davie
Chapter 1 (B. A Forouzan)
Section 1.1, 1.2, 1.3,1.4
Chapter 2 (B.A Forouzan)
Section 2.1