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Computer Communication & Networks
Lecture # 01
Introduction
Nadeem Majeed Choudhary
[email protected]
Grading Policy
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Final Exam:
Mid term Exam
Assignments
Quizzes:
Labs
40%
20%
05%
10%
25%
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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.
Reading
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Text book:
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Data Communications and Networking,
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B.A. Forouzan, McGraw-Hill,
(Latest Available Edition)
ISBN 0-07-292354-7.
Reference books:
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Computer Networking, a top-down approach featuring the
Internet,
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J.K.Kurose, K.W.Ross,Addison-Wesley, 2005,
ISBN 0-321-26976-4.
Computer Networks, A Systems Approach
L. Peterson & Davie
Data and Computer Communication by William Stallings
Prentice Hall.
Data Communications
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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.
Fundamental Characteristics
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The effectiveness of a data communication
system depend on four fundamental
characteristics:
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Delivery
Accuracy
Timelines
Jitter
Five Components of Data Communication
1.
Message
2.
Sender
3.
Receiver
4.
Medium
5.
Protocol
Direction of data flow
Simplex
Half Duplex
Full Duplex
Network design
Before looking inside a computer
network, first agree on what a
computer network is
Computer network ?
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Set of serial lines to
attach terminals to
mainframe ?
Telephone network
carrying voice traffic ?
Cable network to
disseminate video
signals ?
Specialized to
handle:
Keystrokes
Voice
Video
What distinguishes a
Computer network ?
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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
Information, Computers, Networks
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Information: anything that is represented in bits
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Form (can be represented as bits) vs
Substance (cannot be represented as bits)
Properties:
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Infinitely replicable
Computers can “manipulate” information
Networks create “access” to information
Networks
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Potential of networking:
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move bits everywhere, cheaply, and with desired
performance characteristics
Network provides “connectivity”
What is “Connectivity” ?
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Direct or indirect access to every other node in the
network
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Connectivity is the magic needed to communicate if
you do not have a direct pt-pt physical link.
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Tradeoff: Performance characteristics worse than true physical
link!
Building Blocks
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Nodes: PC, special-purpose hardware…
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hosts
switches
Links: coax cable, optical fiber…
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point-to-point
…
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multiple access
Why not connect each node
with every other node ?
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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
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
Switched Networks
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A network can be defined recursively as...
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two or more nodes
connected by a link
white nodes
(switches)
implement the
network
colored nodes
(hosts) use the
network
Switched Networks
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A network can be defined recursively as...
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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”
Switching Strategies
Circuit switching:
carry bit streams
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a.
b.
c.
d.
establishes a dedicated
circuit
links reserved for use
by communication
channel
send/receive bit stream
at constant rate
example: original
telephone network
• Packet switching: storeand-forward messages
a. operates on discrete
blocks of data
b. utilizes resources
according to traffic
demand
c. send/receive messages
at variable rate
d. example: Internet
What next ?
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Hosts are directly or indirectly connected to
each other
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Can we now provide host-host connectivity ?
Nodes must be able to say which host it
wants to communicate with
Addressing and Routing
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Address: byte-string that identifies a node
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Routing: forwarding decisions
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usually unique
process of determining how to forward messages
to the destination node based on its address
Types of addresses
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unicast: node-specific
broadcast: all nodes on the network
multicast: some subset of nodes on the network
Wrap-up
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A network can be constructed from
nesting of networks
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An address is required for each node
that is reachable on the network
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Address is used to route messages
toward appropriate destination
What next ?
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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
Multiplexing
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Physical links and nodes are shared among users
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(synchronous) Time-Division Multiplexing (TDM)
Frequency-Division Multiplexing (FDM)
L1
R1
L2
L3
Switch 1
Multiple flows
on a single link
Switch 2
R2
R3
Do you see any problem with TDM / FDM ?
What Goes Wrong in the Network?
Reliability at stake
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Bit-level errors (electrical interference)
Packet-level errors (congestion)
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distinction between lost and late packet
Link and node failures
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distinction between broken and flaky link
distinction between failed and slow node
What Goes Undesirable in the
Network?
Required performance at stake
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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
Networks: key issues
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Network criteria
 Performance
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Reliability
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Throughput
Delay
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
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Protecting data from unauthorized access
Terminology
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The throughput or bandwidth of a channel is
the number of bits it can transfer per second
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The latency or delay of a channel is the time
that elapses between sending information and
the earliest possible reception of it
Network topologies
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Topology defines the way hosts are
connected to the network
Network topology issues
a goal of any topology
1. high throughput (bandwidth)
2. low latency
Bandwidth and Latency
Bandwidth
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
Categories of Topology
Mostly used network topologies
bus
mesh
star
ring
A hybrid topology: a star backbone with three bus networks
Hierarchical organization of the Internet
LAN, WAN & MAN
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Network in small geographical Area (Room, Building or a
Campus) is called LAN (Local Area Network)
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Network in a City is call MAN (Metropolitan Area Network)
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Network spread geographically (Country or across Globe) is
called WAN (Wide Area Network)
Layering & Protocol Stacks
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
Protocol
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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:
Hi
Hi
Got the
time?
2:00
time
TCP connection
req.
TCP connection
reply.
Get http://gaia.cs.umass.edu/index.htm
<file>
Standard
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Essential in creating and maintaining an open
and competitive market for equipment
manufacturers
Guaranteeing national & international
interoperability of data & telecommunication
technology & process.
Layered Tasks
An example from the everyday life
Hierarchy?
Services
Why layered communication?
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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
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makes easier maintenance & updating
Each layer has its own task
Each layer has its own protocol
Reference Models
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OSI reference model
TCP/IP
OSI Reference model
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1.
2.
3.
4.
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
Seven layers of the OSI model
Exchange using OSI Model
Issues, to be resolved by the layers
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Larger bandwidth at lower cost
Error correction
Flow control
Addressing
Multiplexing
Naming
Congestion control
Mobility
Routing
Fragmentation
Security
....
Applications
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E-mail
Searchable Data (Web Sites)
E-Commerce
News Groups
Internet Telephony (VoIP)
Video Conferencing
Chat Groups
Instant Messengers
Internet Radio
Research areas in Networking
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Routing
Security
Ad-hoc networks
Wireless networks
Protocols
Quality of Service
…
Readings
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Chapter 1: 1.1, 1.2
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Computer Networks, A Systems Approach
L. Peterson & Davie
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Chapter 1 (B. A Forouzan)
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Section 1.1, 1.2, 1.3,1.4
Chapter 2 (B.A Forouzan)
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Section 2.1