Computer Networks
Course Instructor:
Dr. Fawad Salam Khan
Assistant Professor
Email:
fawad.salam@mail.au.edu.pk
Computer Networks (BSDS-IV)
Course Grading Policy:
Grading Policy:
No. Weightage Policy
Quizzes: 05 10% The quizzes may be announced or
unannounced. The questions will be framed to
test the concepts involved in the last few
lectures.
Assignments: 05 10% The GCR will be the primary source for
announcements and submitting assignments.
Projects: 01 10% Team-based project assignment and evaluation
will be carried out. The project will be assigned
in week 09 after midterm exam.
Midterm 01 25% As per AU Policy
Exam:
Final Exam: 01 45% As per AU Policy
Answers to FAQs
◼ All home works are due at the beginning of the class
indicated on the course calendar
❑ After that 10% penalty: only if submitted before solutions
are posted.
◼ Exams are closed-book and extremely time limited.
◼ Exams consist of design questions, numerical,
maybe true-false and short answer questions.
3
Reading
◼ Text book:
❑ Data Communications and Networking, 4/e
◼ B.A. Forouzan,
McGraw-Hill, 2003,
ISBN 0-07-292354-7.
◼ Reference books:
❑ Computer Networking, a top-down approach
featuring the Internet (3rd edition),
◼ J.K.Kurose, K.W.Ross,
Addison-Wesley, 2005,
ISBN 0-321-26976-4.
❑ Computer Networks, A Systems Approach
L. Peterson & Davie
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Required Skills
◼ The course does not assume prior knowledge
of networking.
5
My Requirement from YOU
◼ I require YOU to take active part during lectures
❑ Which means Lot of Questioning in the class –
(Interactive session)
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Aim of the Course
◼ Aim of the course is to introduce you to the
world of computer networks, so that you
could
❑ know the science being used in running this
network
❑ Use this knowledge in your professional field
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Network Design
Before looking inside a computer
network, first agree on what a
computer network is?
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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
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Networks
◼ Potential of networking:
❑ move bits everywhere, cheaply, and with desired
performance characteristics
◼ Network provides “connectivity”
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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 point-point physical link.
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Building Blocks
◼ Nodes: PC, special-purpose hardware…
❑ hosts
❑ switches
◼ Links: coax-cable, optical fiber…
❑ point-to-point
…
❑ multiple access
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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
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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
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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”
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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
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Switching Strategies
◼ Circuit Switching: • Packet Switching: store-
carry bit streams and-forward messages
a. establishes a dedicated a. operates on discrete
circuit blocks of data
b. links reserved for use
b. utilizes resources
by communication
channel according to traffic
c. send/receive bit stream
demand
at constant rate c. send/receive messages
d. example: original at variable rate
telephone network d. example: Internet
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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
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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
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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
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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
21
Multiplexing
◼ Physical links and nodes are shared among users
❑ (synchronous) Time-Division Multiplexing (TDM)
❑ Frequency-Division Multiplexing (FDM)
L1 R1
L2 R2
Multiple flows
on a single link
Switch 1 Switch 2
L3 R3
Do you see any problem with TDM / FDM ?
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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
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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
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