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Computer Networks (CSC 345)


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Title: Computer Networks (CSC 345)

Computer Networks (CSC 345)
  • Fall 2004
  • Professor Haimeng Zhang
  • IVERS 234F
  • x4742

Course Objectives
  • Motivation What is the Internet and how it works
  • To present a comprehensive view of the principles
    and fundamental concepts in Computer Networks
  • To learn about the basics in design and
    implementation of network protocols
  • To provide an understanding of the components of
    a network and how they are connected.
  • To acquire some hands-on experience

Course Requirements
  • Prerequisites
  • Programming experience with C/C, equivalent to
    CSC 225
  • Good to have the knowledge on OS
  • Required Textbook
  • Douglas Comer Computer Networks and Internets
    with Internet Applications, 4th ed. Prentice
    Hall, 2004
  • Reference Book
  • R. Stevens, TCP/IP Illustrated, Volume 1 The
    Protocols, Addison-Wesley, 1994.
  • Supplementary class notes available on line
  • Course web page http//

Course Organization
  • Lectures TH 1030am 1210pm, IVERS 218
  • Homework assignments once every two weeks
  • Programming project one group project
  • Reading project individual project
  • Midterm
  • Final

Course Outline
  • Introduction
  • Fundamental concepts
  • Basic definitions
  • Network architecture
  • Communication Basics
  • Media and signals
  • Asynchronous and synchronous communication
  • Relationship among bandwidth, throughput, and
  • Frequency-division and time-division multiplexing

Course Outline (Continued)
  • Networking and network technologies
  • Packing switching
  • Framing, parity, and error detection
  • Local and wide area technologies
  • Network addressing
  • Connection, wiring and extension (repeaters,
    bridges, hubs, switches)
  • Forwarding and measuring of delay and throughput

Course Outline (Continued)
  • Internets and Internetworking
  • Motivation and concept
  • Internet Protocol (IP) datagram format and
  • Internet routers and routing
  • Address binding (ARP)
  • Internet Control Message Protocol (ICMP)
  • User Datagram Protocol (UDP)
  • Transmission Control Protocol (TCP)

Course Outline (Continued)
  • Network Applications
  • Domain Name System (DNS)
  • File Transfer Protocol (FTP)
  • Remote Login Protocol (TELNET)
  • Email Transfer (SMTP)
  • Web technologies and protocol (HTTP)
  • Putting all pieces together

Schedule of Topics
  • Signals, media, bandwidth, throughput and
    multiplexing 2 weeks
  • Packet transmission concepts, technologies 5
  • Internetworking fundamentals 5 weeks
  • Internet applications 2 weeks

What is a Computer Network?
  • A collection of computers (PCs, workstations) and
    other devices (e.g. printers, credit card
    readers) are all interconnected
  • Components
  • Hosts (computers)
  • Links (coaxial cable, twisted pair, optical
    fiber, radio, satellite)
  • Switches/routers (intermediate systems)
  • Goal provide ubiquitous access to resources
    (e.g., database servers, Web), allow remote users
    to communicate (e.g., email)
  • User runs applications

What is a Computer Network?
  • Major Network Categories
  • The global Internet
  • Internal corporate networks
  • The worldwide telephone system

What is a Computer Network?
  • Telecommunications spans two concerns
  • Voice and video communication versus
  • Data communication
  • At least one party is a computer
  • The two are converging

What is a Computer Network?
What is a Computer Network?
What is a Computer Network?
What is a Computer Network?
What is a Computer Network?
What is a Computer Network?
What is a Computer Network?
  • In summary, a network is a system of hardware,
    software and transmission components that
    collectively allow two application programs on
    two different stations connected to the network
    to communicate well

What is a Computer Network?
  • Direct links (connectivity)
  • Point-to-point communication
  • Multiple-access

What is a Computer Network?
  • Switched Networks
  • Circuit - switched network public telephone
  • Packet switched network Internet (collection of

  • Set up a connection path (circuit) between the
    source and the destination (permanent for the
    lifetime of the connection)
  • All bytes follow the same dedicated path
  • Used in telephony
  • Advantages dedicated resources
  • Disadvantages not very efficient (lower
    utilization, e.g., a person talks lt 35 of the
    time during a call)
  • While A talks to C, B cannot talk to D on the
    same line.

  • Packets from different sources are interleaved
  • Efficient use of resources (since they are used
    on a demand) statistical multiplexing. Nobody
    reserves a lane on a freeway
  • Can accommodate bursty traffic (as opposed to
    circuit-switching where transmission is at
    constant rate).

Features of a Packet-Switching
  • Store and forward intermediate nodes (e.g.,
    routers) store (buffer) incoming packets, process
    them and forward them to the appropriate outgoing
  • Allows for flexibility and robustness. Packets
    can travel through alternate paths (adaptive
  • Undesired situations such congestion, long delays
    may occur.

Packet Switched Networks Example
  • Packets can travel on different networks/links
    that may have different line speeds

Packet-Switched Networks Topologies
What is the Internet?
  • In the 60s and 70s the Internet (ARPANET) was a
    small network connecting universities, research
    labs and government agencies. Main application
    email, FTP. Motivation share research
  • Today it is a global, non-regulated
    communications network with millions of hosts and
    users. Main applications Web, multimedia
    (audio/video), email. Motivation
  • A large number of different network technologies
    and standards exist LANs, WANs, B-ISDN, Optical
    Nets, Wireless, Satellite.

The Internet Today-- Complicated
  • A huge and arbitrary collection of heterogeneous
    nets. A network of networks!
  • More than 70 million hosts
  • Growing exponentially doubling every 18 months
  • Hierarchically structured
  • LANs (e.g., Ethernet)
  • CANs (e.g., FDDI)
  • National/global (e.g., ATM or optical backbone)
  • Fully distributed operation (i.e., no centralized
    system or computer)

An Internet
Probing the Network-Example
  • Concordia campus network
  • http//
  • Minnesota State Network
  • http//
  • Ping - sends message that is echoed by remote
  • Traceroute - reports path to remote computer

Internet Today
  • Packet - switched network
  • Packets
  • Data are chopped up into small blocks called
    packets (e.g., 4500 bytes)
  • Each packet carries extra information to allow it
    to reach its destination
  • Each intermediate node processes the packet and
    forward it to the next node

  • Resource sharing (i.e., accommodate many users
    over the same link or through the same router)
  • Addressing and routing (i.e., how does an email
    message finds its way to the receiver)
  • Reliability and recovery guarantee end-to-end
  • Traffic management monitoring and policing the
    network! Regulate traffic

Network Performance
  • There is a number of measures that characterize
    and capture the performance of a network
  • It is not enough that networks work
  • They must work well
  • Quality of service (QoS) defines quantitative
    measures of service quality
  • Speed
  • Delay (Latency)
  • Reliability
  • Security (not a QoS measure but crucial)

Network Performance
  • Speed
  • Bits per second (bps)
  • Multiples of 1,000 (not 1,024)
  • Kilobits per second (kbps) ? Note the lower case
  • Megabits per second (Mbps)
  • Gigabits per second (Gbps)
  • Terabits per second (Tbps)
  • Related to link bandwidth

Network Performance
  • Congestion and Latency
  • Congestion because traffic chronically or
    momentarily exceeds capacity
  • Latency delay measured in milliseconds (ms),
    microseconds ( ).
  • Especially bad for some services such as voice
    communication or highly interactive applications

Network Performance
  • Delay
  • Transmission time time it takes to transmit a
    packet (depends on the link speed) packet size/
  • Propagation delay time for a bit to travel
    across a link (depends on the distance, physical
  • Queuing delay waiting time inside a buffer
  • Processing delay time to process a packet
  • RTT (round-trip time) time for a bit to travel
    to the destination and come back

Network Performance
  • Example consider a 100 Mbps link which is 4,000
    miles long, if data travels at 40,000 miles/sec
    and a packet is 1MB ( Bytes
  • bits), then
  • Transmission delay 1MB/100 Mbps
  • ms 0.080 sec
  • Propagation delay 4,000/40,000 0.1 sec

Reliability and Recovery
  • Reliability
  • Availability percentage of time the network is
    available to users for transmission and reception
  • Error rate percentage of lost or damaged
    messages or bits. (For example, bit error rate of
  • Examples
  • Bit errors (bits are flipped, e.g., due to
    electrical signal interference.)
  • Packet loss (packets may be dropped due to
    insufficient buffer space.)
  • Packet delays (e.g., due to large queue size)
  • Nodes or links can fail (go down)
  • Malicious users

Reliability and Recovery
  • As a consequence
  • Packets delivered to the wrong destination
  • Long delays on packets
  • Packets delivered out-of-order
  • Duplicate packets
  • Recovery
  • Implement error-control mechanism
  • Hop by hop (I.e., between nodes)
  • End-to-end (source-to-destination).
  • Retransmissions
  • End-to-end security (e.g., encryption,

User Applications
  • Users run application programs (web, email, ftp)
    at the hosts interconnected through a network
  • Hosts need to communicate in a meaningful way.
    User should not be concerned with the underlying
  • Network supports process-to-process (uni- or
    bi-directional) communication among the hosts
  • Applications need to take into consideration
    limitations imposed by the networks physical

What is a Protocol?
  • Set of rules that specify the format and meaning
    of messages exchanged between computers across a
  • Format is sometimes called syntax
  • Meaning is sometimes called semantics
  • Example from everyday life traffic laws!

One Or Many Protocols?
  • Computer communication across a network is a very
    hard problem
  • Complexity requires multiple protocols, each of
    which manages a part of the problem
  • May be simple or complex must all work together

Protocol Suites
  • A set of related protocols that are designed for
    compatibility is called a protocol suite
  • Protocol suite designers
  • Analyze communication problem
  • Divide problems into subproblems
  • Design a protocol for each subproblem

Layered Protocol Design
  • Layering model is a solution to the problem of
    complexity in network protocols
  • Model suggests dividing the network protocol
    into layers, each of which solves part of the
    network communication problem
  • These layers have several constraints, which
    ease the design problem
  • Network protocol designed to have a protocol or
    protocols for each layer

Layered Network Architecture
  • Application data need to be transformed into
    packets (the basic transmission unit)
  • Peer entities in layer N1 communicate with each
    other by communication services provided by layer
    N (below them)
  • Each layer has specific tasks and functionality.
    It also provides services to the layers above and
    below it
  • Peer entities communicate by exchanging messages

ISO 7-Layer Reference Model
  • International Organization for Standards (ISO)
    defined a 7-layer reference model as a guide to
    the design of a network protocol suite

ISO 7-Layer Reference Model
  • Layers are named and numbered reference to
    layer n'' often means the nth layer of the ISO
    7-layer reference model
  • many modern protocols do not exactly fit the ISO
    model, and the ISO protocol suite is mostly of
    historic interest

ISO 7-Layer Reference Model
  • Layer 7 Application
  • Application-specific protocols such as FTP and
    SMTP (electronic mail)
  • Layer 6 Presentation
  • Common formats for representation of data
  • Layer 5 Session
  • Management of sessions such as login to a remote
  • Layer 4 Transport
  • Reliable delivery of data between computers

ISO 7-Layer Reference Model
  • Layer 3 Network
  • Address assignment and data delivery across a
    physical network
  • Layer 2 Data Link
  • Format of data in frames and delivery of frames
    through network interface
  • Layer 1 Physical
  • Basic network hardware media transmission

Layering Principle
Layering Principle
  • Application data need to be transformed into
    packets (the basic transmission unit)
  • Peer entities in layer N1 communicate with each
    other by communication services provided by layer
    N (below them)
  • Each layer has specific tasks and functionality.
    It also provides services to the layers above and
    below it
  • Peer entities communicate by exchanging messages

Data Communications
  • On the sender, each layer
  • Accepts an outgoing message from the layer above
  • Adds a header and other processing
  • Passes resulting message to next lower layer
  • On the receiver, each layer
  • Receives an incoming message from the layer below
  • Removes the header for that layer and performs
    other processing
  • Passes the resulting message to the next higher

Data Communications
  • The software at each layer communicates with the
    corresponding layer through information stored in
  • Each layer adds its header to the front of the
    message from the next higher layer
  • Headers are nested at the front of the message as
    the message traverses the network

Data Communications
Internet Protocol Architecture
  • Originally it was based on the ISO reference
  • Currently, Internet is mostly based on the TCP/IP
    protocol suite (designed in late 70s)
  • TCP/IP became popular as it was bundled with the
    UNIX/C environment
  • ISO is still influential in designing networks
  • Other architectures ATM. Frame Relay

Reading Materials
  • Textbook
  • Chapters 1, 2 and Sections 3.1, 3.2 of Chapter 3
  • Chapter 16