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Lecture 1 Introduction. Basic Concepts.

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Lecture 1


basic concepts
Basic Concepts
  • Distributed System: It is a collection of independent computers or electronic devices, which appears to its users as a single coherent system. Usually, it has a single model or paradigm that it presents to the users. Often, a layer of software on the top of the operating system, called middleware, is responsible for implementing this model.
  • Example: WWW. Essentially, a distributed system is a software system that is built on the top of a network.
  • Distributed Processing: When a task is divided among multiple computers, we call it distributed processing.
  • Computer Network: A collection of computers interconnected by a communications technology.
basic concepts3
Basic Concepts
  • Protocols: It is a set of rules that governs data communications. Basically, it is an agreement between communicating parties on how communication is to proceed.
  • Packets: Packets are discrete units of potentially variable-length blocks of data.
  • Broadcasting: Packets are sent to all possible destinations – every machine in the network receives it.
  • Multicasting: Some broadcast systems can send packets to a subset of the machines in a network, which is known as multicasting.
  • Unicasting: Point-to-point transmission with one sender and one receiver.
basic concepts4
Basic Concepts
  • Categories of Networks :
    • Local Area Network (LAN)
    • Metropolitan Area Network (MAN)
    • Wide Area Network (WAN)
    • Personal Area Network (PAN)
    • Wireless Network
  • Network Operating System (NOS): Loosely coupled operating system for heterogeneous multi-computers (LAN, WAN) that offers local services to remote clients.

Network topology

Fully connected mesh topology (for five devices)

Q: How many links are needed for a full mesh network with 20 nodes?


Network topology

Star topology


Network topology

Bus topology


Network topology

Ring topology

basic concepts14
Basic Concepts
  • Hierarchical Structure: To reduce the design complexity, most networks are organized as a stack of layers or levels, each one built upon the one below it. Between each pair of adjacent layers is an interface. The interface defines which primitive operations and services the lower layer makes available to the upper layer.
  • OSI Reference Model: 7 layer-model: Physical, Data-link, Network, Transport, Session, Presentation, Application
  • The TCP/IP Model: 5 layer-model - Physical, Data-link, Network, Transport, Application
  • We will mainly focus on the TCP/IP (Internet) model.
functions of each layer
Functions of Each Layer
  • Thephysical layerisresponsible for transmitting individualbits from one node to the next. It coordinates the functions required to transmit a bit stream over a physical medium. The major duties are:
    • Physical characteristics of interfaces and media
    • Representation of bits
    • Data rate
    • Synchronization of bits
functions of each layer17
Functions of Each Layer
  • Thedata link layerisresponsible for transmitting frames from one node to the next. The major duties are:
    • Framing
    • Physical addressing
    • Flow control
    • Error Control
    • Access Control
functions of each layer18
Functions of Each Layer
  • Thenetwork layerisresponsible for delivery of packets from source to destination. The major duties are:
    • Logical addressing
    • Routing
functions of each layer19
Functions of Each Layer
  • Thetransport layerisresponsible for process-to-process delivery of the entire message. The major duties are:
    • Port addressing
    • Segmentation and reassembly
    • Connection control
    • Flow control
    • Error control
functions of each layer20
Functions of Each Layer
  • Theapplication layerisresponsible for proving services to the end user. The major duties are:
    • Mail services
    • File transfer and access
    • Remote login
    • Accessing the WWW


The physical layer is responsible for transmitting individual bits from one node to the next.



The data link layer is responsible for transmitting frames from one node to the next.



The network layer is responsible for the delivery of packets from the original source to the final destination.



The transport layer is responsible for delivery of a message from one process to another.



The application layer is responsible for providing services to the user.

  • Network Access Points (NAPs), where individual ISPs would interconnect, as suggested in the previous Fig. The NSF originally funded four such NAPs: Chicago (operated by Ameritech), New York (really Pensauken, NJ, operated by Sprint), San Francisco (operated by Pacific Bell, now SBC), and Washington, D.C. (MAE-East, operated by MFS, now part of Worldcom).
  • The very High Speed Backbone Network Service, a network interconnecting the NAPs and NSF-funded centers, operated by MCI. This network was installed in 1995 and operated at OC-3 (155.52 Mbps); it was completely upgraded to OC-12 (622.08 Mbps) in 1997.
  • The Routing Arbiter, to ensure adequate routing protocols for the Internet.