Tuesday, September 16, 2008

Lecture 4

Lecture 4


Functions required in communication networks


  • user services
    • Including smtp, ftp, telnet, http, video conferencing
  • Switching
    • Transfer information between communication lines
  • Transmission
    • Ability to transmit information across a medium
  • Addressing
    • Identify communication lines and stations
  • Multiplexing    
    • Means for coupling information from different sources together
  • Routing
    • Identify the shortest path between the source and destination
  • Congestion control
    • Identify congestion of data and / or ways to prevent it
  • Flow control
    • Prevent overwhelming of a slower computer
  • Quality of service (QOS)
    • Allocate different class of service to different users
  • Compatibility
    • Connect heterogeneous networks
  • Error detection
    • Identify errors and / or correct them
  • Security
    • Prevent eavesdropping
  • Management
    • Monitor and recover from faults, manage bills etc.

Types of Networks


  • Networks are typically classified in three types: LAN, MAN and WAN
  1. Local Area Networks
    1. Small networks confined to a few kilometre ( <= 1km )
    2. Speeds confined to 100mps. Newer LANs run up to 10Gbps
    3. Uses the principal of broadcasting (one transmits, others listen)
    4. Various Topologies including Token Bus and Token Ring are possible
  2. Metropolitan Area Networks
    1. Covers up to a city (<=10km), Example: Cable TV network, IEEE 802.16
    2. Cable network were initially designed for TV and later extended to Internet
  3. Wide Area Networks
    1. Spans a continent (<=10000 km), Example: internet
    2. Interconnects various LAN using switches (routers) and transmission lines
    3. Uses packet switching in conjunction with the store and forward technology


Chapter 2




  • A set of rules and conventions used by two communicating parties
  • How a communication will be initiated and terminated
  • How data and control information are arranged in a datagram
  • What control information is included, etc.
  • Examples
    • http
    • ftp
    • smtp
    • tcp

Client / Server configuration


  • a server is a computer which may store information / control a network, may store information this is required by all nodes on the network
    • usually a server is more powerful than other hosts on the network
  • a client is another machine connected to the server that retrieve information stored on the server
  • client / server protocol
    • client makes a request over the network to the server
    • the client waits for a response from the server and be ready to receive an answer
    • the servers gets the request, performs the tasks required of it based on the request and returns a reply
  • basic definitions
    • Port is a process on the server that is waiting for requests that come in, and listen on particular ports for particular requests. Ports used quite often are usually known by number
    • Daemon runs on a machine and listens for requests
    • Networks consist of two components
      • Hardware that forms the infrastructure connecting the computers, example twisted pair wire, optical fibre
      • Software that forms a cohesive connection such that the user sees the entire network as a single coherent system. The design of software is highly structured and is the focus of our discussion this presentation
        • Meant to integrate the whole system seamlessly, making the network invisible to the user
        • Structure is the relationship between various entities

    • Browsing a website
      • User clicks a URL
      • Client process determines the IP address corresponding to the host name using the domain name server (DNS) query
        • Using the IP address, client process sets up a 2-way TCP connection with port 80 with the WWW server
        • TCP connection is reliable and connection oriented
      • Client HTTP daemon sends a request (GET) for the document specified in the url. HTTP version used by the browser is HTTP/1.1
      • Server http daemon receives the GET command by listening at CP port 80 and interprets the message

Address Resolution


  • The http example requires a DNS query to resolve the address of server. In other words, an ip address is to be retrieved from the domain name
  • The address resolution is performed b a domain name system(DNS) which is a distributed database used to convert names into addresses
  • A process in the host called 'resolver' composes the question for the DNS
  • The resolver contacts the local DNS server first. Only if it fails to resolve an address, a higher level DNS server is accessed
  • The communication between resolver and DNS server is carried out using the UDP protocol of the transport layer. UDP protocol is unreliable and provides connectionless service.



  • The mail client contacts a local SMTP server for delivery of an email
  • The user prepares a message with recipients email address, subject and body
  • The mail client contacts the local smtp server (may require dns resolution if ip address isn't known) using the TCP protocol of the transport layer and transmits the file to the local SMTP server
  • The local smtp server repeats the above process with the destination smtp server, which in turn repeats the process with the destination SMTP

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