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Showing posts with label Computer Networks. Show all posts
Showing posts with label Computer Networks. Show all posts

Explain how to configure DHCP Server.

 
  • DHCP (Dynamic Host Configuration Protocol) helps to assign IP addresses to clients in a distributed manner. It is centrally configured server who specifies a range of IPs to be allotted to connected clients on request. It cannot assign IP to any Ethernet aliases if exists.
  • Following are the steps to configure DHCP Server
    • Verify the software package is installed on your system as follow:
      • $ ps ax | grep dhcp
    • If not already installed, then install on your system as follow:
      • $ sudo apt-get install isc-dhcp-server
    • There are two main files that are probably need to change /etc/dhcp/dhcpd.conf and /etc/default/isc-dhcp-server to specify the interfaces dhcp should listen to.
    • Open the configuration file of dhcp in editor, i.e. /etc/dhcp/dhcpd.conf
      • And perform the following:
      1. Edit the line 'option domain-name' and write your domain name:
        2. option domain-name "example.com"
      2. Edit the line 'option domain-name-servers' and write the name of your name server as shown below, for eg the name is tyit.example.com
        3. option domain-name-servers tyit.example.com;
      3. Edit the line subnet to provide your subnet and netmask inormation as shown below:
        4. subnet 192.168.1.0 netmask 255.255.255.0
      4. Specify the range in which IPs are to be assigned to DHCP clients on request as shown below:
        5. range 192.168.1.150 192.168.1.200;
    • Restart the service of dhcp, as shown below
      • $ sudo service isc-dhcp-server restart
    • Enable dhcp on boot, as shown below
      • $ chkconfig dhcpd on
    • As the server is configured, verify the setup on the client system.

OSI Reference Model

 
The model is called the ISO OSI (Open System Interconnection) model because it deals with connecting open systems, that is, system that are open for communication with other systems.
The following figure shows the overview of OSI Reference Model:

Fig. OSI Reference Model

The OSI model has seven layers. The principles that were applied to arrive at the seven layers can be summarised as follows:
  • A layer should be created where a different abstraction is needed.
  • Each layer should perform a well-defined function.
  • The function of each layer should be chosen with an eye toward defining an intentionally standardised protocol.
1. Physical Layer.
  • The physical layer is concerned with transmitting raw bits over a communication channel.
  • The design issues have to do with making sure that when one side sends a 1 bit, it is received by the other side as a 1 bit, not as 0 bit.
  • The design issues here largely deal with mechanical, electrical and timing interfaces and the physical transmission medium which lies below the physical layer.
2. Data Link Layer.
  • The main task of the data link layer is to transform a raw transmission facility into a line that appears free of undetected transmission errors to the network layer.
  • It accomplishes the task by having the sender break up the input data into data frames (typically a few hundred or thousand bytes) and the transmit the frame sequentially.
  • If the service is reliable the receiver confirms correct receipt of each frame by sending back an acknowledgement frame.
3. Network Layer.
  • The network layer handles routing among nodes within a packet switched network. 
  • At this layer, the unit of data exchange among nodes is typically called a packet.
4. Transport Layer.
  • The transport layer then implements what we have to do up to this point been calling a process-to-process channel.
  • Here, the unit of data exchanged is commonly called a message rather than a packet or a frame.
  • It runs on the end hosts and not on intermediate switches or routers.
5. Session Layer.
  • The session layer allows users on different machines to establish a session between them.
  • Session after various services includes dialog control (keeping track of whose turn is to transmit), token management (preventing two parties from attempting the same critical operation at the same time), Synchronization (checkpointing long transmission to allow them to continue from where they were after a crash).
6. Presentation Layer.
  • Unlike lower layer, which is mostly concerned with moving bits around, the presentation layer is concerned with the system and semantics of the information transmitted.
  • It manages abstract data structures and allows higher level data structures to be defined and exchanged.
7. Application Layer.
  • The application layer contains a variety of protocols that are commonly needed by users.
  • One widely used application protocol is HTTP (Hyper Text Transfer Protocol) which is the basis for the World Wide Web(www).

Fibre Optic Cable

 
A fibre optic cable is made of glass or plastic and transmits signals in the form of light. Optical fibre use reflection to guide light through a channel. A glass or plastic core is surrounded by a cladding of less dense glass or plastic. The difference in density of the two materials must be such that a beam of light moving through the core is reflected off the cladding instead of being refracted into it.
Following are two major types of fibre optic cable:

  • Single Mode: Uses a specific light wavelength. The cable's core diameter is 8 to 10 micrometre. Single mode fibre is often used for intercity telephone trunks and video applications.
  • Multi Mode: Uses a large number of frequencies. The cable core is larger than that of single mode fibre. Multimode fibre is the type usually specified for LAN's and WAN's.

Advantages:

  1. Lower cost in the long run.
  2. Low loss of signal.
  3. Large data carrying capacity.
  4. No crosstalk between cables.
  5. Immunity to electrical noise.
  6. Reduced size and weight cables.

Disadvantages:

  1. High investment cost.
  2. Need for most expensive optical transmitters and receivers.
  3. Unidirectional light propagation.
  4.  More expensive to repair/maintain.

Coaxial Cable

 
Coaxial cable carries signals of higher frequency ranges than those in twisted pair cable, in part because the two media are constructed quite differently.
Instead of having two wires, coax has a central core conductor of solid or stranded wire enclosed in an insulating sheath, which is in turn enclosed in an outer conductor of metal foil.


The outer metallic wrapping serves both as a shield against noise and as the second conductor, which completes the circuit. This outer conductor(shield) is also enclosed in an insulating sheath, and the whole cable is protected by a plastic cover.

Advantages:

  1. Sufficient frequency rate to support multiple channels.
  2. Lower error rates.
  3. Greater spacing between amplifier.

Disadvantages:

  1. More expensive to install compared to twisted pair cable.
  2. Thicker the cable, the more difficult to work with.

  

Twisted Pair Cable

 

    A twisted pair consists of two conductors (normally copper), each with its own plastic insulation twisted together.


  • One of the wire is used to carry signals to a receiver, and the other is used only as a ground reference. The receiver uses the difference between the two.
  • In addition to the signal sent by the sender on one of the wire, interference (noise) and crosstalk may affect both wires and create unwanted signals.
  • If the two-wire are parallel, the effect of these unwanted signals is not the same on both wires because they are at different locations relative to the noise or crosstalk resources. This results in a difference at the receiver. By twisting the pairs, a balance is maintained and will reduce the effect of noise or external interference.

   Advantages:

  1. Easy to implement and terminate.
  2. It can be used to carry both analog as well as digital data.
  3. Less expensive for short distances.

  Disadvantages:

  1. Attenuation is high.
  2. Very poor security.
  3. Supports only lower bandwidth.
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