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WEEK 2 DAY 2 : CONTINUING WHAT WE LEFT


2.2 CONTINUING

       
    1) HUB

        Hub is a network device which acts as a port to join multiple computers in a network. It is a central device through which all communication are done.


Some common Definitions

        1) PACKET :- When a big data is transferred from one device to other it is divided into small portions and then send. Each portion is called a packet 

       2) BROADCAST DOMAIN :-  A broadcast domain is a logical division of a computer network, in which all nodes can reach each other by broadcast at the data link layer. Any computer connected to the same Ethernet repeater or switch is a member of the same broadcast domain.Routers and other higher-layer devices form boundaries between broadcast domains.

      3) COLLISION DOMAIN :- A collision domain is a section of a network where data packets can collide with one another when being sent on a shared medium or through repeaters, particularly when using early versions of Ethernet.

       4) I.P ADDRESS :- An Internet Protocol address (also known as an IP address) is a numerical label assigned to each device (e.g., computer, printer) participating in a computer network that uses the Internet Protocol for communication.An IP address serves two principal functions: host or network interface identification and location addressing. Its role has been characterized as follows: "A name indicates what we seek. An address indicates where it is. A route indicates how to get there."

       5) MAC ADDRESS :- explained before

       6) ARP TABLE (Address Resolution Protocol) :- It is a table kept by computers or end devices which keeps destination I.P and MAC address of computers on a network system. It is by default blank and can be filled by two methods i.e manually which is called static and the device fills it itself also called dynamic.

       7) ARP Packet :- When communicating a computer/device first ping the receiver by sending an ARP packet  in which IP address of destination and its MAC address are stored.

       8) Collision :- It happens in half duplex systems. It happens when multiple packets or data is sent from receiver as well as source at same time. It gets corrupted in middle of channel and this is called collision.


Communication types

       Let us talk about types of communications. There are three types of communication according to number of channel and method of communication. They are :-

       1) SIMPLEX :- It is one way communication from sender device to receiver device, Receiver cannot sent anything back to sender. e.g :- Radio.
       2) HALF DUPLEX :- It is two way communication but has only one channel for receiving and sending. So one has to wait when the other is sending. There is a chance of collision in these systems. e.g :- Walkie Talkie.
       3) FULL DUPLEX :-  It is two way communication and has two channels for receiving and sending data. There is no chance of collision.

Types of Packet

       There are three types of packet. They are :-

       1) Broadcast :- It is meant for all devices in network i.e one to all.
       2) Unicast :- It is meant for a group of devices in a network i.e one to group.
       3) Unicast :- meant for only one computer in the network. i.e one to one.


Types of Ethernet connections
  • ETHERNET :- 10 Mbps speed
  • FAST ETHERNET :- 100 Mbps
  • GIGABIT ETHERNET :- 1 Gbps
Properties of hub
  • It always does broadcast.
      It means that whatever data packet the hub gets it broadcasts it i.e it sends the data packet to all devices in the network except the sender. Then the packet is rejected or accepted by the receiver. There are two drawbacks of this system. First the system is insecure
and second problem is that for broadcasting it has to copy the data packets into each port and then send it. So, this thing takes time hence it is used only  for networks having 4-8 computers like cybercafe otherwise the speed will get low.
  • It is unmanageable device.
      It cannot be managed or programmed to work according to our needs. For example if we want some computer to be grouped and a message should be sent only to those computer we cannot do it.
  • It is unintelligent device.
       Hub does not know that the packet sent by sender is meant for which computer in network. So, it is an unintelligent device.
  • It has one broadcast domain.
  • It has one collision domain.
  • It supports half duplex communication. ( explained above )
  • It is a shared media.
       It means that the speed of the hub is shared among its members. For example :- If the speed of hub is 8 Mbps and it is connected to 4 computers then each computer will get a speed of 8/4 = 2 Mbps. So, it is a slow device. 
  • Used only for LAN network.
Communication using hub

       Let us assume the following network of hubs





       Let us assume the above configuration of network. Ok let's start. Do read the "Some common Definitions" above before proceeding.

       Before sending any data the computer first ping the machine to establish connection. It requires the I.P address of destination machine for that. We can ping manually also by running ping command on our command window. e.g ping 1.1.1.1 or ping www.google.co.in. Then it looks into ARP table and search for MAC address corresponding that IP address. If it is there it copies it into frame packet
 ( explained later ) else it puts FFFF in destination MAC address. 

       When a device ping other device it forms two packet one is I.P packet in which it puts source and destination I.P address and the other one is frame packet in which it stores source and destination MAC addresses. Then it encapsulates them to form ARP packet.

        OK, let us assume according to above diagram that Comp A ping Comp D. Command would be "ping 1.1.1.4" in Comp A. Now computer will make an I.P packet as follows. We are assuming that they are pinging for first time

Source I.P                       Destination I.P
1.1.1.1                                  1.1.1.4

      Then, it will look into ARP table which is empty. So, frame packet will be as follows

Source MAC                       Destination MAC
A                                         FFFF

       Then it encapsulates it into ARP packet and sends it to HUB.It is also called the eco packet The HUB opens the ARP packet then closes it copies it and paste into ports (broadcasts). The other computer receives it again opens it and compares it with its own I.P. If it matches it accepts else rejects it. (Note :- I.P is given preference over MAC so even when you change the I.P the MAC gets automatically updated against that I.P ). But all computer B,C,D updates their ARP table with source's IP address and MAC. Then Comp D will then make a reply as folows

Source I.P                       Destination I.P
1.1.1.4                                  1.1.1.1

      Then, it will look into ARP table which contains the entry corresponding to A . So, frame packet will be as follows

Source MAC                       Destination MAC
D                                           A

       It will then encapsulate it into ARP packet and send it to HUB. This packet is also called Eco-reply. Again the same procedure happens and source computer then accepts it. It again makes an Eco packet and the procedure now repeats four times total. Note that eco-reply updates the ARP table of all computers i.e A,B,C with MAC and IP of comp D.

Collision and its protection

       Since HUB is a half duplex system collision can happen in it at any time which corrupts the data. To counter this two protocols called collision avoidance protocol are used. They are Carrier sense multiple access with collision avoidance (CSMA/CA) and Carrier Sense Multiple Access With Collision Detection (CSMA/CD).

       CSMA/CA detects a collision and then activates CSMA/CD which then analyse the time of sending of packet and allows the first packet and rejects the other. It sends a message to all computer on network to delay the sending of packets for specified time period. 

        2) SWITCH

       Well, it is a normal on and off switch for household bulbs to which you can connect your computer. Don't take it on heart it was a joke. NOTE :- dangerous don't try this at home.

      HE HE HE HE!!!!! well jokes apart let's get down to business. Switch is a substitute for hub and has more advanced features to support more than 20 computers easily unlike hub which have become absolute in industry and big offices. Switches have replaced hub on all big LAN due to its speed.

Properties of hub

  •  It is used for LAN network only.
  • It first time broadcasts. After that it performs unicast
       Well switch contains their own table called cam table in which they store the MAC address of all computers in network by pinging them. It broadcasts for the first time so that the devices in the network can update their ARP table.
  • It has CAM table to contain MAC address.
  • It supports full duplex communication.
  • It is a manageable device.
       You can program the device as per the needs and its capabilities. e.g :- You can group computers in it and send data only to that group.

  • It is an intelligent device.
       It can read MAC addresses as well as store them. It sends data to destination port only unlike hub which copies it to all port

  • It has one broadcast domain.
  • It has per port collision domain.
  • It is not a shared medium i.e a 16 Mbps switch will give 16 Mbps to all ports.
  • It understands only MAC address.
       

       Communication using Switch

       If we take the above example the procedure at comp A remains the same. When eco-packet reaches the switch the switch updates its CAM table and broadcast it. The procedure at destination side and comp D side remains same. But when eco-reply reaches the switch it unicast it to comp A since now cam table has MAC address of A. The CAM table will be as follows

PORT                   MAC

E 0/1                      A   
E0/2                       -   
E0/3                       -   
    E0/4                      D      

       That concludes what they actually taught us. For more information about router ( which he left ), he told us to go to the site www.routeralley.com. So, enjoy your trip there. 

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