WHAT IS ALOHA ? IN NETWORKING | PART - I

 ALOHA  means Hello.

It is first professional channel.It gives the rule that how to share the channel.It is based on satellite based communication.

EXAMPLE:

LEO cover 8000 diameter of area.

Both Up link and Down link frequency are different.

By the help of above diagram we can easily understand Up link and Down link Frequency, When station A will send any message to Up means towards satellite than it use Up link frequency.

When any station B receive any message downward means from satellite to station than it uses down link frequency.

In ALOHA message is broad castes over the entire area but the message will receive  only destination station and other station are ignore it.

CHARACTERISTICS OF ALOHA:

1. NO OF STATIONS:

 There are two type of ALOHA are used .

a) Infinite population ALOHA : Where infinite number of user / stations are available.

b) Finite population ALOHA    : Where finite number of user / stations are available.

2. SINGLE CHANNEL:

All the station are sharing common up link frequency.

i.e Single channel is available and there is no multiplexing technique used.

3. COLLISION:

In ALOHA because of single channel if two or more station transmit in overlapping time , than collision will occurs and information of transmitting station will occur and information of transmitting station will be lost.  

4. TIME :

There are two possibilities to consider a time.

a)  CONTINUOUS TIME   (or)  PURE ALOHA

b) SLOTTED TIME  (or)  SLOTTED ALOHA

CONTINUOUS TIME   (or)  PURE ALOHA : In continuous time any time station can transmit the message. For example when ever message is created than switch on and send the message. 

SLOTTED TIME  (or)  SLOTTED ALOHA: In slotted time / aloha we can transmit in time slot.For example when the bell is ON than we send the message after some time slot.

5. TRANSMIT BUFFER : Each station has one buffer where a standard size frame is stored for transmission. until unless the stored frame is not transmitted successfully subsequent frames can not be generated.

i.e There is no queue of frame in various station.

EXPLANATION OF ALOHA:

All station is used common channel.So if more than one channel send data at a time then collision will occur. 

For overcome this collision problem each station have random number generator,which generate random number that mean , when A send message and B , C , D ----, also send data at same time frame, then collision will occur and again they re-transmit than again collision will occur. If you do same thing infinitely than collision will occur.

So one provision is that each station generate random number, on the basis of this random number they send the data. Random number like for A it is 4 sec , for B it is 14 sec, for C it is 10 sec etc.

one thing is noted that each random number are not same or different , if the random number are same than again collision will occur.

Random number is just like a bell , when bell is ringing than station will send the message.

NOTE: 

❓One question is that how any source station can detect that his message is received by receiver or not.. Is there is any need for Acknowledgement ?

✔ The Answer is no there is no need for acknowledgement because in this scheme.A satellite cover whole area and all station in that area.

So when satellite send message to destination than this message will come at all stations including source station. But except destination station all other station will ignore that message.

Here source station will detect the message and match that message with original message that it was send , if the message is same than it is confirm that destination will received  message.

Otherwise some collision will occur and the destination will not receive the correct message.

Then we again re-transmit the message.

This all over what is ALOHA? in networking. I hope you all understand . This is end of PART-I

Now in PART-II we will learn about Derivation of PURE AND SLOTTED ALOHA.