Aloha,Csma,Csma CD

MULTIPLE ACCESS PROTOCOLS (

ALOHA, CSMA,CSMA/CD )

RANDOM ACCESS PROTOCOLS  Any

station can send data, depending upon channel is busy or idle.

 No

fix time for station to send data.

 More

chances of collision (damage of frames due to traffic).

 Following

are random access protocols :---1. ALOHA 2. CSMA 3. CSMA/CD

ALOHA  Was   



developed at univ. of Hawaii in 1970 by Norman  Abramson. In this method,stations share common channel. 2 stations when transmit at same time, collisions occurs. So frames destroyed.

 ALOHA has 2 versions---------1. Pure ALOHA 2. Slotted ALOHA

Pure ALOHA  When   

station has frame, it sends. Frame damages if 2 stations send at same time. If ACK, fine. If not, retransmit of frames. Sender waits for random amount of time and sends frame again to avoid collision.

Pure Aloha

Slotted ALOHA      

More efficiency than pure ALOHA. Time of shared channel is divided into slots. Frame can be sent at beginning of slot. Only one frame/slot can be sent. If station forgot to place frame in beginning of  slot,it waits for beginning of next slot. DISADVANTAGE:-col DISADVANTAGE:-collision lision may be there,because 2 senders may start sending at same slot.

Slotted Aloha

CSMA ( carrier sense multiple access) protocol  Reduces

collision, so efficiency.  Station listens ( senses) channel before transmitting whether it is free or busy. Collision n may be------be--------- if frame frame transmitte transmitted d  Collisio from station1 takes some time to reach station2, in mean time station2 senses it idle and transmits its frame, this causes collision.  3 types of CSMA protocols -------1-persistent CSMA , non-persistent CSMA, ppersistent CSMA.

1-persistent     

CSMA

Station senses channel regularly. If channel busy, station waits. Whenever it is idle, it immediately transmits frame with probability 1. Chances of collisions, because 2 stations may find channel idle at same time. Disadvantage:--station1 signal may not reach station2 , causes collision.

1-Persistent

Non-persistent CSMA      

Station senses channel . If channel idle, it sends frame. If busy , it waits for random amount of time. Then again senses channel . Dont continuously senses channel. Dis isad adva vant ntag age: e: ----- chan channe nell ma may y be be idl idle e in in mea mean n time. This reduces network efficiency.

Non-persistent

P-Persistent CSMA      

This method used when,channel has time slots. Station senses channel before transmitting. If channel busy, station waits till next slot. If channel free, it transmits with probability p. With probability q=1-p, station waits for beginning of next time slot. This method reduces chances of collisions.

P-persistent (CSMA)

CSMA/CD( carrier sense multiple access with Collision Collisi on detection) Station senses channel before transmissions.  Stations detect collisions, and abort  transmissions.  CSMA/CD senses channel even after data transmission.   As soon as collision is detected, sending station releases jam signal.  Jam signal alerts other stations .   After jam, wait random time then start again. 

Token Passing  Used     

in networks where systems are arranged in ring

form.  A packet known as token is circulated circulated through whole ring. System Who has token , can use channel and transmits data. If system has data to send , he waits for token.  After data transmission system releases token and passes it to next system in ring. If system has token but no data to send, it passes token to next system in the ring.

Toke passing

token