ALOHA based wireless random access technique

1. ALOHA based wireless random access technique ECE431/Wireless communication

2. Multiple Access Protocols Many algorithms exist for allocation of Multiple Access Channel. To begin with let us investigate representative algorithms: Pure ALOHA Slotted ALOHA Reservation ALOHA ECE431/Wireless communication

3. Pure ALOHA In 1970s, Norman Abramson and his team at University of Hawaii devised the algorithm The basic idea used in the algorithm is applicable to any system in which uncoordinated users are competing for the use of single shared channel The algorithm is referred to as Random Multiple Access Protocol or Pure ALOHA ECE431/Wireless communication

4. Random Multiple Access Protocol Users transmit whenever they have data to be sent There will be collisions and the colliding frames are destroyed However, due to the broadcasting nature of the channel, a sender can always find out whether or not its frame was destroyed by listening to the channel, the same way other users do ECE431/Wireless communication

5. Random Multiple Access Protocol With a LAN, the feedback is immediate However, with a satellite, there is a delay of 270 ms, before the sender knows if the transmission was successful If the frame was destroyed, the sender just waits a random amount of time and sends it again This kind of system where users share a common channel resource is referred to as Contention System ECE431/Wireless communication

6. Pure ALOHA System ECE431/Wireless communication

7. Pure ALOHA System Frames have the same length Whenever two frames try to occupy the channel at the same time, there will be collision and both fames are garbled Question: Can this system work? If yes, What is the efficiency or throughput of the system? ECE431/Wireless communication

8. Formal Description of the Algorithm Transmission Mode: Users transmit at any time they desire, encoding their transmission with an error detection code Listening Mode: After a message transmission, the user listens for the acknowledgement (ACK) from the receiver. Transmissions from different users will sometimes overlap in time, causing errors in the data in each of the colliding partners. The user then receives a negative acknowledgement (NAK) ECE431/Wireless communication

9. Formal Description of the Algorithm Retransmission Mode: When NAK is received, the messages are simply retransmitted. Colliding users retransmit after a random amount of delay Timeout Mode: If, after a transmission, the user does not receive either an ACK or NAK within a specified time, the user retransmits the message ECE431/Wireless communication

10. Slotted ALOHA By introducing a small amount of coordination among users, the performance of the pure ALOHA can be improved Such a scheme is referred to as S-ALOHA or Slotted-ALOHA system As with pure ALOHA system, in S-ALOHA the packet size is constant Packets are required to be sent in the slot time between synchronization pulses and can be started only at the beginning of the time slot. ECE431/Wireless communication

11. Slotted ALOHA This simple change reduces the rate of collisions by half, since only packets transmitted in the same slot can interfere with one another. Normalized throughput of S-ALOHA system is thus given by: ECE431/Wireless communication

12. Slotted ALOHA Operation ECE431/Wireless communication

13. Slotted ALOHA Throughput The maximum value of throughput = 1/e=0.37 This maximum occurs at G =1.0 In S-ALOHA 37% of the Channel Resource can be utilized There exists tradeoff between Channel utilization and Coordination ECE431/Wireless communication

14. Reservation ALOHA (R-ALOHA) Significant improvement in performance can be achieved over ALOHA system by using Reservations The R-ALOHA system has two basic modes: Unreserved Mode (Quiescent State) 1. A time frame is established and divided into small reservation subslots 2. Users use these subslots to reserve message slots 3. After requesting a reservation, the user listens for an acknowledgement and a slot assignment ECE431/Wireless communication

15. Reservation ALOHA (R-ALOHA) Reserved Mode: 1. The time frame is divided into M+1 slots whenever a reservation is made 2. The first M slots are used for message transmission 3. The last is subdivided into subslots to be used for reservations 4. Users send message packets only in their assigned portions of the M slots ECE431/Wireless communication

16. 5 Slots, 6 Subslots R-ALOHA System ECE431/Wireless communication

17. R-ALOHA System In the quiescent state, with no reservations, time is partitioned into short subslots for making reservations Once reservation is made, the system is configured so that 5 message slots followed by 6 reservation subslots becomes the timing format In the Figure the station seeks to reserve three message slots The reservation acknowledgement advises the station where to locate its data packets. ECE431/Wireless communication

18. R-ALOHA System Since the control is distributed, all stations receive the downlink transmission and are aware of the reservation format The acknowledgement need not disclose any more than the location of the first slot to use. When there are no reservations taking place, the system reverts back to its quiescent mode ECE431/Wireless communication