1. 7C29822.038-Cimini-9/97 RANDOM ACCESS TECHNIQUES ALOHA Carrier-Sense Techniques Reservation Protocols PRMA

2. ALOHA Data is packetized. Pure ALOHA – send packet whenever data is available – a collision occurs for any partial overlap of Slotted ALOHA – packets sent during predefined timeslots – A collision occurs when packets overlap, but there is no partial overlap of packets – Packets received in error are retransmitted after random delay interval. packets (nonorthogonal slots) – Packets received in error are retransmitted after random delay interval (avoids subsequent collisions). Packets occupy a given time interval (TD technique) – same as ALOHA but with packet slotting

3. Efficiency* Assumptions Slotted ALOHA – Retransmission required for any packet overlap – Normalize slot time to 1 Pure ALOHA *Data Networks, 2 nd Ed. Bertsekas and Gallager – Infinite number of nodes – Poisson packet arrivals at rate. – For randomized retransmissions, the sum of new and backlogged packet arrivals is Poisson with rate G> :

4. Efficiency Plot.40.30.20.10 00.51.01.52.0 3.0 G (Attempts per Packet TIme) S (Throughput per Packet Time) Slotted Aloha Pure Aloha Note that there are two equilibrium points for both slotted and unslotted ALOHA

5. Comments – inefficient for heavily loaded systems – capture effect improves efficiency – combining SS with ALOHA reduces collisions – Two equili

6. CARRIER-SENSE TECHNIQUES Channel is sensed before transmission to determine if it is occupied. More efficient than ALOHA  fewer retransmissions Carrier sensing is often combined with collision detection in wired networks (e.g., Ethernet).  not possible in a radio environment Collision avoidance is used in current wireless LANs. (WaveLAN, IEEE802.11, Spectral Etiquette) 8C32810.40-Cimini-7/98 Wired Network Busy Tone Wireless Network

7. Demand–Based Assignment – a common reservation channel is used to assign bandwidth on demand – reservation channel requires extra bandwidth – very efficient if overhead traffic is a small percentage of the message traffic Packet Reservation Multiple Access (PRMA) – similar to reservation ALOHA – uses a slotted channel structure – all unreserved slots are open for contention – a successful transmission in an unreserved slot effectively reserves that slot for future transmissions RESERVATION PROTOCOLS 7C29822.041-Cimini-9/97

8. EXAMPLES ARDIS – slotted CSMA RAM Mobile Data – slotted CSMA CDPD – DSMA/CD - Digital Sense Multiple Access – collisions detected at receiver and transmitted back WaveLAN – CSMA/CA 8C32810.126-Cimini-7/98

9. Retransmissions are power and spectrally inefficient. ALOHA cannot satisfy high-speed data throughput requirements. Reservation protocols are also ineffective for short messaging. Delay constraints impose throughput limitations. IMPLICATIONS FOR HIGH SPEED WIRELESS DATA 7C29822.042-Cimini-9/97