RANDOM ACCESS TECHNIQUES ALOHA CarrierSense Techniques Reservation Protocols

RANDOM ACCESS TECHNIQUES • ALOHA • Carrier-Sense Techniques • Reservation Protocols • PRMA 7 C 29822. 038 -Cimini-9/97

ALOHA • Data is packetized. • Packets occupy a given time interval (TD technique) • Pure ALOHA – send packet whenever data is available – a collision occurs for any partial overlap of packets (nonorthogonal slots) – Packets received in error are retransmitted after random delay interval (avoids subsequent collisions). • Slotted ALOHA – same as ALOHA but with packet slotting – 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.

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

S (Throughput per Packet Time) Efficiency Plot . 40 Slotted Aloha . 30. 20 Pure Aloha . 10 0 0. 5 1. 0 1. 5 2. 0 3. 0 G (Attempts per Packet TIme) Note that there are two equilibrium points for both slotted and unslotted ALOHA

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

CARRIER-SENSE TECHNIQUES • Channel is sensed before transmission to determine if it is occupied. • More efficient than ALOHA Þ fewer retransmissions • Carrier sensing is often combined with collision detection in wired networks (e. g. , Ethernet). Þ not possible in a radio environment Busy Tone Wired Network Wireless Network • Collision avoidance is used in current wireless LANs. (Wave. LAN, IEEE 802. 11, Spectral Etiquette) 8 C 32810. 40 -Cimini-7/98

RESERVATION PROTOCOLS • 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 7 C 29822. 041 -Cimini-9/97

EXAMPLES • ARDIS – slotted CSMA • RAM Mobile Data – slotted CSMA • CDPD – DSMA/CD - Digital Sense Multiple Access – collisions detected at receiver and transmitted back • Wave. LAN – CSMA/CA 8 C 32810. 126 -Cimini-7/98

IMPLICATIONS FOR HIGH SPEED WIRELESS DATA • 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. 7 C 29822. 042 -Cimini-9/97