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RANDOM ACCESS TECHNIQUES

• ALOHA • Carrier-Sense Techniques • Reservation Protocols • PRMA. RANDOM ACCESS TECHNIQUES. 7C29822.038-Cimini-9/97. packets (nonorthogonal slots). – Packets received in error are retransmitted after random delay interval (avoids subsequent collisions).

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RANDOM ACCESS TECHNIQUES

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

  2. packets (nonorthogonal slots) – Packets received in error are retransmitted after random delay interval (avoids subsequent collisions). – 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. 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 • Slotted ALOHA – same as ALOHA but with packet slotting

  3. 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, 2nd Ed. Bertsekas and Gallager

  4. .40 Slotted Aloha S (Throughput per Packet Time) .30 .20 Pure Aloha .10 0 0.5 1.0 1.5 2.0 3.0 G (Attempts per Packet TIme) Efficiency Plot Note that there are two equilibrium points for both slotted and unslotted ALOHA

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

  6. Busy Tone 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 Wired Network Wireless Network • Collision avoidance is used in current wireless LANs. (WaveLAN, IEEE802.11, Spectral Etiquette) 8C32810.40-Cimini-7/98

  7. 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 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. 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. 7C29822.042-Cimini-9/97

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