Muhammet Ali KARABULUT, A.f.m. Shahen SHAH, Hacı İLHAN

IEEE 802.11 MAC Protokolünün VANET ağlardaki performans modellemesi ve analizi

IEEE 802.11 standardı, araçlar arası geçici ağlar (Vehicular Ad Hoc Networks, VANETs) için ortam erişim kontrolü (Medium Access Control, MAC) ve fiziksel (PHY) katmanının özelliklerini tanımlamaktadır. IEEE 802.11 standardında, dağıtılmış koordinasyon işlevi (Distributed Coordination Function, DCF) MAC katmanında kullanılan bir tekniktir. DCF, ortama erişmek için çarpışmadan kaçınma ile taşıyıcı duyarlı çoklu erişim (Carrier Sense Multiple Access with Collision Avoidance, CSMA/CA) tekniğini kullanmaktadır. Ayrıca, gönderim isteği (Request To Send, RTS)/ gönderime uygun (Clear To Send, CTS) mekanizması gizli araç düğümleri problemlerinin üstesinden gelmek için kullanılır. Bu makalede, VANET ağlarda önerilen CSMA/CA tabanlı protokolün performansını değerlendirmek için araç hızı, araç yoğunluğu, çekişme penceresi (Contention Window) ve yeniden iletim limiti (Maximum Retransmision Limit) gibi parametreleri dikkate alan Markov zincir modeline dayalı bir analitik model önerilmiştir. Performans ölçüleri ve parametreleri arasındaki ilişki incelenmiştir. Veri hızı, paket bırakma oranı (Packet Dropping Rate, PDR) ve gecikme ifadeleri elde edilmiştir. Ayrıca, benzetim sonuçları ile performans analizi doğrulanmıştır. CSMA/CA ve RTS/CTS mekanizmalarının karşılaştırılması sunulmuştur.

Anahtar Kelimeler:

CSMA/CA, DCF, IEEE 802.11, MAC, Markov zincir modeli, RTS/CTS, VANETs

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