Topic 5: Medium Access Control (Data Link Layer) • Data link layer deals with medium access and packet delivery over a single link (same physical layer) • This chapter focuses on protocols that coordinate the access to the channel media in the broadcast network • Reference: Alberto Leon-Garcia and Indra Widjaja, Communication Networks - Fundamental Concepts and Key Architectures, 2nd Ed., pp. 370-386, 2004. (Reserved in the DC library. Call No. TK5105. L46 2004.) 5.1 Multiple Access Communications • Shared media basis for broadcast networks – Inexpensive: radio over air; copper or coaxial cable – M users communicate by broadcasting into medium • Key issue: How to share the medium? 3 2 4 1 Shared multiple access medium M 5 … • Sharing a transmission medium • Static sharing (channelization schemes) • Dynamic sharing (MAC schemes) Medium sharing techniques Static channelization Dynamic medium access control scheduling Random access • Random access methods constitute the first major class of MAC procedures 3 MAC classification Contention vs Contention-free • Contention-free MAC – Channel partitioning • TDMA, FDMA, CDMA, W-CDMA, OFDMA, etc. – Turn-taking protocols • Polling (IEEE 802.11 PCF), Token based (WTRP), etc. Contention free period Contention period SIFS PIFS SIFS Bea CF- Data + CF-poll CF- Bea- con poll +CF-ACK end con AP SIFS SIFS Data + CF- CF-ACK ACK station null Token passing polling • Contention-based MAC – Radom access • Aloha, S-Aloha, Non-/1-/p-persistent CSMA, CSMA/CA, etc. – Reservation access • Reservation-Aloha, Reservation-TDMA, DRP, etc. 1 0 1 1 0 0 1340 1 0 0 1 0 2 reservation Data transmission period period Other classifications • Centralized vs distributed • Asynchronous vs Synchronous: – Global synchronization required • Single-channel vs multiple-channel • QoS-aware • Power-aware • … … MAC design principles • High throughput • QoS provisioning • Reliability • Robustness • Fairness • Power consumption • Cost .