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Unformatted text preview: MAC and Interaction with IP Chapter Overview Common Multiple Access Control (MAC) protocols Common Interactions between IP network layer and MAC layer Interactions Hubs, Bridges, Ethernet Switches and Routers Hubs, Multiple Access Links and Protocols Three types of links: (a) Point-to-point (single wire) (b) Broadcast (shared wire or medium; eg, Ethernet, wireless, etc.) (c) Switched (eg, switched Ethernet, ATM links) We start with Broadcast links. Main challenge: Broadcast links. Multiple Access Protocol Multiple Multiple Access Control (MAC) Protocols MAC protocol: coordinates transmissions from different stations MAC in order to minimize/avoid collisions: (a) Channel Partitioning MAC protocols (a) Channel MAC (b) Random Access MAC protocols (b) Random MAC (c) “Taking turns” MAC protocols (c) Goal: efficient, fair, simple, decentralized Goal: ef Channel Partitioning MAC protocols TDM (Time Division Multiplexing): channel divided into N time TDM slots, one per user; inefficient with low duty cycle users and at light load. FDM (Frequency Division Multiplexing): frequency subdivided. FDM Other examples include Code Division Multiple Access (CDMA) Other Random Access protocols A node transmits at random (ie, no prior coordination among node at nodes) at full channel data rate R. full channel If two or more nodes “collide”, they retransmit at random times If The random access MAC protocol specifies how to detect The random protocol collisions and how to recover from them (via delayed retransmissions, for example) Examples of random access MAC protocols: Examples (a) SLOTTED ALOHA (b) ALOHA (c) CSMA and CSMA/CD Slotted Aloha Time is divided into equal size slots (= full packet size) Time a newly arriving station transmits at the beginning of the next slot newly if collision occurs (assume channel feedback, e.g. the receiver if informs the source of a collision), the source retransmits the packet at each slot with probability P, until successful. Success (S), Collision (C), Empty (E) slots Success S-ALOHA is channel utilization efficient; it is fully decentralized. S-ALOHA What s the disadvantage of having a constant retry probability P ? the Pure (unslotted) ALOHA Slotted ALOHA requires slot synchronization Slotted A simpler version, pure ALOHA, does not require slots simpler A node transmits without awaiting for the beginning of a slot node Collision probability increases (packet can collide with other Collision packets which are transmitted within a window twice as large as in S-Aloha) Throughput is redu...
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