Distributed systems Module 1 -Basic networking Teaching unit 1 – LAN standards Ernesto Damiani University of Bozen-Bolzano Lesson 2 – LAN Medium Access.

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Presentation transcript:

Distributed systems Module 1 -Basic networking Teaching unit 1 – LAN standards Ernesto Damiani University of Bozen-Bolzano Lesson 2 – LAN Medium Access Control

Contention MAC protocols (reminder) CSMA/CD (Carrier Sensing Multiple Access/Collision Detection) It listens to the channel in advance to avoid collisions, but when they happen it resends the frame (used by Ethernet LAN)  1-persistent: the station resends with probability 1 when the channel is idle  p-persistent: when the channel becomes idle, it transmits with probability p  non-persistent: when the channel idle waits a random time before transmitting

Unifying view and adaptive solution Mixed strategies are adaptive Division of stations into groups; every group may contend for a different slot –Limiting case: only one member in group (collision free protocol) –Limiting case: everybody for one slot (slotted ALOHA) –Intermediate case: they choose the group size depending on the load (adaptive protocol)  For example: at first all are entered, then if there is a collision they are reduced in half and half and so on.

Summary table of MAC solutions ContextRequirementsA solution Low loadMinimized latency Contention protocols (e.g. ALOHA) Variable loadMaximum efficiency Collision free protocols Variable loadDifferent problems Hybrid protocols (limited contention protocols)

Wireless networks

Wireless LAN (1) In a wireless LAN not all the stations are within the range of one another –On one hand, this is an advantage because multiple transmissions can be executed at one time –On the other hand, some problems may arise

No difference

Wireless LAN (2) Hidden station problem –When A transmits to B, C senses the medium as free and starts the transmission Exposed station problem –When B transmits to A, C senses the medium as busy and does not transmit to D

MACA (Multiple Access with Collision Avoidance) RTS (Request To Send) –Any station hearing the RTS is close to A and must keep silent until a CTS is sent back to A CTS (Clear to Send) –Any station hearing the CTS is close to B and must keep silent during the upcoming data transmission

MACA for Wireless ACK data Carrier sense to avoid RTS collision Info on congestion Back-off algorithm for each data stream FINE