EE450 notes p2.docx - MEDIUM ACCESS CONTROL o Broadcast...

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MEDIUM ACCESS CONTROL o Broadcast channels o Point to point does not need mac address but broadcast channels do o Two or more nodes attempt to transmit at the same time: collision o MAC medium access control Ethernet Wi-fi March 2, 2017 o When a medium is shared, we need Medium Access Control Protocol o Multipoint link (bus network) o Wireless sharing the air – will also have collisions o Rules are implemented in the Network interface card o Classifications of MAC protocols Channel Partitioning (Channelization Protocols) Controlled-access protocols Random-access protocols o Channel Partitioning Protocols FDMA Users all over the place but they can transmit at the same time because they use different frequencies Every time the mobile makes a call he will be assigned 2 frequencies – one from mobile unit to base station, one from base station to mobile unit (uplink and downlink) Divided band into uplink and downlink TDMA Sources all over the place Rigid, pre-assigned time slot Users generating traffic simultaneously Cellphone is sending message interleaved in time CDMA Code division multiple access Every cell phone has a certain code The codes are orthogonal Rigid o Controlled-Access Protocols Not as rigid Reservation scheme I will reserve the channel for 10 seconds Broadcast that information so that all other nodes can hear him Polling Central control will poll you o Do you have something to send to me?
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o No o Ask another node You cannot send data unless you are polled Token passing Analogous to a special coin that allows you to use the restroom After you finish you give the token back and the central control passes it on Taking turns o Random-access Protocols Least structured protocol, least strict ALOHA Initial base of the internet (ALOHA is the precursor to the internet) Many nodes distributed in the west coast Sharing the air I am a node, I have a frame to send, I send it and start praying LOL please let me be the only one who is sending You know that a collision has occurred if you do not get a response within a certain time Choose a random certain of time to resend 18% throughput partial collision also means that the frame is damaged total vulnerable time is 2Tf. Slotted ALOHA If a node has a frame to send, it has to send it at the beginning of a timeslot Total vulnerable time is Tf. More constraints -> throughput goes up but complexity also goes up CSMA/CD Carrier sense multiple access with collision detection Ethernet o Proprietary solution Ethernet was based on buss topology Cable was maximum 500 meters long CSMA/CD Mac layer tells physical layer: I have a frame to send, I want you to sense the medium, tell me if there is activity in the medium or the medium is quiet.
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