6610-l10 - Wireless Scheduling 1 MAC for WWANs WWANs such...

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1 Wireless Scheduling
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MAC for WWANs WWANs, such as GSM, LTE and Wimax, use a channel partitioned approach for Medium Access Control: TDMA, FDMA, CDMA. TDMA is a centralized MAC protocol where time is divided into a number of slots and slots are assigned to users. FDMA is a centralized MAC where each user is assigned a different frequency. CDMA employs spread-spectrum technology and a special coding scheme to allow multiple users operate on the same physical channel.
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TDMA: Basics TDMA is a centralized protocol and a base station usually acts as the controlling entity. The base station and all clients are synchronized to a common clock. The TDMA divides each channel into a number of frames made of timeslots. Slots are assigned to clients using an on-demand schedule. Separate slots are assigned for both uplinks and downlinks. Uplink Downlink 1 1 2 2 3 3 4 4 Time slots Frames
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Example: GPRS - Data Transmission PDP is initiated by an MS, when a browser is opened. Different messages are sent on different logical channels. The following are the various messages in a PDP session: The MS sends a request to activate the PDP session. The BS responds with an authentication request. The MS authenticates itself. The BS activates the PDP session and allocates resources for the MS. The MS and BS exchange DATA packets.
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QoS in GPRS Quality of service is provided in GPRS by assigning different number of timeslots for an MS. If a particular MS wants a higher utilization, then the MS is given more slots at the expense of other MSs (that can allow for a lower utilization).
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Review OSI MAC IP TCP Wireless MAC Wireless scheduling 6
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7 Wireless Fair Queuing Wireless channel capacities are scarce Fair sharing of bandwidth becomes critical Both short-term and long-term fairness important
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8 Wireless FQ & Wireless Environment Location dependent and bursty errors For the same wireless channel, a mobile station might experience a clean channel while another might experience high error rates. Why? In wireline fair queuing, the channel is either usable by all flows or unusable by any of the flows …
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9 Wireless Channel Model Base station performs arbitration Schedules both uplink and downlink traffic Neighboring cells use different channels Every mobile host has access to base- station
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10 Wireless Channel Characteristics
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