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NS-2 NIST add-on IEEE 802.16 model (MAC+PHY) Miray Kas 28 Jan 2008
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Outline MAC Layer overview Basic Data about PHY Layer List of Available/Unavailable Features Available Features Convergence Sublayer MAC Common Part PHY Layer Comments on Unavailable Features
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Basic Data on IEEE 802.16 Standards (1)
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Basic Data on IEEE 802.16 Standards (2)
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Why NS-2 & WiMAX Ns-2: De facto Standard for Simulation of Networks Not so many WiMAX modules available: NIST Most reliable Only one that has mobility functionality Chang Gung University Taiwan State University of Campinas Brazil
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ns-2 vs. ns-3 environment ns-2 since 1996 ns-3 in progress, no initial alpha release ns-3 : Rewrite of core simulator 802.11 PHY cleanup ns-2 integration with ns-3 tools to parse output data (tcpdump and Ethereal) (Dynamic) Calendar Queue NIST WiMAX ns-2 port to ns-3
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Main Features of WiMAX Module PHY: PHY: OFDM MAC: MAC: PMP mode TDD Builds on ns2 legacy code Implemented in C++
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Convergence Sublayer Classifying higher layer PDUs into appropriate connections Delivering resulting CS PDUs to MAC SAP associated with the service flow. Suppressing / Rebuilding payload header information Used for Internet Protocol (IP) Point-to-Point Protocol (PPP) Ethernet (IEEE 802.3)
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Convergence Sublayer
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MAC Sublayer - Addressing & Connection Initialization of BS Initialization of BS Initial Ranging( ) Padding( ) Broadcast( ) Adaptive Antenna System( ) Initialization of SS Initialization of SS Initial Ranging( ) Padding( ) Broadcast( ) During network entry, following are set up: During network entry, following are set up: Basic CID( ) Exchange time urgent MAC mng. msg Primary CID( ) Longer delay tolerant MAC mng. msg Secondary CID( ) DHCP, TFTP, SNMP Data CIDs Current model supports only one
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MAC PDU Format 0 or more sub-headers 0 or more fragments Fixed Length Virtual PHY header : Used to carry physical information such as frequency, modulation…
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Mutually Exclusive HT = 1 HT = 0
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Construction & Transmission of MAC PDUs 3 steps: Receive, classify, put in the connection queue Scheduling Transmission Construction: Fragmentation & Packing CRC calculation
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Scheduler Service Flow classes are implemented UGS, nrtPS, rtPS, BE Current one does not use service classes Best Effort Scheduler with Round-Robin algorithm Allows custom implementations One default for SS and BS each Replace current one by a TCL command $mac set-scheduler $scheduler
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Scheduler
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QoS Structure supports Service Flows ertPS not implemented Scheduler does not use them No Admission Control during creation of flows Accept all requests from MSs List of flows in each SS Static Stored by ServiceFlowHandler
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Contention Resolution BS has full control on downlink Collisions possible in uplink Initial Ranging Request Bandwidth Request BS decides contention slots Binary exponential backoff window SS decides how many minislots to defer
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Contention Slot
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Contention Request
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TDD Uplink transmissions occur after downlink transmissions at each frame
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Frame Structure
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Network Entry & Initialization Steps: Scan downlink channel Obtain transmit parameters Initial ranging Registration Following can be configured: Timers to perform channel scanning Frequency of the DCD/UCD messages Parameters for initial ranging (backoff window size & # of slots per frame) Channel allocation
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Network Entry
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Scanning Related to Handover procedure When link quality weakens MS scans to find another BS Send MOB-SCN_REQ to current BS to request scanning interval
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Scanning
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Scanning Modes Without association MS attempts itself Association Level 0 – w.o. Coordination Target BS has no information about scanning MS Association Level 1 – with Coordination Serving BS negotiates with target BS Association Level 2 – Network Assisted Not implemented yet.
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Scanning WiMAXCtrlAgent Extends Agent Has three functions: Exchange DCD/UCD info btw. BSs Trigger sending of NBR-ADV msg. to MSs Synchronize BSs (serving & target)
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Ranging Definition A mechanism to allow an SS to maintain link quality by adjusting its 1-) Transmission power 2-) Modulation Uplink Ranging = Initial Ranging + Periodic Ranging Downlink Ranging = No periodic ranging
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Ranging (Current Situation in the Module) No algorithm to make use of ranging capabilities Periodic ranging not implemented CDMA request not implemented Only to add latency to the network entry Mechanism implemented not applicable to OFDMA
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Timers WimaxFrameTimer WimaxRxTimer WimaxDCDTimer WimaxUCDTimer WimaxRngIntTimer WimaxLostDLMAPTimer WimaxLostULMAPTimer DLTimer ULTimer WimaxT1Timer WimaxT2Timer WimaxT3Timer WimaxT6Timer WimaxT9Timer WimaxT12Timer WimaxT16Timer WimaxT17Timer WimaxT21Timer WimaxMobNbrAdvTimer WimaxScanIntervalTimer WimaxRdvTimer WimaxT44Timer MobilityExtensions
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PHY Layer Only OFDM is implemented OFDMA WirelessMAN-SC WirelessMAN-SCa WirelessHUMAN Missing Yet!!!
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PHY Layer Configurable parameters (TCL): Transmission power Cyclic Prefix Frequency Frequency bandwidth Modulation Computed values: Sampling frequency OFDM symbol time duration Xmission time of a packet acc. to its size and modulation Max packet size for a given modulation and # of available OFDM symbols
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PHY Layer Default: Default: OFDM_BPSK_1_2 modulation OFDM_IDLE Supported Modulations: Supported Modulations: States: States: OFDM_IDLE OFDM_SEND OFDM_RECV OFDM_RX2TX OFDM_TX2RX
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PHY Layer
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Available/Unavailable Features
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Open to Improvement WirelessMAN-OFDMA physical layer ARQ (Automatic Repeat Request) Error Correction Service Flow and QoS scheduling Call Admission Control Periodic ranging and power adjustments
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Will be Still Missing… ATM Convergence Sublayer Mesh Mode Physical Layers WirelessMAN-SC WirelessMAN-SCa WirelessHUMAN Frequency Division Duplexing (FDD) Authentication
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