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WiMAX 簡介

WiMAX 簡介. Ming-Tsung Huang Fu Jen Catholic University Computer Science. Outline.

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WiMAX 簡介

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  1. WiMAX簡介 Ming-Tsung Huang Fu Jen Catholic University Computer Science

  2. Outline • 􀂃 I. Introduction • 􀂃 II. 802.16 Physical Layer • 􀂃 III. 802.16 MAC Layer • 􀂃 IV. QoS Support in IEEE 802.16 • 􀂃

  3. I. Introduction

  4. Many new services are based on multimedia applications: • voice over IP (VoIP) • video conferencing • video on demand (VoD) • massive online gaming • peer-to-peer.

  5. Unlike traditional TCP/IP services, multimedia applications usually require strict network guarantees • reserved bandwidth • bounded delays

  6. Broadband wireless Access • The International Telecommunication Union(ITU), which reported that Broadband WirelessAccess (BWA), although still in the early stage ofits growth, is one of the most promising solutionsfor broadband access. • Standards for BWA are being developed withinIEEE 802.16.

  7. To promote 802.16-compliant technologies, theWorldwide Interoperability for Microwave Access(WiMAX) Forum was founded, with more than300 member companies.

  8. It is envisaged that the first 802.16-compliantproducts to be deployed will very likely be aimedat providing last-mile Internet access forresidential users mainly high-speed Internetaccess and small and medium-sized enterprises(SMEs).

  9. For the SME market, 802.16 will provide acost-effective alternative to existingsolutions based on very expensive leasedlineservices.

  10. 802.16 family

  11. Deployment • 802.16 systems can be deployed effectively where: • Users are dissatisfied with the current packet and/ornetwork interface. • Network operators need to reach customers costeffectively. • New service offerings are available for 2.5G/3Gaugmentation.

  12. Salient Advantages • Some of the more salient advantages of a wirelesssystem for broadband access based on the 802.16standard are as follows: • Bandwidth on demand (BOD) • Higher throughput • Scalable system capacity • Coverage • Quality (CBR or UBR) • Cost (investment risk and end-user fee)

  13. II. 802.16 Physical Layer

  14. Physical Layer • The physical layer of the IEEE 802.16 airinterface operates: • 10–66 GHz band for IEEE 802.16 (LOS) • 2–11 GHz band for IEEE 802.16a (NLOS) • In the 10–66 GHz, the air interface used for thisband is Wireless-SC (single carrier).

  15. In the 2–11 GHz band three different airinterfaces can be used as follows: • WirelessMAN-SCa for single-carrier modulation • WirelessMAN-OFDM. The MAC scheme for thesubscriber stations is TDMA. • WirelessMAN-OFDMA for OFDM-based transmissionusing2048 subcarriers.

  16. OFDMA

  17. III. 802.16 MAC Layer

  18. Medium Access Control Layer • The 802.16 standard specifies two modes forsharing the wireless medium: point-to-multipoint(PMP) and mesh (optional).

  19. IEEE 802.16/WiMAX uses a connection-orientedMAC protocol that provides a mechanism for thesubscriber stations to request bandwidth fromthe BS. • A 16-bit connection identifier (CID) is used primarily to identify each connection to the BS.

  20. 802.16 MAC layer • Sublayer • Service Specific Convergence Sublayer (CS) • MAC-Common Part Sublayer (CPS) • Privacy Sublayer (PS)

  21. On the downlink, the BS broadcasts data to all subscriber stations in its coverage area. Each subscriber station processes only the MAC protocol data units (PDUs) containing its own CID and discards the other PDUs.

  22. For TDD-based access, a MAC frame (i.e., transmission period) is divided into uplink and downlink subframes. • The lengths of these subframes are determined dynamically by the BS and broadcast to the subscriber stations through downlink and uplink MAP messages (DL-MAP and UL-MAP) at the beginning of each frame. • The MAC protocol in the standard supports dynamic bandwidth allocation.

  23. subframe

  24. Subframe(2) Ex. TDD subframe

  25. PMP mode: • The BS serves a setof SSs within thesame antennasector in abroadcast manner.

  26. Mesh Operation Mode: • In addition to the single-hop PMP operation scenario,the WiMAX standard (e.g., IEEE 802.16a) alsodefines the multihop mesh networking scenario among the subscriber stations (i.e., client meshing). • Meshing among the BSs (i.e., infrastructure meshing)has not been standardized yet.

  27. The mesh mode:traffic can berouted through other SSs andcan occur directly among SSs.

  28. IV. QoS Support in IEEE 802.16

  29. QoS in wireless networks is usually managed at the medium access control (MAC) layer.

  30. In PMP mode, uplink (from SS to BS) and downlink (from BS to SS) data transmissionsoccur in separate time frames. • In the downlink subframe, the BS transmits a burst ofMAC protocol data units (PDUs). • Since the transmission is broadcast, all SSs listen tothe data transmitted by the BS. • In the uplink subframe, any SS transmits a burst ofMAC PDUs to the BS in a time-division multipleaccess (TDMA) manner.

  31. Based on measurements at the physical layer, any SS adapts over time the interval usage code (IUC) in use. • That is, modulation, rate, and forward error correction (FEC) scheme, for both downlink (downlink IUC, DIUC) and uplink (uplink IUC, UIUC) transmissions.

  32. Duplex • Downlink and uplink subframes are duplexed using one of the following techniques: • Frequency-division duplex (FDD) • Time-division duplex (TDD)

  33. Frame structure with FDD and TDD

  34. The MAC protocol is connection-oriented: alldata communications, for both transport andcontrol, are in the context of a unidirectionalconnection. • At the start of each frame, the BS schedules theuplink and downlink grants in order to meet theQoS requirements.

  35. Each SS learns the boundaries of itsallocation within the current uplinksubframe by decoding the UL-MAPmessage.

  36. The DL-MAP message contains the timetable ofthe downlink grants in the forthcoming downlinksubframe. Downlink grants directed to SSs withthe same DIUC are advertised by the DL-MAPas a single burst. • Both maps are transmitted by the BS at thebeginning of each downlink subframe for bothFDD and TDD modes.

  37. QoS functions within the BS and SSs

  38. Since the BS controls the access to the mediumin the uplink direction, bandwidth is granted toSSs on demand. • Bandwidth-request mechanisms : • Unsolicited granting • Unicast poll • Broadcast polls

  39. Unsolicited granting: • A fixed amount of bandwidth on a periodicbasis is requested during the setup phase ofan uplink connection. After that phase,bandwidth is never explicitly requested.

  40. Unicast poll • A unicast poll consists of allocating to a polled uplinkconnection the bandwidth needed to transmit abandwidth request. • If the polled connection has no data awaitingtransmission (backlog, for short), or if it has alreadyrequested bandwidth for all of its backlog, it will notreply to the unicast poll, which is thus wasted.

  41. Broadcast polls : • A collision occurs whenever two or moreuplink connections send a bandwidth requestby responding to the same poll, in which casea binary exponential backoff algorithm isemployed.

  42. Bandwidth requests can be piggybackedon a PDU. • Bandwidth requests are used on the BSfor estimating the residual backlog ofuplink connections.

  43. Although bandwidth requests are per connection,the BS nevertheless grants uplink capacity toeach SS as a whole. • When an SS receives an uplink grant, it cannotdeduce from the grant which of connections itwas intended for by the BS.

  44. The 802.16 MAC specifies four differentscheduling services in order to meet the QoSrequirements of multimedia applications: UGS,rtPS, nrtPS, and BE.

  45. Unsolicited Grant Service (UGS) • UGS is designed to support real-timeapplications (with strict delay requirements) thatgenerate fixed-size data packets aperiodicintervals • Ex: T1/E1 and VoIP without silence suppression.

  46. Unsolicited Grant Service (UGS) • The guaranteed service is defined so as toclosely follow the packet arrival pattern, with thebase period equal to the unsolicited grantinterval and the offset upper bounded by thetolerated jitter. • The grant size is computed by the BS based onthe minimum reserved traffic rate.

  47. real-time Polling Service (rtPS) • rtPS is designed to support real-timeapplications (with less stringent delayrequirements) that generate variable-size datapackets at periodic intervals • Ex: MPEG video and VoIP with silencesuppression.

  48. real-time Polling Service (rtPS) • The key QoS parameters for rtPS connectionsare the minimum reserved traffic rate and themaximum latency. • The BS periodically grants unicast polls to rtPSconnections.

  49. non-real-time Polling Service (nrtPS) and Best Effort (BE) • nrtPS and BE are designed for applications thatdo not have any specific delay requirement. • The main difference between the two is thatnrtPS connections are reserved a minimumamount of bandwidth, which can boostperformance of bandwidth-intensive applications,Ex: FTP.

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