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  1. 1. 13PIT101 Multimedia Communication & Networks UNIT - V Dr.A.Kathirvel Professor & Head/IT - VCEW
  2. 2. Unit - V End to End QoS provisioning in Wireless Multimedia Networks – Adaptive Framework – MAC layer QoS enhancements in Wireless Networks – A Hybrid MAC protocol for 10 Multimedia Traffic – Call Admission Control in Wireless Multimedia Networks – A Global QoS Management for Wireless Networks
  3. 3. The Two Successful Domains • Wireless networks (Cellular) – – – – Supports voice Total coverage in many countries Decreasing cost The boon – user mobility • Wireless extension to the Internet (Wi-Fi) – – – – Information content Supports multimedia services Global penetration – millions of nodes Decreasing cost • IEEE 802.16 based WiMax • LTE (Long Term Evolution)
  4. 4. General Problems in Wireless Networks • Resource scarcity – Limited bandwidth • Unreliable wireless link – Error prone channels (BER 10-4 to 10-3) • Varying channel conditions – Channel models fluctuates In spite of all these problems, voice services are well supported. Can it support multimedia services?
  5. 5. Characteristics of Multimedia Services A picture is worth thousand words Combination of various medium – text, audio/video, graphics – Audio/video conferencing, shared whiteboard, surfing, email, etc. • Varied requirements – Low bit error rate – High bandwidth – Low delay • Synchronization of multiple data types – Proper scheduling • Different coding schemes for different types – Source coding
  6. 6. Data on Wireless Networks! What are the Problems? • True characterization of data traffic is yet unknown – Traffic modeling needs to be done • Data services cannot tolerate bit errors – Corrupt packets need to be recovered • Unpredictable nature of wireless medium – QoS provisioning becomes difficult • Bottleneck due to the bandwidth limitation – Proper buffering / filtering required • No differentiated service plans for customers – Class based services required
  7. 7. What is QoS?  Specified by <bandwidth, delay, reliability>  Ability of a network element (e.g. an application, host or router) to have some level of assurance that its traffic and service requirements can be satisfied  Predictable service for the traffic from the network e.g., CPU time, bandwidth, buffer space  Acceptable end-to-end delay and minimum delay jitter  What is QoE (Quality of Experience)?  Human subjectivity associated with quality  How happy is a user with respect to the service he gets
  8. 8. End-to-End QoS  Requires cooperation of all network layers from top-to-bottom, as well as every network element  Knowledge of application at end points decides QoS functions implemented at every layer of the network protocol stack  Type of Services - Best-effort: the Internet (lack of QoS) - Differentiated service (soft QoS) : partial to some traffic but most effective - Guaranteed service (hard QoS) : absolute reservation of resources (RSVP), more expensive
  9. 9. Wireless QoS Challenges  A limited spectral bandwidth to be shared, causes interference  Communication links are time varying, frequency selective channels  User mobility in wireless networks makes QoS provisioning complex because routes from source to destination cells are different, thus causing varying packet delays and delay jitters  Error rate of wireless channel is higher due to mobility, interference from other media, multi-path fading. So mobile hosts may experience different channel rates in the same or different cells  Different applications have different requirements for bandwidth, delay, jitter (e.g., 9.6Kbps for voice and 76.8Kbps for packetized video)
  10. 10. Wireless QoS: Desirable Features  Adapt to dynamically changing network and traffic conditions  Good performance for large networks and large number of connections (like the Internet)  Higher data rate  Modest buffer requirement  Higher capacity utilization  Low overhead in header bits/packet  Low processing overhead/packet within network and end system
  11. 11. Bandwidth Requirement for Multimedia Traffic Application bandwidth requirements on log-scale axis in bits per second (bps) Vertical dashed lines show the bandwidth capability of a few network technologies
  12. 12. Multi-rate Traffic Scenario Base Station C channels Mobile Users  Real-time traffic (voice, video)  Non real-time traffic (TCP/IP packets)
  13. 13. Evolution of Wireless Data Networks  2G wireless systems ( voice-centric, data loss unimportant) - IS-95 CDMA, TDMA, GSM  2.5G systems (voice and low data rate) - CDPD, GPRS, HSCSD, IS-99 CDMA, IS-136+ - Date rates: CDPD (19.2Kbps), HSCSD (76.8Kbps), GPRS (114Kbps)  3G proposed standards (data-centric, high data rate) - UMTS, EDGE, W-CDMA, cdma2000, UWC 136, IMT-2000 - Data rates: EDGE (384Kbps), cdma2000 (2Mbps), W-CDMA (10Mbps)
  14. 14. Last Hop Communication ISDN/PSTN/Internet WIRE-LINE NETWORK Cell Base Station (BS) Mobile unit Wireless Links Wired Links Mobile Switching Center (MSC)
  15. 15. Cellular Framework HLR BSC MSC/VLR MSC/VLRBSC BTS Cellular Network Mobile Terminal Air Link Local Switch Terms to remember MSC: Mobile Switching Center VLR: Visiting Location Register HLR: Home Location Register BSC: Base Station Controller BTS: Base Transmitter Station Mobile Terminal Air Link BTS PSTN Network
  16. 16. Cell: geometric representation of areas. Geographic area is divided into cells, each serviced by an antenna called base station (BS) Mobile Switching Center (MSC) controls several BSs and serves as gateway to the backbone network (PSTN, ISDN, Internet) WHY CHANNEL REUSE?  Limited number of frequency spectrum allocated by FCC and remarkable growth of mobile (wireless) communication users  Frequency band allocated by FCC to the mobile telephone system is 824-849 MHz for transmission from mobiles (uplink) and 869-894 MHz for transmission from base stations (downlink)  With a channel spacing of 30 KHz, this frequency band can accommodate 832 duplex channels  Frequency Reuse: use same carrier frequency or channel at different areas (cells) avoiding co-channel interference  Number of simultaneous calls (capacity) greatly exceeds the total number of frequencies (channels) allocated
  17. 17. Hand-off Problem  Hand-off is the process of switching from one frequency channel to another by the user in midst of a communication  Normally induced by the quality of the ongoing communication channel parameters: Received Signal Strength (RSS), Signal-to-Noise Ratio (SNR) and Bit Error Rate (BER)  RSS attenuates due to the distance from BS, slow fading (shadow or lognormal fading), and fast fading (Rayleigh fading)  Hand-offs are triggered either by the BS or the mobile station itself BS-1 BS-2
  18. 18. Handoff Types Intra-Cell Inter-Cell Soft Handoff Hard Handoff
  19. 19. Hand-off: Who Triggers?  The quality of the RSS from the mobile station is monitored by the BS. When the RSS is below a certain threshold. BS instructs the mobile station to collect signal strength measurements from neighboring BSs  Case 1: mobile station sends the collected information to the BS. BS conveys the signal information to its parent MSC (mobile switching center) which selects the most suitable next BS for the mobile station Both the selected BS and the mobile station are informed when new BS assigns an unoccupied channel to the mobile station  Case 2: mobile station itself selects the most suitable BS. The mobile station informs the current BS, who conveys information about the next BS to its MSC The selected BS is informed by the MSC which assigns a new channel
  20. 20. Hand-off Policies  BS handles hand-off requests in the same manner as originating calls - Disadvantage: Ignores the fact an ongoing call has higher priority for a new channel than originating calls - Solution: Prioritize hand-off channel assignment at the expense of tolerable increase in call blocking probability  Guard channel concepts (Prioritizing Handoffs) - Reserve some channels exclusively for hand-offs. Remaining channels shared equally between hand-offs and originating calls - For fixed assignment. Each cell has a set of guard channels. While for dynamic assignment, channels are assigned during hand-off from a central pool - Disadvantages: -- Penalty in reduction of total carried traffic. Since fewer channels are available for originating calls. Can be partially solved by queuing up blocked originating calls -- Insufficient spectrum utilization – need to evaluate an optimum number of guard channels.
  21. 21. Capacity Improvement and Interference Reduction  There is a close correspondence between the network capacity (expressed by N) and the interference conditions (expressed by C/I)  Cell sectoring reduces the interference by reducing the number of co- channel interferers that each cell is exposed to. For example, for 60 degrees sectorization, only one interferer is present, compared to 6 in omnidirectional antennas. But, cell sectorization also splits the channel sets into smaller groups  Cell splitting allows to create more smaller cells. Thus, the same number of channels is used for smaller area. For the same probability of blocking, more users could be allocated
  22. 22. Cell Splitting: Example 2 2 3 1 2 1 1 77 7 6 4 6 5 3 3 4 6 5 4 5  Advantages: more capacity, only local redesign of the system  Disadvantages: more hand-offs, increased interference levels, more infrastructures
  23. 23. QoS Provisioning at the MAC Layer
  24. 24. View point • IEEE 802.11 experiences serious challenges in meeting the demands of multimedia services and applications. • IEEE 802.11e standard support quality of service at MAC layer. • The viewpoint – 802.11 QoS schemes – 802.11e
  25. 25. Introduction(1/2) • WLANs are becoming ubiquitous and increasingly relied on 802.11 • Wireless users can access real-time and Internet services virtually anytime, anywhere. • In wireless home and office networks, QoS and multimedia support are critical. • QoS and multimedia support are essential ingredients to offer VOD audio on demand and high-speed Internet access.
  26. 26. Introduction(2/2) • The lack of a built-in mechanism for support of real time services makes it difficult to provide QoS guaranteed for throughput-sensitive and delaysensitive multimedia applications. • IEEE 802.11e is being proposed as the upcoming standard for the enhancement of the vice differentiation.
  27. 27. An Overview of IEEE 802.11 Task Group Responsibility 802.11a—OFDM 5GHz 54Mbs 802.11b—HR/DSSS 2.4GHz 22Mbs 802.11c—Bridge Operation Procedures Bridge 802.11d—Global Harmonization Additional regulatory domains 802.11e—MAC Enhancements for QoS EDCF 802.11f—Inter Access Point Protocol Interoperability 802.11g—OFDM 2.4GHz 36/54Mbs 802.11h—DFS Dynamic channel selection 802.11i—security WEP HCF
  28. 28. 802.11MAC (1/4) 免競爭式服務 (具時限傳輸) 競爭式服務 (非同步傳輸) Point Coordination Function (PCF) MAC Extent Distributed Coordination Function (DCF)
  29. 29. 802.11MAC (2/4) • Distributed Coordination Function (DCF) – Defines a basic access mechanism and optional RTS/CTS mechanism. – Shall be implemented in all stations and APs. – Used within both ad hoc and infrastructure configurations. • Point Coordination Function (PCF) – An alternative access method – Shall be implemented on top of the DCF – A point coordinator (polling master) is used to determine which station currently has the right to transmit. – Shall be built up from the DCF through the use of an access priority mechanism
  30. 30. 802.11MAC (3/4) • Different accesses to medium can be defined through the use of different values of IFS (inter-frame space). – PCF IFS (PIFS) < DCF IFS (DIFS) – PCF traffic should have higher priority to access the medium, to provide a contention-free access. – This PIFS allows the PC (point coordinator) to seize control of the medium away from the other stations. • Coexistence of DCF and PCF – DCF and PCF can coexist through superframe. – superframe: a contention-free period followed by a contention period. 超級訊框 免競爭訊框 需競爭訊框
  31. 31. 802.11MAC (4/4) Figure:Coexistence of DCF and PCF
  32. 32. Distributed Coordination Function (1/3) • Allows sharing of medium between PHYs through – CSMA/CA – random backoff following a busy medium. • All packets should be acknowledged (through ACK frame) immediately and positively. – Retransmission should be scheduled immediately if no ACK is received.
  33. 33. Distributed Coordination Function (2/3) • Carrier Sense shall be performed through 2 ways: – physical carrier sensing: provided by the PHY – virtual carrier sensing: provided by MAC • by sending medium reservation through RTS and CTS frames – duration field in these frames • The use of RTS/CTS is under control of RTS_Threshold. • An NAV (Net Allocation Vector) is calculated to estimate the amount of medium busy time in the future. • Requirements on STAs: – can receive any frame transmitted on a given set of rates – can transmit in at least one of these rates – This assures that the Virtual Carrier Sense mechanism work on multiple-rate environments
  34. 34. Distributed Coordination Function (3/3) • MAC-Level ACKs – Frames that should be ACKed: • Data • Poll • Request • Response – An ACK shall be returned immediately following a successfully received frame. – After receiving a frame, an ACK shall be sent after SIFS (Short IFS). • SIFS < PIFS < DIFS • So ACK has the highest priority
  35. 35. DCF: the Random Backoff Time (1/2) • Before transmitting asynchronous MPDUs, a STA shall use the CS function to determine the medium state. • If idle, the STA – defer a DIFS gap – transmit MPDU • If busy, the STA – defer a DIFS gap – then generate a random backoff period (within the contention window CW) for an additional deferral time to resolve contention.
  36. 36. DCF: the Random Backoff Time (2/2) Backoff time = CW* Random() * Slot time where CW = starts at CWmin, and doubles after each failure until reaching CWmax and remains there in all remaining retries CWmax (e.g., CWmin = 7, CWmax = 255) Random() = (0,1) Slot Time = Transmitter turn-on delay + medium propagation delay + medium busy detect response time 255 255 8 127 63 31 15 CWmin 7 第三次重送 初始值 第二次重送 第一次重送
  37. 37. Duration Reservation Strategy (1/2) • Each Fragment and ACK acts as a “virtual” RTS and CTS for the next fragment. • The duration field in the data and ACK specifies the total duration of the next fragment and ACK. • The last fragment and ACK will have the duration set to zero.
  38. 38. Duration Reservation Strategy (2/2) • Goal of fragmentation: – shorter frames are less suspectable to transmission errors, especially under bad channel conditions
  39. 39. Point Coordination Function (1/6) • The PCF provides contention-free services. • One STA will serve as the Point Coordinator (PC), which is responsible of generating the Superframe (SF). – The SF starts with a beacon and consists of a Contention Free period and a Contention Period. – The length of a SF is a manageable parameter and that of the CF period may be variable on a per SF basis. • There is one PC per BSS. – This is an option; it is not necessary that all stations are capable of transmitting PCF data frames
  40. 40. Point Coordination Function (2/6) • The PC first waits for a PIFS period. – PC sends a data frame (CF-Down) with the CF-Poll Subtype bit = 1, to the next station on the polling list. – When a STA is polled, if there is a data frame (CF-Up) in its queue, the frame is sent after SIFS with CF-Poll bit = 1. – Then after another SIFS, the CF polls the next STA. – This results in a burst of CF traffic. – To end the CF period, a CF-End frame is sent.
  41. 41. Point Coordination Function (3/6) • If a polled STA has nothing to send, after PIFS the PC will poll the next STA. • NAV setup: – Each STA should preset it’s NAV to the maximum CFPeriod Length at the beginning of every SF. – On receiving the PC’s CF-End frame, the NAV can be reset (thus may terminate the CF period earlier).
  42. 42. Point Coordination Function (4/6) 超級訊框 免競爭週期 PIFS 媒介忙碌中 CF-D1 CF-D2 CF-U1 SIFS 競爭週期 PIFS SIFS SIFS CF-D3 SIFS CF-D4 CF-U2 SIFS Dx = Down Traffic Ux = Up Traffic CF-U4 SIFS NAV CF-End 重設 NAV CF-邊界
  43. 43. Point Coordination Function (5/6) • When the PC is neither a transmitter nor a recipient: – When the polled STA hears the CF-Down: • It may send a Data frame to any STA in the BSS after an SIFS period. • The recipient (.neq. PC) of the Data frame returns an ACK after SIFS. – Then PC transmits the next CF-Down after an SIFS period after the ACK frame. • If no ACK is heard, the next poll will start after a PIFS period
  44. 44. Point Coordination Function (6/6) 超級訊框 免競爭週期 競爭週期 PIFS 媒介忙碌中 SIFS SIFS CF-D1 CF-D2 S-To-S SIFS CF-End ACK CF-U2 SIFS SIFS NAV Dx = Down Traffic Ux = Up Traffic 重設 NAV CF-邊界
  45. 45. QoS Mechanisms • QoS mechanisms for 802.11 can be classified into three categories: – Service differentiation – Admission control and bandwidth reservation – Link adaptation
  46. 46. BETTER THAN BEST EFFORT SCHEMES: SERVICE DIFFERENTIATION (1/3) • Enhanced DCF (EDCF) – prioritizes traffic categories by different contention parameters, including • arbitrary interframe space (AIFS), • maximum and minimum backoff window size • (CWmax/min), and a multiplication factor for expanding the backoff window. • Persistent Factor DCF (P-DCF) – each traffic class is associated with a persistent factor P – a uniformly distributed random number r is generated in every slot time – Each flow stops the backoff and starts transmission only if (r > P)
  47. 47. BETTER THAN BEST EFFORT SCHEMES: SERVICE DIFFERENTIATION (2/3) • Distributed Weighted Fair Queue (DWFQ) – the backoff window size CW of any traffic flow is adjusted based on the difference between the actual and expected throughputs. – a ratio (Li′ = Ri/Wi) is calculated, where Ri is the actual throughput and Wi the corresponding weight of the ith station. • Distributed Fair Scheduling (DFS) – differentiate thebackoff interval (BI) based on the packet length and traffic class – For the ith flow, BIi = ρi × scaling × factor × Li/ϕi, • Distributed Deficit Round Robin (DDRR) – the ith throughput class at the jth station is assigned with a service quantum rate (Qi,j) equal to the throughput it requires
  49. 49. QOS MECHANISMS FOR ADMISSION CONTROL AND BANDWIDTH RESERVATION (1/2) • Measurement-based approaches • Calculation-based approaches • Scheduling and reservation-based approaches
  51. 51. QOS MECHANISM FOR LINK ADAPTATION (1/2) • • • • • Received signal strength (RSS) PER-prediction MPDU-based link adaptation Link adaptation with success/fail (S/F) thresholds Code Adapts To Enhance Reliability (CATER)
  53. 53. IEEE 802.11E • Main new features of 802.11e: – The Enhanced DCF – THE CONTROLLED HCF
  54. 54. The Enhanced DCF (1/2)
  55. 55. The Enhanced DCF (2/2)
  56. 56. DISTRIBUTED ADMISSION CONTROL FOR EDCF • TXOPBudget[i] =Max(ATL[i] – TxTime[i]*SurplusFactor[i],0) • If TXOPBudget[i] = 0 –TxMemory[i] shall be set to zero all other QSTAs TxMemory[i] remains unchanged • If the TXOPBudget[i] >0 –TxMemory[i] = f*TxMemory[i] + (1 – f)* (TxCounter[i]*SurplusFactor[i] + TXOPBudget[i]) –TxCounter[i] = 0 –TxLimit[i] = TxMemory[i] + TxRemainder[i]
  57. 57. THE CONTROLLED HCF • Controlled channel access function • allows reservation of transmission opportunities (TXOPs) with a hybrid coordinator (HC) • a type of PC handling rules defined by the HCF
  58. 58. ADMISSION CONTROL AND SCHEDULING FOR THE CONTROLLED HCF • The behavior of the scheduler is as follows: – The scheduler shall be implemented – if a traffic stream is admitted by the HC, the scheduler shall send polls anywhere between the minimum service interval and the maximum service interval within the specification interval.
  60. 60. Queries