WIRELESS MULTIMEDIA COMMUNICATION

1. 13PIT101
Multimedia Communication & Networks
UNIT - V
Dr.A.Kathirvel
Professor & Head/IT - VCEW

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. 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. 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. 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. 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. 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. 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. 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. 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. 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. Multi-rate Traffic Scenario
Base Station
C channels
Mobile Users
 Real-time traffic (voice, video)
 Non real-time traffic (TCP/IP
packets)

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. Last Hop Communication
ISDN/PSTN/Internet
WIRE-LINE
NETWORK
Cell
Base Station (BS)
Mobile unit
Wireless Links
Wired Links
Mobile Switching Center (MSC)

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. 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. 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. Handoff Types
Intra-Cell
Inter-Cell
Soft Handoff
Hard Handoff

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. 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. 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. 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. QoS Provisioning at the MAC
Layer

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. 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. 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. 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. 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. 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. 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. 802.11MAC (4/4)
Figure：Coexistence of DCF and PCF

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. 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. 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. 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. 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. 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. Duration Reservation Strategy (2/2)
• Goal of fragmentation:
– shorter frames are less suspectable to transmission
errors, especially under bad channel conditions

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. 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. 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. 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. 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. 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. QoS Mechanisms
• QoS mechanisms for 802.11 can be classified into three
categories:
– Service differentiation
– Admission control and bandwidth reservation
– Link adaptation

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. 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

48. BETTER THAN BEST EFFORT SCHEMES:
SERVICE DIFFERENTIATION (3/3)

49. QOS MECHANISMS FOR ADMISSION CONTROL
AND BANDWIDTH RESERVATION (1/2)
• Measurement-based approaches
• Calculation-based approaches
• Scheduling and reservation-based approaches

50. QOS MECHANISMS FOR ADMISSION CONTROL
AND BANDWIDTH RESERVATION (2/2)

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)

52. QOS MECHANISM FOR LINK ADAPTATION (2/2)

53. IEEE 802.11E
• Main new features of 802.11e:
– The Enhanced DCF
– THE CONTROLLED HCF

54. The Enhanced DCF (1/2)

55. The Enhanced DCF (2/2)

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. 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. 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.

59. PRELIMINARY RESULTS

60. Queries