Multimedia Communication & Networks
UNIT - V
Professor & Head/IT - VCEW
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
The Two Successful Domains
• Wireless networks (Cellular)
Total coverage in many countries
The boon – user mobility
• Wireless extension to the Internet (Wi-Fi)
Supports multimedia services
Global penetration – millions of nodes
• IEEE 802.16 based WiMax
• LTE (Long Term Evolution)
General Problems in Wireless
• 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?
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
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
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
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
- Guaranteed service (hard QoS) : absolute reservation of
resources (RSVP), more expensive
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)
Wireless QoS: Desirable Features
Adapt to dynamically changing network and traffic
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
Bandwidth Requirement for
Application bandwidth requirements on log-scale axis in bits per second
Vertical dashed lines show the bandwidth capability of a few network
Multi-rate Traffic Scenario
Real-time traffic (voice, video)
Non real-time traffic (TCP/IP
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)
Last Hop Communication
Base Station (BS)
Mobile Switching Center (MSC)
Terms to remember
MSC: Mobile Switching Center
VLR: Visiting Location Register
HLR: Home Location Register
BSC: Base Station Controller
BTS: Base Transmitter Station
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
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
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
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
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
-- 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
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
Cell Splitting: Example
Advantages: more capacity, only local redesign of the system
Disadvantages: more hand-offs, increased interference levels, more
• IEEE 802.11 experiences serious challenges in
meeting the demands of multimedia services and
• IEEE 802.11e standard support quality of service at
• The viewpoint
– 802.11 QoS schemes
• 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
• 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
An Overview of IEEE 802.11
802.11c—Bridge Operation Procedures
Additional regulatory domains
802.11e—MAC Enhancements for QoS
802.11f—Inter Access Point Protocol
Dynamic channel selection
• Distributed Coordination Function (DCF)
– Defines a basic access mechanism and optional RTS/CTS
– 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
• 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
Figure：Coexistence of DCF and PCF
Distributed Coordination Function (1/3)
• Allows sharing of medium between PHYs through
– 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.
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
Distributed Coordination Function (3/3)
• MAC-Level ACKs
– Frames that should be ACKed:
– An ACK shall be returned immediately following a successfully
– After receiving a frame, an ACK shall be sent after SIFS (Short
• SIFS < PIFS < DIFS
• So ACK has the highest priority
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
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
(e.g., CWmin = 7, CWmax = 255)
Random() = (0,1)
Slot Time = Transmitter turn-on delay +
medium propagation delay +
medium busy detect response time
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
Duration Reservation Strategy (2/2)
• Goal of fragmentation:
– shorter frames are less suspectable to transmission
errors, especially under bad channel conditions
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
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.
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).
Point Coordination Function (4/6)
Dx = Down Traffic
Ux = Up Traffic
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
• 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
Point Coordination Function (6/6)
Dx = Down Traffic
Ux = Up Traffic
• QoS mechanisms for 802.11 can be classified into three
– Service differentiation
– Admission control and bandwidth reservation
– Link adaptation
BETTER THAN BEST EFFORT SCHEMES:
SERVICE DIFFERENTIATION (1/3)
• Enhanced DCF (EDCF)
– prioritizes traffic categories by different contention parameters,
• arbitrary interframe space (AIFS),
• maximum and minimum backoff window size
• (CWmax/min), and a multiplication factor for expanding the
• 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
– Each flow stops the backoff and starts transmission only if (r > P)
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
BETTER THAN BEST EFFORT SCHEMES:
SERVICE DIFFERENTIATION (3/3)
QOS MECHANISMS FOR ADMISSION CONTROL
AND BANDWIDTH RESERVATION (1/2)
• Measurement-based approaches
• Calculation-based approaches
• Scheduling and reservation-based approaches
QOS MECHANISMS FOR ADMISSION CONTROL
AND BANDWIDTH RESERVATION (2/2)
QOS MECHANISM FOR LINK
Received signal strength (RSS)
MPDU-based link adaptation
Link adaptation with success/fail (S/F) thresholds
Code Adapts To Enhance Reliability (CATER)
DISTRIBUTED ADMISSION CONTROL
=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]
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
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