multimedia communication & networks

13PIT101

Multimedia Communication & Networks

UNIT - V

Dr.A.Kathirvel

Professor & Head/IT - VCEW

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

Unit - V

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)

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?

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

Characteristics of Multimedia Services A picture is worth thousand words

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

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

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

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

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

Multi-rate Traffic Scenario

Mobile Users

Base Station

C channels

Real-time traffic (voice, video)

Non real-time traffic (TCP/IP

packets)

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

WIRE-LINE

NETWORK

Base Station (BS)

Cell

Wireless Links

Wired Links

Mobile Switching Center (MSC)

Mobile unit

ISDN/PSTN/Internet

Cellular Framework

MSC/VLR MSC/VLR BSC BSC

BTS

BTS

Local Switch Cellular Network

PSTN Network

Mobile Terminal

Air Link

HLR

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

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 Problem

BS-1 BS-2

Handoff Types

Intra-Cell Inter-Cell

Soft Handoff Hard Handoff

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

Hand-off: Who Triggers?

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.

Hand-off Policies

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

3

2

1

4

7

6

5

3

2

1

4

7

6

5

3

2

1

4

7

6

5

Advantages: more capacity, only local redesign of the system

Disadvantages: more hand-offs, increased interference levels, more

infrastructures

QoS Provisioning at the MAC

Layer

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

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.

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 delay-

sensitive multimedia applications.

• IEEE 802.11e is being proposed as the upcoming

standard for the enhancement of the vice

differentiation.

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 HCF

802.11f—Inter Access Point Protocol Interoperability

802.11g—OFDM 2.4GHz 36/54Mbs

802.11h—DFS Dynamic channel selection

802.11i—security WEP

802.11MAC (1/4)

MAC Extent

免競爭式服務

(具時限傳輸)

Distributed Coordination Function (DCF)

競爭式服務

(非同步傳輸)

Point Coordination Function (PCF)

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

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.

免競爭訊框

超級訊框

需競爭訊框

802.11MAC (4/4)

Figure：Coexistence of DCF and PCF

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.


• 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


• 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

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.

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

8

CWmax

CWmin 7

15

31

第二次重送

第一次重送

第三次重送

初始值

63

127

255 255

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.

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


• 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.


• 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 CF-

Period 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).


NAV

SIFS

SIFS

媒介忙碌中 CF-D1

CF-U1

CF-D2

SIFS

CF-U2

CF-D3

SIFS

PIFS SIFS

CF-D4

CF-U4

PIFS

SIFS

CF-End

免競爭週期

超級訊框

競爭週期

重設 NAV

CF-邊界

Dx = Down Traffic

Ux = Up Traffic


• 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


NAV

SIFS

媒介忙碌中 CF-D1

S-To-S

SIFS

ACK

CF-D2

SIFS

PIFS SIFS

CF-U2

SIFS

CF-End

免競爭週期

超級訊框

競爭週期

重設 NAV

CF-邊界

Dx = Down Traffic

Ux = Up Traffic

QoS Mechanisms

• QoS mechanisms for 802.11 can be classified into three

categories:

– 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,

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)



• 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

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

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)

QOS MECHANISM FOR LINK ADAPTATION (2/2)

IEEE 802.11E

• Main new features of 802.11e:

– The Enhanced DCF

– THE CONTROLLED HCF

The Enhanced DCF (1/2)

The Enhanced DCF (2/2)

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]

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

interval.

PRELIMINARY RESULTS

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