The Two Successful DomainsThe Two Successful Domains

Wireless networks (Cellular)Wireless networks (Cellular)– Supports voiceSupports voice– Total coverage in many countriesTotal coverage in many countries– Decreasing costDecreasing cost– The boon – user mobilityThe boon – user mobility

Wireless extension to the Internet (Wi-Fi)Wireless extension to the Internet (Wi-Fi)– Information contentInformation content– Supports multimedia servicesSupports multimedia services– Global penetration – millions of nodesGlobal penetration – millions of nodes– Decreasing costDecreasing cost

IEEE 802.16 based WiMaxIEEE 802.16 based WiMax

LTE (Long Term Evolution)LTE (Long Term Evolution)

General Problems in Wireless NetworksGeneral Problems in Wireless Networks

Resource scarcityResource scarcity

– Limited bandwidthLimited bandwidth

Unreliable wireless link Unreliable wireless link

– Error prone channels (BER 10Error prone channels (BER 10-4 -4 to 10to 10-3-3))

Varying channel conditionsVarying channel conditions

– Channel models fluctuates 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, graphicsCombination of various medium – text, audio/video, graphics

– Audio/video conferencing, shared whiteboard, surfing, email, etc.Audio/video conferencing, shared whiteboard, surfing, email, etc.

Varied requirements Varied requirements

– Low bit error rateLow bit error rate

– High bandwidthHigh bandwidth

– Low delayLow delay

Synchronization of multiple data typesSynchronization of multiple data types

– Proper schedulingProper scheduling

Different coding schemes for different typesDifferent coding schemes for different types

– Source codingSource coding

Characteristics of Multimedia ServicesA picture is worth thousand words

Data on Wireless Networks!Data on Wireless Networks!What are the Problems?What are the Problems?

True characterization of data traffic is yet unknownTrue characterization of data traffic is yet unknown

– Traffic modeling needs to be doneTraffic modeling needs to be done

Data services cannot tolerate bit errorsData services cannot tolerate bit errors

– Corrupt packets need to be recoveredCorrupt packets need to be recovered

Unpredictable nature of wireless mediumUnpredictable nature of wireless medium

– QoS provisioning becomes difficultQoS provisioning becomes difficult

Bottleneck due to the bandwidth limitationBottleneck due to the bandwidth limitation

– Proper buffering / filtering requiredProper buffering / filtering required

No differentiated service plans for customersNo differentiated service plans for customers

– Class based services requiredClass based services required

What is QoS?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 QoSEnd-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 ChallengesWireless 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 FeaturesWireless 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 Bandwidth Requirement for Multimedia TrafficMultimedia 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 ScenarioMulti-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 CommunicationLast Hop Communication

WIRE-LINE

NETWORK

WIRE-LINE

NETWORK

Base Station (BS)Cell

Wireless Links

Wired Links

Mobile Switching Center (MSC)

Mobile unit

ISDN/PSTN/Internet

Cellular FrameworkCellular Framework

MSC/VLR MSC/VLRBSC BSC

BTS

BTS

Local SwitchCellular 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

CellCell: geometric representation of areas. Geographic area is divided into : geometric representation of areas. Geographic area is divided into cells, each serviced by an antenna called cells, each serviced by an antenna called base stationbase station (BS) (BS)

Mobile Switching CenterMobile Switching Center (MSC) controls several BSs and serves as (MSC) controls several BSs and serves as gateway to the backbone network (PSTN, ISDN, Internet)gateway to the backbone network (PSTN, ISDN, Internet)

WHY CHANNEL REUSE?WHY CHANNEL REUSE? Limited number of frequency spectrum allocated by FCC and Limited number of frequency spectrum allocated by FCC and

remarkable growth of mobile (wireless) communication usersremarkable growth of mobile (wireless) communication users

Frequency band allocated by FCC to the mobile telephone system is Frequency band allocated by FCC to the mobile telephone system is 824-849 MHz for transmission from mobiles (uplink) and 869-894 MHz 824-849 MHz for transmission from mobiles (uplink) and 869-894 MHz for transmission from base stations (downlink)for transmission from base stations (downlink)

With a channel spacing of 30 KHz, this frequency band can With a channel spacing of 30 KHz, this frequency band can accommodate 832 duplex channelsaccommodate 832 duplex channels

Frequency ReuseFrequency Reuse: use same carrier frequency or channel at different : use same carrier frequency or channel at different areas (cells) avoiding co-channel interferenceareas (cells) avoiding co-channel interference

Number of simultaneous calls (Number of simultaneous calls (capacitycapacity) greatly exceeds the total ) greatly exceeds the total number of frequencies (channels) allocatednumber 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 ProblemHand-off Problem

BS-1BS-1 BS-2BS-2

Handoff TypesHandoff Types

Intra-CellIntra-Cell Inter-CellInter-Cell

Soft HandoffSoft Handoff Hard HandoffHard 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 stationBoth 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 MSCThe selected BS is informed by the MSC which assigns a new channel

Hand-off: Who Triggers?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 PoliciesHand-off Policies

Capacity Improvement and Interference ReductionCapacity 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: ExampleCell Splitting: Example

33

22

11

44

77

66

55

33

22

11

44

77

66

55

3322

11

4477

6655

Advantages: more capacity, only local redesign of the system Disadvantages: more hand-offs, increased interference levels, more

infrastructures