Quality of Service in Mobile Networks - HTW Berlininka.htw-berlin.de/wci/05/praes/Prof.Dr.ChrisGuy_web.pdfQuality of Service in Mobile Networks ... • via a wireless network hotspot

Quality of Service in Mobile Networks

Chris GuySchool of Systems Engineering

The University of Reading

© C.G.Guy 2005QoS in Mobile Networks

Outline of presentation

• Overview of networks which mobile applications could use• 3rd Gen telephone network (UMTS)• Wi-Fi (IEEE 802.11)• WiMax (IEEE 802.16)• IEEE 802.20

• Quality of Service Issues• Different approaches• Which should a mobile user use?• Conclusions


Overview of Mobile Networks

• in the future the mobile user will have a choice of how to connect to the fixed infrastructure• via the telephone network

• 3rd generation (3G)• via a wireless network hotspot or base station

• WiMax (= IEEE 802.16)• Wi-Fi (= IEEE 802.11)• IEEE 802.20

• each has a different approach to offering Quality of Service• and (probably) a different tariff structure


3rd Generation mobile telephony

• now widely available• at least in most major cities

• offers data rates up to 2 Mbps• waiting for HSDPA

• there is talk of data rates up to 30 Mbps

• no ‘range’ issues as cells are dimensioned to provide full coverage• hand-over between cells for mobile users is

transparent and works well• there are still issues about hand-over to GSM

networks when 3G coverage is absent


IEEE 802.16 (WiMax)

• a new standard offering wireless access over a wide-area

• only 802.16d (also known as 802.16-2004) is fully defined• offers data rates up to 75 Mbps• at ranges of up to 8 Km from base station

• this is intended for fixed installations

• support for mobility (802.16e) is not yet well defined• standard expected to be ratified in early 2006


Summary of IEEE 802.16Standard 802.16 802.16-2004 802.16e

completed End 2001 802.16a: Jan 2003802.16-2004: End 2004

Early 2006?

Spectrum 10 to 66 GHz 2.5 – 11 GHz 2.5 - 6 GHz

Channel Line-of-sight Non line-of sight Non line-of-sight

Bit Rate 32 to 134 Mbps up to 75 Mbps up to 15 Mbps

Modulation QPSK, 16 QAM, 64 QAM

OFDM 256, OFDMA, 64 QAM, 16 QAM, QPSK, BPSK

OFDM 256, OFDMA, 64 QAM, 16 QAM, QPSK, BPSK

Mobility Fixed Fixed and portable Mobility, regional roaming

Channel BW 20, 25 or 28 MHz 1.25 to 20 MHz with up to 16 logical sub-channels

1.25 to 20 MHz with up to 16 logical sub-channels

Typical Cell Radius 2 to 5 Km 5 to 8 Km (max 50 Km) 2 to 5 Km


IEEE 802.11 (Wi-Fi)

• well known and readily available as a wireless LAN

• many locations (hotels, cafes, railway stations) offer hotspots• offers data rates up to 54 Mbps

• soon to increase to 200+ Mbps (802.11n)• at a range up to 100 m from hotspot

• a hotspot is really an access point giving an interface to the traditional wired network

• a mechanism for hand-over between access points is defined• but support for mobility does not work well


IEEE 802.20

• a new proposed standard specifically designed for mobile wireless access• due 2007 or 2008

• targets mobile users at speeds of up to 250 Km/hr• cell ranges up to 15 Km• data rates > 1Mbps

• very limited information available• will use frequency bands below 3.5 GHz


User-centric Delay and Packet Loss requirements – ITU G.1010*

0%

Packet Loss

Command/ control

(eg Telnet,Interactive

games)

Conversationalvoice and video

Voice/videomessaging

Streamingaudio/video

Transactions(eg E-commerce,Web-browsing, E-

mail access)

Messaging,Downloads

(eg FTP,still image)

FaxBackground

(eg Usenet)

5%

100 msec 1 sec 10 sec 100 sec

Zeroloss

Delay

Source: ITU G.1010 [“Draft New Recommendation G.QoSRQT – End-user Multimedia QoS Categories”, ITU-T study group 12, contribution 37, August 2001]


QoS requirements for different traffic types

Application Throughput Delay Jitter Packet Loss

Audio Streaming

64 kbps + no strict limits can be compensated for by increased delay

can tolerate some packet loss




Video Streaming

0.5 – 20 Mbps no strict limits can be compensated for by increased delay

Voice over IP 32 – 64 kbps low (less than 100 mS)

low

Video conferencing

128 kbps ++(depends on quality)

low (less than 100 mS)

low


QoS in 3G networks

• as these are ‘traditional’ telephony networks they provide guarantees for levels of service• throughput, delay, jitter • everything is connection-oriented

• main mechanism to ensure QoS is by admission control• numbers of users are restricted so that

service level guarantees can be maintained• forward-error-control is used to protect aspects of

the transmission


QoS in WiMax networks• MAC layer is based on a combination of Time-

Division-Multiplex (TDMA) and Frequency-Division-Duplex (FDD)• with admission control

• QoS can be guaranteed in fixed networks• the MAC layer is connection-oriented

• even connectionless services like IP are mapped to a connection

• uses the concept of service-flows on uplink or downlink• to guarantee QoS parameters such as throughput, delay and jitter• each service flow is then mapped to a MAC layer connection• resources are only committed when flow actually starts

• service flow classes are based on ATM definitions of QoS requirements• e.g. Constant Bit Rate (CBR), Variable Bit Rate (VBR)

etc


QoS in WiMax networks (cont)

• 2 classes of mobile station are defined• based on their ability to accept bandwidth grants

• is the acceptance of a bandwidth grant for the whole station or just for the individual connection?

• affects how management messages are carried• details on how bandwidth is granted are not part of

the standard• contention, polling and explicit requests for

connections (with specific QoS requirements) are all supported• decision is then up to the management software• allows manufacturers to differentiate their products


QoS in WiMax networks (cont)

• the physical layer (called the radio-interface-layer) is adaptable for different QoS needs• as extra bandwidth demands are made or as

conditions change• e.g. OFDM is used with a varying number of

carriers (and hence bandwidth) determined by• QoS needs• channel conditions• total bandwidth demand on base-station


Support for mobility in WiMax networks

• mobility is supported in the 802.16e standard• hand-over between base-stations is based on similar

mechanisms to Wi-Fi• need to re-authenticate and re-associate

• so there will be a delay

• the standard allows for dynamic hand-over to other physical layers within the 802.16 standard• and to Wi-Fi or even 3G networks

• assumes the use of mobile-IP and that you have a subscription agreement with the new network

• security will be a big headache


QoS in Wi-Fi networks

• the normal MAC layer protocol is contention based• gives no mechanism for priority or QoS

• new standard IEEE 802.11e • changes MAC layer to give priority

mechanisms• can prioritise video or voice traffic but there

are no guarantees of QoS• studies have shown that one access point can

support several video or voice calls with reasonable QoS


the Wi-Fi MAC layer• MAC contention mechanism is called the distributed-

coordination-function (DCF)• when a station senses the medium is free it waits a random

amount of time (slot-based) before transmitting• continues to listen whilst waiting• if another station is also waiting to transmit, one or the other

will have a shorter wait-time and so will grab the medium and a collision will be avoided

• if a station has to defer access a second time the contention window (number of slots the random wait is chosen from) is doubled• and doubled again for another deferral

• the DCF cannot guarantee any QoS nor even that a station will ever get access to the medium


Overview of the DCF

successfultransmission

CW

DIFS back-off after defernext frame Ack

SIFS

station wishes to transmit so commences listeningand checks NAV (virtual carrier sense)

Ack

SIFS

NAV = 0Other stations may start to transmit during this time. Frame will contain a new value of NAV which must be counted down to 0 beforere-starting CW.

DIFS = DCF inter-frame spaceSIFS = short inter-frame spaceCW = contention windowNAV = network allocation vector


Changes to the Wi-Fi MAC layer to offer QoS

• the main change is that traffic (not stations) can be prioritised• 4 service levels

• higher priority traffic has (on average) shorter deferral times, so has a greater chance of grabbing the medium• based on fewer slots in the initial contention window• but no guarantees are given

• this is also known as Wireless Multimedia (WMM)• from the Wi-Fi organisation


The use of different wait times to enhance QoS

• QSTA(1) has a higher priority than QSTA (2)• CW(1) starts before CW(2) and has fewer slots

• which increases the chance that QSTA(1) will start to transmit whilst QSTA(2) is still in its back-off period

AIFS(1)successful

transmission CW (1)

CW (2)

slot times

AIFS(2)

SIFS

ordinary traffic has to wait a DIFSbefore trying to transmit

DIFS = DCF inter-frame spaceSIFS = short inter-frame spaceAIFS = arbitration inter-frame spaceCW = contention windowQSTA = QoS aware wireless station


Changes to come to the Wi-Fi MAC layer

• the standard IEEE 802.11e also offers a fully guaranteed access mechanism based on the use of admission control• part of the time is given over to a period where the

access point has total control of the medium and determines which mobile stations can transmit

• this will be known as WMM2 when (and if) it is implemented. • a similar mechanism (PCF) was built in to the original

802.11 MAC layer, but very few manufacturers implemented it


Simulated traffic with or without WMM

taken from: Wi-Fi CERTIFIED™ for WMM™ - Support for Multimedia Applications with Quality of Service in Wi-Fi® NetworksWi-Fi Alliance, September 1, 2004, http://www.wi-fi.org

red line is video stream, blue and green lines are lower priority traffic

without WMM

with WMM


Mobility in Wi-Fi networks• mobility (the ability to move between base-stations without losing

traffic) is built in to all WiFi networks• historically it has not worked well• known problems if base stations and mobile users have wireless

interfaces from different manufacturers• each time a move is made the mobile station must authenticate

and associate with the new base-station• authentication may be done prior to the move, if the new base

station is already in range• no mechanism to ensure QoS is built in to the mobility functions

• extra delay is inevitable as the hand-over process takes time• even if it works a glitch may be seen/heard

• the maximum range (100 m) can be increased by proprietary solutions but this will depend on base station and mobile user having the appropriate technology• can roam between Wi-Fi and GSM/GPRS networks to extend range

• known as Unlicensed Mobile Access (UMA)


QoS in IEEE 802.20 networks

• not yet well defined• only performance targets have been set• mechanisms will be based on IETF DiffServ

architecture• at the IP layer• which will then be mapped to separate MAC layers

• how the targets (see next slide) will be met is not yet set


QoS performance targets for 802.20 networks

QoS class Expedited forwarding

Assured forwarding Best effort

Latency 30 ms 30 ms – 1s 10s

Error tolerant 3 x 10-2 10-2 – 2.5 x 10-1 2.5 x 10-1

Error intolerant 5 x 10-13 5 x 10-13 – 8 x 10-5 8 x 10-5

NB: Assured forwarding can be further divided into 4 sub-classes


Mobility in IEEE 802.20 networks

• the story is similar• only targets have been set• there are no details of how these targets will

be met• the main target is full mobility at speeds

up to 250 km/hr• a speeding train

• this assumes full coverage as (almost by definition) high-speed trains go through un-populated areas


Which service should you choose for your mobile application?

• need to ask• what is your application?

• what are its QoS needs?• are you really mobile?

• if so, how fast must you move?• what services are available?

• in the whole area you wish to roam over• what is the tariff structure?

• do you subscribe to all the available services• do you have the appropriate hardware?


Conclusions• in the near future the only realistic service for truly mobile

QoS aware traffic will be 3G• has demonstrated its ability to carry voice (obviously) and

video with true roaming capability• Wi-Fi connections are a good choice for static users

• or those roaming over small areas• WiMax has yet to be tried out with moving mobile stations

• the hand-over time between base stations will be critical• IEEE 802.20 promises to be the ‘holy-grail’ for mobile

users with demanding applications but deployment is many years away

Documents

Quality of Service in Mobile Networks - HTW Berlininka.htw-berlin.de/wci/05/praes/Prof.Dr.ChrisGuy_web.pdfQuality of Service in Mobile Networks ... • via a wireless network hotspot