Broadband WirelessBroadband WirelessMedia Access ControlMedia Access ControlFrom 802.11 to 802.16From 802.11 to 802.16

Justin ComroeJustin Comroe

ASU CSE 535ASU CSE 535

Nov 27, 2007Nov 27, 2007

WiFi Today at ASUWiFi Today at ASU

WiFi is used in-buildings as IEEE 802.11 was WiFi is used in-buildings as IEEE 802.11 was originally designed, as a wireless LANoriginally designed, as a wireless LAN

WiFi is also being used outdoors as a MAN right WiFi is also being used outdoors as a MAN right now at ASU and many places around the nationnow at ASU and many places around the nation

IEEE has completed development of a new IEEE has completed development of a new standard for outdoor MANstandard for outdoor MAN IEEE 802.16 has departed dramatically from the well-IEEE 802.16 has departed dramatically from the well-

known & used 802.11 techniques for MACknown & used 802.11 techniques for MAC

So what’s wrong with the 802.11 MAC for outdoor applications, especially when everyone is using it and we already know it works?

So what’s wrong with the 802.11 MAC for outdoor applications, especially when everyone is using it and we already know it works?

Collisions KillCollisions Kill

WiFi is basically a wireless EthernetWiFi is basically a wireless Ethernet The Medium is radioThe Medium is radio A fundamental A fundamental MMedium edium AAccess ccess CControl (ontrol (MACMAC) )

function is to prevent transmissions from function is to prevent transmissions from collidingcolliding when a station transmission overlaps another, one or when a station transmission overlaps another, one or

both transmissions will often failboth transmissions will often fail Without collision control >81.6% of total airtime Without collision control >81.6% of total airtime

is wastedis wasted Pure ALOHA maximum throughput is only 18.4%Pure ALOHA maximum throughput is only 18.4%

Collisions KillCollisions KillTransmission 1

Transmission 2

Transmission 1 Transmission 2x

timeDestructive interference at destination

Wasted airtime

IEEE 802.11 MACIEEE 802.11 MAC

Stations listen to all transmissions and Stations listen to all transmissions and transmit when the channel appears idletransmit when the channel appears idle This is This is CCarrier arrier SSense ense MMultiple ultiple AAccess, or ccess, or CSMACSMA

CSMA CSMA works a lot better than Aloha for all cases works a lot better than Aloha for all cases where all the stations can hear each other.where all the stations can hear each other.

Source for following material on 802.11:IEEE P802.11-96/49C“802.11 Tutorial”, from March 1996http://grouper.ieee.org/groups/802/11/Tutorial/MAC.pdfThis is IEEE’s official website for the 802.11 taskgroup

Source for following material on 802.11:IEEE P802.11-96/49C“802.11 Tutorial”, from March 1996http://grouper.ieee.org/groups/802/11/Tutorial/MAC.pdfThis is IEEE’s official website for the 802.11 taskgroup

Carrier Sense – Basic PrincipleCarrier Sense – Basic PrincipleTransmission 1

Transmission 2

Transmission 1

Transmission 2

timeLaptop 2 “hears” Laptop1 transmission

1

2

… and waits until done before transmitting

3

[1]->[3]

[2]->[3]

IEEE 802.11 MACIEEE 802.11 MAC

802.11 further adds several methods meant for 802.11 further adds several methods meant for CCollision ollision AAvoidancevoidance IEEE 802.11 MAC is described as IEEE 802.11 MAC is described as CSMA/CACSMA/CA

802.11 has a MAC rule to preventing overlap, 802.11 has a MAC rule to preventing overlap, called the called the DDistributed istributed CCoordination oordination FFunction, or unction, or DCFDCF

DDistributed / istributed / SShort hort IInternterFFrame rame SSpacepaceand Backoff Intervaland Backoff Interval

Transmission 1

Transmission 2

time

A device wanting to transmit during Idle channel must first see a minimum DIFS interval

A device wanting to transmit during a transmission-in-progress must wait an additional randomly selected number of slot times forming a Backoff Interval

DIFS

AckSIFS

The receiving unit gets to Acknowledge after an SIFS which is < DIFS

DIFS

[1]->[3]

[2]->[3]

[3]->[1]

Deferredaccess

NAVNAV

Transmission 1

Transmission 2

timeLaptop 2 doesn’t “hears” end of Laptop1 transmission

… but still waits until done before transmitting because of NAV

NA

V

NA

V

Under DCF, each unit places a Under DCF, each unit places a NNetwork etwork AAllocation llocation VVector (ector (NAVNAV) in front of each transmission) in front of each transmission identifies the length of its transmission identifies the length of its transmission

Hidden NodeHidden Node

Transmission 1Transmission 21 2

3

Presume [1] and [2] can both hear [3] and vice-versa,But [1] and [2] cannot hear each other.

Transmission 1Transmission 2

time

DIFS

[1]->[3][2]->[3]

DIFS

x

Destructive interference at [3]

RequestToSend / ClearToSendRequestToSend / ClearToSend

Transmission 1

time

DIFSAck

Every transmission can begin with an RTS, which is responded to with a CTS. Transmission and Ack follow the RTS/CTS exchange. Further, the NAV values project enough time for the entire transaction to complete.

[1]->[3]

[3]->[1]NA

V RTS

NA

V CTS

[1]->[3]

[3]->[1] NA

V

NA

V

Even if unit [2] does not hear the RTS from [1]->[3], hearing the NAV preceding the CTS from [3] would defer access until the end of the entire transaction!

802.11 MAC Summary802.11 MAC Summary

All devices independently pick their own time to All devices independently pick their own time to transmittransmit There is no central authority … There is no central authority … DDCF stands for CF stands for

““DistributedDistributed””

DIFS, SIFS, Backoff Intervals … and DIFS, SIFS, Backoff Intervals … and mainly mainly collisionscollisions … all subtract from airtime that could … all subtract from airtime that could successfully deliver payloadsuccessfully deliver payload As more units compete, collisions increaseAs more units compete, collisions increase As collisions increase, capacity goes downAs collisions increase, capacity goes down Therefore capacity goes down as more units competeTherefore capacity goes down as more units compete

MAN CoverageMAN Coverage

This is not a Peer-Peer NetworkThis is not a Peer-Peer Network Units only transmit to Access PointUnits only transmit to Access Point The access point has a physically dominant locationThe access point has a physically dominant location

All units can see the access pointAll units can see the access point Each unit might at best see only a few nearby unitsEach unit might at best see only a few nearby units

MAN usage of 802.11MAN usage of 802.11

Violates primary DCF assumptionsViolates primary DCF assumptions Units cannot hear other unitsUnits cannot hear other units

• NAV cannot function when a peer user NAV cannot function when a peer user transmission is never heardtransmission is never heard

• CSMA and CA both break downCSMA and CA both break down Hidden Node CA doesn’t work either even if Hidden Node CA doesn’t work either even if

employing RTS/CTSemploying RTS/CTS• Mobile initiated RTS never heard by other mobilesMobile initiated RTS never heard by other mobiles• CSMA and CA both break downCSMA and CA both break down

CSMA Breaks DownCSMA Breaks Down

When an access point has too many When an access point has too many users, to the non-engineering user it users, to the non-engineering user it simply appears to stop workingsimply appears to stop working

The engineering observer might presume The engineering observer might presume that capacity is simply being dividedthat capacity is simply being divided

The truth is, when an access point has too The truth is, when an access point has too many users, the capacity actually goes to many users, the capacity actually goes to zero, just as simple ALOHA when zero, just as simple ALOHA when overloadedoverloaded

Full DisclosureFull Disclosure

A handful of companies specifically create A handful of companies specifically create 802.11 based MAN products, but they often alter 802.11 based MAN products, but they often alter the 802.11 MAC (like Alvarion)the 802.11 MAC (like Alvarion)

Virtually all 802.11 uses Virtually all 802.11 uses DDCF, meaning CF, meaning DistributedDistributed

There is a relatively unutilized mode of 802.11 There is a relatively unutilized mode of 802.11 called called PCFPCF ( (Point Configuration FunctionPoint Configuration Function)) PCF permits the Access Point to control accessPCF permits the Access Point to control access PCF is useful for polling applicationsPCF is useful for polling applications

IEEE MAN Standard Takes a IEEE MAN Standard Takes a Radically Different DirectionRadically Different Direction

IEEE 802.16 designed to be a MAN, not a IEEE 802.16 designed to be a MAN, not a LANLAN

Source for following material on 802.16:IEEE C802.16-02/05Ecklund, Marks, Stanwood, & Wang, “IEEE Standard 802.16: A Technical Overview of the WirelessMAN Air Interface for Broadband Wireless Access”, IEEE Communications Magazine, June 2002, pp.98-107http://grouper.ieee.org/groups/802/16/docs/02/C80216-02_05.pdf This is IEEE’s official website for the 802.16 taskgroup

Source for following material on 802.16:IEEE C802.16-02/05Ecklund, Marks, Stanwood, & Wang, “IEEE Standard 802.16: A Technical Overview of the WirelessMAN Air Interface for Broadband Wireless Access”, IEEE Communications Magazine, June 2002, pp.98-107http://grouper.ieee.org/groups/802/16/docs/02/C80216-02_05.pdf This is IEEE’s official website for the 802.16 taskgroup

IEEE 802.16 FundamentalsIEEE 802.16 Fundamentals

Not a LAN … Not a LAN … not Peer-Peer basednot Peer-Peer based Instead: Access Point central control Instead: Access Point central control

(point-multipoint)(point-multipoint)

No contention … No contention … for Payloadfor Payload Instead: request/grant based schedulingInstead: request/grant based scheduling

Contention MACContention MAC

802.11 is like a 802.11 is like a meeting, where meeting, where anyone can talk at anyone can talk at any timeany time

DCF rules are like DCF rules are like manners, manners, recognizing who is recognizing who is currently speaking currently speaking and being politeand being polite

Too many people, and everyone tries to speak-up with people talking over each-other

Too many people, and everyone tries to speak-up with people talking over each-other

Scheduled MACScheduled MAC

802.16 Request/Grant is like a classroom 802.16 Request/Grant is like a classroom controlled by a teachercontrolled by a teacher

Students are not allowed to talk until they Students are not allowed to talk until they raise their hand and are CALLED ON by raise their hand and are CALLED ON by the teacherthe teacher

Collisions can’t happenCollisions can’t happen

Requests – Almost The Only Requests – Almost The Only Contention in 802.16Contention in 802.16

A regular time is set-aside every 0.5 msec to A regular time is set-aside every 0.5 msec to 2msec for any Student (user) to raise their hand 2msec for any Student (user) to raise their hand (request access)(request access)

If the Teacher (Access Point) hears the request, If the Teacher (Access Point) hears the request, they are called upon (sent a Grant)they are called upon (sent a Grant) If their request collides (no grant) they request again If their request collides (no grant) they request again

in a following slotin a following slot

Users select a random position within the Users select a random position within the available request times, and the system can available request times, and the system can allocate enough request time to insure it’s never allocate enough request time to insure it’s never overloadedoverloaded

Registration – The Other Registration – The Other Contention in 802.16Contention in 802.16

Students may ENTER or EXIT at any timeStudents may ENTER or EXIT at any time Entering students must make their presence Entering students must make their presence

knownknown Since students may exit without notice, all Since students may exit without notice, all

present students must periodically confirm their present students must periodically confirm their presencepresence

enter

exit

FramingFraming

The channel is divided into repeated Uplink/Downlink time-frames The channel is divided into repeated Uplink/Downlink time-frames by the Access Pointby the Access Point

Frame rate is system defined to be 0.5msec, 1msec, or 2msec in Frame rate is system defined to be 0.5msec, 1msec, or 2msec in lengthlength

DL UL DL UL DL UL

Both TDD (single channel) and FDD (separate UL & DL Both TDD (single channel) and FDD (separate UL & DL frequencies) are standardizedfrequencies) are standardized

DL

UL

DL

UL

DL

UL

DL

UL

DL

UL

DL

UL

TDD Format

FDD Format

Single channelSeparate UL & DL channels

When / Where is the Contention?When / Where is the Contention?

Contention is allocated Contention is allocated pre-defined subframes pre-defined subframes within any/every UL within any/every UL FrameFrame

Units randomly pick a Units randomly pick a sub-slot within an Initial sub-slot within an Initial Maintenance Opportunity Maintenance Opportunity slot to registerslot to register

Units randomly pick a Units randomly pick a sub-slot within a Request sub-slot within a Request Contention Opportunity to Contention Opportunity to request servicerequest service

DL UL DL UL DL UL

IMO RCO SD SD SD

Each Uplink Frame

802.16 Channel Frames

Scheduled Data slots

Initial Maintenance Opportunities

Request Contention Opportunities

Finding the Contention OpportunitiesFinding the Contention Opportunities

All units must locate / All units must locate / lock onto the lock onto the Preamble that starts Preamble that starts every DL frameevery DL frame

The very next frame The very next frame is is BBroadcast roadcast CControl ontrol containing DL & UP containing DL & UP mapsmaps

The UL map identifies The UL map identifies the makeup of the the makeup of the next uplink framenext uplink frame

DL UL DL UL DL UL

Pre BC TDM TDM TDM

Each Downlink Frame

802.16 Channel Frames

Time Division Multiplex slots

Broadcast Control

DLMAP

ULMAP

RegisteringRegistering

When a unit enters a cell, When a unit enters a cell, the unit locates the next the unit locates the next opportunity from the opportunity from the preceeding frame UL-MAPpreceeding frame UL-MAP

The unit can then transmit a The unit can then transmit a registration on a randomly registration on a randomly selected sub-slot in the next selected sub-slot in the next UL frameUL frame

The unit monitors for an Ack The unit monitors for an Ack in a following DL framein a following DL frame

DL UL DL UL DL UL

RxUL-MAP

Tx Registration

RxAck

Sending DataSending Data

When there is inbound data When there is inbound data to send, the unit locates the to send, the unit locates the next opportunity from the next opportunity from the preceeding frame UL-MAPpreceeding frame UL-MAP

The unit can then transmit a The unit can then transmit a request on a randomly request on a randomly selected sub-slot in the next selected sub-slot in the next UL frameUL frame

The unit monitors for a The unit monitors for a grant in a following DL grant in a following DL frame, and using a new UL-frame, and using a new UL-MAP identifies an assigned MAP identifies an assigned scheduled data slotscheduled data slot

The unit then monitors for The unit then monitors for an Ack in a following DL an Ack in a following DL frameframe

DL UL DL UL DL UL

RxUL-MAP

Tx Request

RxUL-MAPand Grant

Tx Data

RxAck

Bounding ContentionBounding Contention

IEEE 802.16 limits contention to IEEE 802.16 limits contention to registration and service requestsregistration and service requests This is a limited portion of UL FramesThis is a limited portion of UL Frames

Only the shortest control messages Only the shortest control messages contendcontend Registrations & requests basically contain a Registrations & requests basically contain a

unit identity and only a single argumentunit identity and only a single argument Everything else on the channel is Everything else on the channel is

scheduledscheduled ScheduledScheduled means means Contention FreeContention Free

Scheduling vs CapacityScheduling vs Capacity

Because contention is not permitted when Because contention is not permitted when transmitting payload, payload collision transmitting payload, payload collision does not exist (does not exist (cannot happencannot happen))

Payload capacity is guaranteed regardless Payload capacity is guaranteed regardless of user loadof user load

Capacity trade-off for scheduling is bound Capacity trade-off for scheduling is bound by the amount of channel time allocated to by the amount of channel time allocated to request / grant contention request / grant contention

SummarySummary

802.11 is a LAN, that is often used as a MAN802.11 is a LAN, that is often used as a MAN 802.11 uses contention with 802.11 uses contention with CCarrier arrier SSense and ense and

CCollision ollision AAvoidance algorithmsvoidance algorithms Neither Neither CSCS nor nor CACA work properly as a MAN work properly as a MAN When overloaded, 802.11 ceases to function when When overloaded, 802.11 ceases to function when

used as a MAN (capacity approaches zero)used as a MAN (capacity approaches zero) A new standard has been developed for MAN A new standard has been developed for MAN

applications (802.16)applications (802.16) 802.16 eliminates the basic shortcoming of 802.16 eliminates the basic shortcoming of

802.11 used as a MAN by eliminating virtually all 802.11 used as a MAN by eliminating virtually all contentioncontention