MAC Research Highlight - ocw.nctu.edu.twocw.nctu.edu.tw/course/ws992/D2.pdf · – K. Kanodia et al., “Ordered Packet Scheduling in Wireless Ad Hoc Networks: Mechanisms and Performance

國立交通大學國立交通大學國立交通大學國立交通大學資訊工程系資訊工程系資訊工程系資訊工程系曾煜棋曾煜棋曾煜棋曾煜棋教授教授教授教授

MAC Research HighlightMAC Research Highlight

Y.C. TsengY.C. Tseng

Outline: 3 Main Research IssuesOutline: 3 Main Research Issues

• Analysis:– G. Bianchi, “Performance Analysis of the IEEE 802.11 Distributed

Coordination Function”, IEEE J-SAC, 2000.– K. Kanodia et al., “Ordered Packet Scheduling in Wireless Ad Hoc

Networks: Mechanisms and Performance Analysis”, ACM MobileHoc2002.

• Protocols:– R. Garces and J. J. Garcia-Luna-Aceves, "Collision Avoidance and Resolution Multiple Access with Transmission Groups", INFOCOM 2007.

– B. P. Crow, J. G. Kim, & P. Sakai, "Investigation of the IEEE 802.11 Medium Access Control (MAC) Sublayer Functions", INFOCOM'97.

– R. O. Baldwin, N. Davis, and S. F. Midkiff, "A Real-time Medium Access Control Protocol for Ad Hoc Wireless Local Area Networks", ACM MC2R, Vol. 3, No. 2, 1999, pp. 20-27.


• Handover latency reduction:– H. Kim, S. Park, C. Park, J. Kim, and S. Ko, “Selective

Channel Scanning for Fast Handoff in Wireless LAN using Neighbor Graph”, ITC-CSCC 2004, July 2004.

– S. Shin, A. S. Rawat, H. Schulzrinne, "Reducing MAC Layer HandoffLatency in IEEE 802.11 Wireless LANs", ACM MobiWac'04, Oct, 2004.

– C.C. Tseng, K.H. Chi, M.D. Hsieh, and H.H. Chang, “Location-based fast handoff for 802.11 networks”, IEEE Communications letters, vol. 9, issue 4, pp. 304-306, April 2005.



Research Highlight:

DCF Performance Analysis

� Ref: G. Bianchi, “Performance Analysis of the IEEE

802.11 Distributed Coordination Function”, IEEE J-SAC,

2000.

� Assuming saturation situation (stations always have

packets to transmit), the work analyze the DCF

performance.

� state of a station: (s(t), b(t))

� s(t): backoff stage (0, 1, …, m) of the station

�CWmax = 2m Wmin

�Let Wi = 2i W.

� b(t): backoff counter value

� p: colliding probability (a constant)


State Transition Diagram of Backoff


Some Important Transitionsstart backoff

failure, next stage

failure, max stage

backoff 1 step

succ

essf

ul

tran

s.


Research Highlight: Unfair Access

� Ref: K. Kanodia et al., “Ordered Packet Scheduling in

Wireless Ad Hoc Networks: Mechanisms and Performance

Analysis”, ACM MobileHoc 2002.

� As there are multiple wireless links coexisting, some

unfairness problem may arise.

� Scenario 1: Asymmetric Information

�throughputs ratio of A to B = 5% : 95%

�reason: B knows more information than A does

AB


� Scenario 2: Perceived Collision

�throughputs of A : B : C = 36% : 28% : 36%

�reason: Due to spatial reuse, flow A and C can capture the

channel simultaneously, thus causing flow B to reserve

consecutive NAVs.

� Proposed solution: “Distributed Wireless Ordering

Protocol”

�an ordered distributed packet scheduling for MAC

�can be based on any reference scheduler, such as FIFI,

Virtual Clock, Earliest Deadline First.

AB

C


Research Highlight:

Collision Avoidance and Resolution

Multiple Access with Transmission Groups

R. Garces and J. J. Garcia-Luna-Aceves

INFOCOM’97


Abstract

� a CARMA-NTG protocol for accessing wireless media

�CARMA-NTG = Collision Avoidance and Resolution

Multiple Access Protocol with Non-persisitent Trees and

transmission Group

�Based on transmission group

�Once obtaining the medium, a station will have its right to

keep on sending.

�based on RTS/CTS messages


Concept of Cycles

� Dynamically divide the channel into cycles of variable

length.

�Each cycle contains a contention period and a group-

transmission period.

�The group-transmission period is a train of packets sent by

users already in the group.

� New users contend to join transmission group by

contending during the contention period.

A, B, C Y, A, B, C

X Y Z

Z, Y, A, B, C X, Z, Y, A, B, Cmedia

: contention period

: group trans. period


Each STA Needs to Keep Track of …

� To send in the transmission period, each station must know

the following environment parameters:

�the number of members in the transmission group

�its position within the group

�the beginning of the each group-transmission period

�the successful RTS/CTS exchange of new users in the

previous contention period


Group-Transmission Period

� A station transmits once the previous station’s packet is

received.

�The spacing is twice the propagation delay.

� If this is not heard during this period,

�assume that the previous station fails

�its membership is removed from the group

�the failed station has to contend to join the group later.

A B C A C A C

B contend later

B’s transmission exceeds

propagation delay


Contention Period

� Contending based on RTS/CTS exchange.

� The contention period terminates once the first station successfully join the group.

� Each station runs the NTG scheme (non-persistent tree and transmission group)

� Each station keeps the following variables:

�a unique ID

�LowID and HiID: to denote the current contention windowin the current contention period

�contention window: the allowable ID’s that can contend

�an ID not within this range can not contend

�a stack: the future potential contention windows


NTG Scheme

� Initially, LowID=1 and HiID=(max. ID in the system)

� On RTS conflict, all stations divide (LowID, HiID) into

�(LowID, (LowID+HiID)/2)

�((LowID+HiID)/2 + 1, HiID) // i.e., binary split

� PUSH the first part into STACK

� Contend if its ID is within the latter part.

� If no RTS is heard after channel delay, POP the stack and repeat recursively.

� ONLY stations in the RTS state can persist in trying.

�new stations: backoff and wait until the next period

�already-in-group stations: not until they leave the group


Contention Example

� A system with 4 stations: n00, n01, n10, n11.

� n00 and n01 are contending.

n11

idle

n10

idle

n01

RTS

n00

RTS

(00, 11)

before 1st

collision

(10, 11)

after 1st

collision

allowed interval

(00, 01)

after idle

(00, 01)

after 2nd

collision

(01, 01)

(00, 00)

after n01

success

(00, 11)

n01

RTS

n00

RTS

n01

RTSpackets

(a)

(a) (b)

(b) (c)

(c) (d)

(d)


Short Summary

� propose the concept of group transmission

�Only one RTS/CTS exchange is used for transmitting a train

of packets

�better fairness than IEEE 802.11

� NTG (non-persistent tree group) keeps the contention cost

low.

� Performance:

�on high load, similar to TDMA

�on low load, better than TDMA by getting rid of empty slots


Research Highlight:

Polling Issue in IEEE 802.11

Research Highlight:

Polling Issue in IEEE 802.11

�“Investigation of the IEEE 802.11 Medium Access Control (MAC) Sublayer Functions”, B. P. Crow,

J. G. Kim, & P. Sakai, INFOCOM’97.


Problem Statement

� In the PCF function of IEEE 802.11, it is NOT specified how to poll STAs.

� Problem: how to do voice communication using PCF?

�Assuming that all voice packets have the

same priority.

� Voice stream characteristic:

�ON-and-OFF process

�ON = talking;

�OFF = listening

talk silent

low probability

low probability


A “Round Robin” Approach

� AP keeps track of the list of STAs to be polled.

�When CFP begins, the AP polls the STAs

sequentially.

�If the AP has an MPDU to send, the poll and MPDU are combined in one frame to be sent.

�O/w, a sole CF-Poll is sent.

�When CFP ends, the AP keeps track of

the location where the polling stops.

�Then resume at the same place in the next

CFP.


(cont.)

� Within a CFP_Repetition_Interval, if an STA sends no payload in k polls, the STA is dropped from the polling list.

� k is an tunable parameter

� In the next CFP, the STA will be added back to the list again.

� Basic Idea: to avoid useless polling.


� Simulation results:

�Smaller k gives better data throughput (Fig.

14).

� k = 1~5 does not affect the voice delay

(Fig. 15).


Short Summary

� An interesting polling mechanism based on specific applications.

� Future directions: how to support other types of media.


� In ACM Mobile Computing and

Communication Review,

� 1999, Vol. 3, No. 2, pp. 20-27,

� by R. O. Baldwin, N. Davis, and S.

Midkiff.

A Real-Time Medium Access Control

Protocol for Ad Hoc Wireless Local

Area Networks


Goal

� An enhancement of IEEE 802.11 for real-time

communication.

� less mean delay

� less misses of deadline

� less packet collisions

� In RT applications, each packet has a

deadline.

� After the deadline, sending this packet is useless.

� Ex: Military personnel in the field communicate with their weapons remotely and wirelessly.


Review of IEEE 802.11

� The CW (contention window) is initially CWmin, and is doubled after each failure, until CWmax is reached.

�BV (backoff value) randomly in [0..CW-1].

� The BV is decreased after each idle slot.


Drawback of IEEE 802.11

� Can not meet the requirements of real-time communication.

�When a packet has missed its deadline,

the packet will still be buffered and sent.

� Thus, this causes more contention,

collisions, ...

�more packets may miss their deadlines.


Basic Idea of RT-MAC

(Real-Time MAC)

� Each packet is associated with a deadline when passed to the MAC layer.

�Note: The deadline value does not need to

be sent along with the packet.

�After the deadline, the packet will not be

sent.

� There are 4 rules (next few pages).


Rule 1:

Enhanced Collision Avoidance� Announcing the next BV:

�When a packet is transmitted, the next BV

to be used is placed in a field of the packet.

�Stations who hear this packet will avoid

selecting this BV as their next backoff

timer.

�BV is a random number in [0..CW-1].


� Details:

�Prior to transmitting a packet, a station will

select its next BV from the range of

[0..CW-1], excluding those BV’s already

chosen by other stations.

�A station will indicate in its data packet the

next BV value to be used.

�A station should keep a table of BV values

used by other stations.

�After an idle slot, a station should decrease its own BV, as well as others’ BVs in its table.


� Example:

�A: 3 � 1 � 8

�B: 1 � 6 � ...

�C: 5 � 2 (collides with B’s, changed to 3)

B(6) A(1) A(8) C(3) B(...) C(...)0 1 2 3 4 5 6 7 …


Rule 2:

Transmission Control� A station must send when its BV value

has expired.

� If the packet experiences transmission failure, it will be reexamined to see if its deadline has been missed.

�Note: another backoff still has to be taken.


Rule 3:

Contention Window Size

� CW is set to 8N, where N is the estimated

number of “real-time” stations.

� N: can be estimated by counting the number of

unique addresses for a period of time.

� [alternative] N: a function of current channel load.

� “8” is chosen by instinct.

� Note: CW is thus not doubled after a

transmission failure

� (compared the original IEEE 802.11 of doubling each time).


Rule 4: Collision of BV�Due to mobility, transmission error, and

collisions, a station may receive a packet indicating a BV equal to its own BV.�The station must select another BV value;

otherwise, collision will occur.

� To avoid the station being unduly penalized, the new BV should be selected from [0..CBV-1].�CBV = its current BV.

�I.e., the station is given higher priority.

� If all values in [0..CBV-1] are chosen, then we double it (i.e., [0..2*CBV-1]).


Collision Ratio

� RT-MAC is quite stable in collision prob. with respect to the number of stations.


Short Summary

� A new RT-MAC protocol.

� broadcasting the next BV value

�BV depends on the current number of

stations

� Results:

� The network behavior is quite stable in

terms of mean delay, missed deadline

ratio, and collision ratio.

� The mean delay is quite independent of

the number of stations.


Research Highlights

How to reduce handover time?


How to reduce handover time?

� Channel scanning in 802.11

is very time-consuming if

all channels need to be

scanned.

�If scanning one channel

takes 30 ms, the toally

300-400 ms is needed.


Research Highlight:

Fast Channel Scanning by Neighbor Graph

� Ref: H. Kim, S. Park, C. Park, J. Kim, and S. Ko,

“Selective Channel Scanning for Fast Handoff in Wireless

LAN using Neighbor Graph”, ITC-CSCC 2004, July 2004.

� Method:

�A concept called neighbor graph (NG) is proposed. From the

NG provided by an external server, a MH only needs to scan

the channels that are used by its current AP’s neighbors.

About 10 ms are needed to scan a specific neighbor.


Research Highlight:

Fast Channel Scanning by Caching

� Ref: S. Shin, A. S. Rawat, H. Schulzrinne, "Reducing

MAC Layer HandoffLatency in IEEE 802.11 Wireless

LANs", ACM MobiWac'04, Oct, 2004.

� Method:

�MH maintains a cache which contains a list of APs adjacent

to its current AP.

�The cached data was established from its previous scanning.

�Only the two APs with the best RSSI were cached.

�During handoff, the cached APs are searched first. If this

fails, scanning is still inevitable.


Research Highlight:

Fast Channel Scanning by Location Information

� Ref: C.C. Tseng, K.H. Chi, M.D. Hsieh, and H.H. Chang,

“Location-based fast handoff for 802.11 networks”, IEEE

Communications letters, vol. 9, issue 4, pp. 304- 306, April

2005.

� Method:

�MH can predict its movement path and select the potential

AP.

�A location server is needed to provide information of APs.

�So a MH can re-associate with its new AP directly without

going through the probe procedure.

�However, this scheme relies on a precise localization method.

Other ReadingsOther Readings

•• Medium Access ControlMedium Access Control–– R. R. GarcesGarces and J.J. Garciaand J.J. Garcia--LunaLuna--AcevesAceves, , ““Floor Acquisition Multiple Floor Acquisition Multiple

Access with Collision Resolution,Access with Collision Resolution,”” Proc. ACM/IEEE Proc. ACM/IEEE MobiComMobiCom 96, Rye, 96, Rye, New York, November 11New York, November 11--12, 1996.12, 1996.

–– Z. Tang and J.J. GarciaZ. Tang and J.J. Garcia--LunaLuna--AcevesAceves, , ““HopHop--Reservation Multiple Access Reservation Multiple Access (HRMA) for Ad(HRMA) for Ad--Hoc Networks,Hoc Networks,”” Proc. IEEE INFOCOM '99, New York, Proc. IEEE INFOCOM '99, New York, New York, March 21New York, March 21----25, 1999.25, 1999.

–– V. V. BharghavanBharghavan, A. Demers, S. , A. Demers, S. ShenkerShenker and and LixiaLixia Zhang, Zhang, ““MACAW: A MACAW: A Media Access Protocol for Wireless LAN's,Media Access Protocol for Wireless LAN's,”” Proceedings of SIGCOMM 94, Proceedings of SIGCOMM 94, pp.212pp.212--225.225.

–– P. P. KarnKarn, , ““MACA MACA -- A New Channel Access Method for Packet Radio,A New Channel Access Method for Packet Radio,””ARRL/CRRL Amateur Radio 9th Computer Networking Conference, ApriARRL/CRRL Amateur Radio 9th Computer Networking Conference, April l 1990, pp.1341990, pp.134--140.140.

–– RomitRomit Roy Roy ChoudhuryChoudhury, , XueXue Yang, Ram Yang, Ram RamanathanRamanathan, and , and NitinNitin VaidyaVaidya, , ““Using Directional Antennas for Medium Access Control in Ad Hoc Using Directional Antennas for Medium Access Control in Ad Hoc Networks,Networks,”” ACM International Conference on Mobile Computing and ACM International Conference on Mobile Computing and Networking (Networking (MobiComMobiCom), September 2002.), September 2002. 國立交通大學國立交通大學國立交通大學國立交通大學資訊工程系資訊工程系資訊工程系資訊工程系曾煜棋曾煜棋曾煜棋曾煜棋教授教授教授教授

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MAC Research Highlight - ocw.nctu.edu.twocw.nctu.edu.tw/course/ws992/D2.pdf · – K. Kanodia et al., “Ordered Packet Scheduling in Wireless Ad Hoc Networks: Mechanisms and Performance