mac research highlight - ocw.nctu.edu.twocw.nctu.edu.tw/course/ws992/d2.pdf · – k. kanodia et...
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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.
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
• 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.
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
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
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)
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
State Transition Diagram of Backoff
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
Some Important Transitionsstart backoff
failure, next stage
failure, max stage
backoff 1 step
succ
essf
ul
tran
s.
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
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
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
� 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
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
Research Highlight:
Collision Avoidance and Resolution
Multiple Access with Transmission Groups
R. Garces and J. J. Garcia-Luna-Aceves
INFOCOM’97
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
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
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
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
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
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
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
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
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
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
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
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
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
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)
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
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
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
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.
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
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
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
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.
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
(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.
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
� Simulation results:
�Smaller k gives better data throughput (Fig.
14).
� k = 1~5 does not affect the voice delay
(Fig. 15).
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
Short Summary
� An interesting polling mechanism based on specific applications.
� Future directions: how to support other types of media.
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
� 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
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
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.
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
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.
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
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.
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
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).
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
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].
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
� 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.
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
� 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 …
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
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.
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
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).
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
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]).
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
Collision Ratio
� RT-MAC is quite stable in collision prob. with respect to the number of stations.
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
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.
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
Research Highlights
How to reduce handover time?
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
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.
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
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.
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
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.
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
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