MAC Research Highlight

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MAC Research Highlight

• Y.C. Tseng

2
Outline

• 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 MobileHoc 2002.
• 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.

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• 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.

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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)

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State Transition Diagram of Backoff
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Some Important Transitions
start backoff
backoff 1 step
failure, next stage
successful trans.
failure, max stage
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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

B
A
8

• 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.

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Research HighlightCollision Avoidance and
Resolution Multiple Access with Transmission
Groups

• R. Garces and J. J. Garcia-Luna-Aceves
• INFOCOM97

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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

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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
Z, Y, A, B, C
X, Z, Y, A, B, C
media
contention period group trans. period
X Y Z
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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

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Group-Transmission Period

• A station transmits once the previous stations
  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.

Bs transmission exceeds propagation delay
A
B
C
A
C
A
C
B contend later
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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
  window in the current contention period
• contention window the allowable IDs that can
  contend
• an ID not within this range can not contend
• a stack the future potential contention windows

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NTG Scheme

• Initially, LowID1 and HiID(max. ID in the
  system)
• On RTS conflict, all stations divide (LowID,
  HiID) into
• (LowID, (LowIDHiID)/2)
• ((LowIDHiID)/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

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Contention Example

• A system with 4 stations n00, n01, n10, n11.
• n00 and n01 are contending.

(a)
(c)
(b)
(d)
n01 RTS
n00 RTS
n11 idle
n10 idle
after idle
after 2nd collision
after n01 success
before 1st collision
after 1st collision
(a)
(b)
(c)
(d)
(00, 01)
(00, 00)
(00, 11)
(10, 11)
allowed interval
(00, 01)
(01, 01)
(00, 11)
n01 RTS
n00 RTS
n01 RTS
packets
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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

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Research HighlightPolling 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, INFOCOM97.

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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

low probability
talk
silent
low probability
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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.

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(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.

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• Simulation results
• Smaller k gives better data throughput (Fig. 14).
• k 15 does not affect the voice delay (Fig. 15).

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Short Summary

• An interesting polling mechanism based on
  specific applications.
• Future directions how to support other types of
  media.

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A Real-Time Medium Access Control Protocol for Ad
Hoc Wireless Local Area Networks

• In ACM Mobile Computing and Communication Review,
  
• 1999, Vol. 3, No. 2, pp. 20-27,
• by R. O. Baldwin, N. Davis, and S. Midkiff.

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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.

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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.

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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.

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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.

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Rule 1Enhanced 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.

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• Details
• Prior to transmitting a packet, a station will
  select its next BV from the range of 0..CW-1,
  excluding those BVs 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.

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• Example
• A 3 ? 1 ? 8
• B 1 ? 6 ? ...
• C 5 ? 2 (collides with Bs, changed to 3)

B(6)
A(1)
A(8)
C(3)
B(...)
C(...)
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Rule 2Transmission 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.

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Rule 3Contention 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).

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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..2CBV-1).

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Collision Ratio

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

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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.

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Research Highlights How to reduce handover time?
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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.

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Research HighlightFast 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 APs neighbors. About 10 ms are
  needed to scan a specific neighbor.

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Research HighlightFast 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.

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Research HighlightFast 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.

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Other Readings

• Medium Access Control
• R. Garces and J.J. Garcia-Luna-Aceves, Floor
  Acquisition Multiple Access with Collision
  Resolution, Proc. ACM/IEEE MobiCom 96, Rye, New
  York, November 11-12, 1996.
• Z. Tang and J.J. Garcia-Luna-Aceves,
  Hop-Reservation Multiple Access (HRMA) for
  Ad-Hoc Networks, Proc. IEEE INFOCOM '99, New
  York, New York, March 21--25, 1999.
• V. Bharghavan, A. Demers, S. Shenker and Lixia
  Zhang, MACAW A Media Access Protocol for
  Wireless LAN's, Proceedings of SIGCOMM 94,
  pp.212-225.
• P. Karn, MACA - A New Channel Access Method for
  Packet Radio, ARRL/CRRL Amateur Radio 9th
  Computer Networking Conference, April 1990,
  pp.134-140.
• Romit Roy Choudhury, Xue Yang, Ram Ramanathan,
  and Nitin Vaidya, Using Directional Antennas for
  Medium Access Control in Ad Hoc Networks, ACM
  International Conference on Mobile Computing and
  Networking (MobiCom), September 2002.