MAC Research Highlight
Y.C. Tseng
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.
• 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. 304306, 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 Transitions
successful trans.
backoff 1 step
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
A
B
A

B
C
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.
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 grouptransmission 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.
media
A, B, C
Y, A, B, C
Z, Y, A, B, C
X, Z, Y, A, B, C
: contention period
X
Y
Z
: 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.
B’s transmission exceeds
propagation delay
A
B
C
A
C
A
C
B contend later
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 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.
(a)
(b)
n11
idle
(a) before 1st
collision
after 1st
collision
(c)
(d)
n10
idle
n01
RTS
n00
RTS
(b) after idle
(00, 01)
allowed interval (00, 11)
packets
n01
RTS
n00
RTS
(10, 11)
(c) after 2nd
collision
(d) after n01
success
(00, 00)
(00, 01)
(01, 01)
(00, 11)
n01
RTS
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
 “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.
low probability

Voice stream characteristic:
 ON-and-OFF process
 ON = talking;
 OFF = listening
talk
silent
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.
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.
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 realtime 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.
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(...)
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 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.