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1
National Research Council - Pisa - Italy
WLAN and WPAN technologies for Mobile ad hoc
Networks Marco Conti Italian National Research
Council (CNR) IIT Institute marco.conti
_at_iit.cnr.it
Joint work with G. Anastasi, E. Gregori and A.
Passarella
2
Emerging WLAN and WPAN technologies
The availability of appropriate networking
standards is a major factor for a successful
network technology

• Bluetooth a de-facto standard for WPAN
• IEEE 802.11 the family of IEEE standard for
  WLAN
• HiperLAN 1 and 2 the ETSI standards for WLAN

3
IEEE 802.11 and Ad-hoc Networking
infrastructure-based
ad-hoc networking
Due to the flexibility of the CSMA/CA algorithm
stations synchronization (to a common clock) is
sufficient to receive or transmit data correctly
4
Bluetooth and Ad-hoc Networking
AT HOME
IN OFFICE
OUTDOOR
5
Performance indices
CAPACITY

• Maximum throughput normalized to the channel
  speed
• The capacity gives a good indication of the
  overhead required by the protocol to perform its
  coordination task

DELAY

• MAC delay the best figure to measure the
  behavior of a protocol
• Response time the best figure to measure the QoS
  perceived by the users

FAIRNESS
6
IEEE 802.11 Standards

• In 1997, the IEEE adopted the first wireless
  local area network standard, named IEEE 802.11,
  with data rates up to 2Mbps
• Since then, several task groups have been created
  to extend the IEEE 802.11 standard (designated by
  a, b, c, etc.)
• The 802.11b task group produced in 1999 a
  standard for WLAN operations in 2.4 GHz band,
  with data rates up to 11 Mbps. This standard has
  been very successful (Wi-Fi)
• The 802.11a task group created a standard for
  WLAN operations in the 5 GHz band, with data
  rates up to 54 Mbps.

7
IEEE 802.11 Architecture
Fundamental Access Method
8
Distributed Coordination Function (DCF)

• Carrier Sense Multiple Access/Collision Avoidance
• Interframes spaces SIFS lt DIFS
• Access modes
• Basic Access
• RTS/CTS mechanism
• Immediate positive ACK
• Binary Exponential Backoff

9
DCF basic access overview

• Successful transmission

Source
Destination
Other

• Collision

LA
There is no collision detection each collided
packet is completely transmitted
Collision Length collided packet maximum
length (LA)
10
Backoff procedure

• Selection of a Contention Window
• The CW is doubled after each retransmission till
  a CWMAX is reached
• CW-value increasing sequence
• (time_slots number)

• Selection of a random Backoff Time

• Reduction of the Backoff Time
• After an idle DIFS period from the last
  transmission, a station decrements its
• Backoff Time by a Slot_time for each slot
  where no activity is sensed on the
• medium.
• Frozen
• As soon as the medium is determined to be
  busy, the backoff procedure is
• suspended
• Transmission
• When the Backoff Time reaches zero, the
  station starts the transmission

11
IEEE 802.11 capacity steady state analysis
The protocol efficiency decreases with the
increase of the number of active stations (M)

• Pseudo-bayesian algorithms() stabilizes the
  protocol and     achieves the theoretical
  throughput

() F. Calì, M. Conti, E. Gregori, "Dynamic
Tuning of the IEEE 802.11 Protocol to Achieve a
Theoretical Throughput Limit, IEEE/ACM
Transactions on Networking, December 2000.
12
IEEE 802.11 MAC delay
Typical behavior of random access protocols

• Low delays under light load
• Unbounded delays when approaching the protocol
  capacity
• Stabilizing algorithms stochastically solve the
  problem

13
IEEE 802.11 and hidden stations
Sender S1
Receiver
Sender S2
Carrier Sensing fails with hidden stations
Request To Send (RTS) / Clear To Send (CTS)
mechanism
14
Request To Send (RTS) / Clear To Send (CTS)
mechanism

• Virtual Carrier Sensing
• RTS packet frozes the channel in the sender
  coverage area
• CTS packet frozes the channel in the receiver
  coverage area

15
IEEE 802.11, hidden stations and TCP

• The behavior of TCP/IP protocols on top of IEEE
  802.11 is an open research issue
• Some simulative studies seem to point out that
  may exist Capture phenomena and severe Unfairness
  problems
• These studies are highly dependent on the
  interference  model

Measurements studies are required
16
IEEE 802.11 measurements
Indoor Experiments. In this case the experiments
were performed in a scenario characterized by
hidden stations
17
IEEE 802.11 behavior
The Transmission Range (TX_Range) represents the
range (with respect to the transmitting station)
within which a transmitted packet can be
successfully received. The Physical Carrier
Sensing Range (PCS_Range) is the range (with
respect to the transmitting station) within which
the other stations detect a transmission. Interfer
ence Range (IF_Range) is the range within which
stations in receive mode will be interfered
with by a transmitter, and thus suffer a loss.
18
IEEE 802.11 behavior
The following relationship exists between the
ranges TX_Range lt IF_Range ltPCS_Range in
the Indoor experiments even though transmitting
nodes are outside the transmission range of each
other, they are inside the same carrier sensing
range. Therefore, the physical carrier sensing is
effective, and hence adding a virtual carrier
sensing (i.e., RTS/CTS) is useless. () in NS2
the following values are used TX_Range250m,
IF_RangePCS_Range550m
19
IEEE 802.11 measurements
Outdoor Experiments. Two ftp sessions are
contemporary active. The arrows represent the
direction of the ftp sessions.

• by varying the distance d, the couples of nodes
  are
• in the same transmitting range (Exp1)
• out of the transmitting range, but inside the
  same carrier sensing range (Exp2)
• out of the same carrier sensing range (Exp3).

20
IEEE 802.11 measurements
The achieved results, summarized in the Table,
show that i) Exp1. All stations are inside the
same TX_Range, and a fair bandwidth sharing is
almost obtained. The RTS/CTS mechanism is
useless ii) Exp3. In this case the two sessions
are independent and both achieve the maximum
throughput. iii) Exp2. In the intermediate
situation a capture of the channel by one of
the two TCP connections is observed. In this case
the RTS/CTS mechanism provides a little help in
solving the problem.
21
IEEE 802.11 measurements
The table reports results obtained in the Exp2
configuration when the traffic flows are either
TCP or UDP based. As shown in the table, the
capture effect disappears when the UDP protocol
is used.
22
A Bluetooth Network Piconet and Scatternet

• Piconet
• Is the bulding block
• Capacity 1 Mbps
• Two Bluetooth units (a master and one slave) can
  form a piconet
• A Piconet contains one master and up to 7 slaves
• The units of a Piconet are synchronized on the
  master clock

• Scatternet
• Two (or more) partially overlapping piconets
• A Bluetooth units can be master of one Piconet
  only

23
Bluetooth Architecture
24
Bluetooth the Physical channel

• The channel is slotted
• The synchronization is provided by the Master
  clock
• Time-Division Duplex (TDD) transmission
• The Master (Slave) can start its transmissions in
  the even (odd) slots only
• Frequency hopping (79 RF channels in the ISM
  band)
• Frequency hopping sequence is piconet dependent

25
The Bluetooth packet

• The access code (channel access code) identifies
  all the packets belonging to the same piconet
• The header contains the Slave address, the type
  of packet and the ARQ protocol information

26
Bluetooth Multi-slot Packets

• A packet is transmitted in 1, 3 or 5 consecutive
  slots
• A packet is transmitted on a single frequency
  the one corresponding to the starting slot

27
Bluetooth Services

• SCO Link Synchronous Connection Oriented Link
• symmetric, point-to-point between the master and
  one slave
• periodic transmission with period equal to 2, 4
  or 6 time-slots
• circuit-switched
• Rate equal to 64 kbps if used for voice traffic

• ACL Link Asynchronous Connection-Less Link
• point-to-multipoint link between the master and
  all the slaves
• transmission in the slots not reserved to the SCO
  links
• packet-switched
• symmetric and asymmetric data transmission

28
Bluetooth Behavior

• Polling system a slave can transmit only after
  the master polling

29
Bluetooth limiting performance
30
Bluetooth and TCP
655.36 kbps
569 kbps
Ideal conditions inside the Internet
Capacity reduction is mainly due to the polling
mechanism
31
Performance evaluation of a Bluetooth Piconet
Traffic patterns
Symmetric traffic
Asymmetric traffic
32
Bluetooth performance the capacity
33
Increasing Bluetooth efficiency

• Bluetooth performance are highly affected by the
  polling mechanism
• The Round Robin scheduling algorithm is affected
  by the traffic pattern
• We are currently working to the definition of a
  scheduling algorithm whose performance are almost
  independent from the traffic pattern
• The scheduling algorithm inside a Scatternet is
  an open issue

34
IEEE 802.11 vs. Bluetooth
IEEE 802.11 Bluetooth

• Random access (flexible)

• Token based (fair)

• best effort traffic

• service integration

• variable QoS

• QoS guarantees

• Speed up to 11Mbps

• Speed equal to 1Mbps

• Campus coverage

• Room coverage

• Scatternets ?

• hidden stations problem

• unfairness with TCP ?

• Efficiency with TCP ?