‫دانشکده‌مهندس ی‌کامپیوتر‬
‫شبکه‌های‌بی‌سیم‌(‪)40-873‬‬
‫شبکه‌های‌محلی‌بی‌سیم‌با‌دسترس ی‌تصادفی‬
‫ّ‬
‫نیمسال‌دوم‌‪92-93‬‬
‫ّ‬
‫افشین‌همتیار‬
Introduction
 FDM-TDMA Networks:
• Stream traffic over circuit multiplexed cellular networks.
• A centrally coordinated mechanism for sharing the channels provides
capacity on demand to a call.
• The call is blocked if the requisite resource is not available.
 CDMA Networks:
• Allocating resources
to stream traffic to satisfy in-call
requirements like BER.
• An arriving call is blocked if the in-call QoS cannot be met.
• Resource allocation for packet multiplexed elastic traffic.
QoS
 OFMA-TDMA Networks:
• Packet multiplexing with a centralized resource allocation of the
OFDMA system.
• The carrier and timeslots are the resources and, if a packet cannot be
transmitted on arrival, it is queued and not dropped or blocked.
 WLAN Networks:
• Packet multiplexed wireless networks with queueing of packets, but
with distributed resource sharing mechanisms.
• Several nodes share a wireless medium with possibly no central
coordination.
• Distributed multiplexing using random access techniques.
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Overview (1)
• All the nodes of network use the same part of the spectrum.
• Random access based Medium Access Control (MAC)
• Protocols for distributed access control.
• Aloha MAC protocol analysis based on elementary probability
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theory and Markov chains.
Some current applications of the Aloha protocol in cellular
networks and in VSAT networks.
MAC protocols for Wireless Local Area Networks (WLANs).
Hidden and exposed nodes in multihop WLANs.
Handshake mechanism for collision avoidance.
Carrier Sense Multiple Access with Collision Avoidance
(CSMA/CA) protocol of the IEEE 802.11 WLAN MAC standard.
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Overview (2)
• ETSI HIPERLAN .
• A simplified version of a popular saturation throughput
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analysis of the IEEE 802.11 MAC protocol.
Renewal reward theorem and a fixed point theorem.
Service differentiation mechanisms in IEEE 802.11 networks.
The performance of TCP-based data and voice traffic over
WLANs.
Optimal association of an 802.11 node to an access point.
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Preliminaries (1)
• A minimum SINR (θ) is required by a receiver to decode a
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packet that is being received.
Like in CDMA networks, in WLANs also all nodes use the
entire allocated spectrum.
We use a simplified model of channel usage.
Collision: Whenever two or more transmissions in the same
frequency
band
and
of
sufficient
strength
arrive
simultaneously at a receiver, neither can be detected.
A collision can occur even after a receiver has successfully
decoded a part of a transmission.
Capture: It can happen that if a receiver is simultaneously
receiving signals from one or more transmitters, one of them
is strong enough for the resulting SINR to be above the
threshold, and received packets can be decoded.
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Preliminaries (2)
• Lower bound of the throughput and capacity obtained by not
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accounting for capture.
Interference region: carrier sense region
Decode region: a subset of carrier sense region.
If the transmitter is in the interference region of the receiver,
the received power at the receiver is significant and causes a
collision at the receiver if another transmission is being
received at the same time.
If the transmitter is also in the decode region, then the SNR
at the receiver is greater than the prescribed threshold (θ),
and the receiver can decode the transmission.
The received power from transmitters outside of the
interference region is assumed insignificant.
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Preliminaries (3)
• The solid line is the boundary of the decode region and the
dashed line is that of the interference region.
• When no other node is transmitting, A can decode
transmissions from B (or C).
• Transmissions from D or E (or both) when receiving from B
(or C) can cause a collision at A. However, the transmissions
from D and E cannot be decoded at A.
• Transmissions from G or H do not cause a collision at A.
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Preliminaries (4)
• The decode and interference regions depend on the locations
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of the other nodes that are transmitting.
We can also define the interference and decode regions for a
transmitter.
The interference regions (decode regions) for transmission
and reception may not be equal.
In single hop networks (also called broadcast networks or
co-located networks), every node is within the decode region
of every other nodes.
We will consider only single hop networks.
The channel is also called the medium and a Medium Access
Control (MAC) protocol regulates use of the medium by
prescribing the rules to initiate a transmission and continue
with it.
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Preliminaries (5)
• In random access networks, collisions may occur and the
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MAC protocol has to resolve collisions; it has to arbitrate
among the nodes contending to use the medium.
The arbitration is a distributed algorithm that typically
prescribes forced silences on the nodes.
Some amount of transmission time is lost to collisions and
arbitration.
The fraction of time so lost is a measure of the efficiency of
the protocol.
Simple protocols, even if of low efficiency, are useful if the
per node throughput that the protocol obtains is significant
compared to the throughput required by the nodes in the
network.
Two key wireless MAC protocols: Aloha and CSMA/CA.
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Random Access (1)
• An example:
 A 100-node multiple access network using a 10 Mbps
channel.
 Each node requires an average throughput of 1 Kbps, but
in bursts. A node may have to transmit 1000-bit packets,
on an average once every second.
 In a TDM scheme, each node would be statically allocated
every 100th slot.
 If each slot corresponds to a packet transmission time,
the waiting time before the packet transmission is
completed could be as high as 10.1 ms and the expected
delay would be 5.1 ms. This is assuming that the queue is
empty when this packet arrives at the node.
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Random Access (2)
• Polling scheme: polling overhead and a corresponding delay.
• Random access:
 Transmission rate is 10 Mbps
 Transmission time is just 100 μs
 Offered load to the network is low (1% of the maximum
possible throughput)
 Access delay, the delay between the packet being ready
and the beginning of transmission, will also be low.
• In TDM and polling schemes, the exact number of nodes
needs to be known.
• In wireless networks, at any time, number of nodes is
typically a random number. Thus random access is possibly
the only option, which is not a bad option.
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Aloha (1)
• Earliest random access MAC protocol: Aloha (pure Aloha)
• Simple idea: If a node has a packet to transmit, it just
transmits!
• Example of a satellite network:
 Every node transmits to a satellite, which then reflects the
transmission back for every node to receive it.
 Data rate is 1 Mbps and the packets are 1000 bits long
so the packet transmission time is 1 ms.
 Propagation delay is approximately 250 ms.
 What a node that wants to transmit is hearing on
the channel at time t is actually a transmission from
(t−250) ms.
 There is no use deferring to a carrier and it is best just to
transmit the packet and hope that no other node is
transmitting at the same time.
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Aloha (2)
• Of course, if there indeed was another transmission at the
same time, there would be a collision and neither packet can
be decoded correctly by the corresponding receivers.
• The nodes will have to use additional mechanisms to
determine if the packet was successfully received.
• If the packet is not correctly received, the packets will have
to be retransmitted using a suitable retransmission
algorithm.
Space-time diagram of a transmission and reception
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Aloha (3)
• Simple model:
 Fixed length packets .
 Unit of time: packet transmission time.
 Nodes are located along a straight line of length η.
 Distances are measured in terms of the propagation delay.
 Maximum propagation delay in the network is also η.
 Poisson process of rate G attempts per second.
 Uniform distribution of transmitting nodes in [0, η].
 Independent transmissions.
 Each packet transmission attempt is characterized by an
ordered pair (t, y), where t ≥ 0 is the time at which the
transmission started, and 0 ≤ y ≤ η is the location of the
transmitting node.
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Aloha (4)
A sample realization of this space-time attempt process
• Number of attempts in two non-overlapping areas, A and B
are independent. Thus, the number of attempts in A has a
Poisson distribution with mean ((G/η) × (Area of A)).
• Thus the space-time attempt process in the region
([0,∞) × [0, η]) is a two-dimensional Poisson point process
of rate (G/η) attempts per meter-second.
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Aloha (5)
• Consider a node at location T transmitting a packet to a node
at location R.
• For this transmission, we can define a collision window in
time at each location in [0, η]. If a transmission is begun at
that location in the collision window, then it will arrive at R
when it is receiving the packet from T, thus causing a
collision.
Space-time diagram for Aloha (collision cone ” bcdefgb”)
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Aloha (6)
• The space-time arrival rate, G/η, multiplying by the area of
collision cone,2η, gives the mean of the Poisson distribution
of the number of arrivals in the collision cone.
• Thus, the probability that a reception is successful, Ps, is
given by:
Ps = Pr(No transmission attempt in collision cone)
= e−(G/η×2η) = e−2G
• Defining
the throughput, S, as the mean number of
successful attempts per unit time, we get:
S = GPs = Ge−2G
• The maximum value of S is achieved for G = 0.5 and
Smax = 1/(2e) ≈ 0.18
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Slotted Aloha (1)
• To make pure Aloha more efficient, let time be slotted and
the nodes be allowed to begin transmission only at the
beginning of a slot.
• The slot length is made equal to the sum of the packet
transmission time (unity) and the maximum propagation
delay in the network (η).
• Nodes begin transmission only at slot boundaries and the
transmission and reception of a packet is completed in one
slot.
• Packets that arrive in a slot are transmitted at the beginning
of the next slot.
Time slotting in slotted Aloha
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Slotted Aloha (2)
• Thus, for a collision at a receiver, a second packet should
begin transmission in the same slot; that is, another packet
should have arrived in the previous slot.
• This means that the collision cone is now a rectangle of sides
(1 + η) and η.
• The Poisson rate of packet arrivals that can cause a collision is
the expected number of Poisson arrivals in a slot, G(1 + η).
• The probability that a transmission is successfully received is:
Ps =e−G(1+η)
• We can obtain the throughput as before and it is:
S = GPs = Ge−G(1+η)
• The maximum achievable throughput is:
Smax = 1/(e(1 + η)) = (0.487760)
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Slotted Aloha (3)
• The slot length was made equal to (1 + η), rather than 1, to
absorb the variations in the propagation delays between the
nodes.
• In many networks, such as a satellite network the
propagation delays between any pair of nodes are very
nearly the same, approximately 250 ms, and we can use a
slot length of one unit.
• In terrestrial networks, like in the cellular and the cable
networks, the nodes usually transmit to a central node.
• The nodes use ranging to determine the propagation delay to
the central node and advance or delay their transmission
times to approximate a time slotted link and absorb the
differences in the propagation delays.
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Slotted Aloha (4)
• An Example: GSM Cellular Network
 Random Access Channel (RACH), on the reverse link from
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the mobile node to the base station, is used by the GSM
mobile stations to send messages to the network.
The messages are small and are generated at a very low
rate compared to the capacity of the RACH channel.
The number of mobile nodes in a cell is not fixed and also
quite large and signaling bandwidth cannot be allocated
statically to these nodes.
Hence, slotted Aloha is used on this channel.
After transmitting on the RACH, the mobile station waits for
a fixed duration to know if the transmission was successful.
If an acknowledgment is not received before this duration,
a retransmission is attempted.
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Slotted Aloha (5)
• Another Example: VSAT Network
 Very Small Aperture Terminal (VSAT) network is a satellite
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network in which there are several geographically
widespread, small terminals.
These terminals are attached to individual computers or to
the local area networks of small organizations through the
Digital Interface Unit (DIU).
The nodes can communicate only with the hub, through a
shared satellite channel, and all inter-node communications
are over two hops via the hub.
The terminals request for reservations of time on the
inbound channel, using the slotted Aloha protocol.
This reservation scheme can be very efficient if the
bandwidth allocated for the reservation requests is small
and the amount of reserved bandwidth is large.
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CSMA (1)
• If in a network, the propagation delay is small compared to
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the packet transmission time, it is possible to infer channel
state (busy or idle) through carrier sensing.
In such networks, if a node senses the channel to be busy
and yet transmits, it can cause a collision at the receiver of
the ongoing transmission, and both transmissions are lost.
Hence, a node should listen to the channel before beginning
to transmit and defer to an ongoing transmission.
This is the principle of the Carrier Sense Multiple Access
(CSMA) protocol.
In this protocol, once a node begins transmitting, it transmits
the complete packet.
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CSMA (2)
• The collision window for the CSMA is the time since the
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beginning of a transmission during which another node (not
having heard the ongoing transmission) can begin its own
transmission, and hence collide with the first transmission.
The maximum collision window is equal to the maximum
propagation delay in the network.
Because after this interval, the carrier would have reached
every node in the network and all nodes will defer a
transmission attempt until the end of the packet transmission
that is in progress.
Two or more nodes can begin transmission within a short
time of each other (less than the collision window) and
collide.
In this case, all the colliding transmissions will be lost.
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CSMA (3)
• The duration of a collision in the network is the time from the
beginning of the first transmission in the collision until the
earliest time at which a fresh transmission can begin. Then:
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Maximum duration of a collision = ttrans + 2tpropgn
where ttrans is the packet transmission time and tpropgn is the
maximum propagation delay.
• Collision can be detected if the node continuing to monitor
the channel after beginning transmission.
• If it senses a collision on the channel, then the node can
immediately stop transmission and minimize the loss of
channel capacity.
• This is called CSMA with Collision Detect or CSMA/CD. Then:
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Maximum duration of a collision = 3tpropgn
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CSMA (4)
Collisions in CSMA and CSMA/CD networks
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CSMA (5)
• In wireless networks, spatial reuse allows the spectrum to be
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simultaneously used in different parts of the network and
significantly increases the traffic carrying capacity.
Spatial reuse requires that the interference region of the
transmitters be much smaller than the geographical spread of
the network.
This allows different transmitter-receiver pairs to be active in
geographically different parts of the network.
Challenges of carrier detection:
 Hidden and exposed nodes.
 Difficulty to design reliable collision detection hardware.
Hence in wireless LANs, the emphasis is on avoiding
collisions, rather than detecting them.
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Collision Avoidance (1)
Node b is hidden from
node a with reference to
a transmission to node c.
Node d is exposed to
a transmission from node a.
Hidden and exposed nodes in a wireless network
(Propagation delays are assumed to be zero.)
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Collision Avoidance (2)
• Hidden nodes reduce the capacity by causing collisions at
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receivers without the transmitter knowing about it.
Exposed nodes force a node to unnecessarily defer in its
transmission attempts, thus reducing spatial reuse.
Collision Avoidance (CA) mechanisms prevent collisions due
to transmissions by hidden node.
These mechanisms assume that the interference regions, and
also the decode regions, for transmission and reception are
identical.
A simple CA mechanism is to have a narrowband auxiliary
signaling channel in addition to the data channel.
A node actively receiving data on the data channel transmits
a busy tone on the signaling channel to enable the hidden
nodes to defer to receiving nodes in their interference
regions.
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Collision Avoidance (3)
• Dividing
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the available spectrum into two parts is both
cumbersome and inefficient.
The effect of the busy tone is achieved by preceding the
actual data transfer by a handshake between the transmitter
and the receiver.
This handshake is used to convey an imminent reception to
the hidden nodes.
Before transmitting a data packet, a source node transmits a
(short) Request To Send (RTS) packet to the destination.
If the destination receives the RTS correctly, it means that it
is not receiving any other packet and it acknowledges the
RTS with a Clear To Send (CTS) packet.
CTS informs the neighborhood of a receiver about an
impending packet reception.
The source then begins the packet transmission.
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Collision Avoidance (4)
• If CTS is not received within a specified timeout period, the
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source assumes that the RTS had a collision at the receiver
(most likely with another RTS packet) and a retransmission is
attempted after a random back-off period.
The RTS serves to inform nodes in the decode region of the
transmitter about the imminent transmission of a packet.
The CTS serves the same purpose for nodes in the decode
region of the receiver.
Thus nodes that are not in the interference region of the
transmitter, but are in the decode region of the receiver, are
informed of the imminent packet transmission.
If the transmission duration information is included in the
RTS and CTS packets, then the nodes in the decode region of
both the transmitter and the receiver can maintain a Network
Allocation Vector (NAV).
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Collision Avoidance (5)
• NAV indicates the remaining time in the current transmission
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and schedule their own transmissions to avoid collision.
Thus this handshake is a collision avoidance scheme and the
protocol is the Carrier Sense, Multiple Access with Collision
Avoidance (CSMA/CA).
After completion of the RTS/CTS exchange, the medium is
reserved in the region that is the union of the decode regions
of the transmitter and the receiver.
Hence this basic channel access mechanism was called
Multiple Access with Channel Acquisition (MACA) when it was
first proposed.
It is an adaptation of the handshake protocols used in RS232-C and Apple talk.
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Collision Avoidance (6)
• Collision avoidance helps to reduce the inefficiency that is
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introduced by not being able to do collision detection in
wireless networks.
Only short RTS packets collide, and hence the time lost due
to collisions is small.
The RTS/CTS scheme helps ameliorate the hidden terminal
problem but does not eliminate it.
Only the nodes in the decode region of the receiver have
been alerted by the CTS. Those in the interference region,
but not in the decode region of the receiver, have just sensed
a carrier but they do not know of an impending packet
transmission.
During the packet transmission, they do not sense the carrier
and can still cause a collision.
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Collision Avoidance (7)
• In the CSMA/CA scheme, it seems difficult to be able to allow
an exposed node to transmit.
• Any node in the interference region of the transmitter of the
ongoing packet is exposed. Even if such a node were allowed
to transmit a RTS to a node (outside the interference region
of the transmitter of the ongoing packet), it will itself not be
able to receive the subsequent CTS and hence it will not
know if it can transmit.
• Improvements: MACA for Wireless (MACAW)
 An acknowledgment from the receiver after the successful
reception of a packet.
 Transmission of a short Data Sending (DS) packet
preceding the actual data transfer.
This is also useful in networks where the nodes do not
have carrier sensing capability.
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Collision Avoidance (8)
Handshake and data exchange sequence in MACAW
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IEEE 802.11 WLAN Standard (1)
• The basic ideas of the CSMA/CA protocol of MACA and
MACAW have been formalized in IEEE 802.11 (Wi-Fi) wireless
LAN standards.
• Two modes of network configuration:
 Independent or ad hoc network mode
o Nodes form an independent multi-hop wireless network
and they communicate directly with one another.
o A routing protocol and a corresponding routing
algorithm will need to be used so that the packets find
paths to the destinations.
An IEEE 802.11 ad hoc network
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IEEE 802.11 WLAN Standard (2)
 Infrastructure mode
o Data communication is always between a mobile station
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(MS) and an Access Point (AP).
The AP is connected to the wired network and provides a
service similar to the base station of a cellular network.
MSs need to associate with an AP using an association
protocol.
The AP and the MSs associated with it form a Basic
Service Set (BSS), and a set of BSSs is called an
Extended Service Set (ESS).
The association, and dissociation, allows the MSs to be
mobile within the ESS.
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IEEE 802.11 WLAN Standard (3)
A typical IEEE 802.11 ESS architecture
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IEEE 802.11 WLAN Standard (4)
PHYsical (PHY) layer
• Different frequency bands and data rates
• The initial 802.11 standard had three PHY standards:
1) Infrared
2) 1 and 2 Mbps over Frequency Hopping Spread Spectrum
(FHSS) in the 2.4 GHz band
3) 1 and 2 Mbps over Direct Sequence Spread Spectrum
(DSSS) in the 2.4 GHz band.
• The transmitter and the receiver can choose the data rate to
suit the channel conditions.
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IEEE 802.11 WLAN Standard (5)
• A second version of the IEEE 802.11 standard defined the
following physical layer standards:
 802.11a
o 5.8 GHz band
o OFDM (52 carriers, 48 for data)
o 20 MHz bandwidth
o Data rates: 6, 9, 12, 18, 24, 36, 48, or 54 Mbps
 802.11b
o 2.4 GHz band
o DSSS
o 11MHz bandwidth
o Data rates: 1, 2, 5.5, 11 Mbps
 802.11g (an extension of 802.11b)
o DSSS and OFDM
o Data rates: 5.5 to 54 Mbps
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IEEE 802.11 WLAN Standard (6)
2.4 GHz Wi-Fi channels (802.11b,g WLAN)
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IEEE 802.11 WLAN Standard (7)
Medium Access Control (MAC) layer
• Two basic protocols:
1) Polling-based: Point Coordination Function (PCF)
2) Random-access: Distributed Coordination Function (DCF)
• PCF and DCF can coexist in the same BSS.
• Time is divided into super-frames and each super-frame has
two parts:
(1) Contention Free Period (CFP)
(2) Contention Period (CP)
• PCF is used in the CFP and DCF in the CP.
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IEEE 802.11 WLAN Standard (8)
• PCF in CFP:
 AP polls all the nodes in the BSS by transmitting a beacon
frame.
 Data that need to be transmitted to them are transmitted
along with the polling message.
 If any polled node has packets to transmit, it will also
transmit them in response to the polling packet.
 When all the nodes are polled by the AP, the end of CFP is
signaled using the “End frame”.
• DCF in CP:
 Using the DCF-based MAC, until the end of the super-frame
period.
• In a super-frame , the CFP and the CP alternate.
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IEEE 802.11 WLAN Standard (9)
• Short Inter-Frame Space (SIFS) timer starts at the end of the
data transmission, and allows the receiving node to turn
around its radio and send back a MAC level ACK packet.
• DCF Inter-Frame Space (DIFS) timer starts by all the nodes,
after completion of ACK transmission and the channel is
sensed to be idle.
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• DIFS duration is more than SIFS.
Beacon frame, PCF, and DCF periods in an IEEE 802.11 network
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IEEE 802.11 WLAN Standard (10)
• Target Beacon Transmission Time (TBTT) specifies the period
of the super-frame.
• AP will begin trying to initiate a new CFP, after TBTT has
elapsed from the time that the previous one was initiated.
• In addition to the RTS and CTS based handshake mechanism,
the standard specifies the following:
o Minimum silence periods between transmissions is different
for different kinds of packets according to their priorities.
o Back-off mechanism is used to resolve collisions.
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IEEE 802.11 WLAN Standard (11)
• All nodes enter a backoff phase, when the DIFS timers
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expire.
Random backoff is used to avoid collision.
If a node was already in a backoff when another node
started its data transmission, then during the transmission
backoff timer is frozen.
Upon completion of a node transmission, each other node
that had deferred to the node continues the remainder of its
backoff.
The backoff durations are multiples of the basic slot time.
When a new backoff is sampled, this multiple is sampled
uniformly from the integers {0, 1, . . . ,CWmin − 1}.
In the standard, CWmin =32.
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IEEE 802.11 WLAN Standard (12)
• When a backoff period of some node expires, then this node
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transmits an RTS packet to its destination node.
Upon hearing activity on the medium, all other nodes freeze
their backoff timers.
The node to which the RTS was directed then sends back a
CTS packet (a SIFS elapses in between the two).
Transmitter node then waits for a SIFS and sends its data
packet, after which an ACK is sent by the receiver node.
A collision occurs if two nodes finish their backoffs within one
slot of each other.
It is assumed that the maximum propagation delay in the
network is such that all nodes are able to sense a
transmission within one slot time.
In this case, both RTS packets collide.
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IEEE 802.11 WLAN Standard (13)
• A CTS time-out then follows, after which the colliding nodes
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sample a backoff from a doubled collision window; that is,
from the window {0, 1, 2, . . . , 2 · CWmin − 1}.
After the collision event, the nodes that were not involved in
the collision continue their backoffs with their residual backoff
timers.
Repeated collisions lead to a doubling of the collision window
until it reaches CWmax, after which the collision window
remains fixed.
In the standard, CWmax = 1024.
Extended Inter-Frame Space (EIFS) is equal to sum of the
transmission times of SIFS, CTS (and ACK), and DIFS
packets and the PHY headers.
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IEEE 802.11 WLAN Standard (14)
• EIFS is longer than DIFS and is used by nodes that cannot
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decode a transmission, because of a collision or because the
received power is such that the SNR is below the decoding
threshold.
This prevents collision during reception of CTS and ACK.
Also, when the point coordinator wants to initiate the CFP, it
waits for PCF Interframe Space (PIFS) after the end of an
ongoing transmission before transmitting a beacon or the
polling packet.
SIFS < PIFS < DIFS implies that the initiation of the CFP has
priority over new transmission initiations.
Multi-valued DIFS can be used to prioritize among different
traffic classes.
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IEEE 802.11 WLAN Standard (15)
Specification of 802.11a, b, and g
• RTS packet: 20 octets
• CTS packet: 14 octets
• ACK packet: 14 octets
• All transmitted at the lowest transmission rate.
• Payload packet: Up to 2312 bytes
• MAC header and trailers: 34 octets
• PHY headers: 192 bits.
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IEEE 802.11 WLAN Standard (16)
Events during data transfer in the IEEE 802.11 DCF MAC protocol
with the RTS/CTS mechanism
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