COSC4301/5340: Fund. Wireless and Mobile Prot.

Spring 2020

HOMEWORK I

QUESTION 1. By using geometric arguments, show that the co-channel reuse factor for cellular deployments based on hexagonal cells is given by D/R = sqrt(3N).

QUESTION 2. A new wireless service provider decided to employ a cluster of N=19 cells as the basic module for frequency reuse.

a) Can you identify one such cluster structure?

b) Repeat a) for N=28

c) Can you get an alternative cluster structure for part a)?

d) What is the reuse distance for the system of part c)?

e) Can you find the worst case co-channel interference in such a system?

QUESTION 3.

a) If you use an AMPS cell phone in GSM environment what minimum changes do you have to make so that you can communicate?

b) Why is a smart card needed in GSM and while not required for AMPS? Explain the logic behind this.

c) What is the frame size and the number of TDMA slots per frame in GSM and how does it compare with the IS-54 format?

QUESTION 4. The GSM system uses a 270.833 kbps data rate to support 8 users per frame.

a) What is the raw data rate provided fro each user?

b) If guard time, ramp up time, and synchronization bits occupy 10.1 kbps, determine the traffic efficiency for each user.

QUESTION 5. Why is the near far problem present in CDMA and not in FDMA?

QUESTION 6. Is the hidden terminal problem solved in IEEE 802.11? Explain the rational behind your answer.

QUESTION 7. Design a new location update scheme other than the movement-based, distance-based, and time-based schemes covered in the class. Briefly describe the location update and paging procedures of your new scheme.

Fund. Wireless & Mobile Protocol

COSC 4301/5340

Wireless Local Area Networks (WLANs)

Instructor: Dr. Xingya

Liu

Department of Computer Science

Lamar University

Office: MAES 8

6

Phone: 409-880-8677

Email: xliu@lamar.edu

Fund. Wireless & Mobile Protocol

IEEE 802.11 Protocol

Architecture

Liu 2

Physical Layer (PHY)

Distributed Coordination Function (DCF)

Point Coordination
Function (PCF)

Normal Data Traffic
(Asynchronous)
Contention Service

Real Time Traffic
Contention Free Service

MAC

Fund. Wireless & Mobile Protocol

IEEE 802.11 –

Medium Access Control
• Distributed Mode: Distributed Coordination Function (DCF)

– Based on CSMA/CA protocol

– Uses a contention algorithm to provide access to all traffic.

– Ordinary traffic uses DCF directly.

• Coordinated Mode: Point Coordination Function (PCF)

– Supports real-time traffic

– Based on polling which is controlled by a centralized point coordinator.

– Uses a centralized MAC algorithm and provides contention-free service.

– PCF is built on top of DCF and exploits features of DCF to assure
access for its users.

• NOTE: Both the DCF and PCF can operate concurrently within
the same BSS to provide alternative contention and contention-
free periods

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Fund. Wireless & Mobile Protocol
IEEE 802.11 –

MAC Layer Overview

• DFWMAC-DCF CSMA (Mandatory)

– Distributed Foundation Wireless Medium Access Control –
Distributed Coordination Function CSMA

• DFWMAC-DCF w/ RTS/CTS (Optional)

– Distributed Foundation Wireless MAC

– Avoids Hidden Terminal problem

• DFWMAC- PCF (Optional)

– Access point polls terminals according to a lis

t

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Fund. Wireless & Mobile Protocol

IEEE 802.11 Access Method –

Interframe Spaces (IFS)

• Priorities

– Defined through different interframe spaces (IFS)

– SIFS (Short Inter Frame Space)

• highest priority, for ACK, CTS, polling response

• SIFS: 16 µsec

• SIFS required for turn around of Tx to Rx and vice versa

– PIFS (PCF IFS) – Point Coordination Function Interframe Space

• medium priority, for real-time services using PCF

• SIFS + one slot time (9 µsec) = 25 µsec

– DIFS (DCF IFS) – Distributed Coordination Function Interframe Space

• lowest priority, for asynchronous data services

• SIFS + two slot times (9 µsec) = 34 µsec

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Fund. Wireless & Mobile Protocol
IEEE 802.11 –

DFWMAC-DCF CSMA

• A station with a frame to transmit senses the medium.

• If the medium is idle, it waits to see if the medium remains idle
for a time equal to DIFS. If so, the station may transmit
immediately.

• Receivers acknowledge at once (after waiting for SIFS) if the

packet was received correctly

• Receiver transmits ACK without sensing the medium

6
t

SIFS

DIFS

data

ACK

waiting time

Other

Stations

Receiver

Sender
data

DIFS

contention

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Fund. Wireless & Mobile Protocol
IEEE 802.11 –
DFWMAC-DCF CSMA

• If the medium is busy (either because the station initially finds
the medium busy or because the medium becomes busy during
the DIFS idle time), the station defers transmission and continues
to monitor the medium until the current transmission is over.

• Once the current transmission is over, the station delays another
DIFS.

• If the medium remains idle for this period, the station backs off
using a binary exponential backoff scheme and again senses the
medium.

• Backoff Timer start decreasing after an idle time of DIFS
following the medium busyness

• When Backoff Timer is 0, station may transmit immediately.

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Fund. Wireless & Mobile Protocol
IEEE 802.11 –

DFWMAC-DCF CSMA
• Maintain a value CW (Contention-Window)

• If Busy

– Wait until channel is idle for DIFS

– Then MAC runs a random number generator to choose a
random number between 0 and CW to set a BACKOFF
CLOCK for every contending station and starts a backoff
timer for proportional amount of time

8

DIFS
Contention Window

Slot time

Defer Access

Backoff-Window Next Frame

Select Slot and Decrement Backoff as long as medium is idle.

SIFS

PIFSDIFS

Free access when medium
is free longer than DIFS

Busy Medium

IFS: Inter Frame Space

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Fund. Wireless & Mobile Protocol
IEEE 802.11 –

DFWMAC-DCF CSMA
– The first station that expires its clock starts transmission

– Other terminals sense the new transmission and freeze their
clocks to be reactivated after the completion of the current
transmission in the next contention

period

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Fund. Wireless & Mobile Protocol
IEEE 802.11 –

DFWMAC-DCF CSMA
• If Collisions (Control or Data)

– Binary exponential increase (doubling) of CW

– Length of backoff time is exponentially increased as the
station goes through successive retransmissions

• CWmin<=CW<=CWmax

– CWmin = 31slots, CWmax = 1023slots (for 802.11b)

10Liu

Fund. Wireless & Mobile Protocol
IEEE 802.11 –

DFWMAC-DCF CSMA
– Up to CWmax, CW = (CWmin+1)* 2n-1, where n = 0, 1, 2,

… is retransmission number

11Liu

Fund. Wireless & Mobile Protocol
IEEE 802.11 –

DFWMAC w/ RTS/CTS

12

t
SIFS
DIFS
ACK

defer access

Other
Stations
Receiver
Sender
data
DIFS
contention

RTS

CTS
SIFS SIFS

NAV (RTS)

NAV (CTS)

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RTS …

Fund. Wireless & Mobile Protocol
IEEE 802.11 –
DFWMAC w/ RTS/CTS

• RTS/CTS is used for reserving channel for data transmission so that the

collision can only occur in control message

• Station can send RTS (request to send) with reservation parameter after

waiting for DIFS (reservation determines amount of time the data packet

needs the medium)

• Every node receiving the RTS has to set its

Network Allocation Vector

(NAV) in accordance with the duration of the field (NAV specifies the

earliest point at which the station can try to access the medium

• If receiver receives RTS, it sends CTS (Clear to Send) after medium has been

idle for SIFS. CTS again contains duration field and all stations receiving this

packet need to adjust their

NAV

• Sender can now send data after SIFS, acknowledgement via ACK by receiver

after SIFS

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Fund. Wireless & Mobile Protocol
Network Allocation Vector

(NAV)

• Both Physical Carrier Sensing and Virtual Carrier Sensing used

in 802.11

• If either function indicates that the medium is busy, 802.11 treats

the channel to be busy

• Virtual Carrier Sensing is provided by the NAV (Network

Allocation Vector)

• Virtual Carrier Sensing:

– Most 802.11 frames carry a duration field which is used to reserve the

medium for a fixed time period

– Tx sets the NAV to the time for which it expects to use the medium

14Liu

Fund. Wireless & Mobile Protocol
Network Allocation Vector
(NAV)

– Other stations start counting down from NAV to 0

– When NAV > 0, the medium is busy

– Channel virtually busy a NAV SIGNAL is turned on

– The transmission will be delayed until the NAV signal has

disappeared

– Provides a unique access right for a station without any

contention.

• When the channel is virtually available, then MAC

checks for PHY condition of the channel

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Fund. Wireless & Mobile Protocol
IEEE 802.11 –

DFWMAC-PCF (Optional)

• The access mechanisms presented so far cannot
guarantee a maximum access delay

• To provide a time bounded service, the standards
specify a Point Coordination Function (PCF) on top of
the DCF mechanisms.

• Using PCF requires an access point that can control
medium access and poll the single nodes. Ad Hoc
networks cannot use this function.

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Fund. Wireless & Mobile Protocol
IEEE 802.11 –
DFWMAC-PCF (Optional)

• Point Coordination Function

– PCF is an alternative access method implemented on top of
the DCF.

– The operation consists of polling with the centralized polling
master (point coordinator).

– The network is configured such that a number of stations
with time-sensitive traffic are controlled by the PC while
remaining traffic contends for access using CSMA/CA

– The point coordinator resides in the access point (AP). It
provides contention free frame transfer.

– The PC issues polls in a round-robin fashion to all stations
configured for polling

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Fund. Wireless & Mobile Protocol
IEEE 802.11 –
DFWMAC-PCF (Optional)

• Point Coordination Function (Cont.)

– The PC makes use of PIFS when issuing polls.

– Because PIFS is smaller than DIFS, the point coordinator can

seize the medium and lock out all asynchronous traffic while

it issues polls and receives responses.

– When a poll is issued, the polled station may respond using
SIFS.

– If the point coordinator receives a response, it issues another
poll to another station using SIFS.

– If no response is received during the expected turnaround
time, the coordinator issues another poll.

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Fund. Wireless & Mobile Protocol
IEEE 802.11 –
DFWMAC-PCF (Optional)

19

PIFS

Stations‘

NAV

Wireless

Stations

Point

Coordinator

D1

U1

SIFS
NAV
SIFS

D2

U2

SIFS
SIFS

SuperFrame
t0

medium busy

t1

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Fund. Wireless & Mobile Protocol
IEEE 802.11 –
DFWMAC-PCF (Optional)

• At time t0 the contention-free period should start, but

another station is transmitting data

• After the medium has been idle, the PCF has to wait for

PIFS before accessing the medium.

• The point coordinator now sends data D1 to the first

station. The station can answer after SIFS. After

waiting for SIFS, the point coordinator can poll the

second station by sending D2.

• The second station replies with U2

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Fund. Wireless & Mobile Protocol
IEEE 802.11 –
DFWMAC-PCF (Optional)

21

t
Stations‘
NAV
Wireless
Stations
Point
Coordinator

D3

NAV
PIFS

D4

U4

SIFS
SIFS

CFend

contention
period

contention free period

t2 t3 t4

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Fund. Wireless & Mobile Protocol
IEEE 802.11 –
DFWMAC-PCF (Optional)

22

• Polling continues with the third node which has
nothing to answer.

• After waiting for PIFS, the point coordinator can issue
another poll.

• At the end of contention-free period, an end marker
(CFend) is issued indicating that the contention period
may start again.

• The cycle starts again with the next superframe

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Fund. Wireless & Mobile Protocol
IEEE 802.11 –
DFWMAC-PCF (Optional)

• The figure illustrates the use of the superframe.

23

NAV
NAV

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Fund. Wireless & Mobile Protocol
IEEE 802.11 –

DFWMAC-PCF (Optional)
• The AP organizes a periodical contention-free period (CFP) for

the real-time information.

• The AP coordinates real-time data to be transmitted at the
beginning of each superframe and during those periods it
arranges an NAV signal for all other stations.

• The length of the PCF period is variable because of the variable
frame size issued by responding stations, and it only occupies a
portion of the superframe.

• The rest of the superframe is released for contention and DCF
packets.

• If a DCF packet occupies the channel and does not complete
before the start of the next superframe, the starting time of the
superframe will defer.

• However, the NAV signal for all other terminals goes to
operation at the beginning of the CFP.

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Fund. Wireless & Mobile Protocol
IEEE 802.11 –
DFWMAC-PCF (Optional)

• At the end of the superframe interval, the point
coordinator contends for access to the medium using
PIFS.

• If the medium is idle, the point coordinator gains
immediate access and a full superframe period follows.

• However, the medium may be busy at the end of a
superframe.

• In this case, the point coordinator must wait until the
medium is idle to gain access; this results in a
foreshortened superframe period for the next cycle.

25Liu

Fund. Wireless & Mobile Protocol
IEEE 802.11 –

Power Saving Mode (PS)

• IEEE 802.11 stations can maximize battery life by shutting

down the radio transceiver and sleeping periodically

• During sleeping periods, access points buffer any data for

sleeping stations

• The data is announced by subsequent beacon frames

• To retrieve buffered frames, newly awakened stations use PS-

poll frames

• Access point can choose to respond immediately with data or

promise to delivery it later

26Liu

Fund. Wireless & Mobile Protocol

IEEE 802.11 – More Developments

• 802.11aa: Streaming of audio video transport streams –
completed (June 2012)

• 802.11ae: Prioritization of management frames –
completed (March 2012)

• 802.11ac: Very high throughput improvements over
802.11n

– Better modulation, wider channels, multi user MIMO.

• 802.11ad: Very high throughput at 60GHz

• 802.11af: TV Whitespace

• 802.11ah: Smart metering, 1GHz sensor network

• 802.11ai: Fast initial link setup

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Fund. Wireless & Mobile Protocol

Performance Metrics

• Overall coverage area

– Can be evaluated in terms of received signal strength
intensity (RSSI)

• Throughput

– Can be evaluated by measuring TCP connection throughputs
since WLANs establish a client-server communication link
via TCP connection

• Implementations of handoff and dropping are the
responsibility of manufacturers, since they vary
according to different equipments

28Liu

Fund. Wireless & Mobile Protocol

Interference Issue

• An important issue in all wireless systems because
nearby users occupy the same bandwidth and cause co-
channel interference.

• For WLANs, in addition to co-channel interference,
other types of interference exist mainly due to the use
of unlicensed ISM band. Interference to WLANs
comes from the following major sources:

– Co-channel interference.

– Interference from non-WLAN devices in the same frequency
band

– Interference between different WLANs in the same
frequency band.

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Fund. Wireless & Mobile Protocol

Environmental Issue

• Has been proved that WLAN is safe for health

– Radiation used by this technology falls well within the limits

of safety guidelines (both in terms of frequency content and

power level) specified by Radio Frequency Safety Standards

and Recommendations.

– Radiation in this frequency range is non-ionizing (as they do

not have enough energy to break the chemical bonds of

genetic material of body cells).

– Vendors designing their products to operate within the power

limit set by the Safety Standards.

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Fund. Wireless & Mobile Protocol

Challenging Issues

• Interference between different types of WLANs

• Lack of Interoperability between WLANs

• Relatively low data rate

• Lack of support for real-time services.

– IEEE 802.11 products, which are based on the CSMA/CA
protocol, are unable to provide QoS guarantees for voice,
video, and other real time services. (IEEE 802.11 working

31Liu

Fund. Wireless & Mobile Protocol

COSC 4301/5340
Wireless Local Area Networks (WLANs)

Instructor: Dr. Xingya

Liu

Department of Computer Science

Lamar University

Office: MAES 86
Phone: 409-880-8677

Email: xliu@lamar.edu

Fund. Wireless & Mobile Protocol

The ubiquitous WLAN
• Today’s road worriers require access to the Internet everywhere.
• WLAN is more than just cable replacement, it provides hassle-

free broadband Internet access everywhere.

• Coverage in ‘hot-spots’ sufficient.
• WLAN meets the expectations for easiness, cost and bandwidth.

Liu 2

Public
WLAN

Airport

Railway
Station

Campus

Plant

Semi-public
WLAN

Office

Hospital

Congress hall,
Hotel

Corporate
WLAN

Office

Home
WLAN

Remote
Access

Fund. Wireless & Mobile Protocol

Wireless LANs

3

Infrastructure
Network

AP

AP

AP

wired network

AP: Access

Point

Ad-hoc Network

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Fund. Wireless & Mobile Protocol

Overview of WLANs
• Wireless Internet access cheaper and faster

• The number of public Wi-Fi hot spots worldwide will be 5.8
million by 2015, up from virtually nil in 2001. (The Wireless
Broadband Alliance projections)

• 350% increase compared to the 1.3 million WiFi hot spots that
are live today (Nov. 2011)

• Wireless public area access service revenue grows from $3.9
million in 2001 to $224.7 million in 2005.

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Fund. Wireless & Mobile Protocol

Applications
• Home wireless networks

• Enterprise wireless networks

• Public access: Airport, Convention Centers, Cafes, Train Stations

• Hospitals

• Warehouses
• Consulting and audit teams

• Dynamic environments, ad agencies, etc.

• Universities

• Historic buildings, older buildings

• Meeting rooms
• Retail stores

• Restaurants and car rental agencies

5Liu

Fund. Wireless & Mobile Protocol

Basics of Wireless LANs
• Advantages:

– Generally works in industrial, scientific, and medical (ISM)
band, which is un-licensed and available for public.

– Users can access high speed multimedia applications, with
easy implementation, low cost, and wide user acceptance.

• Disadvantages:
– Interference problem

• Co-channel interference
• Interference between different WLANs in the same frequency band
• Interference from non-WLAN devices in the same frequency band

– Weak to provide real-time services, QoS guarantees

6Liu

Fund. Wireless & Mobile Protocol

Topologies –
Single-Cell Wireless LAN

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Fund. Wireless & Mobile Protocol

Topologies –
Single Cell Wireless LAN

• In the figure, there is a backbone wired LAN, such as
Ethernet, that supports servers, workstations, and one
or more bridges or routers to link with other networks.

• There is a control module (CM) (Access Point (AP))
that acts as an interface to a wireless LAN. (CM = AP)

• The control module includes either bridge or router
functionality to link the wireless LAN to the backbone.

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Fund. Wireless & Mobile Protocol
Topologies –
Single Cell Wireless LAN

• CM includes some sort of access control logic, such as
a polling or token-passing scheme, to regulate the
access from the end systems.

• Note that some of the end systems are stand-alone
devices such as a workstation or a server.

• In addition, hubs or other user modules (UM)
(PORTAL) that control a number of stations off a
wired LAN may also be part of the wireless LAN
configuration.

9Liu

Fund. Wireless & Mobile Protocol

Topologies –
Multiple Cell Wireless LAN

10Liu

Fund. Wireless & Mobile Protocol
Topologies –
Multiple Cell Wireless LAN

• There are multiple control modules interconnected by a
wired LAN.

• Each control module supports a number of wireless end
systems within its transmission range.

• For example, with an infrared LAN, transmission is
limited to a single room; therefore, one cell is needed
for each room in an office building that requires
wireless support.

11Liu

Fund. Wireless & Mobile Protocol

Family of Wireless LAN
Standards – IEEE 802.11

• 802.11-1997: released in 1997, clarified in 1999, many
amendments
– 802.11a – 54Mbps 5GHz- Ratified in 1999
– 802.11b – 11Mbps 2.4GHz- ratified in 1999
– 802.11d – International (country-to-country) roaming
– 802.11e – Quality of Service

• Enhance the 802.11 MAC to expand support for applications with Quality of
Service requirements

– 802.11F – Inter-Access Point Protocol (IAPP), withdrawn later
• Establish an Inter-Access Point Protocol for data exchange via the distribution

system
– 802.11g – Higher Data rate (54Mbps) 2.4GHz
– 802.11h – Dynamic Frequency Selection and Transmit Power Control

mechanisms for 802.11a
– 802.11i – Authentication and security

• Enhance the 802.11 MAC to provide improvements in security
– 802.11j – Extensions for Japan

12Liu

Fund. Wireless & Mobile Protocol
Family of Wireless LAN
Standards – IEEE 802.11

• 802.11-2007: a new release of the standard, merged amendments
a , b, d, e, g, h, i, and j with the base standard (July 2007)
– 802.11k – Radio resource measurement
– 802.11n – Higher data rate using MIMO, both 2.4GHz and 5GHz
– 802.11p – WAVE (wireless access for the vehicular environment)
– 802.11r – Fast BSS transition
– 802.11s – Mesh networking
– 802.11u – Third-part authorization of clients, e.g., cellular offload
– 802.11v – Wireless network management
– 802.11w – Protected management frames
– 802.11y – 3.6GHz (3650-3700MHz) operation in the U.S.
– 802.11z – Extensions to Direct Link Setup (DLS)

• 802.11-2012: a new release of the standard, merged amendments
k , n, p, r, s, u, v, w, y, and z with the 2007 standard (March 2012)

13Liu

Fund. Wireless & Mobile Protocol

IEEE 802.11 Reference
Architecture

• Station (STA)
– terminal with access mechanisms

to the wireless medium and radio
contact to the

access point

• Basic Service Set (BSS)
– group of stations using the same

radio frequency
• Access Point

– station integrated into the wireless
LAN and the distribution system

•

Distribution System

– interconnection network to form

one logical network (ESS:
Extended Service Set) based on
several BSS

•

Portal

– bridge to other (wired) networks

14

Distribution System
Portal

Access
Point

BSS2

BSS1
Access

Point

STA1

STA2
STA3

ESS

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Fund. Wireless & Mobile Protocol
IEEE 802.11 Reference
Architecture

• The smallest building block of a wireless LAN is a
basic service set (BSS), which consists of some number
of stations executing the same MAC protocol and
competing for access to the same shared medium.

• A basic service set may be isolated or it may connect to
a backbone distribution system through an access point.

• The MAC protocol may be fully distributed or
controlled by a central coordination function housed in
the access point.

• The access point functions as a bridge.

15Liu

Fund. Wireless & Mobile Protocol
IEEE 802.11 Reference
Architecture

• The basic service set generally corresponds to what is
referred to as a cell in the literature.

• An extended service set (ESS) consists of two or more
basic service sets interconnected by a distribution
system.

• Typically, the distribution system is a wired backbone
LAN.

• The extended service set appears as a single logical
LAN.

16Liu

Fund. Wireless & Mobile Protocol

Protocol Architecture

17

mobile terminal

access point

fixed
terminal

application

TCP

PHY

MAC

IP

802.3 MAC

802.3 PHY

application
TCP
802.3 PHY
802.3 MAC
IP
MAC
PHY

LLC

infrastructure
network

LLC LLC

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Fund. Wireless & Mobile Protocol

WLAN: IEEE 802.11b
• Data rate

– 1, 2, 5.5, 11 Mbit/s, depending on SNR
– User data rate max. approx. 6 Mbit/s

• Transmission range
– 300m outdoor, 30m indoor Max. data rate ~10m indoor

• Frequency: Free 2.4 GHz ISM-band
• Availability: Many products, many vendors

• Quality of Service
– Typically Best effort, no guarantees (unless polling is used, limited support

in products)

• Special Advantages/Disadvantages
– Advantage: many installed systems, lots of experience, available

worldwide, free ISM-band, many vendors, integrated in laptops

– Disadvantage: heavy interference on ISM-band (Industrial, Scientific,
Medical band), no service guarantees, slow relative speed

Limited, WEP (Wired Equivalent Privacy) insecure 18Liu

Fund. Wireless & Mobile Protocol

WLAN: IEEE 802.11a
• Data rate

– 6, 9, 12, 18, 24, 36, 48, 54 Mbit/s, depending on SNR
– User throughput (1500 byte packets): 5.3 (6), 18 (24), 24 (36), 32 (54)
– 6, 12, 24 Mbps mandatory

• Transmission range
– 100m outdoor, 10m indoor

• E.g., 54 Mbps up to 5 m, 48Mbps up to 12 m, 36Mbps up to 25 m, 24Mbps up to 30m,
18Mbps up to 40 m, 12Mbps up to 60 m

• Frequency
– Free 5.15-5.25, 5.25-5.35, 5.725-5.825 GHz ISM-band

• Availability
– Some products, some vendors

• Quality of Service
– Typically best effort, no guarantees (same as all 802.11 products)

• Special Advantages/Disadvantages
– Advantage: fits into 802.x standards, free ISM-band, uses less crowded 5 GHz band
– Disadvantage: data rates may drop fast depending on SNR, propagation condition,

and the distance between sender and receiver, no QoS
• Security

– Limited, WEP insecure
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Fund. Wireless & Mobile Protocol

WLANs – 802.11 Compatibility

• 802.11a and 802.11b share the same MAC layer

• Significant differences at the physical layer.
– 802.11b: 2.4 GHz, DSSS
– 802.11a: 5 GHz, OFDM
– Possible to operate both on the same network concurrently

(using the same access points)

• Interoperability
– WECA (Wireless Ethernet Compatibility Alliance):

organization behind Wi-Fi (Wireless Fidelity) that certifies
products meeting the 802.11b specification through
compatibility testing

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Fund. Wireless & Mobile Protocol 21Liu

Fund. Wireless & Mobile Protocol 22Liu

Fund. Wireless & Mobile Protocol

IEEE 802.11 Protocol
Architecture

23

Physical Layer (PHY)

Distributed Coordination Function (DCF)

Point Coordination
Function (PCF)

Normal Data Traffic
(Asynchronous)
Contention Service

Real Time Traffic
Contention Free Service

MAC
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Fund. Wireless & Mobile Protocol

MAC Protocol Review
• Medium Access Control (MAC)

– Resolve contentions to the shared medium
– Especially important for LAN

• Wired LAN
– ALOHA

• Send without waiting
• Simple, no synchronization

– Slotted ALOHA
• Time is divided into equal size slots

– CSMA
• Listen before transmit
• 1-persistent, non-persistent, p-persistent

– CSMA/CD
• Abort colliding transmission

24Liu

Fund. Wireless & Mobile Protocol

Wireless LAN MAC

• CSMA (Carrier Sense Multiple Access) as Wireless MAC?

– Collision detection is difficult
– Hidden and Exposed Terminal Problem makes the use of CSMA an

inefficient technique

• Hidden Terminal Problem
– A talks to B
– C senses the channel
– C does not hear A’s transmission (out of range)
– C talks to B
– Signals from A and B collide

25

A B C

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Fund. Wireless & Mobile Protocol
Wireless LAN MAC

• Exposed Terminal Problem
– B talks to A
– C wants to talk to D
– C senses channel and finds it to be busy
– C stays quiet (when it could have ideally transmitted)

26

A B C D
Not
possible

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Fund. Wireless & Mobile Protocol

Hidden and Exposed Terminal
Problems

• Hidden Terminal Problem
– More collisions
– Waste of resources

• Exposed Terminal Problem
– Under-utilization of channel
– Lower effective throughput

27Liu

Fund. Wireless & Mobile Protocol

Multiple Access with Collision
Avoidance (MACA)

• MACA (Multiple Access with Collision Avoidance) uses short
signaling packets for collision avoidance

– RTS (request to send) (20 byte): a sender request the right to send
from a receiver with a short RTS packet before it sends a data
packet

– CTS (clear to send) (16 byte): the receiver grants the right to send
as soon as it is ready to receive

• Signaling packets contain

– sender address
– receiver address
– packet size

• Variants of this method can be found in IEEE802.11 as
DFWMAC (Distributed Foundation Wireless MAC)

28Liu

Fund. Wireless & Mobile Protocol
Multiple Access with Collision
Avoidance (MACA)

• Hidden Terminal Revisited

– A sends RTS
– B sends CTS
– C overhears CTS
– C inhibits its own transmitter
– A successfully sends DATA to B

29

A B C
RTS CTS

DATA
CTS

Liu

Fund. Wireless & Mobile Protocol
Multiple Access with Collision
Avoidance (MACA)

• Hidden Terminal Revisited

– How does C know how long to wait before it can attempt a
transmission?

– A includes length of DATA that it wants to send in the RTS
packet

– B includes this information in the CTS packet

– C, when it overhears the CTS packet, retrieves the length
information and uses it to set the inhibition time

30Liu

Fund. Wireless & Mobile Protocol
Multiple Access with Collision
Avoidance (MACA)

• Exposed Terminal Revisited
– B sends RTS to A (overheard by C)
– A sends CTS to B
– C cannot hear A’s CTS
– C assumes A is either down or out of range
– C does not inhibit its transmissions to D

31

A B C D
RTS RTS

CTS Cannot hear CTS

Tx not
inhibited

Liu

Fund. Wireless & Mobile Protocol
Multiple Access with Collision
Avoidance (MACA)

• Collision

– Still possible RTS packets can collide!
– Binary exponential backoff performed by stations that

experience RTS collisions
– RTS collisions not as bad as data collisions in CSMA since

RTS packets are typically much smaller than DATA packets
– If DATA packets are of the same size as RTS/CTS packets,

significant overheads

32Liu