Wireless LAN (WLAN) concepts: Modulation to Aggregation

802.11 Wireless LAN
Concepts
By
Chaitanya T K

To repeat what others have said,
requires education; to challenge it,
requires brains.

Mary Pettibone Poole, A Glass
Eye at a Keyhole, 1938

Objectives:

 Evolution-Why Wireless ?
 Overview of 802.11 standards
 Basics of RF Medium and wireless
concepts
 Different RF Access Technologies
used in 802.11 PHY layer
 Wireless Standards(802.11
b/a/g/n)
 802.11 MAC layer in Detail
 Roaming
 802.11e-QOS

Evolution of Networks:

 PC-PC wired, LAN (Ethernet-Fast
Ethernet-Gigabit Ethernet), MAN
and WAN
 PC-PC wireless, WLAN,WiMAX and
LTE

Why Wireless?
 Advantages

 Disadvantages

Wireless Standard Bodies

 FCC– Frequency,BW,Usage and EIRP (Only
U.S and some others)
 ITU-T– RF spectrum management through 5
standard bodies per continent (Global)
 IEEE– Developing standards for PHY and
MAC layers (L2 and L1 only)
 Wi-fi Alliance – Maintaining
interoperability b/w vendors
 ISO- For developing a general framework
for data communication (OSI Model)

Overview of wireless
Standards:
 802.11 Legacy-IR/DSSS/FHSS - Up
to 2 Mbps
 802.11b- CCK-HR_DSSS-Up to 11
Mbps
 802.11a-OFDM-Up to 54 Mbps (5Ghz)
 802.11g-OFDM-up to 54 Mbps (2.4
GHz)
 802.11n-MIMO-OFDMA-Up to 600
Mbps(Both 2.4 and 5 GHz)
 802.11y-OFDM-up to 54 Mbps (3.7
GHz)

Wireless Terminology:

 Key Parameters in understanding
wireless channels
 Delay Spread
 Coherence bandwidth
 Doppler spread
 Coherence time

Modulation techniques:

 PHY layer Modulation techniques
used in WLAN
 Infra Red OFDM
 DHSS OFMDA
 FHSS Scalable-OFDMA
 HR-DHSS MIMO
 CCK MIMO-OFDM(A)
 CCK-OFDM and other
common
 PBCC QPSK/BPSK/QAM..

Spread Spectrum techniques:
 Highly resistant to narrowband
interference.
 Spread-spectrum signals are
difficult to intercept , it
appears as a white noise
 BW utilization by sharing the BW
with zero interference.
 Two Types:
 Direct Sequence spread spectrum
(DSSS)
 Frequency Hopping spread spectrum
(FHSS)

Direct Sequence spread
spectrum (DSSS)
 It phase-modulates a sine wave
pseudo randomly with a continuous
string of pseudo-noise (PN) code
symbols called "chips“.
 Multiplies the data being
transmitted by a "noise" signal.
This noise signal is a
pseudorandom sequence of 1 and −1
values, at a frequency much
higher than that of the original
signal, thereby spreading the
energy of the original signal
into a much wider band.

Frequency Hopping spread
spectrum (FHSS)

 The initiating party sends a
request via a predefined
frequency or control channel.
 The receiving party sends a
number, known as a seed.
 The initiating party uses the
number as a variable in a
predefined algorithm, which
calculates the sequence of
frequencies that must be used.

 Most often the period of the
frequency change is predefined,
as to allow a single base station
to serve multiple connections.
 The initiating party sends a
synchronization signal via the
first frequency in the calculated
sequence, thus acknowledging to
the receiving party it has
correctly calculated the
sequence.
 The communication begins

Complementary Code Keying:
 Complementary codes, first
introduced by Golay in 1961.
 They are sets of finite sequences
of equal length, such that the
number of pairs of identical
elements with any given
separation in one sequence is
equal to the number of pairs of
unlike elements having the same
separation in the other
sequences.

Mathematics Involved:
 complementary codes are
characterized by the property
that their periodic auto
correlative vector sum is zero
everywhere except at the zero
shift.

Polyphase Complementary
Codes:

 Polyphase complementary code is a
sequence having complementary
properties, the elements of which
have phase parameters.

 Eg: c = {1,– 1, j,j,–j,j ,– 1,-1}

Representation of CCK:

 Uses I/Q Modulator and M-Ary
Signaling schemes

I/Q Modulators:
 Quadrature modulators are used to
conserve bandwidth for a given
data rate. This is accomplished
by modulating two orthogonal data
streams onto a common carrier. If
the phases and amplitudes of both
data stream (in-phase "I" and
quadrature "Q"), then one of the
sidebands is completely cancelled
out. If there is no DC bias feed
through, then the carrier itself
is completely cancelled out.

 In practice, complete
cancellation is never
accomplished, but without too
much work, achieving 40 dB of
sideband cancellation is not hard
to do. Even 60 dB is relatively
easy; however, preventing drift
due to thermal and mechanical
effects is not so easy, and the
result is that a "textbook"
quadrature alignment during
alignment can look pretty bad
over time.

CCK modulator:

I OUT
3 Pick One 1
of 8 Walsh
Functions

MBOK 3 Pick One 1
Q OUT

of 8 Walsh
Functions
1
Data Input MUX
1:8 1

1.375 MHz 11 MHz

Modulation is Bi-orthagonal keying on both I and Q channels
Data Rate = 8 bits/symbol * 1.375 MSps = 11 MBps

I OUT
Pick One of

Differential Mod
6 1
64 Complex
CCK Codes
Q OUT
1

1
MUX
Data Input 1:8 1

Code Set is defined by
formula:
1.375 MHz c = e j(ϕ + 2 + 3 + 4 ) , e j (ϕ + 3+ 4 ) , e j (ϕ + 2 + 4 ) ,
{ 1 ϕ ϕ ϕ 1 ϕ ϕ 1 ϕ ϕ

11 MHz − j(ϕ + 4 ) , e j (ϕ + 2 + 3 ) , e j(ϕ + 3 ) , − j (ϕ + 2 ) , e jϕ }
e 1 ϕ 1 ϕ ϕ 1 ϕ e 1 ϕ 1

Data Rate = 8 bits/symbol * 1.375 MSps = 11 MBps

Comparison:

Modulation Technique and Data rates
c = e j(ϕ+ 2+ 3+ 4) , e j(ϕ+ 3+ 4) , e j(ϕ+ 2+ 4) ,
{ 1ϕ ϕ ϕ 1 ϕ ϕ 1 ϕ ϕ

− j(ϕ + 4) , e j(ϕ + 2+ 3) , e j(ϕ+ 3) , − j(ϕ+ 2) , e jϕ }
e 1ϕ 1 ϕ ϕ 1 ϕ e 1ϕ 1
11 Bit Barker Word 22 MHz Code set

802.11 DSSS BPSK 802.11 DSSS QPSK 5.5 MBps 11 MBps
CCK CCK
1 MBps 2MBps
Barker Barker
BPSK QPSK

1 bit used to 2 bits encoded to 6 bits encoded to
2 bits used to 64 complex code
BPSK code word 4 complex code
QPSK code word words; 2-QPSK
words; 2-QPSK

I, Q I, Q
I, Q I, Q

11 chips 11 chips 8 chips 8 chips
1 MSps 1 MSps 1.375 MSps 1.375 MSps

Packet Binary Convolutional
Coding:
 In its simplest form, PBCC works by
reducing overhead (bandwidth used to
transmit non-data) through the removal of
extraneous information and
 optimizing transmission by using smaller
data packets,cutting the response time in
processing those packets.
 Thus, allowing for a greater amount of
data to be transmitted between networked
devices.
 Specific examples of overhead include:

• Protocol headers – carry information
for controlling the device
• Frame interval times – allow users to
gain access to the frequency band
• Error and flow control – ensure the
integrity of the wireless transmission
• Received message acknowledgements –
verify that messages were received
correctly or need to be resent due to
errors and/or message collisions

OFDM:
 Modulation and Multiplexing
 Orthogonality
 Sub carriers
 Sub carrier spacing
 Multipath and ISI
 Cyclic Prefix
 Flat Fading and Performs well in
frequency Selective fading

 High PAPR
 Local Oscillator Frequency Offset

OFDM Symbol Time and
Frequency Representation:

Protocol Stack for 802.11:

 PHYsical Layer:

 PLCP (Physical Layer Convergence
Protocol)
 PMD (Physical Medium Dependent)
sub layer.
 The PMD takes care of the
wireless encoding.

 The PLCP presents a common
interface for higher-level
drivers to write to and provides
carrier sense and CCA (Clear
Channel Assessment), which is the
signal that the MAC (Media Access
Control) layer needs so it can
determine whether the medium is
currently in use.

PHY concepts:
 The PLCP consists of a 144 bits
preamble that is used for
synchronization to determine radio gain
and to establish CCA.
 The preamble comprises 128 bits of
synchronization, followed by a 16 bits
field consisting of the pattern
1111001110100000.
 This sequence is used to mark the start
of every frame and is called the SFD
(Start Frame Delimiter).

 The next 48 bits are collectively known
as the PLCP header. The header contains
four fields: signal, service, length
and HEC (header error check). The
signal field indicates how fast the
payload will be transmitted (1, 2, 5.5
or 11 Mbps).
 The service field is reserved for
future use. The length field indicates
the length of the ensuing payload, and
the HEC is a 16 bits CRC of the 48 bits
header.

 In a wireless environment, the PLCP is
always transmitted at 1 Mbps. Thus, 24
bytes of each packet are sent at 1
Mbps.
 The PLCP introduces 24 bytes of
overhead into each wireless Ethernet
packet before we even start talking
about where the packet is going.
Ethernet introduces only 8 bytes of
data. Because the 192 bits header
payload is transmitted at 1 Mbps,
802.11b is at best only 85 percent
efficient at the physical layer.

Working of Different Modes
in WLAN:

 802.11 MAC functions much the
same way in both ADHOC
(or)independent and
Infrastructure modes.
 Only difference is a centralized
controller is to elected in the
absence of an Access point.

Modes of Access Point:

 Root Mode,
 Bridge Mode,
 Repeater Mode

 BSSID
 ESSID
 SSID

Carrier Sensing:
 Physical Sensing-Hardware-PHY
 Virtual sensing-Software-MAC-NAV

Clear Channel Assessment
Modes (DSSS):

Clear Channel Assessment
Modes (CCK):

Scanning Procedures:
 Active Scanning
-Use of Probe Request
-Can be a
unicast/multicast/broadcast
- Optional
 Passive Scanning
- Waits for beacon
- It may miss a beacon
- Mandatory

Inter frame Spacing
(Contd..):

802.11 frame: addressing

Internet
H1 R1 router
AP

R1 MAC addr AP MAC addr
dest. address source address

802.3 frame

AP MAC addr H1 MAC addr R1 MAC addr
address 1 address 2 address 3

802.11 frame

Problems and Solutions in
MAC:

Channel Placement in 2.4 &
5 GHz bands:

802.11n:
 Requirements:
 MAC layer throughput > 100Mbps
 Support 20MHz Channel
 Support 5GHz Band
 Backward compatible with 802.11g (if
support 2.4GHz Band) and 802.11a
 Support 802.11e (QoS)
 Spectral Efficiency >= 3 bps / Hz
 Control of support for legacy STA from
802.11n AP

Timing Diagram For
Aggregation:

802.11y:
 Operates in 3650-3700 MHz band
 Novel Licensing
 High Power compared to normal 20
Watts EIRP.
 Hence Higher coverage area (5 Km)
 3 Major changes compared to
traditional 802.11:
 Contention based protocol
 Extended Channel switch
announcement
 Dependent station enablement

Terminology in 802.11y:
 Dependent STA: A station that
must receive and successfully
decode an enabling signal before
it is allowed to transmit.
 Registered STA: A station that
is certified to operate at a
given location after registering
with the required authorities.
Registered STAs do not need to
receive an enabling signal before
commencing operation.

Terminology in 802.11y
(Contd..):
 Fixed STA: a station that is
authorized to operate only at a
specific location.
 Enabling point(EP): A fixed
station that transmits coded
signals that contain the
necessary information and
authorization for a dependant
STAs to transmit in a restricted
band for the purpose of engaging
in the DSE process.

Terminology in 802.11y
(Contd..):

 Dependent STA enablement
(DSE):
The process by which a
dependant STA gains
permission to transmit on a
restricted channel.

Roaming in WLAN:
 Seamless Roaming
 Nomadic Roaming
 A roaming domain is defined as APs that
are in the same broadcast domain and
configured with the same SSID.
 Stated another way, a client can only
roam between APs in the same VLAN and
with the same SSID.
 As WLAN deployments expand within an
organization, roaming domains might
need to scale beyond a single Layer 2
VLAN.

Layer 2 Roaming:
 The client must decide to roam—Roaming
algorithms (signal strength, frame
acknowledgment, missed beacons..)
 The client must decide where to roam—
 preemptive AP discovery,
 roam-time AP discovery.
 The client initiates a roam—The client
uses 802.11 (re-)association frames to
associate to a new AP.
 The client can resume existing
application sessions.

Roam Types:

 Preemptive AP discovery:
 Effects application Throughput
 Reduces roaming time
 Better use for Power save clients
 Not suitable for fast roaming
client
 Roam-time AP discovery:
 High roaming delay

Roaming Processes:
 The previous AP must determine that
the client has roamed away from it.
 The previous AP should buffer data
destined for the roaming client.*
 The new AP should indicate to the
previous AP that the client has
successfully roamed. This step usually
happens via a unicast or multicast
packet from the old AP to the new AP
with the source MAC address set to the
MAC of the roaming client.*
 The previous AP should send the
buffered data to the new AP.

 The previous AP must determine
that the client has roamed away
from it.
 The AP must update MAC address
tables on infrastructure
switches to prevent the loss of
data to the roaming client.
Note: * Tasks are not mandatory
because they are not specified
in the 802.11 standard.

Effects of roaming on Data
Flow:

Mobile IP to the rescue:
 Mobile node (MN)—The MN is the roaming
station.
 Home agent (HA)—The HA exists on
routers or Layer 3 switches and ensures
that a roaming MN receives its IP
packets.Foreign agent (FA)—The FA
exists on router or Layer 3 switches
and aids the MN notifying the HA of the
new MN location by receiving packets
from the HA destined for the MN.
 Care-of address (CoA)—The CoA is a
locally attached router that receives
packets sent by the HA, destined for
the MN. Co-located care-of address
(CCoA)—A CoA that exists on the mobile
node itself.

Steps in Layer 3 Roaming:
 A station is on its home subnet
if the station's IP address
belongs to the subnet of the HA.
 As the MN roams to a foreign
subnet, the MN detects the
presence of the FA and registers
with the FA or with the MN CCoA.
 The FA or MN CCoA communicates
with the HA and establishes a
tunnel between the HA and a CoA
for the MN.

 Packets destined to the MN are
sent to the HA (via normal IP
routing.
 The HA forwards the packets via
the tunnel to the MN.
 Any packets the MN transmits are
sent via the FA as if the MN were
local on the subnet. (A "reverse
tunnel" mode is available when
the edge routers use ingress
packet filtering.)

Effect on data flow in L3
roaming:

Overview of Mobile IP in
WLAN:

 Agent discovery:
 Uses IRDP (ICMP+router discovery
Protocol) Multicast/Broadcast
 The agent advertisement contains two
fields that allow the MN to determine
whether it has roamed to a new subnet:
 The lifetime field from the agent
advertisement
 The prefix-length extension

 Upon determining it is on a
foreign subnet, the MN gleans the
CoA from the agent advertisement.
The CoA can take two forms:
 The address of the FA.
 CCoA (Note that the CCoA is not
advertised by the FA, but it is
probably acquired by the MN as a
Dynamic Host Configuration
Protocol [DHCP] option.)

MN Registration Process:
 The MN sends a registration
request to the FA. If the MN has
a CCoA, this step is skipped.
 The FA processes the
registration request and
forwards the request to the HA.
 The HA accepts or declines the
registration and sends a
registration reply to the FA.
 The FA processes the
registration reply and relays it
to the MN.

Understanding 802.11e-
QoS

802.11e:

 HCF: Hybrid of DCF & PCF
 NO ACK
 BLOCK ACK
 TX OP

EDCA:
 EDCA contention access is an
extension of the legacy
CSMA/CA DCF mechanism to
include priorities

EDCA (Contd..):
 After resolving internal and
external collisions an STA is
given a TXOP.
 If the frame exchange sequence
has been completed, and there is
still time remaining in the TXOP,
the QSTA can may extend the frame
exchange sequence by transmitting
another frame in the same access
category. The QSTA must ensure
that the transmitted frame and
any necessary acknowledgement can
fit into the time remaining in
the TXOP.

EDCA Admission control:
 A station specifies its traffic
flow requirements (data rate,
delay bounds, packet size, and
others) and requests the QAP to
create a TSPEC by sending the
ADDTS (add TSPEC) management
action frame
 The QAP calculates the existing
load based on the current set of
issued TSPECs

 If the TSPEC is denied, the high
priority access category inside
the QSTA is not permitted to use
the high priority access
parameters, but it must use lower
priority parameters instead.
 Admission control is not
intended to be used for the "best
effort" and "background" traffic
classes

HCCA:

 The HCF controlled channel access
(HCCA) mechanism uses a hybrid
coordinator (HC) to centrally
manage medium access. The intent
of HCCA is to increase efficiency
by reducing the contention on the
medium.
 Delay for HC will be the least
 HC control Admission control
using TSPEC and grants the TXOP

Optional 802.11e features:
 Contention-free bursts,
-When time remains in a given TXOP
-To save 11g from 11b clients by
TXOP(g)=1 frame length of 11b
 Block acknowledgements,
 Direct link protocol,
 Active power mode save delivery
-Automatic Schedule in terms of beacon
periods
-APSD operation is invoked by a
station by establishing a TSPEC with
the APSD flag set.

Block acknowledgements:
Immediate BLOCK ACK

Delayed BLOCK ACK

My method is to take the utmost
trouble to find the right thing to say,
and then to say it with the utmost
levity.
.
George Bernard Shaw,
"Answers to Nine Questions"
Irish dramatist & socialist (1856 -
1950)

Wireless LAN (WLAN) concepts: Modulation to Aggregation

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