WIRELESS LAN
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36
IT 2402\MOBILE COMMUNITCATION \ U-2 \Page
CONTENTS
- Infrastructure and Ad Hoc networks
- IEEE 802.11 WLAN - Advantages, Disadvantages, Infrared Vs
radio transmission
-System Architecture
-Protocol Architecture
-Physical layer - FHSS, DSSS, Infrared
-MAC layer - DFWMAC-DCF, RTS / CTS, PCF with polling
- MAC Management Synchronization, Registration, Handoff,
Power
Management, Roaming, Security
- Wireless local loop
- IEEE 802.16 WiMAX
1. Infrastructure and Ad Hoc networks
1.1. Infrastructure based networks:
Provides Wireless Devices access to infrastructure network
Contains forwarding function, medium access control function
etc.
Communication between wireless nodes and the access points but
not directly between nodes.
Access points, with network in between, can connect several
wireless networks to form a larger network.
The network is simpler as most of the network functionality is
located at the Access Point and client remains quite simple.
Coordination is required for medium access to avoid
collision.
Typical infrastructure based wireless network is shown
below:
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If access point controls the medium access by polling, collision
may be avoided and this may also result in guaranteeing minimum
bandwidth for certain nodes.
This architecture has less flexibility in case of disaster,
system collapses.
Mobile cellular network, satellite cellular phones, are example
of this type.
1.2 Ad Hoc Network: Following diagram depicts the simple ad hoc
network concept.
- Nodes within an ad hoc network can only communicate if they
are radio distant close to each other.
The device complexity is higher as they have to implement,
medium access mechanism, mechanism handle hidden terminal, priority
mechanism to provide certain quality service etc.
Exhibits greatest flexibility.
There are other types of system that are mix of these two types.
That is, network infrastructure is used for basic services such as
authentication, control of medium access for data with associated
quality service, management functions etc, but also provide direct
communication between wireless nodes.
2. IEEE 802.11 WIRELESS LAN (WiFi)
2.1 .Introduction
IEEE 802. x specifies a number of standards, like Ethernet,
token ring etc. Wireless is also clubbed along with these as the
protocol structure is similar. In all these, only the physical
layer and Data Link Control layer are different leaving all other
upper layers same. The data link layer is divided into Logical Link
Control Layer and Medium Access Control Layer as shown below:
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General functionality of Physical layer is to provide encoding /
decoding of signals, synchronization and bit transmission and
reception. Similarly the functionality of MAC layer is - on
transmission, to assemble data into frame with address and error
detection fields, on reception , to disassemble frame and perform
address recognition and error detection. Govern access to the LAN
transmission medium and to provide interface to higher layers and
perform flow and error control.
IN addition to the above functionality, WLAN inlcude the support
of power management, handling hidden nodes ability to operate world
wide. To create world wide operability, the ISM bands at 900MHz and
2.4 MHz are selected in addition to IR transmission reception.
Advantages of Wireless LAN:
Flexibility: Within radio coverage, nodes can access each other
as radio waves can penetrate even partition walls.
Planning: No prior planning is required for connectivity as long
as devices follow standard convention
Design: Allows to design and develop mobile devices.
Robustness: wireless network can survive disaster eg
earthquakes. If the devices survive, communication can still be
established.
Disadvantages:
Quality of Service: Lower than the wired counterparts due to low
bandwidth (1 10 Mbps), higher error rates due to interference ( 10
exp 4 rather than 10 exp 10 as in the case of wired network)
Cost: Wireless LAN adapters are costly compared to wired
adapters.
Proprietary Solution: Due to slow standardization process, many
solution are proprietary that limit the homogeneity of
operation.
Restriction: Individual countries have their own radio spectral
policies. This restricts the proliferation of the technology
Safety and Security: Wireless Radio waves may interfere with
other devices. Eg; In a hospital radio waves may interfere with
high tech equipment.
Competing Requirement:
Global Operation: For global operation many national and
international frequencies have to be considered so that LAN devices
can be carried across the globe.
Low Power : As the devices will operate with batteries, the
design should take care of these facts.
License Free operation: Should be able to operate in the ISM
band of frequencies so that no license need be applied for its
operation.
Robust Transmission Technology: Must be capable of operating in
difficult condition where in high interference is expected from
other electrical devices.
Simplified Spontaneous Co Operation: Must be able to network
after power up without much complication.
Easy to Use: Must be easy to use by a common man without
complicated procedure.
Protection to Investment: Must be able operate with existing
system without modification.
Safety and Security: Should incorporate safe operation in places
like hospital and other critical areas like armament depot etc. NO
user must be able to read personal data during transmission that is
encryption mechanism should be integrated. Must not be possible to
collect roaming profile of any user.
Transparency for application: Existing application must continue
to run over wireless LAN, may be with higher delay and lower
bandwidth.
There are two technologies based on which the WLAN are set up.
One based on IR technology (around 900nm) and other based on radio
transmission at 2.4 GHz. Brief description of the same is given
below:
IR System
Wireless LAN Technology uses, infrared or radio transmission
technologies. Infrared technology (900 nano meters)uses diffused
light reflected at walls or directed light if a line of sight
exists between sender and receiver. LEDs or Laser diodes are used
as source for transmission while photodiodes are used as receivers.
The main advantages of infrared are, these are very cheap and all
mobile devices (PDA, Laptos, Mobile phones) are fitted with IrDA
Infra red Data Association - interface (Data rate of 115 kbps for
1.0 version and 1.152 4 Mbps for IrDA 1.1 Version). The
disadvantage, is its lack of penetration into walls and other
obstacles. Works with LOS for high data rate.
Radio System:
Advantages of radio transmission are it can cover large areas,
can penetrate walls furniture, trees. RF does not need LOS unless
the frequencies are very high. Current Transmission rates possible
are 10 mbps.
Disadvantage: As the radio waves can penetrate the walls,
shielding is not very simple and can interfere with other
electrical devices. Radio transmission is permitted in specific
band only. Very limited ranges of license free bands are available
worldwide.
2.2 System Architecture:
2.2.1 Infrastructure based :
Following is the figure of infrastructure based network.
Nodes known as stations are wirelessly connected to access
points (AP) that are within the radio coverage. The radio coverage
area of the Access Point is known as Base Service Set (BSS). The
association between the station and a BSS is dynamic. Station may
turn off, come within range and go out of range etc. Two or more
BSS are connected via a Distribution System (DS). This process
extends the reachability of the nodes in a BSS. This network is
called Extended Service Set (ESS). The ESS appears a single logical
LAN to the logical control level (LLC). The DS connects the BSS /
ESS AP with a portal - which is implemented in a device such as a
bridge or router that is part of a wired LAN - which forms the
Interworking units to other LANs.
2.2.2 Adhoc based:
IEEE 802.11 also allows to build Ad Hoc Networks as shown in the
following figure:
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IN this case a BSS comprises a group of stations that use the
same frequency. There is connectivity between the station within a
BSS but not to nodes of other BSS. However if the radio frequency
do not overlap, there can be more number of BSS in the same
geographical area Also 802.11 does not specify any special nodes
that support routing, forwarding of data or exchange of topology
information.
2.3 Protocol Architecture: It is intended that IEEE 802.11
protocol architecture fits seamlessly into the other 802.x
standards. Following figure depicts the 802.11 integrated to
Ethernet, that is 802.3 protocol via abridge.
As it can be seen, higher layers look the same for wired as well
as wireless nodes. Also, applications should not notice any
difference apart from the lower bandwidth and higher access time
from the wireless LAN. The upper part of the Data Link Control
Layer - the LLC covers the differences of the medium access control
layers needed for different media (IR, Radio- FHSS, DSSS CCK).
The PHYSICAL layer is subdivided into a PLCP Physical Layer
Convergence Protocol and the PMD Physical Medium dependent Sub
layer. As shown below:
The basic task of MAC layer comprises of medium access,
fragmentation of user data and encryption.
The PLCP sub layer provides a carrier sense signal called Clear
Channel Assessment(CCA) and provides a common PHY Service Access
Point (SAP)
PMD sub layer handles modulation and encoding / decoding of
signals.
The MAC Management layer supports, association and re
association of a station to an access point and roaming between
different access points. It also controls, authentication mechanism
, encryption, synchronization of a station with regard to an access
point and power management.
MAC Management also maintains MAC Management Information
Base.
PHY Management includes channel tuning and PHY MIB
maintenance.
Station management interacts with both the management layers and
is responsible for additional layer functions such as - control of
bridging and interaction with the distribution system in case of
access point.
2.3.1 Physical Layer:
IEEE 802. 11 supports different type of physical layer. One
layer based on Infra red and two layers based on the radio
transmission (primarily in the ISM bands at 2.4 GHz). These were
introduced in the year 1997. In addition , two more variants with
higher data rates were introduced in the year 1999. These were IEEE
802.11 a and 802.11b . The details are shown below:
App PHY variants include the provision of the clear channel
assessment (CCA). The purpose of this signal is to provide medium
access mechanism by indicating if the medium is busy or idle. The
transmission technology determines exactly how this signal is
obtained.
Also, the physical layers offers a service access point (SAP)
with a 1 or 2 Mbps transfer rate to the MAC layer.
Following chart gives the details of the PHYSICAL Layer
specifications:
Direct Sequence Spread Spectrum:
In this up to seven channels, each with a bandwidth of 5 MHz can
a be used Number of channel available in each country depends on
the spectrum allocated by that country. The encoding schemes that
is used is DBPSK for the 1 Mbps and DQPSK for 2 Mbps rate. DSSS
makes use of the chipping code or pseudo noise sequence, to spread
the data rate and hence the bandwidth of the signal. For IEEE
802.11, a 11 bit barker sequence is used. Maximum transmit power is
limited to 100 mw EIRP(Equivalent Isotropically Radiated Power) All
bits are transmitted by the DSSS PHY layer are scrambled with the
polynomial s(z) =
z7 + z4 + 1.
Following figures shows a frame of the physical layer using
DSSS:
The frame consists of two parts, the PLCP part (preamble and
header) and the payload part. PLCP part is always transmitted at 1
Mbps where as the pay load, that is the MAC data can use either 1
Mbps or 2 Mbps. The details of the field is given below:
Synchronisation : The first 128 bits are for synchronization,
gain setting, energy detection (for the CCA) and frequency offset
compensation. These are scrambled 1 bits.
Start Frame Delimiter (SFD) : This 16 bit files is used for
synchronization at the beginning of a frame and consists of pattern
1111001110100000.
Signal : As on now only two values have been specified . 0x 0A
indicates 1 Mbps and Ox 14 indicates 2 Mbps. Other values have been
reserved for future use.
Service : This fields is reserved for futures use. 0x 00
indicates an IEEE 802.11 compliant frame.
Length: 16 bits are used for length indication.
Header Error Check : Signal, service and length fields are
protected by this checksum using CRC 16 polynomial.
Frequency Hopping Spread Spectrum:
Frequency hopping spread spectrum technique allows for
coexistence of multiple networks in the same area by separating
different networks using different hoping sequences. The standard
defines 79 hopping channels for NA and 23 hopping channels for
Japan (each with a bandwidth of 1 MHz in the 2.4 MHz ISM band.) For
modulation, FHSS schemes uses two level Gaussian FSK for the 1 Mbps
system and for four level GFSK for 2 Mbps system. In this bits one
and zero are encoded as deviations from the current carrier
frequency. In case of four level, four different deviations form
the carrier frequency. While sending and receiving is mandatory at
1 Mbps for all devices, operation at 2 Mbps is optional. Following
diagram shows the frame of the physical layer used with FHSS.
In this also there two parts - PLCP part and Payload part.
Functions of various fields are described below:
Synchronisation: The PLCP preamble starts with 80 bit
synchronization which is 010101 .. bit pattern. This is used for
synchronization and for CCA.
Start Frame Delimiter (SFD): These 16 bits indicate that start
of the frame and this provide frame synchronization. The SFD
pattern is 0000110010111101
PLCP _PDU Length Word (PLW): This fields indicates the length of
the payload in bytes including the 32 bit CRC at the end of the
payload. PLW can range between 0 4095.
PLCP Signaling Files (PSF) : This four bit field indicates thje
data rate of the payload following 1 or 2 Mbps.
Header error check : The PLCP header is protected by a 16 bit
checksum with the standard ITU_ T generator Polynomial .
Infrared:
The IEEE 802.11 infrared scheme is omni directional rather than
point to point. A range of up to 20 m is possible. The modulation
scheme for the 1 Mbps data rate is known as 16 PPM (Pulse Position
Modulation). In this scheme group of 4 data bits is mapped into one
of the 16 PPM symbols, each symbol is a string of 16 bits. Each 16
bit string consists of fifteen 0 and one binary 1. For the 2 Mbps
data rate, each group of 2 data bits is mapped into one of four 4
bit sequences. Each sequence consists of a three 0 and one binary
1. The actual transmission uses an intensity modulation schemes in
which the presence of a signal corresponds to a binary 1 and the
absence of a signal corresponds to a binary 0.
IEEE 802.11 a:
This makes use of 5 GHz band. In this orthogonal frequency
division multiplexing (OFDM) is used. This is also called multi
carrier modulation. Uses multiple carrier signals at different
frequencies, sending some of the bits on each channel. This is
similar to FDM. But in the case of OFDM, all of the sub channels
are dedicated to a single data source. The possible data rates are
6, 9, 12, 18, 24, 36, 48, and 54 Mbps. The systems uses up to 52
sub carriers that are modulated using BPSK, QPSK 16 QAM or 64 QAM,
depending on the data rate required.
IEEE 802.11b:
It is an extension of the IEEE 802.11 DS-SS scheme, providing
rates of 5.5 and 11 Mb[s. The chipping rate is 11 MHz, which is the
sme as the original scheme, thereby occupying the same bandwidth.
To achieve higher data rate in the same bandwidth at the same
chipping rate, a modulation scheme known as Complementary Code
Keying (CCK) is used.
In CCK, input data are treated in blocks of 8 bits at a rate of
1.375 MHz ( 8 bits / symbol * 1.375 MHz = 11 Mbps). Six of these
bits are mapped into one of the 64 codes sequences based on the use
of 8 x 8 Walsh matrix. The out put of the mapping and two
additional bits forms the input to a QPSK modulator
2.3.2 MEDIUM ACCESS CONTROL:
a) MAC Mechanisms:
Three methods defined in IEEE 802.11 are known as Distribution
Foundation Wireless MAC.
Basic version based on CSMA/CA (Broadcast method)
Optional method to avoid hidden terminal problem. RTS / CTS
(Unicast method)
Contention free polling method for time bounded service. PCF
(Point Coordinated Function)
First two are summarized as Distributed Coordinated Function
(DCF) the third method is called Point Coordinated Function.
(PCF)
MAC mechanism is also called DFWMAC (Distributed Foundation
Wireless Medium Access Control).
For all access methods, several parameters are defined that
control the waiting time for the nodes. The duration is defined as
multiples of slot time. Slot time is derived from medium
propagation delay, transmitter delay and other PHY layer dependent
parameters (50 micro sec for FHSS & 20 Micro sec for DSSS)
Three different IFS (Inter Frame Space) parameters define the
priority of the medium. These are DIFS (DCF IFS) , PIFS(PCF IFS)
and SIFS (Short IFS).
Short IFS (SIFS)
Shortest IFS, Highest priority, (DSSS = 10 Micro sec and FHSS =
28 Micro sec)
Used for Acknowledgement, CTS and poll Response which are all
immediate response actions
Point Coordination Function IFS (PIFS)
Mid-length IFS
Used by centralized controller in PCF scheme when using polls
(SIFS + 1 slot time)
Distributed coordination function IFS (DIFS)
Longest IFS
Used as minimum delay of asynchronous frames contending for
access
Used for all ordinary asynchronous traffic
i)Basic Method DFWMAC:
Basic and mandatory access method based on CSMA /CA. If the
medium is idle for a certain duration of DIFS(ascertained with
CCA), a station accesses the medium immediately. (short delay light
load). If medium is busy, nodes enter contention phase by choosing
Radom Backoff time. After this period, if the medium is still busy,
it has to wait once again fir DIFS. To provide fairness, the
backoff timer is stopped, and continued from remained duration in
the next cycle. Backoff timer values increased exponentially from
0-7,15,31,63,127,255 depending load being light or heavy.
Station 3,1,are ready to transmit. Station 3 waits for DIFS and
with channel being free, starts transmission. Station 2 defer
transmission and waits for DIFS after channel busy period. By this
time station2 and station 5 become ready to transmit. Station 1,2,5
wait for DIFS period after busy period and generate backoff time.
Station 2 having shortest backoff time, accesses the medium first
and station 1 and 5 enter the contention phase in the cycle with
the residual back off timer. By this time station 4 joins and
generates backoff time equal to that of station 5. After DIFS
station 2,4,5 stars counting backoff timer and as station4, 5 have
same back off timer, the transmission clashes. Once again in the
next cycle, station1 completes the backoff timer successfully and
occupies the medium. Station 4,5 enter the contention phase once
again.
In case of Unicast transmission, receiver answers directly with
ACK after SIFS period which has the shortest duration. This ACK
ensures correct reception (correct checksum at the receiver). This
is important in the error prone environment. If no ACK is received
after SIFS, the sender resends the data after going the through
contention cycle.
ii))Contention Based RTS/CTS to avoid hidden node problem
Hidden node problem arises in case of WLAN, when two nodes that
are not within the radio range of each other try to communicate
with a node that is within radio range of each of them. To avoid
collision at the receiver, RTS CTS based contention mechanism is
used.
In this, after waiting for DIFS plus random backoff time, the
sender can issue a RTS control packet. RTS is not given higher
priority. RTS will contain the address of the receiver and the
duration of data. All station receiving this information will set
the Net Allocation Vector- NAV in accordance with duration field .
NAV is the earliest time when the station can access the medium.
The receiver receiving the RTS, sends CTS after SIFS. The CTS
control packet contains duration field and the address of the node
from which it will receive the data. All nodes listening to this
transmission will set their NAV till the acknowledgement is
received. As all the nodes closer to both transmitter and receiver
are set to NAV, this will avoid the hidden node problem.
iii)PCF Access Mechanism
PCF (Point Coordination Function) mechanism sits over DCF to
support contention free time transmission operation. (As the
operation is complicated, most manufacture have not opted for it).
PCF operation is available only for infrastructure network. AP
playing the point of coordinator, stops all other terminals and
polls other stations in semi periodic manner as shown below
Contention based methods do not guarantee minimum bandwidth to
all nodes. The medium access is not fair. In order to provide equal
opportunity to all nodes, IEEE 802.11 specifies an optional methods
called PCF. PCF provides time bounded service. Ad Hoc network
cannot use this provision. Point Coordinator splits the access time
- super frame period - into contention free period and contention
based period. In the contention free period, the point coordinator
(Access Point) waits for PCF IFS period (instead of DIFS) and
access the medium and starts the polling with each station. This is
shown in the following figure.
Access Point sends data1 to first station. Station 1 sends ACK
after SIFS. After SIFS duration, PC sends data to station2. In case
there is no data to be sent by the AP to a node, the AP waits for a
duration equal to SIFS and polls the next station as shown below.
This process continues, thus providing equal time to all the
station. At the end, it generates CFend (Contention free period
End) control pulse at which time, contention based medium access
takes place. At the end of contention based period, the super frame
repeats itself.
This ensures mini duration for each node in the system to access
the Access Point.
2.3.3 MAC FRAME FORMAT:
The frame transmission in case of WLAN can be categorised into
three types: Data, Management and Control. These are indicated in
the MAC frame. Following figure shows the MAC frame format.
This is a general format : (a) in octets while (b) in bits
Overall frame structure is shown in the(a). Details are as
follows:
FC : Frame Control indicates the type of frame : Data/Management
/ control
D/I : Duration / connection ID: When used as a duration field,
indicates the time in microseconds the channel will be used by the
source. In some cases, it may indicate connection identifier.
Addresses: These are Source/ Destination/ Sender and Receiver
addresses.
SC: Sequence Control: Four bits are used for numbering
fragmentation of a message while the 12 bits are used sequence
number sent between a transmitter and a receiver.
Frame Body: These are MAC SDU limited to a length of 2312
bytes.
Frame Check Sequence: a 32 bit Cyclical Redundancy Check.
Details of the frame control fields are shown in part (b) of the
above figure.:
Protocol Version: 802.11 version. Currently it is set to 0
Type: Identifies the frame as control, management or data
frame.
Sub Type: Further identifies various function under each type.
Details of Type and Sub type are shown below:
1. Management Type (00)
0000 / 0001 : Station requesting association with a AP (BSS) and
APs response
0010 / 0011 : Re association request and response. Sent by a
station when moves out of a BSS and moves into a different BSS.
0100 / 0101 : Probe Request and Response Used to obtain
information from another station or AP
1000 : Beacon
1001 : Announcement Traffic Indication Map : ATIM: A station
making an announcement to other station that it has buffered data
to transmit and its power low.
Disassociation: Used by a station to terminate Association.
1011 / 1100 : Authentication and De authentication. Indicates
using secure communication.
2. Control Type : 01
1010 : Power Save Poll: Request the AP or other station to
transmit any buffered data as the station is in power saving
mode.
1011 / 1100 : RTS and CTS;
1101 : ACK
1110 / 1111 : CF End / CF end with ACK.: Announces the end of
contention free period (PCF) / ACK the CF end.
3. Data Frames : 10:
0000 / 0001 : Data / Data with CF ACK. Simple data transfer /
Sends ACK for previously received data along with Data
0010 / 0011 : Data + CF Poll : Used by PCF to deliver data a
mobile station and also to request that the mobile station send a
data frame that it may have buffered.
0100 / 0101 : No data / CF ACK no Data indicates that the frame
carries no data, polls or ACK, but used to carry power management
bit int the frame fields to the AP to indicate the AP that is
changing to low power operation mode.
0110 / 0111 : CF Poll (no data)/ CF Poll ACK (no data) : These
fames with no data.
2.3.4 MAC Management Sub Layer:
This sub layer handles various management functions such as
Synchronization, Registration, Handoff, Power management, roaming
and security. These are explained briefly:
a) Synchronization:
To synchronize all stations, IEEE 802.11 specifies a TCF (Time
Synchronization Function). Synchronized clocks are needed for power
management, for coordinated PCF and (Super frame, CF and Contention
based period synchronization) and for synchronization of FHSS
hopping sequence. A beacon is transmitted periodically which
contains the following information:
time stamp (For Synchronization)
BSS ID
Traffic Indication Map (TIM)
Power Management
Roaming
i)Infrastructure Based Network
Beacon transmission is not periodic. Its transmission is
deferred, if the medium is busy. As it can be seen in the following
figure, second beacon is deferred till the medium is free. In
Infrastructure based network, access point performs the sync
function. All other nodes, adjust their local timer to the time
stamp.
ii)Ad Hoc Network
In case of ad hoc network, beacon is transmitted whichever
station access the medium first using random backoff algorithm. If
the medium is busy at the scheduled interval, its transmission is
deferred by the node. Later, once again each node contends for
access to the medium and which ever node access the medium first,
it sends the beacon. This can be seen in the following figure. In
the first case, station access the medium first and transmits the
beacon. However, at the scheduled repetition, the medium becomes
busy and as the medium becomes free, all nodes contend for access
and this time node 2 access the medium first and transmits the
beacon frame. All other nodes synchronize with the second node
now.
b) Registration: . The beacon is a management frame that is
transmitted quasi periodically by the AP to establish the timing
synchronisation function (TSF). It contains information such as the
BSS ID, Time stamp (for synchronisation), power management , and
roaming. Received Signal Strength (RSS) measurements are made on
the beacon message. Any node receiving the beacon, if it decides to
register with the AP, will a Association Request frame. The AP
receiving the same, will send back Association Response frame back.
With this , the registration will complete and the node will form
the part of the AP. Only on registration, the distribution system
will know to which BSS an MS is attached.
c) Hand Off: There are three types of mobility in WLAN .
No transition : Implies the MS is within or is moving within a
BSA.
BSS Transition: Indicates that the MS moves from one BSS to
another in the same ESS.
ESS Transition: This is the movement of MS from one BSS to
another BSS that is part of new ESS. In this case connection may
break unless it has higher layer IP connection.
The handoff procedure is a WLAN is shown in the following
figure:
The AP broadcast a beacon signal periodically (typically once in
100 ms). An MS that scans the beacon signal and associates itself
with the AP with the strongest beacon. The beacon contains
information corresponding to the AP such as Time Stamp, beacon
interval, capability, ESS Id and traffic indication Map (TIM). The
MS uses this information in the beacon to distinguish between
different APs.
The MS keeps tack of the RSS of the beacon of the AP with which
is it is associated, and when the RSS becomes weak, it starts to
scan for stronger beacons from the neighbouring APs. The scanning
process can be either active or passive. In passive scanning, the
MS simply listens to available beacons. In active scanning, the MS
sends a probe request to a targeted set of APs that are capable of
receiving its probe. Each AP that receives the probes responds with
a probe response that contains the same information that is
available in the beacon except for the TIM. The probe response thus
serves the MS to select the AP with strongest beacon and sends
re-association requests to the new AP. In response, the new AP
sends re association which contains information of MS and that of
old AP. In Response the new AP sends the re association response
that has the information about the supported bit rates, station ID
and so on needed for communication. The old AP is not informed by
the MS about the change of location. The hand off is intimated by
using IAPP (Inter Access point protocol) standard that intimates
the old AP about handoff through wired network.
d) Power Management:
Power Management is carried out when the station is in awake
position during reception of inbound data and sleep position during
idle period. Throughput is traded for battery life. Longer off
period, low throughput and vice versa.
States of a station: sleep and awake and buffering of data in
senders. Sleeping station wakes periodically and stays awake for
certain duration. If it detects that it has to receive data, it
keeps awake. Waking up at right time requires TSF(Timing
Synchronization Function.
A receiver node knows when to transmit data but does not know
when it receive data or when to wake up.
System should be synchronized to sleep and wake up
transparently. (Known to other nodes)
Longer off period saves battery but throughput is reduced and
vice versa.
i)Infrastructure Based Network
AP buffers data for sleeping nodes and sends TIM (Traffic
Indication Map) which contains list of address for which uni-cast
data are buffered in AP. AP sends multicast or broadcast data
periodically at each DTIM (Delivery Traffic Indication Map) DTIM is
multiple time of TIM.
Once the nodes are synchronized, the AP sends Traffic indication
map (TIM), periodically in which it transmits the address of the
nodes for which it has the data buffered. Any node that has a data
to receive will continue to wake up till the time the data is
transmitted by AP and received by the node. At the end, the node
will go to sleep mode. In case of multi cast and broad cast of data
for a number of nodes or to all nodes, the DTIM is transmitted by
the AP at regular intervals which is multiples of TIM interval as
shown in the above figure.
ii) Ad-hoc Network: In case of Ad Hoc Networks, Traffic
Indication map is announced by the node that has a data to
transmit. The node (address) to which this TIM is applicable, sends
a Acknowledgement TIM. On receipt of this, the node transmits data
and receives acknowledgment from the receiver. As there is no
central agency like AP to control it, it is more complicated. In
this there is collision of ATIMs are possible.
e) Roaming:
No or bad connection? Then perform:
i) Scanning
scan the environment, i.e., listen into the medium for beacon
signals or send probes into the medium and wait for an answer
Scanning involves the active search for a BSS. IEEE 802.11
differentiates between passive and active scanning.
Passive scanning - listening into the medium to find other
networks, i.e., receiving the beacon of another network issued by
access point.
Active scanning - sending a probe on each channel and waiting
for a response. Beacon and probe responses contain the information
necessary to join the new BSS.
ii)Reassociation Request
station sends a request to one or several AP(s)
iii)Reassociation Response
success: AP has answered, station can now participate
failure: continue scanning
iv)AP accepts Reassociation Request
signal the new station to the distribution system
the distribution system updates its data base (i.e., location
information)
typically, the distribution system now informs the old AP so it
can release resources
f)Security:
IEEE 802.11 provides both privacy and authentication mechanism.
The mechanism is known as Wired Equivalent Privacy (WEP). To
provide privacy and data integrity, WEP uses an encryption
algorithm based on the RC4 algorithm. Following figure shows the
encryption process.
Above figure shows the encryption process. The integrity
algorithm is the 32 bit CRC that is appended to the end of MAC
frame. For encryption process, a 40 bit secret key is shared by two
participants in the exchange. An Initialisation Vector I(IV ) is I
concatenated to the secret key. The resulting block form the seed
that is input to the pseudorandom number generator (PRNG) defined
in RC4. The PRNG generates a bit sequence of the same length as
that of MAC frame plus its CRC. A bit by bit exclusive OR between
the MAC frame and the PRNG sequence produces the cipher text. The
IV is changed periodically (as often as every transmission). Every
time the IV is changed, the PRNG sequence is changed, which
provides protection against eavesdropper.
At the receiving end, the receiver retrieves the IV from the
data block and concatenates this with the shared secret key to
generate the same key sequence used by sender. This key sequence is
then XORed with the incoming block to recover the plaintext. This
technique makes use of the property A(B (B = A. Finally the
receiver compares the incoming CRC with the CRC calculated at the
receiver to validate integrity.
Authentication:
There are two types of authentication : Open System and Shared
Key.
Open system authentication simply provides a way for two parties
to agree to exchange data and provides no security benefits. IN
this one party sends a MAC control frame, known as authentication
frame to other party. The frame indicates that this is an open
system authentication type. The other party responds with its own
authentication frame and the process is complete.
Shared Key Authentication: This requires that the two parties
share a secret key not shared by any other party. This key is used
to assure that both sides are authenticated to each other. The
procedure is as follows:
a. A sends a MAC authentication frame with an authentication
algorithm identification of Shared Key and with station identifier
that identifies the sending station
b. B responds with an authentication frame that includes a 128
octet challenge text which is generated using WEP PRNG.
c. A transmits an authentication frame that includes the
challenge text just received from B. The entire frame is encrypted
using WEP.
d. B Receives the encrypted frame and decrypts it using WEP and
the secret key shared with A. If decryption is successful (matching
CRC), then B compares the inkling challenge text with the challenge
text that it sent in the second message. B then sends an
authentication message to A with a status code indicating success
or failure.
3.Wireless Local Loop
3.1 Introduction
Wired technologies respond to reliable, high-speed access by
residential, business, and government subscribers requirements For
example, ISDN, xDSL, cable modems etc. However, increasing interest
shown in competing wireless technologies for subscriber access
known as WLL or Fixed Wireless Access. Initially they were
considered for providing quick telephone connection to residents,
office etc based on quick deployment of WLL technology.
Wireless local loop (WLL)
Narrowband offers a replacement for existing telephony services
(MMDS- Multichannel Multipoint Distribution Services)
Broadband provides high-speed two-way voice and data
service(LMDS- Local Multipoint Distribution Services)
Some of the advantages of WLL are listed below:
Cost wireless systems are less expensive due to cost of cable
installation thats avoided
Installation time WLL systems can be installed in a small
fraction of the time required for a new wired system
Selective installation radio units installed for subscribers who
want service at a given time
With a wired system, cable is laid out in anticipation of
serving every subscriber in a given area
3.2 Wireless Local Loop Technologies
*Satellite-Based Systems
Provide Telephony services for rural communities and isolated
areas
*Cellular-Based Systems
Provide high-power, wide-range, median subscriber density and
median circuit-quality WLL services
Offers both mobility and fixed wireless access via the same
platform as cellular
*Low-tier PCS or Microcellular-Based Systems
Provide low-power, narrow-range, high subscriber density and
high circuit-quality WLL services
*Fixed Wireless Access (FWA) Systems
Proprietary radio systems
Disadvantage of the cellular approach
*Limitation on toll-quality voice and signaling transparency
Disadvantage of low-tier PCS and microcellular approaches
*Narrow radio coverage range
FWA addresses these issues
3.3 WLL Configuration:
A simple configuration of WLL is shown below: WLL antennas are
mounted on the top a tallest building and they are connected
through medium to receiver antennas that are mounted on top of
residential buildings, private and public offices as shown
below.
WLL with DECT:
3.4 Wireless Local Loop Architecture
Wireless Access Network Unit (WANU)
Components
BTSs or Radio Ports (RPs)
A Radio Port Control Unit (RPCU)
Access Manager (AM) and HLR
Functions provided by WANU
Authentication, Air Interface Privacy
Over-the-Air Registration of Subscriber Units
Radio Resource Management
InterworkingFunction (IWF)
Operation and Maintenance (OAM)
Routing, Billing, and Switching
Protocol conversion and transcoding of voice and data
Wireless Access Subscriber Unit (WASU)
Functions provided by WASU
Air Interface UWLLtoward the network
A traditional interface TWLLtoward the subscriber
This Interfaces include
Protocol conversion
Transcoding
Authentication Function
OAM
Signaling Functions
Modem Function to support voice-band data
Switching Function (SF)
3.5 Propagation Considerations for WLL
Most high-speed WLL schemes use millimeter wave frequencies (10
GHz to about 300 GHz) as there are wide unused frequency bands
available above 25 GHz band. At these high frequencies, wide
channel bandwidths can be used, providing high data rates. Also,
high frequencies, small size transceivers and adaptive antenna
arrays can be used
Millimeter wave systems have some undesirable propagation
characteristics. These are listed below:
Free space loss increases with the square of the frequency;
losses are much higher in millimeter wave range
Above 10 GHz, attenuation effects due to rainfall and
atmospheric or gaseous absorption are large
Multipath losses can be quite high
Because of these limitations, WLL serves cells of limited
radius. Obstructions including foliage must be avoided along or
near the line of sight. Rainfall and humidity limit the range and
availability of WLL system.
Fresnel Zone
Unlike the case for mobile communication, in case of WLL, direct
line of sight between the transmitter and receiver antenna must be
free of any obstruction At these high frequencies, radio signals
are lost due to any obstruction. Understanding of Fresnels zone
gives importance of obstruction free line of sight between
transmitter and receiver antenna. It s based on the theory that any
small element of space in the path of an EM wave may be considered
the sources of secondary wavelet and radiation can be built up of
super position of all these wavelets. On the basis of this theory,
it can be shown that objects lying within a series of concentric
circles around the direct line of sight between two trans receivers
have constructive or destructive effects on communication. These
are called Fresnels Zone. Out of this, those that fall within the
first circle, that is the first Fresnel Zone, have the most serious
effects.
Consider a point along the path between a transmitter and
receiver, that is a distance S from the transmitter and a distance
D from the receiver, with the total distance along the path equal
to S + D . The radius of the first Fresnel zone at the point is
Where R, S, D are in the same units and is the wavelength of the
signal along the path. For convenience, it can be restated as
follows:
Where is R is expressed in meters and the two distance are in
kilometer and the frequency in gigahertz.
For S & D = 10 KM and for f= 2.5 GHz, Rm=17.3 M
S & D = 10 KM and for f= 28 GHz, Rm=5.17 M
S & D = 1 KM and for f= 28 GHz, Rm=3.1 M
If there is no obstruction within 0.6 times the radius of the
first Fresnels zone, at any point between the two trans-receivers,
then attenuation due to obstruction is negligible. Also, height of
the two antenna must be such that at no point along the path at
which the ground is within 0.6 times the radius of the first
Fresnels zone.
Atmospheric Absorption
Radio waves at frequencies above 10 GHz are subject to molecular
absorption. Peak of water vapor absorption take s place at 22 GHz
and peak of oxygen absorption is near 60 GHz. There are favorable
frequency slots that are useful for WLL communication. These are
given below:
From 28 GHz to 42 GHz
From 75 GHz to 95 GHz
These details can be seen in the figure shown below:
These effects are different for different temperature, relative
humidity and atmospheric pressure.
Effect of Rain :
Radio waves at high frequencies are severely attenuated due to
rain. Presence of raindrops can severely degrade the reliability
and performance of communication links (We can observe this
phenomenon in our DTH operated TV sets. As they operate at above 10
GHz, these signals are subjected to severe absorption during rainy
period and signals are lost). The effect of rain depends on drop
shape, drop size, rain rate, and frequency.
Estimated attenuation due to rain:
Where A = attenuation (dB/km); R = rain rate (mm/hr) ; a and b
depend on drop sizes and frequency
Effects of Vegetation :
Trees near subscriber sites can lead to multipath fading.
Multipath effects from the tree canopy are diffraction and
scattering. Measurements in orchards found considerable attenuation
values when the foliage is within 60% of the first Fresnel zone.
Multipath effects highly variable due to wind
4. WiMAX
4.1 WiMAX - What is WiMAX ?
WiMAX would operate similar to WiFi but at higher speeds over
greater distances and for a greater number of users. WiMAX was
formed in April 2001, in anticipation of the publication of the
original 10-66 GHz IEEE 802.16 specifications. WiMAX is to 802.16
as the WiFi Alliance is to 802.11.
WiMAX is:
Acronym for Worldwide Interoperability for Microwave Access.
Based on Wireless MAN technology.
A wireless technology optimized for the delivery of IP centric
services over a wide area.
A scalable wireless platform for constructing alternative and
complementary broadband networks.
A certification that denotes interoperability of equipment built
to the IEEE 802.16 or compatible standard. The IEEE 802.16 Working
Group develops standards that address two types of usage
models:
A fixed usage model (IEEE 802.16-2004).
A portable usage model (IEEE 802.16e).
4.2 WiMax Speed and Range:
WiMAX is expected to offer initially up to about 40 Mbps
capacity per wireless channel for both fixed and portable
applications, depending on the particular technical configuration
chosen, enough to support hundreds of businesses with T-1 speed
connectivity and thousands of residences with DSL speed
connectivity. WiMAX can support voice and video as well as Internet
data.WiMAX could potentially be deployed in a variety of spectrum
bands: 2.3GHz, 2.5GHz, 3.5GHz, and 5.8GHz
4.3WiMAX - Salient Features
WiMAX is a wireless broadband solution that offers a rich set of
features with a lot of flexibility in terms of deployment options
and potential service offerings. Some of the more salient features
that deserve highlighting are as follows:
Two Type of Services:
WiMAX can provide two forms of wireless service:
Non-line-of-sight: service is a WiFi sort of service. Here a
small antenna on your computer connects to the WiMAX tower. In this
mode, WiMAX uses a lower frequency range -- 2 GHz to 11 GHz
(similar to WiFi).
Line-of-sight: service, where a fixed dish antenna points
straight at the WiMAX tower from a rooftop or pole. The
line-of-sight connection is stronger and more stable, so it's able
to send a lot of data with fewer errors. Line-of-sight
transmissions use higher frequencies, with ranges reaching a
possible 66 GHz.
OFDM-based physical layer:
The WiMAX physical layer (PHY) is based on orthogonal frequency
division multiplexing, a scheme that offers good resistance to
multipath, and allows WiMAX to operate in NLOS conditions.
Very high peak data rates:
WiMAX is capable of supporting very high peak data rates. In
fact, the peak PHY data rate can be as high as 74Mbps when
operating using a 20MHz wide spectrum.More typically, using a 10MHz
spectrum operating using TDD scheme with a 3:1 downlink-to-uplink
ratio, the peak PHY data rate is about 25Mbps and 6.7Mbps for the
downlink and the uplink, respectively.
Scalable bandwidth and data rate support:
WiMAX has a scalable physical-layer architecture that allows for
the data rate to scale easily with available channel bandwidth.For
example, a WiMAX system may use 128, 512, or 1,048-bit FFTs (fast
fourier transforms) based on whether the channel bandwidth is
1.25MHz, 5MHz, or 10MHz, respectively. This scaling may be done
dynamically to support user roaming across different networks that
may have different bandwidth allocations.
Adaptive modulation and coding (AMC):
WiMAX supports a number of modulation and forward error
correction (FEC) coding schemes and allows the scheme to be changed
on a per user and per frame basis, based on channel conditions.AMC
is an effective mechanism to maximize throughput in a time-varying
channel.
Link-layer retransmissions:
WiMAX supports automatic retransmission requests (ARQ) at the
link layer for connections that require enhanced reliability.
ARQ-enabled connections require each transmitted packet to be
acknowledged by the receiver; unacknowledged packets are assumed to
be lost and are retransmitted.
Support for TDD and FDD:
IEEE 802.16-2004 and IEEE 802.16e-2005 supports both time
division duplexing and frequency division duplexing, as well as a
half-duplex FDD, which allows for a low-cost system
implementation.
WiMAX uses OFDM:
Mobile WiMAX uses Orthogonal frequency division multiple access
(OFDM) as a multiple-access technique, whereby different users can
be allocated different subsets of the OFDM tones.
Flexible and dynamic per user resource allocation:
Both uplink and downlink resource allocation are controlled by a
scheduler in the base station. Capacity is shared among multiple
users on a demand basis, using a burst TDM scheme.
Support for advanced antenna techniques:
The WiMAX solution has a number of hooks built into the
physical-layer design, which allows for the use of multiple-antenna
techniques, such as beamforming, space-time coding, and spatial
multiplexing.
Quality-of-service support:
The WiMAX MAC layer has a connection-oriented architecture that
is designed to support a variety of applications, including voice
and multimedia services.
WiMAX system offers support for constant bit rate, variable bit
rate, real-time, and non-real-time traffic flows, in addition to
best-effort data traffic.
WiMAX MAC is designed to support a large number of users, with
multiple connections per terminal, each with its own QoS
requirement.
Robust security:
WiMAX supports strong encryption, using Advanced Encryption
Standard (AES), and has a robust privacy and key-management
protocol.
The system also offers a very flexible authentication
architecture based on Extensible Authentication Protocol (EAP),
which allows for a variety of user credentials, including
username/password, digital certificates, and smart cards.
Support for mobility:
The mobile WiMAX variant of the system has mechanisms to support
secure seamless handovers for delay-tolerant full-mobility
applications, such as VoIP.
IP-based architecture:
The WiMAX Forum has defined a reference network architecture
that is based on an all-IP platform. All end-to-end services are
delivered over an IP architecture relying on IP-based protocols for
end-to-end transport, QoS, session management, security, and
mobility.
4.4 WiMAX - Reference Network Model
4.4.1 WiMAX - Building Blocks
A WiMAX system consists of two major parts:
A WiMAX base station.
A WiMAX receiver.
WiMAX Base Station:
A WiMAX base station consists of indoor electronics and a WiMAX
tower similar in concept to a cell-phone tower. A WiMAX base
station can provide coverage to a very large area up to a radius of
6 miles. Any wireless device within the coverage area would be able
to access the Internet.The WiMAX base stations would use the MAC
layer defined in the standard, a common interface that makes the
networks interoperable and would allocate uplink and downlink
bandwidth to subscribers according to their needs, on an
essentially real-time basis.
Each base station provides wireless coverage over an area called
a cell. Theoretically, the maximum radius of a cell is 50 km or 30
miles however, practical considerations limit it to about 10 km or
6 miles.
WiMAX Receiver:
A WiMAX receiver may have a separate antenna or could be a
stand-alone box or a PCMCIA card sitting in your laptop or computer
or any other device. This is also referred as customer premise
equipment (CPE).WiMAX base station is similar to accessing a
wireless access point in a WiFi network, but the coverage is
greater.
Backhaul:
A WiMAX tower station can connect directly to the Internet using
a high-bandwidth, wired connection (for example, a T3 line). It can
also connect to another WiMAX tower using a line-of-sight microwave
link.Backhaul refers both to the connection from the access point
back to the base station and to the connection from the base
station to the core network.It is possible to connect several base
stations to one another using high-speed backhaul microwave links.
This would also allow for roaming by a WiMAX subscriber from one
base station coverage area to another, similar to the roaming
enabled by cell phones.
4.4.2 System Reference Architecture
IEEE802.16 provides a communication path between a subscriber
site which may either be a single subscriber device or a network of
the subscribers premises (eg LAN, PBX, IP based network) and a core
network, eg. Public Telephone network and the internet.
The IEEE 802.16e-2005 standard provides the air interface for
WiMAX but does not define the full end-to-end WiMAX network. The
WiMAX Forum's Network Working Group (NWG) is responsible for
developing the end-to-end network requirements, architecture, and
protocols for WiMAX, using IEEE 802.16e-2005 as the air interface.
The WiMAX NWG has developed a network reference model to serve as
an architecture framework for WiMAX deployments and to ensure
interoperability among various WiMAX equipment and operators.
The network reference model envisions a unified network
architecture for supporting fixed, nomadic, and mobile deployments
and is based on an IP service model. Below is simplified
illustration of an IP-based WiMAX network architecture. The overall
network may be logically divided into three parts:
Mobile Stations (MS) used by the end user to access the
network.
The access service network (ASN), which comprises one or more
base stations and one or more ASN gateways that form the radio
access network at the edge.
Connectivity service network (CSN), which provides IP
connectivity and all the IP core network functions.
The network reference model developed by the WiMAX Forum NWG
defines a number of functional entities and interfaces between
those entities. Fig below shows some of the more important
functional entities.
Base station (BS): The BS is responsible for providing the air
interface to the MS. Additional functions that may be part of the
BS are micromobility management functions, such as handoff
triggering and tunnel establishment, radio resource management, QoS
policy enforcement, traffic classification, DHCP (Dynamic Host
Control Protocol) proxy, key management, session management, and
multicast group management.
Access service network gateway (ASN-GW): The ASN gateway
typically acts as a layer 2 traffic aggregation point within an
ASN. Additional functions that may be part of the ASN gateway
include intra-ASN location management and paging, radio resource
management, and admission control, caching of subscriber profiles,
and encryption keys, AAA client functionality, establishment, and
management of mobility tunnel with base stations, QoS and policy
enforcement, foreign agent functionality for mobile IP, and routing
to the selected CSN.
Connectivity service network (CSN): The CSN provides
connectivity to the Internet, ASP, other public networks, and
corporate networks. The CSN is owned by the NSP and includes AAA
servers that support authentication for the devices, users, and
specific services. The CSN also provides per user policy management
of QoS and security. The CSN is also responsible for IP address
management, support for roaming between different NSPs, location
management between ASNs, and mobility and roaming between ASNs.
The WiMAX architecture framework allows for the flexible
decomposition and/or combination of functional entities when
building the physical entities. For example, the ASN may be
decomposed into base station transceivers (BST), base station
controllers (BSC), and an ASNGW analogous to the GSM model of BTS,
BSC, and Serving GPRS Support Node (SGSN).
4.5 IEEE 802.16 Protocol Architecture
Protocol Architecture
Physical and transmission layer functions:
Encoding/decoding of signals
Preamble generation/removal (Synchronization)
Bit transmission/reception
Medium access control layer functions:
On transmission, assemble data into a frame with address and
error detection fields
On reception, disassemble frame, and perform address recognition
and error detection
Govern access to the wireless transmission medium
Convergence layer functions:
Encapsulate PDU framing of upper layers into native 802.16
MAC/PHY frames
Map upper layers addresses into 802.16 addresses
Translate upper layer QoS parameters into native 802.16 MAC
format
Adapt time dependencies of upper layer traffic into equivalent
MAC service
4.5.1 WiMAX - Physical Layer
The WiMAX physical layer is based on orthogonal frequency
division multiplexing. OFDM is the transmission scheme of choice to
enable high-speed data, video, and multimedia communications and is
used by a variety of commercial broadband systems, including DSL,
Wi-Fi, Digital Video Broadcast-Handheld (DVB-H), and MediaFLO,
besides WiMAX.
OFDM is an elegant and efficient scheme for high data rate
transmission in a non-line-of-sight or multipath radio
environment.
Adaptive Modulation and Coding in WiMAX:
WiMAX supports a variety of modulation and coding schemes and
allows for the scheme to change on a burst-by-burst basis per link,
depending on channel conditions. Using the channel quality feedback
indicator, the mobile can provide the base station with feedback on
the downlink channel quality. For the uplink, the base station can
estimate the channel quality, based on the received signal
quality.
Following is a list of the various modulation and coding schemes
supported by WiMAX.
Downlink
Uplink
Modulation
BPSK, QPSK, 16 QAM, 64 QAM; BPSK optional for OFDMA-PHY
BPSK, QPSK, 16 QAM; 64 QAM optional
Coding
Mandatory: convolutional codes at rate 1/2, 2/3, 3/4, 5/6
Optional: convolutional turbo codes at rate 1/2, 2/3, 3/4, 5/6;
repetition codes at rate 1/2, 1/3, 1/6, LDPC, RS-Codes for
OFDM-PHY
Mandatory: convolutional codes at rate 1/2, 2/3, 3/4, 5/6
Optional: convolutional turbo codes at rate 1/2, 2/3, 3/4, 5/6;
repetition codes at rate 1/2, 1/3, 1/6, LDPC
PHY-Layer Data Rates:
Because the physical layer of WiMAX is quite flexible, data rate
performance varies based on the operating parameters. Parameters
that have a significant impact on the physical-layer data rate are
channel bandwidth and the modulation and coding scheme used. Other
parameters, such as number of subchannels, OFDM guard time, and
oversampling rate, also have an impact.
Following is the PHY-layer data rate at various channel
bandwidths, as well as modulation and coding schemes.
WiMAX - OFDM Basics
OFDM belongs to a family of transmission schemes called
multicarrier modulation, which is based on the idea of dividing a
given high-bit-rate data stream into several parallel lower
bit-rate streams and modulating each stream on separate carriers,
often called subcarriers or tones. Multicarrier modulation schemes
eliminate or minimize intersymbol interference (ISI) by making the
symbol time large enough so that the channel-induced delays delay
spread being a good measure of this in wireless channels are an
insignificant (typically, < 10 percent) fraction of the symbol
duration.Therefore, in high-data-rate systems in which the symbol
duration is small, being inversely proportional to the data rate
splitting the data stream into many parallel streams increases the
symbol duration of each stream such that the delay spread is only a
small fraction of the symbol duration.
OFDM is a spectrally efficient version of multicarrier
modulation, where the subcarriers are selected such that they are
all orthogonal to one another over the symbol duration, thereby
avoiding the need to have nonoverlapping subcarrier channels to
eliminate intercarrier interference.In order to completely
eliminate ISI, guard intervals are used between OFDM symbols. By
making the guard interval larger than the expected multipath delay
spread, ISI can be completely eliminated. Adding a guard interval,
however, implies power wastage and a decrease in bandwidth
efficiency.
4.5.2 WiMAX - MAC Layer
The IEEE 802.16 MAC was designed for point-to-multipoint
broadband wireless access applications. The primary task of the
WiMAX MAC layer is to provide an interface between the higher
transport layers and the physical layer.The MAC layer takes packets
from the upper layer, these packets are called MAC service data
units (MSDUs) and organizes them into MAC protocol data units
(MPDUs) for transmission over the air. For received transmissions,
the MAC layer does the reverse.The IEEE 802.16-2004 and IEEE
802.16e-2005 MAC design includes a convergence sublayer that can
interface with a variety of higher-layer protocols, such as ATM TDM
Voice, Ethernet, IP, and any unknown future protocol.The 802.16 MAC
is designed for point-to-multipoint (PMP) applications and is based
on collision sense multiple access with collision avoidance
(CSMA/CA).
The MAC incorporates several features suitable for a broad range
of applications at different mobility rates, such as the
following:
Privacy key management (PKM) for MAC layer security. PKM version
2 incorporates support for extensible authentication protocol
(EAP).
Broadcast and multicast support.
Manageability primitives.
High-speed handover and mobility management primitives.
Three power management levels, normal operation, sleep, and
idle.
Header suppression, packing and fragmentation for efficient use
of spectrum.
Five service classes, unsolicited grant service (UGS), real-time
polling service (rtPS), non-real-time polling service (nrtPS), best
effort (BE), and Extended real-time variable rate (ERT-VR)
service.
These features combined with the inherent benefits of scalable
OFDMA make 802.16 suitable for high-speed data and bursty or
isochronous IP multimedia applications.Support for QoS is a
fundamental part of the WiMAX MAC-layer design. WiMAX borrows some
of the basic ideas behind its QoS design from the DOCSIS cable
modem standard.Strong QoS control is achieved by using a
connection-oriented MAC architecture, where all downlink and uplink
connections are controlled by the serving BS.WiMAX also defines a
concept of a service flow. A service flow is a unidirectional flow
of packets with a particular set of QoS parameters and is
identified by a service flow identifier (SFID).
4.6 Summary:
WiMAX is based on a very flexible and robust air interface
defined by the IEEE 802.16 group.
WiMAX is similar to the wireless standard known as Wi-Fi, but on
a much larger scale and at faster speeds.
The WiMAX physical layer is based on OFDM, which is an elegant
and effective technique for overcoming multipath distortion.
The physical layer supports several advanced techniques for
increasing the reliability of the link layer. These techniques
include powerful error correction coding, including turbo coding
and LDPC, hybrid-ARQ, and antenna arrays.
WiMAX supports a number of advanced signal-processing techniques
to improve overall system capacity. These techniques include
adaptive modulation and coding, spatial multiplexing, and multiuser
diversity.
WiMAX has a very flexible MAC layer that can accommodate a
variety of traffic types, including voice, video, and multimedia,
and provide strong QoS.
Robust security functions, such as strong encryption and mutual
authentication, are built into the WiMAX standard.
WiMAX defines a flexible all-IP-based network architecture that
allows for the exploitation of all the benefits of IP.
WiMAX offers very high spectral efficiency, particularly when
using higher-order MIMO solutions
4.7 WiMAX and Wi-Fi Comparison
Feature
WiMax(802.16a)
Wi-Fi(802.11b)
Wi-Fi(802.11a/g)
PrimaryApplication
Broadband WirelessAccess
Wireless LAN
Wireless LAN
Frequency Band
Licensed/Unlicensed2 G to 11 GHz
2.4 GHz ISM
2.4 GHz ISM (g)5 GHz U-NII (a)
ChannelBandwidth
Adjustable1.25 M to 20 MHz
25 MHz
20 MHz
Half/Full Duplex
Full
Half
Half
Radio Technology
OFDM(256-channels)
Direct SequenceSpread Spectrum
OFDM(64-channels)
BandwidthEfficiency