Laboratory – Wi-MAX Technology
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Wi-MAX technology
1. Theoretical Introduction
1.1. WiMAX Network
1.1.1. General Features
WiMAX (Worldwide Interoperability for Microwave Access) is a
telecommunications technology broadband wireless standard based on IEEE 802.16.
The main advantages of this technology are:
o Physical layer based on orthogonal frequency multiplexing (OFDM) scheme
offering good behavior under multipath propagation and allows operation without
direct line of sight;
o Very high transmission rates: for example, using a bandwidth of 10MHz and with
time duplexing (TDD), can be achieved, in good signal conditions, a maximum
rate of 25Mbps on the downward path;
o Adaptive modulation and coding (AMC): selection, by an algorithm, of the best
modulation and coding schemes, supportable in the existing SNR conditions in
the channel at that time;
o Automatic retransmission request (ARQ) at the link layer (level 2 OSI), for
connections requiring high reliability;
o Allocation of resources per user and per Frame in time, frequency and optionally,
in space.
1.1.2. Components of a WiMAX network
WiMAX network reference model provides a unified architecture for fixed,
nomadic and mobile communications and is based on an IP network. The WiMAX
network can logically be divided into three parts:
o Subscriber stations used by end users to access the network;
o Access service network (ASN), which consists of one or more base stations and
one or more ASN GateWays (GW). This is the radio access network. ASN GW is
located at the edge of network access services, and links to network connectivity
services (CSN).
o Connectivity service network (CSN) that provides IP connectivity and all IP
network services underlying the WiMAX network. Here is the server for
authentication, authorization and account (AAA). Here is done the IP address
allocation for subscriber stations, the QoS management, based on user
subscription type and the billing of the subscriber, among others.
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A simplification of this model is shown in Figure 1:
Reţeaua de
acces
ASN
GW Reţea IPReţeaua de servicii
conectivitate (CSN)
AAA
Gateway
Internet
Reţea IP
PSTN
3GPP/
3GPP2
ASP
SS
SS
SS
BS
BS
BSReţeaua de
servicii acces
(ASN) 3GPP/3GPP2: reţele UMTS / CDMA2000
AAA: autentificare, autorizare, conturi
ASN GW: gateway reţea de servicii acces
ASP: furnizor serviciu aplicaţie
BS: staţie de bază
PSTN: reţea comutată publică de telefonie
SS: staţie de abonat
Figure 1: Architecture of an IP-based WiMAX network
1.2. Protocol stack attached WiMAX systems
Taking as reference the OSI reference model (Open Systems Interconnection), the
802.16 standard handles the MAC sub-layer (Medium Access Control) of the level 2
(Data Link) and all of level 1 (physical), as can be seen in Figure 2:
7. Aplicaţie
6. Prezentare
5. Sesiune
4. Transport
3. Reţea
2. Legătură de Date
1. Fizic
Subnivelul LLC
(Control Legătură Logică)
Subnivelul MAC
(Control Acces la Mediu)
Subnivelul CS
(Convergenţă)
Subnivelul SS
(Securitate)
Subnivelul CPS
(Parte Comună)
Subnivelul TCS
(Convergenţă
Transmisie)
Nivelele modelului OSI
Standardul 802.16 ocupă subnivelul MAC al nivelului 2 şi întreg
nivelul 1 din modelul OSI
QPSK 16QAM 64QAM
Figure 2: Framing the 802.16 standard in the OSI reference model
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1.2.1. MAC layer
MAC layer is responsible for controlling and multiplexing of data streams from
different applications and services (HTTP, VoIP, etc.) on the same transmission medium.
The main functions of the MAC level are:
o Segmentation or concatenation of the service data units (SDU) received from
higher levels in MAC protocol data units (MAC PDU), which are creating the
blocks of the payload of the MAC layer;
o Selection of modulation schemes and transmission power level suitable for
transmitting the MAC -protocol data units;
o Retransmission of the MAC - protocol data units that have been received
erroneously by the subscriber station, when the ARQ mechanism is activated;
o Control of the quality of service (QoS) and prioritization of MAC PDUs
belonging to different data streams;
o Allocation of the resources of the physical layer (PHY) to the MAC PDU to be
transmitted;
o Offers support to higher levels for mobility management;
o Management of security and of the encryption keys;
o Offers the possibility of switching the equipment to low power consumption
modes.
The MAC layer is divided into three sub-layers:
Convergence sub-layer (CS): is responsible to receive packages, called service data
units (SDU), from the upper level, and perform on them all the operations dependent on
the type of higher level protocol. Thus, this sub-layer masks the protocol from the upper
level and its requirements in relationship with the lower MAC and PHY sublevels.
Common part sub-layer (CPS): performs on packets all operations that do not depend
on the upper level protocols, such as fragmentation/concatenation of SDU in MAC PDU,
quality-of-service control, and automatic retransmission request (ARQ).
Security Sublevel (SS) is responsible for encryption, authorization and encryption key
exchange between the base station and subscriber station.
1.2.2. The physical layer (PHY)
The physical layer of a WiMAX network is based on orthogonal frequency
multiplexing (OFDM). This scheme is very efficient for high-speed data transmission in
conditions without direct visibility and/or multipath propagation.
OFDM parameters used in WiMAX:
Parameters Fixed WiMAX
OFDM- PHY
Scalable mobile WiMAX
OFDMA -PHY
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FFT size 256 128 512 1024 2048
Number of used subcarriers 192 72 360 720 1440
Number of pilot subcarriers 8 12 60 120 240
Number of null subcarriers/guard
interval 56 44 92 184 368
Cyclic prefix of the guard interval
(Tg/Tb) 1/32 1/16 1/8 1/4
Channel bandwidth (MHz) 3,5 1,25 5 10 20
Distance between subcarriers (KHz) 15,625 10,94
Time Symbol (μS) 64 91,4
OFDM symbol duration (μS) 72 102,9
The number of OFDM symbols in a
frame of 5 μs 69 48
In a WiMAX system, not all OFDM subcarriers are used to transmit data. Some
of them are pilot subcarriers, used for synchronization and channel estimation, another
part are guard subcarriers and the rest are data subcarriers.
1.3 The WiMAX Frame
Subcarriers may be divided into groups called sub-channels. These groups are
defined in the standard. A sub-channel represents the minimum frequency that can be
allocated to a subscriber station. By allocating different sub-channels to various
subscriber stations, the multiple access technique called OFDMA (orthogonal frequency
division multiple access) is performed. The smallest unit in time and frequency that can
be allocated to a connection is called slot. It consists of the use of a sub-channel for the
duration of one, two or three OFDM symbols, depending on the scheme used to share the
sub-channels. A contiguous portion of slots allocated to a user is called the data portion
of that user. Algorithms for allocating resources are based on user demand, quality of
service (QoS) and channel conditions.
Figure 3 illustrates the WiMAX frame structure
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Burst DL nr.2
Burst DL
nr.1
Burst DL
nr.3
Burst DL nr.4
Burst DL nr.5
Burst UL nr.1
Burst UL nr.3
Burst
UL nr.4
Ranging
Burst UL nr.7
Burst UL nr.2
Burst UL
nr.5
Burst UL
nr.6
Pre
am
bu
l
UL-M
AP
DL
-MA
PD
L-M
AP
UL
-MA
P (
con
tinu
are
)
Numărul simbolului OFDM (timp)
Su
bp
urt
ăto
are
(fr
ecv
en
ţă)
Subframa legăturii
descendente
Subframa legăturii
ascendente
Interval de gardă
Figure 3: Structure of a WiMAX frame
As shown in Figure 3, the downlink sub-frame starts with a preamble which is
used by the physical layer for time and frequency synchronization, and for the initial
estimation of the channel. The map with the data portions allocated to each subscriber
station is contained in the MAP messages (DL -MAP and UL -MAP), which are sent to
all stations. MAP messages contain burst profiles, defining the modulation scheme and
the coding used for that link.
Uplink sub-frame consists of multiple burst from different users, and a portion
with competition -based access, used for many purposes. The main goal is ranging, i.e.
realizing frequency, time, and power corrections, in the initial phase (entry in the
network) and then periodically. Ranging channel is used by a subscriber station also to
require allocation of bandwidth on the uplink. Uplink sub-frame also contains a channel
through which the subscriber stations transmit channel quality information (CQICH) to
the base station and a channel through which the base stations transmit acknowledgments
of reception of data on downlink (ACK).
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1.4 Modulation and coding
Figure 4 shows the modulation and coding schemes supported in WiMAX system:
Downlink Uplink
Modulation BPSK modulation (optional for.
OFDMA - PHY), QPSK, 16QAM,
64QAM
BPSK, QPSK, 16QAM,
64QAM (optional)
Coding Required coding: convolutional
codes of rate 1/2, 2/ 3, 3 /4, 5/6
Optional: convolutional turbo code
of rates 1/2, 2/ 3, 3 /4, 5/6, repetitive
codes of rates 1/2, 1/ 3, 1 /6, LDPC
codes, Reed – Solomon.
Required coding: convolutional
codes of rate 1/2, 2/ 3, 3 /4, 5/6
Optional: convolutional turbo code
of rates 1/2, 2/ 3, 3 /4, 5/6, repetitive
codes of rates 1/2, 1/ 3, 1 /6, LDPC
codes.
Figure 4: Modulation and Coding Scheme in WiMAX System
Adaptive modulation and coding (AMC)
Using the CQICH channel, the base station receives from the subscriber- station
information about the quality of the downlink. For the uplink, the base station can
estimate the quality using the received data signals. Thus, taking into account the quality
of the uplink and downlink for each user, the AMC algorithm from each base station
assigns to each user a modulation and coding scheme for uplink and another one for
downlink, that will maximize the capacity of the connections for the existing signal-to-
noise-ratio of the channel at the time.
2. WiMAX equipment overview
In making an experimental WiMAX network, the following equipment can be
used:
• The sector controller AN-100U
• Radio-frequency transceiver
• A subscriber station
All these equipment are manufactured by Redline Communications. The bridge
and sector controller are indoor units, while the RF transceiver and the subscriber station
(elements between which the radio link forms) are outdoor units.
In addition, one will use cables and various RF components, such as splitters and
attenuators, since the radio connections in the lab are on cable and not throughout the air.
Also, for each device’s configuration, will use a computer connected to the unit’s
management interface.
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2.1. AN-100U sector controller
AN-100U is a sector controller. It operates at frequencies between 2-11 GHz, and
supports PTP (point to point) or PMP (point to multipoint) connectivity. AN-100U
provides QoS transfer rates’ control, on both DL/UL, thus enabling predictable transfer
rates. It uses BPSK modulation schemes, as well as QPSK, 16QAM, 64QAM and coding
rate of ½, 2/3, ¾. It also supports ARQ (automatic erroneous packet retransmission) and
encrypts/decrypts the data.
Figure 1: AN-100U sector controller
2.1.1. General description
All connectors, indicators and switches are located on the front panel of the
machine. Here are, as can be seen in the figure below, the following:
• N-type female connector, to communicate with the transceiver;
• 2 SMA connectors for time synchronization;
• LED indicators: they can indicate different problems, their description can be found in
the user manual of the equipment.
• LAN data port;
• Management LAN port;
• Management Serial port (RS-232).
Figure 2: AN-100U front panel
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The management LAN pot can be disabled, in order to configure the equipment
over the data port.
2.1.2. Configuration
Setup is done on the LAN interface; the interface has the default IP address
192.168.101.3. To connect to the equipment, use an Ethernet straight cable. The
computer’s IP address and the equipment’s address must be on the same network
(192.168.101.x).
The equipment has a web interface, accessible from Internet Explorer by entering
the equipment IP address (192.168.101.3) and the following login information username:
admin/password: admin. After successful authentication, one the left side, a general
menu appears, where various submenus can be accessed by clicking on the appropriate
lines of text.
Figure 3: AN-100U Web interface – General Menu
Each submenu’s functions are described in the table below.
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Submenu Sub-submenu Functions
Monitoring General Info View general and RF network settings
Status View radio link, data and management statistics
SS Info View system info, IP settings and active subscriber
stations statistics
Event Log View system activity and recorder error messages
Service Flow
Configuration
Service Classes Service Classes
Service Flows Service Flows
Classifiers Define classifiers for each service flow
Manage Activate service flows
Interfaces Wireless
Interface
RF PHY and MAC settings view/modification
Ethernet
Interface
Ethernet interface settings view/modification
Management IP and DHCP settings view/modification
Admin Tools Advanced
Config
RF advanced settings view/modification
Software
Upgrade
Software settings
Accounts
Management
Add users, change system password
Reboot Reboot AN-100U
We will summarize the main settings (radio frequency, IP, etc.):
RF DL Channel KHz: radio channel’s central frequency;
RF Channel Separation kHz: frequency spacing between the DL and UL
channels (for FDD);
Tx dBm Output Power: the power used for transmission by the RF transceiver;
SS Tx Power Control Enable: enables control of the SS’s (subscriber station)
power transmission in order to get a desired RSSI (Received signal strength indicator)on
UL;
RSS Reference: RSSI average value desired for the UL;
Enable Auto Rx Gain: enables automatic control of the receiver’s sensitivity;
Select MHz Band: select the type of band spreading inside the channels;
Channel Size MHz: channel’s bandwidth;
Guard Interval: cyclic prefix length;
Ms Frame Duration: Frame duration
DL Ratio: The ratio between the downlink's duration and the overall frame’s
duration;
Disable RF: disable the RF transceiver’s output;
IP Address: the terminal’s IP address;
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Subnet Mask: the terminal’s IP address’s subnet mask;
Default Gateway: the IP address of the terminal’s default gateway;
DHCP Server: the address of a DHCP server. AN-100U will forward all requests
to the server DHCP coming from subscriber stations.
In the bottom of the setup window there are three control options:
Save: Save settings;
Cancel: returns to previous settings;
Default: Restore to factory settings.
2.2. RF Transceiver
The radio transceiver is an outdoor equipment. For this reason it is locked in an
aluminum casing, resistant to weather variations and connectors are also protected in this
regard. As shown in the figure below, the transceiver is mounted on a pole together with
the antenna.
Figure 4: RF Transceiver
2.2.1. General description
The radio transceiver has only two ports:
IF port (intermediate frequency) – N-type mother jack, and only through this port,
the transceiver:
o Send/receive data modulated with the intermediate frequency (of) the
indoor terminal (AN-100U)
o Send status information to the indoor unit
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o Get control information from the terminal
o Get power supply from the terminal
RF port (radio-frequency) is also an N-type mother jack, and is used by the
transceiver to send/receive the radio signal(s) to/from the antenna. In the
experimental laboratory platform, this port will be connected to a coaxial cable,
then will connect various radio frequency components (attenuators, splitters) and
finally, at the other end, will be connected subscriber station.
2.3. Subscriber’s station
The subscriber station is an outdoor equipment. For this reason it is closed and in
an aluminum casing resistant to weather variations. As shown in the figure below, the
station has a mounting bracket and the power cable is PoE-type. Also, through this cable,
occur data and management traffic. The station is designed to operate at frequencies
between 2-11 GHz, in PTP and PMP configurations. It supports modulation schemes
such as BPSK, QPSK, 16QAM, 64QAM and coding rates of ½, 2/3, ¾, ARQ mechanism,
QoS, among others.
Figure 5: The subscriber’s station
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2.3.1. General description
The subscriber station has an integrated antenna, an Ethernet connector for power
supply, an N-type female RF Connector, and grounding.
2.3.2. Configuration
The equipment’s configuration is available over the LAN interface, using the
Telnet protocol, the default IP address is 192.168.101.1. Once the BS connection is
established, configuration can be done on the WAN interface.
To connect to the equipment, one has to use an Ethernet crossover cable, from the
computer’s interface, and use an address on the same network as the equipment
(192.168.101.x). When connecting, the user will be prompted login information (Login:
super/Password: super).
After logging, the station's command prompt appears, where you can enter
commands. The command line has the autocomplete function (by pressing TAB) and
command history (clicking↑ and ↓) and help (?).
Commands can be divided into categories, and the command prompt changes
depending on the level at which we find ourselves in the tree order. Eg: RPM#>
RPM#>rfconfig
RPM(rfconfig ->)#>
We summarize the main commands, which helps setting/viewing RF, PHY, IP
parameters:
rfconfig show: Radiofrequency display settings. An example of output of this command
is given below:
Settings --- <<SS Mmgt RF Configuration Parameter>>
Transmit Receive
---------------------------------- ----------------------------------
FixedPower:............10.00 dBm Gain:...................0.00 dB
ActualPower:............1.60 dBm RfRssi:.................0.00 dBm
Lock:......................0 Lock:......................0
FixedGain:.................0 Agc:.......................0
Frequency Others ---------------------------------- ----------------------------------
LoRfFreq1:...........2680000 kHz RfTemp:.................0 Celsius
HiRfFreq1:...........2680000 kHz MaxRngRetries:............10
Priority1:.................0 StickinessTimer:..........30 sec
LoRfFreq2:.................0 kHz MaxTxPower:............27.00 dBm
HiRfFreq2:.................0 kHz Nomadic:.........Disabled(0)
Priority2:.................0
LoRfFreq3:.................0 kHz
HiRfFreq3:.................0 kHz
Priority3:.................0
LoRfFreq4:.................0 kHz
HiRfFreq4:.................0 kHz
Priority4:.................0
LoRfFreq5:.................0 kHz
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HiRfFreq5:.................0 kHz
Priority5:.................0
LoRfFreq6:.................0 kHz
HiRfFreq6:.................0 kHz
Priority6:.................0
LoRfFreq7:.................0 kHz
HiRfFreq7:.................0 kHz
Priority7:.................0
LoRfFreq8:.................0 kHz
HiRfFreq8:.................0 kHz
Priority8:.................0
LoRfFreq9:.................0 kHz
HiRfFreq9:.................0 kHz
Priority9:.................0
LoRfFreq10:................0 kHz
HiRfFreq10:................0 kHz
Priority10:................0
LoRfFreq11:................0 kHz
HiRfFreq11:................0 kHz
Priority11:................0
LoRfFreq12:................0 kHz
HiRfFreq12:................0 kHz
Priority12:................0
LoRfFreq13:................0 kHz
HiRfFreq13:................0 kHz
Priority13:................0
LoRfFreq14:................0 kHz
HiRfFreq14:................0 kHz
Priority14:................0
LoRfFreq15:................0 kHz
HiRfFreq15:................0 kHz
Priority15:................0
LoRfFreq16:................0 kHz
HiRfFreq16:................0 kHz
Priority16:................0
RfFreq:..............2680000 kHz
As shown, there are 16 frequency intervals for the SS to scan in order to to find a
network. These commands are user configurable, using commands such as:
rfconfig set LoRfFreq1 2580000
rfconfig set HiRfFreq1 2600000
rfconfig set LoRfFreq2 2630000
rfconfig set HiRfFreq2 2640000
.................
rfconfig set LoRfFreq16 xxxxxxx
rfconfig set HiRfFreq16 xxxxxxx
If LoRfFreqX and HiRfFreqX are configured with identical values, the SS scans
only that frequency. In the example above, one frequency has been set, since LoRfFreq1
and HiRfFreq1 have the same value, while the remaining intervals are not configured.
rfconfig set txFixedGain 1: sets the SS transmission gain to a fixed value.
rfconfig set txFixedPower x: sets the above gain to a value which achieves a x
dBm transmission power.
rfconfig set txFixedGain 0: sets the tranmission gain to be controlled by the BS.
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interface wman0 show phy: displays PHY settings. An output example is shown
below:
Settings --- <<SS Mmgt PHY Configuration Parameter>>
Bandwidth= 5000 kHz
FftSize= 512
CyclicPrefix= 8 PS
FrameSize= 5000 usec
These parameters can be configured using the following commands:
interface wman0 set phy bandwidth x: sets the bandwidth. Possible values for x are
5000, 7000, 8750 and 10000 (KHz). With this parameter changes automatically and FFT
Size, as follows:
Bandwidth FftSize
5000 512
7000 1024
8750 1024
10000 1024
interface wman0 set phy framesize x: sets the WiMAX frame length. Possible values
for x are 5000 and 10000 (us).
interface eth0 show ip: displays IP settings.
interface wman0 set ip address xxx.xxx.xxx.xxx: sets the station’s IP address.
interface wman0 set ip mask xxx.xxx.xxx.xxx: sets the station’s IP mask.
status show: displays statistics about the link quality (RSSI, PCLNR, packets
sent/received, number of errors HCS/DL CRC). An example of the output of this
command is given below:
Settings --- <<SS Mmgt Status Data>>
Downlink General Uplink General
----------------------------------- -----------------------------------
fpCount:....................0 MapCount:...................0
fpErrCount:.................0 MapErrCount:................0
fpCrcCount:.................0 MapCrcCount:................0
MapCount:...................0 ByteCount:..................0
MapErrCount:................0 SduCount:...................0
MapCrcCount:................0 MpduCount:..................0
ByteCount:.........1621934196
SduCount:.............1577757
MpduCount:..................0
HCrcErrCount:...............1
CrcErrCount:................3
Management Downlink Chan Desc
----------------------------------- -----------------------------------
RxCount:...............541516 RxCount:....................0
ErrCount:...................0 ErrCount:...................0
CrcCount:...................0 CrcCount:...................0
ChangeCount:.............2591
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Uplink Chan Desc Others
----------------------------------- -----------------------------------
RxCount:.................2591 Rssi:...................-72.6
ErrCount:...................0 Snr:.....................35.2
CrcCount:...................0 FreqOffset:..............2609
TCnt:.......................0
ModemResets:................0
LostFrames:.................0
FrameDuration:..............0
RngTimeCorrection:..........0
TotalHCrcErrors:............0
TotalCrcErrors:.............0
TotalTxBurstCount:..........0
TotalRngReqCount:...........0
TotalBwReqCount:............0
TotalMgmSentCount:......10371
TotalPaddingCount:.1318473221
RfRssi:.................-72.6
TxPower:.................16.0
LinkStatus:.................1
exit: jumps to a previous level in the commands’ tree. Example: RPM#>
RPM#>interface
RPM(itf)#>wman0
RPM(itf:wman0)#>exit
RPM(itf)#>exit
RPM#>
3. Practical activities
The equipment described above will connect as follows:
The sector controller will be connected with the transceiver through a coaxial
cable from the controller’s N type signal jack to the transceiver’s IF jack.
The transceiver will be connected to the splitter via a coaxial cable attached to the
RF jack.
At the splitter’s other jack will be connected the subscriber station, using two
attenuators of 30 + 10 = 40 dB and a coaxial cable.
The sector controller will be powered from the 220 power-line via a power cable.
The subscriber station will also be powered from the main 220V AC line via the
power-over-Ethernet adaptor, using a power cord and a straight network cable for
outdoor. This network cable connects to the adaptor’s “Data-and-power” jack.
The sector controller connects to PC 1, using a straight-type network cable, from
the controller’s “Data” jack.
The subscriber’s station connects to PC 2 using a cross-over-type network cable,
from the power-over-Ethernet adaptor’s “Data” jack.
To configure the sector controller first check PC1: Control Panel-Network
Settings that PC1's IP family is 192.168.101.xxx. Then open a web browser window (eg.
Internet Explorer or Mozilla) on PC1 and access the appropriate page of the controller IP
address (192.168.101.3).
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In order to authenticate, use User: 'admin', Password 'admin'. After authentication,
the user has the possibility to modify the operating parameters of the Controller. Students
can change them, but must not save them, so when restarting the Controller, to return to
the original settings.
To study the WiMAX network’s performances, we use the Jperf program, both on
PC1 and PC2. For this, we set Jperf, turned on PC1, which is connected to the controller
sector, to act as server. Then we set Jperf, turned on PC2, which is connected to the
subscriber station, to act as customer. Also, in this window, we will write the server’s IP
address, i.e. PC1’s IP address.
To generate data traffic between two PCs via WiMAX network, select on PC2
Jperf’s window a transmission time of 20 seconds and choose TCP traffic. Traffic will
start between the two PCs after the Run option is selected in both windows. We will
observe the throughput offered, which is a transfer rate metric.
Next, we will perform several sets of measurements obtained with Jperf, in order
to complete the table below. The transfer rate is displayed in real time in Jperf’s center
console.
To activate a service class, activate only the two service flows associated with that
class. These two streams are uplink and downlink. The other flows defined in the list
must be disabled.
To change the power output of the controller, enter the Wireless Interface menu:
Tx Power.
To find the type of modulation used by the Controller, choose Monitoring-SS Info.
Scenario no. Service
Class
Suma
atenuatoarelor
[dB]
Controller’s
transmission
power
Transfer
rate
Modulation
and Coding
1 Class 1 40 0
2 Class 2 40 0
3 Class 3 40 0
4 Class 3 70 0
5 Class 3 80 0
6 Class 3 90 0
7 Class 3 100 0
Indicate in the table the maximum attenuation at which network traffic still occurs.
4. Preliminary questions
1. Enumerate the components of the WiMAX access network.
2. Describe the subcarrier types used in the OFDM modulation by the WiMAX
technology.
3. Explain two functions of the medium access control (MAC) sublevel of the WiMAX
technology.