Transcript IEEE 802.11b - Bill Buchanan

Wireless LAN
Author: Bill Buchanan
Unit 2: Wireless Fundamentals
Wireless connections … which technology?
Areas covered:
Author: Bill Buchanan
Basic radio parameters.
This area covers the main type of wireless
communications.
IEEE 802.11b issues.
This area covers some of the fundamentals of radio
waves.
Sample Aironet Configurations
This shows some simple configuration examples.
Wireless
Access
point
Wireless
Bridge
Author: Bill Buchanan
Wireless
Client
Author: Bill Buchanan
Broadcast span
Author: Bill Buchanan
Defined
by
broadcast
domain
Author: Bill Buchanan
IEEE 802.11b networks
IEEE 802.11 - Wireless
• IEEE 802.11a. 802.11a deals with communications available
in the 5GHz frequency, and has a maximum data rate of 54
Mbps.
• IEEE 802.11b. 802.11b, or Wi-Fi, is the standard that is most
commonly used in wireless LAN communications. It has a
maximum bandwidth of 11Mbps, at a frequency of 2.4GHz.
• IEEE 802.11g. 802.11g is a proposed standard that hopes to
provide 54Mbps maximum bandwidth over a 2.4GHz
connection, the same frequency as the popular 802.11b
standard.
Author: Bill Buchanan
• IEEE 802.11c. 802.11c is a group set up to deal with bridging
operations when developing access points.
• IEEE 802.11f. 802.11f is concerned with standardising access
point roaming which is involved in making sure that
interoperability between access points is guaranteed
Operating Channels:
11 for N. America, 14 Japan, 13 Europe (ETSI), 2 Spain, 4 France
Operating Frequency:
2.412-2.462 GHz (North America), 2.412-2.484 GHz (Japan),
2.412-2.472 GHz (Europe ETSI), 2.457-2.462 GHz (Spain),
2.457-2.472 GHz (France)
Data Rate:
1, 2, 5.5 or 11Mbps
Media Access Protocol:
CSMA/CA, 802.11 Compliant
Range:
11Mbps: 140m (460 feet)
5.5Mbps: 200m (656 feet)
2Mbps: 270m (885 feet)
1Mbps: 400m (1311 feet)
RF Technology:
Direct Sequence Spread Spectrum
Modulation:
CCK (11Mps, 5.5Mbps), DQPSK (2Mbps), DBPSK (1Mbps)
Author: Bill Buchanan
IEEE 802.11b
Maximum bandwidth (IEEE 802.11b)
11Mbps
Max bandwidth
CCK
Available
bandwidth
CCK - Complementary Code Keying
DQPSK - differential quadrature phase
shift keying
BPSK - biphase shift keying
5.5Mbps
CCK
2Mbps
DQPSK
1Mbps
100m
200m
300m
Distance
400m
Author: Bill Buchanan
DBPSK
Maximum bandwidth (IEEE 802.11b)
11Mbps
Max bandwidth
Actual maximum
bandwidth
5.5Mbps
2Mbps
100m
200m
300m
400m
Author: Bill Buchanan
1Mbps
6Mbps
Too many errors
causes the
TCP window
to close, and reduce
throughput.
Linear increase in
actual throughput
against required
throughput
2Mbps
4Mbps
2Mbps
Required
data throughput
More collisions
and errors are
occurring, thus
data frames are
being deleted, causing
wasted bandwidth.
8Mbps
10Mbps
Author: Bill Buchanan
8Mbps
Available
throughput
Maximum bandwidth (IEEE 802.11b)
Author: Bill Buchanan
IEEE 802.11g networks
Maximum bandwidth (IEEE 802.11g)
54Mbps
Available
bandwidth
64-QAM
QAM – Quadrature Amplitude Modulation
CCK - Complementary Code Keying
DQPSK - differential quadrature phase
shift keying
BPSK - biphase shift keying
24Mbps
16-QAM
Author: Bill Buchanan
100m
802.11g
Mbps Modulation
6
BPSK
2Mbps
9
BPSK
QPSK
12
QPSK
1Mbps
18DBPSK
QPSK
24
16-QAM
400m
300m 36
200m
16-QAM
Distance
48
64-QAM
54
64-QAM
Author: Bill Buchanan
Netperf on a 11g link
Author: Bill Buchanan
Windows IP Configuration
Ethernet adapter Wireless Network Connection 3:
Connection-specific DNS Suffix . :
IP Address. . . . . . . . . . . . : 192.168.1.100
Subnet Mask . . . . . . . . . . . : 255.255.255.0
Default Gateway . . . . . . . . . : 192.168.1.1
C:\>netperf -H 192.168.1.101 -p 1001
TCP STREAM TEST to 192.168.1.101
Recv
Send
Send
Socket Socket Message Elapsed
Size
Size
Size
Time
Throughput
bytes bytes
bytes
secs.
10^6bits/sec
8192
8192
8192
10.00
9.60
C:\>netperf -H 192.168.1.101 -p 1001
TCP STREAM TEST to 192.168.1.101
Recv
Send
Send
Socket Socket Message Elapsed
Size
Size
Size
Time
Throughput
bytes bytes
bytes
secs.
10^6bits/sec
8192
8192
8192
10.00
7.60
c:\>netperf -H 192.168.1.101 -p 1001
TCP STREAM TEST to 192.168.1.101
Recv
Send
Send
Socket Socket Message Elapsed
Size
Size
Size
Time
Throughput
bytes bytes
bytes
secs.
10^6bits/sec
8192
8192
8192
10.00
7.60
Author: Bill Buchanan
IEEE 802.11g example
Author: Bill Buchanan
IEEE 802.11g example
Author: Bill Buchanan
IEEE 802.11n
802.11n
Frequency:
Max:
2.4 GHz or 5 GHz
540 Mbit/s
Range:
Same as 11b
MIMO (Multiple-in, multiple-out)
Sends information on two or more antennas.
These signals then reflect off objects, creating
multiple paths creating multiple paths.
Normally these cause interference and fading,
but with MIMO they carry different information,
which are recombined on the receiving side.
Author: Bill Buchanan
802.11n
Author: Bill Buchanan
802.11n
Author: Bill Buchanan
MIMO
Author: Bill Buchanan
IEEE 802.11 networks
CSMA/CA and PCF
• CSMA/CA. CSMA/CA is, like standard Ethernet (IEEE 802.3)
a contention-based protocol, but uses collision avoidance
rather than collision detection. It would be impossible to use
collision detection as a radio wave is always either sending
or receiving and can never do both at the same time. The
nodes will thus not be able to listen on the channel while
they are transmitting.
• Point Coordination Function (PCF). This is an optional
priority-based protocol, which provides contention-free frame
transfer for transmission of time-critical data, such as realtime video or audio. With this, the point coordinator (PC)
operates in the wireless access point and identifies the
devices which are allowed to transmit at any given time.
Each PC then, with the contention-free (CF) period, the PC
polls each of the enabled PCF to determine if they wish to
transmit data frames. No other device is allowed to transmit
while a another node is being polled. Thus, PCF will be
contention-free and enables devices to transmit data frames
synchronously, with defined time delays between data frame
transmissions.
Author: Bill Buchanan
IEEE 802.11 can use two mechanisms for shared access:
CSMA/CD
Listen for no activity
1
ACK
time-out
2
2
• Node has gone.
• Data frame has collided
with another
• Data frame corrupted
with noise.
Author: Bill Buchanan
ACK
Frame Duration/ Address Address Address Sequence Address
control
ID
1
2
3
control
4
Frame
body
FCS
2 Bytes
0-2312
4
2
6
6
6
2
6
Frame control. This contains control information.
Duration/ID. This contains information on how long the data frame will
last.
Address fields. This contains different types of address, such as an
individual address of group addresses. The two main types of group
addresses are broadcast and multicast.
Sequence control. This identifies the sequence number of the data
frames, and allows the recipient to check for missing or duplicate data
frames.
Frame body. This part contains the actual data. The maximum
amount is 2312 bytes, but most implementations use up to 1500
bytes.
FCS (Frame Check Sequence). This is a strong error detection code.
Author: Bill Buchanan
IEEE 802.11 data frame
Frame
body
FCS
2 Bytes
0-2312
4
2
6
6
6
2
6
Author: Bill Buchanan
Frame Duration/ Address Address Address Sequence Address
control
ID
1
2
3
control
4
Author: Bill Buchanan
IEEE 802.11 networks
dot11radio0
(or d0)
bvi 1 port is used
to configure both ports
with the same address
con
e0 (or fa0)
Antenna
connector
Author: Bill Buchanan
# config t
(config)# int bvi1
(config-if)# ip address 192.168.0.1 255.255.255.0
(config-if)# exit
Fixed network
Root
# config t
(config)# int dot11radio0
(config-if)# station role root
(config-if)# station role repeater
(config-if)# end
Author: Bill Buchanan
Repeater
Fixed network
Root
# config t
(config)# ip default-gateway 192.168.1.254
(config)# exit
Author: Bill Buchanan
Repeater
Channel Frequency
Channel. If an ad-hoc network is used, then the nodes which
communicate must use the same channel.
6
11
11
1
6
Author: Bill Buchanan
1
channel 1—2412
channel 2—2417
channel 3—2422
channel 4—2427
channel 5—2432
channel 6—2437
channel 7—2442
channel 8—2447
channel 9—2452
channel 10—2457
channel 11—2462
channel 12—2467
channel 13—2472
channel 14—2484
Channel Frequency
Channel. If an ad-hoc network is used, then the nodes which
communicate must use the same channel.
6
11
11
1
(config)# int dot11radio0
(config-if)# channel 7
(config-if)# no shutdown
(config)# int fa0
(config-if)# no shutdown
6
Author: Bill Buchanan
1
channel 1—2412
channel 2—2417
channel 3—2422
channel 4—2427
channel 5—2432
channel 6—2437
channel 7—2442
channel 8—2447
channel 9—2452
channel 10—2457
channel 11—2462
channel 12—2467
channel 13—2472
channel 14—2484
Fragmentation threshold
Fragmentation threshold. This can be used to split large data
frames into smaller fragments. The value can range from 64 to
1500 bytes. This is used to improve the efficiency when there is a
high amount of traffic on the wireless network, as smaller frames
make more efficient usage of the network.
The large data frames may
allow nodes to ‘hog’ the airwave
Author: Bill Buchanan
Data packets are split into 1500 byte data frames (MTU)
Fragmentation threshold
Fragmentation threshold. This can be used to split large data
frames into smaller fragments. The value can range from 64 to
1500 bytes. This is used to improve the efficiency when there is a
high amount of traffic on the wireless network, as smaller frames
make more efficient usage of the network.
Possibly allows for a smoother and fairer
transmission.
Author: Bill Buchanan
Data frames are fragmented into smaller frames
Network settings
Fragmentation threshold. This can be used to split large data
frames into smaller fragments. The value can range from 64 to
1500 bytes. This is used to improve the efficiency when there is a
high amount of traffic on the wireless network, as smaller frames
make more efficient usage of the network.
Data frames are fragmented into smaller frames
Possibly allows for a smoother and fairer
transmission.
Author: Bill Buchanan
# config t
(config)# int dot11radio0
(config-if)# fragment-threshold ?
<256-2346>
(config-if)# fragment-threshold 700
(config-if)# end
Infrastructure or ad-hoc
Network type. This can either be set to an infrastructure network
(which use access points, or wireless hubs) or Ad-hoc, which
allows nodes to interconnect without the need for an access point.
Infrastructure
Author: Bill Buchanan
Ad-hoc
Infrastructure or ad-hoc
Infrastructure
SSID
defines the
connected nodes
Ad-hoc
Author: Bill Buchanan
SSID
defines the
connected nodes
Infrastructure or ad-hoc
Infrastructure
SSID
defines the
connected nodes
Ad-hoc
# config t
(config-if)# dot11 ssid fred
(config-ssid)# guest-mode
(config-ssid)# exit
(config)# int dot11radio0
(config-if)# ssid fred
(config-if-ssid)# end
Author: Bill Buchanan
SSID
defines the
connected nodes
L
L
Author: Bill Buchanan
Span of network
L
Network settings
Authentication
algorithm. This sets whether the adapter to use
an open system (where other nodes can listen to the
communications), or uses encryption (using either a WEP key, or a
shared key).
Author: Bill Buchanan
# config t
(config)# dot11 ssid fred
(config-ssid)# authentication ?
client
LEAP client information
key-management key management
network-eap
leap method
open
open method
shared
shared method
(config-ssid)# authentication open
(config-ssid)# exit
(config)# exit
Authentication is
a key issue, and
will be covered later
in the module. At
present the authentication
is open, so that any user
and device can connect
without authenticating itself.
Author: Bill Buchanan
Other Factors
Network settings
Preamble – this is sent
before the start of the data
transmission so that nodes
can detect that it is about to transmit.
Author: Bill Buchanan
Preamble mode. This can either be set to Long (which is the
default) or short. A long preamble allows for interoperatively with
1Mbps and 2Mbps DSSS specifications. The shorter allows for
faster operations (as the preamble is kept to a minimum) and can
be used where the transmission parameters must be maximized,
and that there are no interoperatablity problems.
Network settings
Preamble mode. This can either be set to Long (which is the
default) or short. A long preamble allows for interoperatively with
1Mbps and 2Mbps DSSS specifications. The shorter allows for
faster operations (as the preamble is kept to a minimum) and can
be used where the transmission parameters must be maximized,
and that there are no interoperatablity problems.
Preamble – this is sent
before the start of the data
transmission so that nodes
can detect that it is about to transmit.
Author: Bill Buchanan
# config t
(config)# int dot11radio0
(config-if)# preamble-short
(config-if)# end
Hidden node problem
The hidden node problem
occurs when two nodes transmit
to an access point, but they are not
in communication range, thus their
signals can collide, and cause errors.
Author: Bill Buchanan
These nodes cannot
hear each other.
Network settings (cont.)
Author: Bill Buchanan
RTS/CTS threshold. The RTS Threshold prevents the Hidden
Node problem, where two wireless nodes are within range of
the same access point, but are not within range of each other.
As they do not know that they both exist on the network, they
may try to communicate with the access point at the same time.
When they do, their data frames may collide when arriving
simultaneously at the Access Point, which causes a loss of data
frames from the nodes. The RTS threshold tries to overcome
this by enabling the handshaking signals of Ready To Send
(RTS) and Clear To Send (CTS). When a node wishes to
communicate with the access point it sends a RTS signal to the
access point. Once the access point defines that it can then
communicate, the access point sends a CTS message. The
node can then send its data.
Hidden node problem
RTS (Ready To
Send)
RTS (Ready To
Send)
CTS (Clear To
Send)
Author: Bill Buchanan
Data transmitted
Author: Bill Buchanan
RTS
RTS
CTS
Data
CTS
Data
Hidden node problem
RTS threshold
RTS threshold determines the data frame size that is required, in order for it
send an RTS to the WAP. The default value is 4000.
RTS (Ready To
Send)
# config
t
(config)# int dot11radio0
(config-if)# rts threshold 8000
(config-if)# end
RTS (Ready To
Send)
CTS (Clear To
Send)
Author: Bill Buchanan
Data transmitted
Hidden node problem
RTS retries
RTS Retries defines the number of times that an access point will transmit
an RTS before it stops sending the data frame. Values range from 1 to 128.
RTS (Ready
# config
t To
Send)
(config)#
int dot11radio0
(config-if)# rts retries 10
(config-if)# end
RTS (Ready To
Send)
CTS (Clear To
Send)
Author: Bill Buchanan
Data transmitted
Power management
The power of the access point and also
of the client are important as they will
define the coverage of the signal, and must
also be within the required safety limits.
Author: Bill Buchanan
The higher the
transmitting power,
the wider the coverage.
Power management
Author: Bill Buchanan
# config t
(config)# int dot11radio0
(config-if)# power ?
(config-if)# power local ?
(config-if)# power local 30
The higher the
(config-if)# power client 10
transmitting power,
(config-if)# speed ?
the wider the coverage.
(config-if)# speed 1.0
(config-if)# exit
(config)# exit
The power of the access point and also
of the client are important as they will
define the coverage of the signal, and must
also be within the required safety limits.
[1.0]
[11.0]
[2.0] [5.5]
[basic-1.0]
[basic-11.0]
[basic-2.0]
[basic-5.5]
| range |
throughput
Power management
Power saving modes:
Author: Bill Buchanan
CAM (Constant awake mode). Used when power
usage is not a problem.
PSP (Power save mode). Power is conserved
as much as possible. The card will typically go
to sleep, and will only be awoken by the access
point, or if there is activity.
FastPSP (Fast power save mode). This uses
both CAM and PSP, and is a compromise between
the two.
Maximum bandwidth (IEEE 802.11b)
11Mbps
Max bandwidth
CCK
(config)# int dot11radio0
(config-if)# speed 1.0
5.5Mbps
(config-if)# exit
(config)# exit
2Mbps
DQPSK
DBPSK
100m
200m
300m
400m
Author: Bill Buchanan
1Mbps
Maximum associations
... this could be due to an attack, such as
DoS (Denial of Service), or due to poor
planning.
Author: Bill Buchanan
A particular problem in wireless networks
is that the access point may become
overburdened with connected clients...
Maximum associations
For example:
Author: Bill Buchanan
Max bandwidth = 25Mbps
Av rate = 0.5Mbps
Max. associations = 50
Maximum associations
... this could be due to an attack, such as
# show dot11 adjacent-ap
DoS (Denial of Service), or due to poor
planning.
Author: Bill Buchanan
# config t
(config)# dot11 ssid fred
(config-ssid)# max ?
<1-255> association limit
(config-ssid)# max 100
(config-ssid)# exit
(config)# int dot11radio0
(config-if)# ssid fred
(config)# exit
A particular problem in wireless networks
is that the access point may become
# show dot11 association
overburdened
with connected clients...
# show dot11 statistics
client-traffic