Wireless LAN Concept
Wireless LAN Concept
Robin Lin
ICOM Senior Engineer
Glossary
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STA – Station
AP – Access Point
BSS – Basic Service Set
BSSID – Basic Service Set Identifier
IBSS – Independent BSS (Ad hoc)
Infrastructure BSS
ESS – Extended Service Set
Channel
WEP – Wireless Equivalent Privacy
BSS
• Basic Service Set
• A set of wireless devices communicate
with each other
• The basic component of WLAN
Infrastructure BSS
Independent BSS
SSID
• Service Set Identifier
• Name of a wireless local area network
(WLAN).
• All wireless devices on a WLAN must
employ the same SSID in order to
communicate with each other.
• Case sensitive text strings.
• Maximum length is 32 characters.
SSID
• The SSID on wireless clients can be set either
manually, by entering the SSID into the client
network settings, or automatically, by leaving the
SSID unspecified or blank.
• A network administrator often uses a public
SSID, that is set on the access point and
broadcast to all wireless devices in range.
• Some newer wireless APs disable the automatic
SSID broadcast feature in an attempt to improve
network security.
BSSID
• The BSSID is a 48bit identity used to
identify a particular BSS within an area
• In Infrastructure BSS networks, the BSSID
is the MAC address of the AP
• In Independent BSS or ad hoc networks,
the BSSID is generated randomly
• For example:
– 02:D0:01:01:71:18 (IBSS)
– 00:90:4B:0B:14:7E (Infrastructure BSS)
Independent BSS (IBSS)
• An 802.11 networking framework in which
devices or stations communicate directly
with each other, without any AP.
• Independent BSS is also referred to as
peer-to-peer or Ad hoc.
• Useful for establishing a network where
wireless infrastructure does not exist or
where services are not required.
Independent BSS (IBSS)
• To establish a IBSS, all the devices or
stations want to join have to change to
IBSS mode and set the same SSID,
channel and WEP settings.
Independent BSS (IBSS)
SSID: WLAN
Channel: 10
WEP: Open System
SSID: WLAN
Channel: 10
WEP: Open System
IBSS
SSID: WLAN
Channel: 10
WEP: Open System
SSID: WLAN
Channel: 10
WEP: Open System
Infrastructure BSS
• An 802.11 networking framework in which
devices communicate with each other by
first going through an Access Point (AP).
• Bridge a wireless network to a wired
Ethernet network.
• Most corporate wireless LANs operate in
infrastructure mode because they require
access to the wired LAN in order to use
services such as file servers or printers.
Infrastructure BSS
• Compared to the IBSS, infrastructure BSS
offer the advantage of scalability and
centralized security management.
• To establish a infrastructure BSS, you
have to setup a AP with a specific SSID
and all the devices or workstation want to
join have to set the same SSID as the AP.
Infrastructure BSS
SSID: ADVANAP
WEP: Open System
SSID: ADVANAP
WEP: Open System
SSID: ADVANAP
WEP: Open System
SSID: ADVANAP
Channel: 3
WEP: Open System
BSS
SSID: ADVANAP
WEP: Open System
ESS
• Extended Service Set
• A set of two or more BSSs that form a single
sub-network.
• Enables limited mobility within the WLAN.
• All the BSSs have the same SSID or as ESSID.
• When the BSSs have overlap, we can use
different channel.
• Recommend more 5 channel gap. For example:
1, 6, 11
ESS
Move to another
position
BSS
Et
Still can use the network
SSID: BIGWLAN
Channel: 5
he
rn
et
SSID: BIGWLAN
Channel: 1
BSS
Example of ESS
SSID:BIGLAN
Channel:
11
SSID:BIGLAN
Channel:
SSID:BIGLAN
Channel:
SSID:BIGLAN
Channel:
11
6
1
SSID:BIGLAN
Channel:
SSID:BIGLAN
Channel:
SSID:BIGLAN
Channel:
1
6
SSID:BIGLAN
Channel:
11
SSID:BIGLAN
11
Channel:
SSID:BIGLAN
Channel:
6
1
SSID:BIGLAN
Channel:
11
802.11b Channel
Channel
Frequency (GHz)
US/Canada
ESTI
France
1
2.412
ˇ
ˇ
2
2.417
ˇ
ˇ
3
2.422
ˇ
ˇ
4
2.427
ˇ
ˇ
5
2.432
ˇ
ˇ
6
2.437
ˇ
ˇ
7
2.442
ˇ
ˇ
8
2.447
ˇ
ˇ
9
2.452
ˇ
ˇ
10
2.457
ˇ
ˇ
ˇ
11
2.462
ˇ
ˇ
ˇ
12
2.467
ˇ
ˇ
13
2.472
ˇ
ˇ
14
2.475
◎ Most country allow to use channel 10. We choose this channel as default channel.
WEP
• Wireless Equivalent Privacy
• WEP uses the stream cipher RC4 from
RSA Security Inc.
• There are two levels of WEP commonly
available
– based on a 40 bit encryption key and 24bit
initialization vector (64bit encryption)
– based on a 104 bit encryption key and 24bit
initialization vector (128bit encryption).
Advance Configuration Parameters
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•
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Beacon Interval
RTS Threshold
Fragment Threshold
Preamble Type
Beacon
• The "heartbeat" of a WLAN, announcing the
existence of the network, and enabling stations
to establish and maintain communications in an
orderly fashion.
• It carries the following information (some of
which is optional):
– The Timestamp.
– The Beacon interval defines the amount of time
between transmitting beacon frames.
– The Capability Information lists requirements of
stations that want to join the WLAN. For example, it
indicates that all stations must use WEP.
– The Service Set Identifier (SSID).
– The Basic Rate Set.
– The optional Parameter Sets.
Beacon Interval
• The amount of time between beacon
transmissions.
• Before a station enters power save mode,
the station needs the beacon interval to
know when to wake up to receive the
beacon (and learn whether there are
buffered frames at the access point).
RTS Threshold
• RTS Threshold is the frame size above that an RTS/CTS
handshake will be performed before attempting to
transmit.
• RTS/CTS asks for permission to transmit to reduce
collisions, but adds considerable overhead.
• Disabling RTS/CTS can reduce overhead and latency in
WLANs where all stations are close together, but can
increase collisions and degrade performance in WLANs
where stations are far apart and unable to sense each
other to avoid collisions.
• If you are experiencing excessive collisions, you can try
turning RTS/CTS on or (if already on) reduce RTS/CTS
Threshold on the affected stations.
Fragment Threshold
• Fragmentation Threshold is the maximum length of the
frame, beyond which payload must be broken up into
two or more frames.
• Collisions occur more often for long frames because
sending them occupies the channel for a longer period of
time, increasing the chance that another station will
transmit and cause a collision.
• Reducing Fragmentation Threshold results in shorter
frames that "busy" the channel for shorter periods,
reducing packet error rate and resulting retransmissions.
• However, shorter frames also increase overhead,
degrading maximum possible throughput, so adjusting
this parameter means striking a good balance between
error rate and throughput.
Preamble Type
• A preamble is a signal used in network
communications to synchronize the
transmission timing between two or more
systems.
• Proper timing ensures that all systems are
interpreting the start of the information
transfer correctly.
802.11 alphabet
802.11
Base standard. 2.4 GHz
and IR. DSSS and FHSS
Completed
802.11a
5 GHz OFDM 54 Mb/s
Completed
802.11b
2.4 GHz. DSSS. 11 Mb/s
Completed
802.11c
2.4 GHz. DSSS. 11 Mb/s
Completed
802.11d
Global Harmonization
Completed
802.11e
QoS enhancements
Ongoing
802.11f
Inter Access Point
Completed
802.11g
2.4 GHz. DSSS and OFDM. 54 Mb/s
Completed
802.11h Spectrum and transmit power management
Completed
802.11 alphabet
802.11i
Security enhancements
Completed
802.11j
5 GHz operation in Japan
Completed
802.11k
Radio Resource Measurements
Ongoing
802.11m
Standard maintenance
Ongoing
802.11n
High Throughput
Ongoing
802.11p
Wireless access in vehicular environments
Ongoing
802.11r
Fast BSS transition
Ongoing
802.11s
ESS mesh
Ongoing
802.11T
Wireless Performance Prediction
Ongoing
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