Ch. 2 – 802.11 and NICs
Part 1 – 802.11 MAC and Cisco Client
Adapters
Cisco Fundamentals of Wireless LANs version 1.2
Overview
Will not use curriculum.
Additional information provided.
MAC – Two presentations. This is Part I
PHY – Separate presentation.
•
Sections 2.2 and 2.3
– We will not use most of the online curriculum in these
sections.
– This presentation will add additional material.
– However, still please read the online curriculum.
Rick Graziani graziani@cabrillo.edu
2
802.11 Overview and MAC Layer
Part 1 – 802.11 MAC and Cisco
Client Adapters
• 2.1 Online Curriculum
– 802.11 Standards
• Overview of WLAN Topologies
– IBSS
– BSS
– ESS
– Access Points
• 802.11 Medium Access
Mechanisms
– DCF Operations
– Hidden Node Problem
– RTS/CTS
– Frame Fragmentation
Rick Graziani graziani@cabrillo.edu
• 2.4 – 2.6 Online Curriculum
– Client Adapters
– Aironet Client Utility (ACU)
– ACU Monitoring and
Troubleshooting Tools
Part 2 – 802.11 MAC
• (Separate Presentation)
• 802.11 Data Frames and
Addressing
• 802.11 MAC Layer Operations
– Station Connectivity
– Power Save Operations
– 802.11 Frame Formats
• Non-standard devices
3
Recommended Reading and Sources for
this Presentation
Pejman Roshan
Jonathan Leary
ISBN:
1587050773
Matthew S. Gast
ISBN:
0596001835
• To understand WLANs it is important to understand the 802.11
•
protocols and their operations.
These two books do an excellent job in presenting this information and
is used throughout this and other presentations.
Rick Graziani graziani@cabrillo.edu
4
Acknowledgements
• Thanks to Pejman Roshan and Jonathan Leary at Cisco Systems,
•
authors of 802.11 Wireless LAN Fundamentals for allowing me to use
their graphics and examples for this presentation.
Also thanks to Matthew Gast for author of 802.11 Wireless Networks,
The Definitive Guide for allowing me to use their graphics and
examples for this presentation.
Rick Graziani graziani@cabrillo.edu
5
802.11 Standards
Overview of
Standardization
• Standardization of networking functions has done much to further the
•
•
•
development of affordable, interoperable networking products.
This is true for wireless products as well.
Prior to the development of standards, wireless systems were plagued
with low data rates, incompatibility, and high costs.
Standardization provides all of the following benefits:
– Interoperability among the products of multiple vendors
– Faster product development
– Stability
– Ability to upgrade
– Cost reductions
Rick Graziani graziani@cabrillo.edu
7
IEEE and 802.11
• IEEE, founded in 1884, is a nonprofit professional organization
• Plays a critical role in developing standards, publishing technical
•
works, sponsoring conferences, and providing accreditation in the area
of electrical and electronics technology.
In the area of networking, the IEEE has produced many widely used
standards such as the 802.x group of local area network (LAN) and
metropolitan area network (MAN) standards,
Rick Graziani graziani@cabrillo.edu
8
IEEE 802 Architecture
Some you may recognize:
• 802.3 – CSMA/CD (Carrier Sense Multiple Access with Collision Detection),
often mistakenly called Ethernet
• 802.1d – Spanning Tree
• 802.1Q – VLANs
• 802.5 – Token Ring
Rick Graziani graziani@cabrillo.edu
9
IEEE 802.11 Architecture
• 802.11 is a family of protocols, including the original specification,
•
•
•
•
•
802.11, 802.11b, 802.11a, 802.11g and others.
Officially called the IEEE Standard for WLAN MAC and PHY
specifications.
802.11 “is just another link layer for 802.2”
802.11 is sometimes called wireless Ethernet, because of its shared
lineage with Ethernet, 802.3.
The wired network side of the network could be Ethernet, Token Ring,
etc.(we will always use Ethernet in our examples)
Access Points and Bridges act as “translation bridges” between
802.11 and 802.3 (or other other protocol)
Rick Graziani graziani@cabrillo.edu
10
Overview of WLAN Topologies
IBSS
BSS
ESS
Access Points
Quick Preview: Station/AP Connectivity
Overview of WLAN Topologies
•
•
•
•
•
Three types of WLAN Topologies:
– Independent Basic Service Sets (IBSS)
– Basic Service Set (BSS)
– Extended Service Set (ESS)
Service Set – A logical grouping of devices.
WLANs provide network access by broadcasting a signal across a wireless
radio frequency.
Transmitter prefaces its transmissions with a Service Set Identifier (SSID)
A station may receive transmissions from transmitters with the same or
different SSIDs.
Rick Graziani graziani@cabrillo.edu
12
Independent Basic Service Sets (IBSS)
• IBSS consists of a group of 802.11 stations directly communicating
•
•
•
•
with each other.
No Access Point used
Also known as an ad-hoc network.
Usage: Few stations setup up for a specific purpose for a short period
of time. (ex. file transfers.)
We will have a an IBSS lab, but our main focus will be BSSs and
ESSs.
Rick Graziani graziani@cabrillo.edu
13
Basic Service Set (BSS)
• BSS, also known as an Infrastructure BSS (never called IBSS)
• Requires an Access Point (AP)
•
•
– Converts 802.11 frames to Ethernet and visa versa
– Known as a translation bridge
Stations do not communicate directly, but via the AP
APs typically have an uplink port that connects the BSS to a wired
network (usually Ethernet), known as the Distribution System (DS).
Rick Graziani graziani@cabrillo.edu
14
Extended Service Set (ESS)
• Multiple BSSs can be connected together with a layer 2 “backbone
•
•
•
•
network” to form an Extended Service Set (ESS).
802.11 does not specify the backbone network
The backbone network is also known as the Distribution System (DS)
and could be wired or wireless.
Stations are “associated” with only one AP at a time.
The SSID is the same for all BSS areas in the ESS (unless creating
multiple BSSs, i.e. one for Marketing and another for Sales).
Rick Graziani graziani@cabrillo.edu
15
Extended Service Set (ESS)
• What if you want to be able to move between access points without the
•
•
•
latency of re-association and re-authentication (these will be explained)?
Roaming gives stations true mobility allowing them to move seamlessly
between BSSs. (More later)
APs need to be able to communicate between themselves since stations
can only associate with one AP at a time.
IEEE 802.11 working group (Task Group F) is working on standardizing
IAPP (Inter-Access Point Protocol)
Rick Graziani graziani@cabrillo.edu
16
Access Points
•
Access Point (AP)
– Translates (converts) 802.11 frames to Ethernet and
visa versa
– Typically provides wireless-to-wired bridging function
– All BSS communications must go through the AP, even
between two wireless stations
Rick Graziani graziani@cabrillo.edu
17
Quick Preview: Station/AP Connectivity
• This is just a preview.
• Later in this module, we will take
a closer look at the following:
– The hardware/software:
• Wireless NICs
• Client Utilities (Aironet)
• Using Windows to set the
IP Address
– The 802.11 MAC Layer
Operations:
• Station Connectivity
Rick Graziani graziani@cabrillo.edu
18
Quick Preview: Station/AP Connectivity
SSID (Service Set Identifier)
• At a minimum a client station and
the access point must be
configured to be using the same
SSID.
• An SSID is:
– Between 2 and 32
alphanumeric characters
– Spaces okay
– Must match EXACTLY,
including upper and lower
case
– Sometimes called the ESSID
– Not the same as BSSID (MAC
address of the AP, later)
Rick Graziani graziani@cabrillo.edu
19
Quick Preview: Station/AP Connectivity
Can use windows to configure wireless
NIC, but we will use the Cisco client
utility, Aironet
SSID 2 and 3 are used for roaming
where different SSIDs are used (later)
•
•
SSIDs are sent by the APs in beacons (and other frames)
Applications such as NetStumbler or even Windows can
see these beacons and interpret the information in them.
Rick Graziani graziani@cabrillo.edu
20
Quick Preview: Station/AP Connectivity
SSID
• The Cisco APs have the default SSID tsunami.
Rick Graziani graziani@cabrillo.edu
21
Quick Preview:
Station/AP Connectivity
Using Windows
Looking for an AP?
Using NetStumbler
Rick Graziani graziani@cabrillo.edu
Right click
22
Quick Preview: Station/AP Connectivity
Windows Toolbar Icon
Windows
Network
Properties
Aironet Toolbar Icon
• Your operating system (Windows) or wireless NIC client (Aironet) will
tell you whether or not you have successfully connected (associated).
Rick Graziani graziani@cabrillo.edu
23
802.11 Medium Access
Mechanisms
DCF Operations
Hidden Node Problem
RTS/CTS
Frame Fragmentation
802.11 Frames – This isn’t Ethernet (802.3)
Distribution System (DS)
IP Packet
General 802.11 Frame
L IP Packet
L
C
•
•
•
•
•
802.11 has some similarities with Ethernet but it is a different protocol.
Access Points are translation bridges.
From 802.11 to Ethernet, and from Ethernet to 802.11
The “data/frame body” is re-encapsulated with the proper layer 2 frame.
Certain addresses are copied between the two types of frames.
Rick Graziani graziani@cabrillo.edu
25
802.11 Frames
802.11 Frames
• Data Frames (most are PCF)
– Data
– Null data
– Data+CF+Ack
– Data+CF+Poll
– Data+CF+Ac+CF+Poll
– CF-Ack
– CF-Poll
– CF-Cak+CF-Poll
• Control Frames
– RTS
– CTS
– ACK
– CF-End
– CF-End+CF-Ack
Rick Graziani graziani@cabrillo.edu
•
Management Frames
– Beacon
– Probe Request
– Probe Response
– Authentication
– Deauthentication
– Association Request
– Association Response
– Reassociation Request
– Reassociation Response
– Disassociation
– Traffic Indication Map (TIM)
26
Medium Access – CSMA/CA
All stations detect the
collision
ACK
CSMA/CD
CSMA/CA
• Both CSMA/CD and CSMA/CA are half-duplex architectures
• Ethernet uses CSMA/CD – Collision Detection
•
– Ethernet devices detect a collision as when the data is transmitted
802.11 uses CSMA/CA – Collision Avoidance
– 802.11 devices only detect (assume) a collision when the
transmitter has not received an Acknowledgement.
– Stations also use a virtual carrier-sense function, NAV
Rick Graziani graziani@cabrillo.edu
27
Medium Access – CSMA/CA
All stations detect the
collision
ACK
•
•
•
CSMA/CA
CSMA/CD
The 802.11 standard makes it mandatory that all stations implement the DCF
(Distributed Coordination Function), a form of carrier sense multiple access
with collision avoidance (CSMA/CA). More Coming!
CSMA is a contention-based protocol making sure that all stations first sense
the medium before transmitting (physically and virtually). More Coming!
The main goal of CSMA/CA is to avoid having stations transmit at the same
time, which will then result in collisions and eventual retransmissions.
However, collisions may still occur and when they do stations may or may not
be able to detect them (hidden node problem). More Coming!
Rick Graziani graziani@cabrillo.edu
28
DCF
•
IEEE mandated access mechanism for 802.11 is DCF
(Distributed Coordination Function)
– Basis for CSMA/CA
– Discussed in detail next
Rick Graziani graziani@cabrillo.edu
29
DCF Operation
An example will be
coming!
• In DCF operation, a station wanting to transmit :
•
•
– Checks to see if radio link is clear, CS/CCA – Carrier Sense, Clear
Channel Assessment (Later in PHY presentation)
– Checks its NAV timer (coming) to see if someone else is using the
medium.
– If medium is available DCF uses a random backoff timer to avoid
collisions and sends the frame.
Transmitting station only knows the 802.11 frame got there if it receives
an ACK.
May also use RTS/CTS to reduce collisions (coming)
Rick Graziani graziani@cabrillo.edu
30
Duration Field
An example will be coming!
General 802.11 Frame (more on this later)
• Duration/ID field – The number of microseconds (millionth of a
second) that the medium is expected to remain busy for transmission
currently in progress.
– Transmitting device sets the Duration time in microseconds.
– Includes time to:
• Transmit this frame to the AP (or to the client if an AP)
• The returning ACK
• The time in-between frames, IFS (Interframe Spacing)
• All stations monitor this field!
Graziani
graziani@cabrillo.edu
•Rick All
stations
update their NAV (Network Allocation Vector) timer.
31
NAV Timer
An example will be coming!
General 802.11 Frame (more on this later)
•
•
•
•
•
•
•
•
All stations have a NAV (Network Allocation Vector) timer.
Virtual carrier-sensing function
Protects the sequence of frames from interruption.
Martha sends a frame to George.
Since wireless medium is a “broadcast-based” (not broadcast frame) shared
medium, all stations including Vivian receive the frame.
Vivian updates her NAV timer with the duration value.
Vivian will not attempt to transmit until her NAV is decremented to 0.
Stations will only update their NAV when the duration field value received is
greater than their current NAV.
Rick Graziani graziani@cabrillo.edu
32
Broadcast-based shared medium
• Host A is sending
•
•
•
802.11 frames to
another host via the AP.
All other 802.11 devices
in BSS (on this channel)
and within range of the
signal will see the
frame.
802.11 framing provides
addressing, so only the
AP knows it is the nexthop receiver.
Other 802.11 devices
within this BSS can
sense that the medium
is in use and will update
their NAV values.
Rick Graziani graziani@cabrillo.edu
What if a station is in range of the AP but not
the Host A? (Hidden node problem – later)
33
Interframe Spacing (IFS)
An example will be coming!
•
802.11 uses four different interframe spaces used to determine medium
access (note: microsecond = millionth of a second):
– DIFS – DCF Interframe Space
• Minimum amount of medium idle time until contention-based services
begin.
– PIFS – PCF Interframe Space
• Used by PCF
– SIFS – Short Interframe Space
• Used for highest priority transmission, ACKs, RTS, CTS
– EIFS – Extended Interframe Space
• Not a fixed interval and used only when there is an error in frame
transmission.
Rick Graziani graziani@cabrillo.edu
34
Wanting to transmit (1/3)
Random backoff slots
•
•
•
•
Station wanting to transmit.
Carrier Sensing:
– Physical: Physically senses medium is idle (CS/CCA – coming).
– Virtual: NAV timer is 0
Waits DIFS (DCF Interframe Space)
– Minimum amount of medium idle time until contention-based services
begin.
– Once DCF is over, stations can contend for access.
Contention window begins.
– Uses random backoff algorithm to determine when it can attempt to
access the medium. (next)
Rick Graziani graziani@cabrillo.edu
35
Wanting to transmit (2/3)
Contention Window Begins
•
•
•
•
•
(Detail of random backoff algorthim has been left out, but this will be sufficient.)
The random backoff algorithm randomly selects a value from 0 to 255
(maximum value varies by vendor and stored in the NIC).
The random value is the number of 802.11 slot times the station must wait
after the DIFS, during the contention window before it may transmit.
Stations pick a random slot and wait for that slot before attempting to access
the medium.
With several stations attempting to transmit, the station that picks the lowest
slot, lowest random number, wins.
Rick Graziani graziani@cabrillo.edu
36
Wanting to transmit (3/3)
Others
update NAV
General 802.11 Frame (more on this later)
•
•
•
•
•
Station transmits, setting the Duration ID to the time needed to transmit data,
ACK and IFSs.
Other stations with higher slots will see the new transmission and wait to
transmit.
If frame arrives at AP (assuming the transmitter is a station), then an ACK
will be returned (stations have updated their NAVs from original frame).
If there is not an ACK received, the sending station assumes there has been
a collision (stations have not updated their NAVs because of collision).
– If two stations have the same lowest slot time and both transmit, then a
collision occurs.
Stations will update its retry counter (double) to determine a new randomly
selected slot time and process starts all over again.
Rick Graziani graziani@cabrillo.edu
37
Example
I’m
waiting
I’m
waiting
Scenario:
• Both Vivian and George want to transmit frames.
• Both stations have same NAV values and physically sense when the
medium is idle.
• Both are waiting for Martha’s transmission to end and the medium to
become available.
• The medium now becomes available.
Rick Graziani graziani@cabrillo.edu
38
Example
Random backoff slots
• George and Vivian are both wanting to transmit.
• Both perform the following:
• Both sense that medium is available using Physical and Virtual
•
•
Carriers Sensing:
– Physical: Physically senses medium is idle (CS/CCA – coming).
– Virtual: NAV timer is 0
Both waits DIFS (DCF Interface Space)
Contention window begins.
– Uses random backoff algorithm to determine when it can attempt
to access the medium. (next)
Rick Graziani graziani@cabrillo.edu
39
Example
Vivian (7), George (31)
• Both Vivian and George calculate their random backoff algorithm to
•
•
•
randomly selects a value from 0 to 255.
Vivian has a slot time of 7, George a slot time of 31.
Vivian wins.
The destination of her frame is George (could have been a station on
the wired network.)
Rick Graziani graziani@cabrillo.edu
40
Example
Martha and George
receive “broadcastbased” 802.11 frame.
Others
update NAV
(((
)))
General 802.11 Frame (more on this later)
• Vivian transmits, setting the Duration ID to the time needed to
•
•
transmit data, ACK and IFSs.
George with a higher slot will see the 802.11 frame from Vivian and
wait to transmit.
Assuming their was not a collision from another station, Martha and
George update their NAVs.
Rick Graziani graziani@cabrillo.edu
41
Example
•
•
•
•
•
The frame arrives at the AP.
After the SIFS, the AP sends an ACK back to Vivian, which is how Vivian
knows the frame was received by the AP.
The AP now has the frame and must contend for access to the medium
like all other stations.
Once it sends the frame to George, George will send an ACK back to the AP.
Remember, 802.11 uses a half-duplex, shared medium and the AP has to
contend for access just like all other devices!
Rick Graziani graziani@cabrillo.edu
42
Your turn!
•
•
•
•
•
•
•
Get into teams of 2.
Each person is a wireless client contending for access to shared
wireless medium.
Using the example as an example contend for the wireless medium.
Station 1: Backoff slot of 3
Station 2: Backoff slot of 9
Go through the steps of sending the 802.11 frame to the AP
– Use a total NAV value of 1054 microseconds for both stations.
Answer is on the next slide! Don’t look until you need to check!
Rick Graziani graziani@cabrillo.edu
43
Your turn! (Solution)
1.
2.
3.
4.
5.
6.
7.
Both stations count down their NAV timers to 0
Both stations physically sense the wireless medium is available
Both stations wait the DIFS
Station 1 and Station 2 prepare to send their 802.11 by updating the
NAV value in the Duration/ID field to reflect the time needed to
transmit to the AP, the ACK from the AP and IFS for both the originally
transmitted frame and the ACK from the AP.
Station 1 sends its 802.11 frame first because it had to wait less time
with a Backoff slot of 3.
Station 2 “sees” that Station 1 has accessed the shared medium, the
802.11 frame from Station 1 to the AP, and updates its NAV timer.
Station 2 waits for the NAV timer to count down to 0. (Step 1)
Rick Graziani graziani@cabrillo.edu
44
802.11 Medium Access
Mechanisms
DCF Operations
Hidden Node Problem
RTS/CTS
Frame Fragmentation
Hidden Node Problem
• What if a station is in range of the AP but not other hosts, like the
•
•
transmitting host?
Wireless networks have fuzzy boundaries, sometimes where may not
be able to communicate/see every other node.
Hidden nodes can be caused by:
– Hosts are in range of the AP but not each other.
– An obstacle is blocking the signal between the hosts.
Rick Graziani graziani@cabrillo.edu
46
Hidden Node Problem
• The problem is collisions.
•
– Collisions occur at the AP (or another station in an IBSS).
– Both stations assume the medium is clear and transmit near the
same time, resulting in a collision.
– The AP cannot properly receive either signal and will not ACK
either one.
– Both stations retransmit, resulting in more collisions.
Throughput is significantly reduced
Rick Graziani graziani@cabrillo.edu
47
Hidden Node Problem
• Solutions:
– Move the node
– Remove the obstacle
– Use RTS/CTS (Request to Send / Clear to Send)
Rick Graziani graziani@cabrillo.edu
48
802.11 Medium Access
Mechanisms
DCF Operations
Hidden Node Problem
RTS/CTS
Frame Fragmentation
RTS/CTS Solution
• Vivian attempts to reserve the medium using
•
•
•
•
•
•
an RTS control frame to the AP.
The RTS frame indicates to the AP and all
stations within range, that Vivian wants to
reserve the medium for a certain duration
of time, message, ACK, and SIFS.
The hidden node stations cannot see the RTS.
The AP replies to Vivian with a CTS, which all nodes, including the
hidden node can see.
Vivian transmits the frame.
The AP returns an ACK to Vivian.
The AP sends the message to George who returns an ACK to the AP.
Rick Graziani graziani@cabrillo.edu
50
RTS/CTS Solution
The CTS is sent to the AP’s
entire coverage area
Rick Graziani graziani@cabrillo.edu
51
RTS/CTS Solution
• RTS/CTS consumes a fair amount of
•
capacity and overhead, resulting in
additional latency.
Normally used in high capacity
environments.
• The RTS/CTS procedure can be enabled/controlled by setting the RTS
•
threshold on the 802.11 client NIC.
RTS/CTS is also used during frame fragmentation (coming).
Rick Graziani graziani@cabrillo.edu
52
Setting the RTS Threshold on a Cisco Client
RTS
Threshold
•
•
Specifies the data packet size beyond which the low-level RF protocol invokes RTS/CTS
flow control. A small value causes RTS packets to be sent more often, which consumes
more of the available bandwidth and reduces the throughput of other network packets.
However, small values help the system recover from interference or collisions.
Rick Graziani graziani@cabrillo.edu
53
Improving WLAN Performance with RTS/CTS by
Jim Geier (wi-fiplanet.com)
• If you enable RTS/CTS on a particular station (just the hidden
•
node station), it will refrain from sending a data frame until
the station completes a RTS/CTS handshake with another
station, such as an access point.
Keep in mind, though, that an increase in performance using
RTS/CTS is the net result of introducing overhead (i.e.,
RTS/CTS frames) and reducing overhead (i.e., fewer
retransmissions). If you don't have any hidden nodes, then
the use of RTS/CTS will only increase the amount of
overhead, which reduces throughput. A slight hidden node
problem may also result in performance degradation if you
implement RTS/CTS. In this case, the additional RTS/CTS
frames cost more in terms of overhead than what you gain by
reducing retransmissions. Thus, be careful when
implementing RTS/CTS.
Rick Graziani graziani@cabrillo.edu
54
Improving WLAN Performance with RTS/CTS by
Jim Geier (wi-fiplanet.com)
• One of the best ways to determine if you should activate
•
RTS/CTS is to monitor the wireless LAN for collisions. If you
find a large number of collisions and the users are relatively
far apart and likely out of range, then try enabling RTS/CTS
on the applicable user wireless NICs. You can activate the
function by clicking "enable RTS/CTS" somewhere in the user
setup screens. You don't need to enable RTS/CTS at the
access point in this case. After receiving a RTS frame from a
user's radio NIC, the access point will always respond with a
CTS frame.
Of course, keep in mind that user mobility can change the
results. A highly mobile user may be hidden for a short period
of time, perhaps when you perform the testing, then be closer
to other stations most of the time. If collisions are occurring
between users within range of each other, the problem may
be the result of high network utilization or possibly RF
interference.
Rick Graziani graziani@cabrillo.edu
55
RTS/CTS Example
HN-A
RTS/CTS
E
C
AP
D
F
• Stations C, D, E, and F can see traffic (signals) from all stations
HN-B
RTS/CTS
including HN-A and HN-B (and visa versa).
• HN-A and HN-B can not see each other, but can communicate with the
AP.
• RTS/CTS is enabled on HN-A and HN-B, so that the AP will respond
with a CTS that the other HN station will see.
•Rick IfGraziani
it wasn’t
for the other HN station, neither HN would need RTS/CTS 56
graziani@cabrillo.edu
802.11 Medium Access
Mechanisms
DCF Operations
Hidden Node Problem
RTS/CTS
Frame Fragmentation
Frame Fragmentation
• Since we have already discussed RTS/CTS, let’s also discuss frame
•
•
fragmentation.
Later, we will see that RTS/CTS and fragmentation are typically
combined.
Frame fragmentation is a MAC layer function that is designed to
increase the reliability of transmitting frames across a wireless
medium.
Rick Graziani graziani@cabrillo.edu
58
Frame Fragmentation
• In a “hostile wireless medium” (interference, noise) larger frames may
•
•
•
have more of a problem reaching the receiver without any errors.
By decreasing the size of the frame, the probability of interference
during transmission can be reduced.
Breaking up a large frame into smaller frames, allows a larger
percentage of frames to arrive undamaged (without errors).
“Easier to poor sand down a hole than boulders.”
Rick Graziani graziani@cabrillo.edu
59
Frame Fragmentation
• Frame fragmentation can increase the reliability of frame transmissions
but there is additional overhead:
– Each frame fragment includes the 802.11 MAC protocol header.
– Each frame fragment requires a corresponding acknowledgement.
• If a frame fragment encounters errors or a collision, only that fragment
needs to be retransmitted, not the entire frame.
• The frame control field includes information that this is a fragmented
frame.
Rick Graziani graziani@cabrillo.edu
60
Frame
Fragmentation
Fragment Threshold:
Defines the largest RF packet that the client
adapter sends without splitting the packet
into two or more smaller fragments. If a
single fragment experiences interference
during transmission, only that fragment must
be resent. Fragmentation generally reduces
throughput because the packet overhead for
each fragment consumes a higher portion of
the RF bandwidth.
• The “network administrator” (user) can define the fragment size.
• Fragment size – The largest packet that the client adapter sends
•
•
without fragmenting the packet.
Only unicast packets will be fragmented, not broadcasts or multicasts.
WiFiPlanet.com: “The fragment size value can typically be set between
256 and 2,048 bytes… Setting the threshold to the largest value (2,048
bytes) effectively disables fragmentation”
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Frame Fragmentation
From 802.11 Wireless
Networks, by Matthew
Gast
• Frame fragments are sent in a burst, using a single iteration of DCF
•
•
to access the medium.
In other words the NAV is set in the first fragment and later
fragments to reserve the medium for the entire original frame.
FYI – Some of the detail
– The first frame sets the NAV to be long enough to include the
returning ACK, the next fragment, its ACK, and 3 SIFS.
– The following frames set the NAV to include successive ACKs and
SIFS.
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Frame Fragmentation and RTS/CTS
• In practice, the RTS/CTS exchange is usually combined with frame
fragmentation.
• Fragmented frames are usually quite long and therefore will benefit
from the RTS/CTS process.
• This will ensure exclusive access to the medium, free from collisions
with hidden nodes.
• Many vendors set the default fragmentation threshold to be identical to
the RTS/CTS threshold, so when fragmentation occurs so does
RTS/CTS.
Rick Graziani graziani@cabrillo.edu
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Client Adapters
2.4 Online Curriculum
Introduction
• The Cisco Aironet Wireless WLAN Adapters are also referred to as
•
•
•
client adapters.
The client adapters are fully compatible when used in devices
supporting Plug-and-Play (PnP) technology.
The primary function of the client adapters is to transfer data packets
through the wireless infrastructure.
The adapters operate similarly to a standard network product except
that the cable is replaced with a radio connection.
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350 Series PC card
• 802.11b
• Integrated antenna.
• A PCMCIA card radio module can be inserted into any device
•
equipped with an external Type II or Type III PC card slot.
Host devices can include laptops, notebook computers, personal digital
assistants, and hand-held or portable devices.
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350 Series LM card
• 802.11b
• The 350 Series LM card client adapter, also referred to as an LM
•
•
card.
A PCMCIA card radio module, which can be inserted into any
device equipped with an internal Type II or Type III PC card slot.
The primary difference between this and the PC card adapter is
that the LM card does not include a built-in antenna.
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350 Series PCI client adapter
• 802.11b
• A client adapter card radio
•
module, which can be inserted
into any device equipped with an
empty PCI expansion slot, such as
a desktop computer.
These cards are typically shipped
with an antenna that attaches to
an external connector.
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350 Series Mini-PCI
• 802.11b
• The Mini-PCI is available to laptop manufacturers to provide integrated
•
802.11b support.
The Mini-PCI is also used in the Cisco 1100 AP and 1200 AP to
provide 802.11b.
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Cisco Aironet 5 GHz 54 Mbps WLAN card
• 802.11a
• IEEE 802.11a-compliant CardBus Type II adapter.
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Parts of the client adapter
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LED Status
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Windows Drivers
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Linux and Macintosh Drivers
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Downloading Drivers and Software
http://www.cisco.com/public/sw-center/sw-wireless.shtml
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IBSS (Ad-Hoc ) or BSS (Infrastructure)
Client adapters work in both
IBSS and BSS networks.
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Aironet Client Utility (ACU)
ACU Overview
• The following slides are just an overview of the various ACU screens.
• We will discuss many of these features and settings during the
•
semester.
You will also be configuring many of these features and settings in the
various labs.
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Aironet Client Utility: Main Screen
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Windows CE, OS X, and Linux
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Aironet Client Utility: Loading Firmware
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Aironet Client Utility: Profile Manager
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Aironet Client Utility: Adding a Profile
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Profile: System Parameters
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Profile: RF Network
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Profile: Advanced (Infrastructure)
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Profile: Advanced (Ad Hoc)
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Profile: Network Security
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Auto Profile
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Auto Profile
• After creating profiles for the client adapter, they can be included in the
•
•
profile manager's auto profile selection feature.
Then when auto profile selection is enabled, the client adapter
automatically selects a profile from the list of profiles that were included
in auto profile selection and uses it to establish a connection to the
network.
This makes the wireless profile selection invisible to the user and
improves the user experience.
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Auto Profile
•
•
•
•
This saves me time.
At home I have a different SSID and I use WEP.
LuLu Carpenters, has a different SSID and is open authentication (no WEP).
This way I do not have to select a profile before I am connected, by I connect
automatically.
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Aironet Client Monitor (ACM)
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Aironet Client Monitor (ACM)
Right Click
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ACU Monitoring and
Troubleshooting Tools
Aironet Client Utility: Status
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Aironet Client Utility: Statistics
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Aironet Client Utility: Link Test
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Aironet Client Utility: Site Survey
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Link Status Meter
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Installing the ACU Software
Installing the Client Adapter
• Insert the adapter card
into the PCMCIA slot
in the PC
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ACU Install and Setup
From the UTILS
folder on the
driver CD, run
SETUP
SETUP
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ACU Install and Setup (cont.)
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ACU Install and Setup (cont.)
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ACU Install and Setup (cont.)
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ACU Install and Setup (cont.)
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ACU Install and Setup (cont.)
• Aironet Client Utility (ACU)
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ACU Install and Setup (cont.)
Step 14
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ACU Install and Setup (cont.)
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Ch. 2 – 802.11 and NICs
Part 1 – 802.11 MAC and Cisco Client
Adapters
Cisco Fundamentals of Wireless LANs version 1.1
Rick Graziani
Cabrillo College