Wireless LAN Introduction
Omer Ben-shalom
Lecture brief
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This lecture will touch briefly on the following
items:
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WLAN as a disruptive technology
Advantages and disadvantages of WLAN
802.11 (WiFi) technology and main standards
Some WLAN myth and reality
Mobility definitions
(Time permitting)
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Challenges in WLAN deployment and how to
meet them
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The impact of WLAN on client, network and apps.
WLAN as a disruptive technology
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Like previous disruptive technologies (IP, the Internet) WLAN
is changing the way people work and live
WLAN is taking a strong hold in the home. People are very
happy with being ‘Unwired’ and the concept is pushed by
industry looking for more growth engines.
IT is pressured by users to deliver WLAN
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If IT does not deliver people tend to start installing their own
WLAN
The technology is problematic for the corporate and many
would like to avoid implementing it due to various concerns
IT organizations are starting to see the real benefits in WLAN
as well in both cost savings and (more importantly)
productivity gains
As in the past the simpler technology wins over better ones
The advantages of WLAN
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Its ‘sexy’ and hyped, People like new toys
It saves carrying a dongle and/or patch cord 
(more seriously)
Enhances user productivity
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As a primary network the total cost is markedly
lower then the wired LAN
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No clear ROI. Most quote few more hours of work a week
11 minutes more a week will cover costs according to study
Provides improved efficiency/productivity
Need to be able to do Wireless Voice for that
Allows flexible service provisioning
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With the right gear multiple networks can be provided for
different users in the same location opaquely
Disadvantages of WLAN
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Unless used as primary means of connectivity
presents a second infrastructure
Security – signal leakage outside the buildings.
Physical perimeter security is gone
Performance/Reliability
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Back to shared media with relative low speed
Operates in the non-licensed band and therefore open to
interference
Increases the number of managed entities by an
order of magnitude
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At least in the Naïve implementation options
The state of WLAN
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WLAN is starting to mature
Both standard and (many) proprietary solutions
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Getting to become the main connectivity method
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Has to include voice over WLAN
Requires SLA (uptime, performance) much closer to wired
The drive for implementing primary WLAN is two
fold:
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Few of the new startups will likely survive
Improved user productivity
Lower TCO
Lots of new architectures and options are popping
up
802.11b – Physical and MAC layers
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Two WLAN stations conversing on shared
WLAN infrastructure. WLAN is only
concerned with the physical and MAC layers
application
application
TCP
TCP
IP
IP
LLC
LLC
LLC
802.11 MAC
802.11 MAC
802.3 MAC
802.3 MAC
802.11 PHY
802.11 PHY
802.3 PHY
802.3 PHY
Basic 802.11 terminology
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AP – Access Point. A central controller that can
extend the range of the service set
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BSS – Basic service set
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stations in the BSS talk through a central controller (AP)
The AP sets configurable parameters that all must match
Those are carried in special packets called beacons
Group of stations using a single media and coordination
function in a Basic Set Area (BSA)
All stations can communicate with each other directly
If no central controller exist this is an iBSS
Membership in a BSS is defined by the Service Set
Identifier (SSID) and the BSSID (Normally controlled
by the AP)
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Multiple APs per SSID. Potentially multiple SSID per AP
Basic 802.11 terminology
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Different APs connect through a distribution
system (DS). Normally a wired backbone
All the APs connected on the DS and their
BSS form the ESS - Extended service set
The ESS is a single L2 environment
/broadcast domain
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Stations send packets other stations in the same
ESS ‘directly’
Stations can freely move within the ESS
An EBSS environment
DS
Hidden node problem
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not everyone hears everyone
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Distance
Physical barriers (walls etc)
A traffic to B can collide with C traffic to B without
A or C being in the know
A
B
C
Radio standards
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Wireless LAN (WiFi) is a layer 1-2 technology based on Ethernet
Uses CSMA (Collision sense multiple access) but unlike
Ethernet attempts Collision Avoidance (CA) rather than detection
(CD) so considered CSMA/CA
Tailored for the noisy radio band
Supports client to client Ad-Hoc networking and base station
(AP) based connectivity called infrastructure mode
Operates in two main bands:
 The ISM 2.4 Ghz band (about 80 Mhz, 3 non overlapping
channels). Each channel uses some 20mhz.
 The 5 Ghz licensed band. Channels are also 20Mhz
Can use RTS/CTS mechanism to deal with access rights and
solve the ‘hidden node’ problem.
Incurs a lot of overhead due to collision avoidance scheme and
error correction in the noisy medium
The 802.11 main working groups
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802.11 is the IEEE committee working on the WLAN standards
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Focuses on short range, high throughput, relatively low power
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IEEE deals with the lower levels protocols only
PAN focuses on high throughput in lower power
MAN/WAN such as WiMAX will focus on range with higher power
involved
It includes a lot of working groups. Main ones are
Radio standards (802.11, 802.11b, 802.11a, 802.11g, 802.11n)
Other supporting functions (partial list)
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802.11e - MAC Enhancements for QoS (Expected Sep’ 05)
802.11f - Inter Access Point Protocol
802.11i - MAC Enhancements for Enhanced Security
802.11R – Fast roaming
Radio standards in the 802.11
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802.11 – The old FH/DSSS WLAN standard @12mbps in the 2.4 GHz range, 3 channels
802.11b – improved modulation (CCK) @up to
11mbps at the 2.4 GHz range, 3 channels
802.11g – The newest 2.4GHz modulation using
OFDM and able to provide 54Mbps, 3 channels
802.11a – OFDM modulation in the 5GHz licensed
band, not available everywhere. OFDM, up to 54
Mbps and 8-12 usable channels. Uses 802.11h for
transmit power control and channel selection
802.11n – OFDM modulation using multiple antenna
(MIMO) provides >100mbps, not rectified
Different Standards for Different Needs
802.11a
Business
• High performance and scalability
802.11b
• Mature, globally deployed standard
• Good wall penetration and range
802.11g
• Faster speeds than 802.11b, backward
compatible to 802.11b
• Good wall penetration and range
• Early standard and solutions
802.11a/b
• High performance, scalability and
interoperability
802.11a/b/g
• Best overall solution for freedom,
flexibility and interoperability
Auditorium
Hotspots
Campus
SOHO
Home
802.11b
11
802.11a
802.11g
11
11
2.4 GHz
3 non-overlapping
channels
54
11a
Higher throughput
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Existing Infrastructure
11b
54
54
54
11g
54
54
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54
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Better wall penetration
Low wireless interference
54
54
Feature
Higher network capacity
54
54
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5 GHz
8+ non-overlapping
channels§
§Exact
number of 11a channels depends on
individual country restrictions.
WLAN myth - performance
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WLAN BW quoted is client association speed
This is the biggest myth in WLAN because in
actuality the performance that one can get
from the network is way lower than the one
quoted
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802.11b supports 11mbps but stops at 6 mbps
even for optimal size packets
802.11g and 802.11a are supposed to support 54
mbps but in reality support about 22mbps/15
mbps respectively with optimal packet sizes
With small packets this drops significantly
Performance 802.11b/a
30
802.11a
802.11b
25
20
15
~4.5x
10
~2.5x
5
0
0
50
100
150
200
250
Real life throughput of 802.11b/g/a
Throughput dependency on packet
sizes
802.11b TPT vs. Packet Size
(Using Long Preamble)
7,000,000
6,000,000
5,000,000
4,000,000
11
5.5
2
1
3,000,000
2,000,000
1,000,000
0
64
128 192 256 320 384 448 512 576 640 704 768 832 896 960 1024 1088 1152 1216 1280 1344 1408 1472 1500
WLAN myth – BW requirements
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Users do not consume as much BW as expected
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Move from switched 100 mbps to shared ’10’ looks horrible
But – users are pretty happy with their home ADSL
@750kbps down/96kbps up
How much does a user really need?
Today the LAN is ‘non blocking’ and ‘free’
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Bottleneck is usually servers
Clients cannot really use 100mbps normally
Changes with the move to WLAN
Need solutions
WLAN Mobility - basics
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Mobility includes two different usage models
often mixed
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‘Nomadic’ user –able to take laptop/PDA from one
place to another and work there (but not continue
sessions)
‘Roaming’ user – ability to (seamlessly) continue
working while moving. Harder by far
Achieving nomadic status is mainly about
having coverage
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But each time all sessions need to be reestablished
WLAN Mobility - roaming
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Layer 2 roaming happens when a client changes AP
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Challenge – hand over user fast enough to not drop packets or
eve degrade voice quality.
Main problem – 802.1X re-authentication
Solution – fast secure roaming (802.11r in work)
Layer 3 roaming happens when a client moves to a new
EBSS
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There is a limit to how far a single BSS can be stretched
Challenge – keep sessions open. IP change will tear down
sessions
Solution – allow client to keep their IP between BSS. Usually with
mobile IP or L2 overlay network
Same or worse timing problem
Seamless mobility
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The newest hype is about ‘Seamless mobility’
The ability to switch between transports
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WiFi
WiMax
Cellular/3G
LAN?
While not loosing the sessions
Requires solutions in the network layer
(mobileIP) or the application layer
Main challenges in WLAN implementation
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Security, security, security (your standard FUD)
 Wireless is easily tapped. WEP can be broken
 Rogue APs
Capacity/Performance
 LAN is switched 100 mbps. ‘shared 11mbps’ seems problematic
 No real way to stop malicious or innocent interference
 Requires applications to consider limited BW
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Mobility – Roam without impacting quality too much
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Cost
 Creating additional infrastructure for the ‘Wireless’
 TCO for the infrastructure and clients
Management
 Number of managed entities grows significantly
 Users are on the move and difficult to track
WLAN security concerns
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Using no security allows everyone in range to tap in
to your network
Simple security solutions (mac filtering ,hidden SSID
etc) do not work
Everyone knows that WEP can be broken (but
apparently not how hard it really is)
Most people still do not use encryption
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Even businesses are often found unprotected
Rogue APs are a real security threat
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Allow anyone access to your LAN from outside
Not implementing your own WLAN increases risk
WLAN Security - solutions
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WLAN security flaws have been the focus of (too)
many articles and discussions
The hard fact is that the currently available solutions
are quite good (and will be discussed extensively
later in the course)
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WEP is broken but it takes much more than what is
commonly perceived to break it
Existing WPA has never been shown to be broken
The full 802.11i with AES encryption is even stronger
L2 and L3 VPN technologies can easily secure the WLAN
The single worst security threat is rogue APs and
those are just worse if no WLAN is supplied
We will review the security solutions in depth in a
future lecture
Capacity concerns and solutions
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WLAN capacity is much lower than the switched 100mbps
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How much do users actually need?
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Also – the limited number of channels is a big consideration
It is better than the old shared 10mbps unless packets are
extremely small since CA works better than CD with high utilization
Most users are happy enough on ADSL with 750K max.
300 Kbps/user for secondary service
For Primary use is 1 mbps the magic number ?
High impact of VOIP and other real time protocols or ones that use
small packets
Move to higher throughput with 802.11a/g and 802.11n later on
Tailor applications to recognize network conditions
In the future smart antenna solutions will really help
Reliability concerns
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WLAN is wide open to interference
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The common protocols of 802.11b and 802.11g use
the unlicensed 2.4GHz band
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Unless you use a Faraday cage you are open to
interference
Even a normal cordless phone can bring down a WLAN
channel
A microwave can pretty much block the whole range
Any cable tester for 2.4 with directional antenna will do
Very hard to persecute offenders
The security required adds more points of failure
802.11e not rectified yet (QOS)
Dealing with reliability
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Engineer the network for no single point of failure
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Implement interference detection/avoidance
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Redundant coverage for AP and L2 switches
Dual L3 devices
Multiple authentication servers or VPN gateways
Use equipment that knows to automatically switch away
from channels blocked by interference
Implement location services to find interference sources
Move to 802.11a
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Less interference than the ISM band
8-12 channels makes it difficult to block the service - The
wider the band the more difficult it is to block
802.11h allows channel agility and power control
WLAN management concerns
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With WLAN the number of network elements is
vastly larger than normal LAN
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Part due to area coverage and part to the low throughout of
each AP
Managing such a large number of devices is very
problematic
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How do you set them all up
How do you change configuration when needed
dynamically
Updating software and firmware on all the APs is a big
problem
WLAN management solutions
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Smart management servers (available from a
few vendors) that can centrally manage large
number of APs
Removing as much of the managed entities
from the AP to a central location
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Different AP types: ‘Smart’, ‘thin’, ‘hybrid’
Each of the last two moves some of the MAC
layer to a central controller
More on this in later lectures
Cost concerns
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WLAN infrastructure has many more entities
to manage/control/upgrade which contributes
to high TCO
Clients need to be brought to new standards
to gain benefit of better security and radio
protocols
Every new technology is harder to support
and means training the support personnel. It
may also be less stable = more calls
Cost benefits
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APs are very cheap compared to LAN
switches so the cost of the infrastructure
goes way down
No more Add/Move/Change cost for clients
Our observation is that WLAN reduces the
number of support calls rather then increase
them
Move to primary use allows real cost benefit
WLAN Impact on infrastructure
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The ‘one network serves all’ paradigm is broken
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We got used to the LAN delivering any requirent for 95% of
users
This is no longer the case
Different users have different BW requirements and with
WLAN that has to be taken into account
VOIP introduces different requirements and so does Video.
VOIP traffic clogs the network very easily
‘standard’ WLAN does not support multiple networks
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Multiple AP on same location is expensive and causes cohabitation problems
WLAN has no real QoS today
WLAN impact on user
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Users can become mobile
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Nomadic or Roaming user
Users can use the network in places not
available before
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More usable work time
Different use models, especially with roaming
User productivity is much enhanced
WLAN impact on applications
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Applications writers used to consider the
network a ‘non issue’
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With WLAN and mobility that is not true
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Which often proved very wrong on the WLAN but
right for the LAN
Users may lose the network temporarily and/or
their BW may be limited/fluctuating
Applications need to be written to address
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Good example – outlook 2003
Bad example – net meeting