Wireless LAN - Introduction

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Wireless LAN - Introduction

• Omer Ben-shalom

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Lecture brief

• This lecture will touch briefly on the following
  items
• 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)
• Challenges in WLAN deployment and how to meet
  them
• The impact of WLAN on client, network and apps.

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WLAN as a disruptive technology

• 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
• 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

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The advantages of WLAN

• Its sexy and hyped, People like new toys
• It saves carrying a dongle and/or patch cord ?
• (more seriously)
• Enhances user productivity
• 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
• As a primary network the total cost is markedly
  lower then the wired LAN
• Need to be able to do Wireless Voice for that
• Allows flexible service provisioning
• With the right gear multiple networks can be
  provided for different users in the same location
  opaquely

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Disadvantages of WLAN

• Unless used as primary means of connectivity
  presents a second infrastructure
• Security signal leakage outside the buildings.
  Physical perimeter security is gone
• Performance/Reliability
• 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
• At least in the Naïve implementation options

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The state of WLAN

• WLAN is starting to mature
• Both standard and (many) proprietary solutions
• Few of the new startups will likely survive
• Getting to become the main connectivity method
• Has to include voice over WLAN
• Requires SLA (uptime, performance) much closer to
  wired
• The drive for implementing primary WLAN is two
  fold
• Improved user productivity
• Lower TCO
• Lots of new architectures and options are popping
  up

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802.11b Physical and MAC layers

• 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
802.3 MAC
802.3 MAC
802.11 MAC
802.11 PHY
802.3 PHY
802.3 PHY
802.11 PHY
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Basic 802.11 terminology

• AP Access Point. A central controller that can
  extend the range of the service set
• 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
• BSS Basic service set
• 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)
• Multiple APs per SSID. Potentially multiple SSID
  per AP

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Basic 802.11 terminology

• 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
• Stations send packets other stations in the same
  ESS directly
• Stations can freely move within the ESS

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An EBSS environment
DS
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Hidden node problem

• not everyone hears everyone
• Distance
• Physical barriers (walls etc)
• A traffic to B can collide with C traffic to B
  without A or C being in the know

C
B
A
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Radio standards

• 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

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The 802.11 main working groups

• 802.11 is the IEEE committee working on the WLAN
  standards
• IEEE deals with the lower levels protocols only
• Focuses on short range, high throughput,
  relatively low power
• 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)
• 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

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Radio standards in the 802.11

• 802.11 The old FH/DSSS WLAN standard _at_1-2mbps
  in the 2.4 GHz range, 3 channels
• 802.11b improved modulation (CCK) _at_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 gt100mbps, not rectified

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Different Standards for Different Needs
Business
802.11a

• High performance and scalability

Auditorium
802.11b

• Mature, globally deployed standard
• Good wall penetration and range

Hotspots
802.11g

• Faster speeds than 802.11b, backward
  compatible to 802.11b
• Good wall penetration and range
• Early standard and solutions

Campus
SOHO
802.11a/b

• High performance, scalability and
  interoperability

Home
802.11a/b/g

• Best overall solution for freedom,
  flexibility and interoperability

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802.11a
802.11b
802.11g
2.4 GHz 3 non-overlapping channels
Feature 11a 11b 11g
Higher throughput þ þ
Higher network capacity þ
Better wall penetration þ þ
Low wireless interference þ
Existing Infrastructure þ
5 GHz 8 non-overlapping channels
Exact number of 11a channels depends on
individual country restrictions.
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WLAN myth - performance

• 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
• 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

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Performance 802.11b/a
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Real life throughput of 802.11b/g/a
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Throughput dependency on packet sizes
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WLAN myth BW requirements

• Users do not consume as much BW as expected
• Move from switched 100 mbps to shared 10 looks
  horrible
• But users are pretty happy with their home ADSL
  _at_750kbps down/96kbps up
• How much does a user really need?
• Today the LAN is non blocking and free
• Bottleneck is usually servers
• Clients cannot really use 100mbps normally
• Changes with the move to WLAN
• Need solutions

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WLAN Mobility - basics

• Mobility includes two different usage models
  often mixed
• 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
• But each time all sessions need to be
  re-established

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WLAN Mobility - roaming

• Layer 2 roaming happens when a client changes AP
• 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
• 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

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Seamless mobility

• The newest hype is about Seamless mobility
• The ability to switch between transports
• WiFi
• WiMax
• Cellular/3G
• LAN?
• While not loosing the sessions
• Requires solutions in the network layer
  (mobileIP) or the application layer

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Main challenges in WLAN implementation

• 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
• Mobility Roam without impacting quality too
  much
• 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

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WLAN security concerns

• 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
• Even businesses are often found unprotected
• Rogue APs are a real security threat
• Allow anyone access to your LAN from outside
• Not implementing your own WLAN increases risk

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WLAN Security - solutions

• 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)
• 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

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Capacity concerns and solutions

• WLAN capacity is much lower than the switched
  100mbps
• 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
• How much do users actually need?
• 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

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Reliability concerns

• WLAN is wide open to interference
• 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
• The common protocols of 802.11b and 802.11g use
  the unlicensed 2.4GHz band
• Very hard to persecute offenders
• The security required adds more points of failure
• 802.11e not rectified yet (QOS)

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Dealing with reliability

• Engineer the network for no single point of
  failure
• Redundant coverage for AP and L2 switches
• Dual L3 devices
• Multiple authentication servers or VPN gateways
• Implement interference detection/avoidance
• Use equipment that knows to automatically switch
  away from channels blocked by interference
• Implement location services to find interference
  sources
• Move to 802.11a
• 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

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WLAN management concerns

• With WLAN the number of network elements is
  vastly larger than normal LAN
• Part due to area coverage and part to the low
  throughout of each AP
• Managing such a large number of devices is very
  problematic
• 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

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WLAN management solutions

• 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
• 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

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Cost concerns

• 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

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Cost benefits

• 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

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WLAN Impact on infrastructure

• The one network serves all paradigm is broken
• 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
• Multiple AP on same location is expensive and
  causes co-habitation problems
• WLAN has no real QoS today

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WLAN impact on user

• Users can become mobile
• Nomadic or Roaming user
• Users can use the network in places not available
  before
• More usable work time
• Different use models, especially with roaming
• User productivity is much enhanced

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WLAN impact on applications

• Applications writers used to consider the network
  a non issue
• Which often proved very wrong on the WLAN but
  right for the LAN
• With WLAN and mobility that is not true
• Users may lose the network temporarily and/or
  their BW may be limited/fluctuating
• Applications need to be written to address
• Good example outlook 2003
• Bad example net meeting