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Wireless LAN - Introduction


Wireless LAN - Introduction. Omer Ben-shalom. Lecture brief. This lecture will touch briefly on the following items: WLAN as a disruptive technology ... – PowerPoint PPT presentation

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Title: Wireless LAN - Introduction

Wireless LAN - Introduction
  • Omer Ben-shalom

Lecture brief
  • This lecture will touch briefly on the following
  • 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
  • The impact of WLAN on client, network and apps.

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

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
  • 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

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

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
  • The drive for implementing primary WLAN is two
  • Improved user productivity
  • Lower TCO
  • Lots of new architectures and options are popping

802.11b Physical and MAC layers
  • Two WLAN stations conversing on shared WLAN
    infrastructure. WLAN is only concerned with the
    physical and MAC layers

802.11 MAC
802.3 MAC
802.3 MAC
802.11 MAC
802.11 PHY
802.3 PHY
802.3 PHY
802.11 PHY
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
  • Those are carried in special packets called
  • 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
  • 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

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

An EBSS environment
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

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
  • 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
  • 802.11 is the IEEE committee working on the WLAN
  • 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
  • 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
  • 802.11f - Inter Access Point Protocol
  • 802.11i - MAC Enhancements for Enhanced Security
  • 802.11R Fast roaming

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

Different Standards for Different Needs
  • High performance and scalability

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

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

  • High performance, scalability and

  • Best overall solution for freedom,
    flexibility and interoperability

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.
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

Performance 802.11b/a
Real life throughput of 802.11b/g/a
Throughput dependency on packet sizes
WLAN myth BW requirements
  • Users do not consume as much BW as expected
  • Move from switched 100 mbps to shared 10 looks
  • 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

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
  • Roaming user ability to (seamlessly) continue
    working while moving. Harder by far
  • Achieving nomadic status is mainly about having
  • But each time all sessions need to be

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
  • 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
  • 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

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
  • No real way to stop malicious or innocent
  • Requires applications to consider limited BW
  • Mobility Roam without impacting quality too
  • Cost
  • Creating additional infrastructure for the
  • 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
  • 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

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
  • L2 and L3 VPN technologies can easily secure the
  • 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
  • WLAN capacity is much lower than the switched
  • Also the limited number of channels is a big
  • 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
  • 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
  • In the future smart antenna solutions will really

Reliability concerns
  • WLAN is wide open to interference
  • Unless you use a Faraday cage you are open to
  • 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)

Dealing with reliability
  • Engineer the network for no single point of
  • 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
  • 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

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
  • How do you set them all up
  • How do you change configuration when needed
  • Updating software and firmware on all the APs is
    a big problem

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

Cost concerns
  • WLAN infrastructure has many more entities to
    manage/control/upgrade which contributes to high
  • Clients need to be brought to new standards to
    gain benefit of better security and radio
  • Every new technology is harder to support and
    means training the support personnel. It may also
    be less stable more calls

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

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
  • standard WLAN does not support multiple
  • Multiple AP on same location is expensive and
    causes co-habitation problems
  • WLAN has no real QoS today

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

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
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