Introduction to Wireless LANs

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Introduction to Wireless LANs

• David OReilly

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Wireless LANs

• Wireless LANs are not necessarily a replacement
  for wired networks. They may form part of a
  larger solution.
• Radio transmissions are used instead of wires.
• Allows a network to be deployed in awkward
  environments, or where it would otherwise be too
  expensive to have wired connections.
• Wireless LANs are useful where a high level of
  mobility is required.

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Wireless LANs

• A wireless network can be installed without
  altering the building in any way, and can be
  removed when necessary.
• There is a premium for wireless hardware over
  traditional wired hardware, but this is covered
  by the savings in cabling and installation costs
  and the benefits of flexible communications.
• Many businesses reorganise on a regular basis,
  and a wireless network could mean that IT is
  placed far down the list of obstacles.

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Wireless LANs

• Wireless networks can provide access to all
  online resources from anywhere within the
  organisation.
• Wireless networks can be configured in similar
  ways to wired networks from ad hoc or
  peer-to-peer installations from small workgroups,
  to full infrastructure for hundreds or thousands
  of users.
• The user experience of a wireless LAN is exactly
  the same as a wired network. No re-training of
  the user base is required.

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Consider the following questions

• Would you need to install excessive cabling to
  link up distant departments?
• Do your premises present special difficulties
  when it comes to wiring for Ethernet?
• Would your business benefit from having drop in
  facilities for roaming staff?
• Do you reconfigure your office regularly, cater
  for varied staff numbers or allowing people from
  partner companies to work on-site with your
  staff?

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The 802.11b standard

• Previous wireless devices had a reputation for
  being slow and unreliable with poor
  interoperability with other vendor equipment.
• New standards and co-operation are making
  wireless products available to a wide range of
  users

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The 802.11b standard

• The main standard for wireless LANs is the IEEE
  802.11 standard.
• Conceived in 1990, is was approved in 1997.
• It uses the 2.4GHz IMS (Industrial, medical,
  scientific) frequency band which does not require
  a license.
• An 802.11 network is based on a cellular
  architecture.
• An individual cell is referred to as a basic
  service set and is controlled by an access point.

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The 802.11b standard

• Most installations have several cells, with
  access points connected through a backbone. The
  backbone is usually Ethernet.
• There are also situations where no access point
  is required known as ad hoc networks.
• The original 802.11 standard specified data rates
  of 1Mbit/sec and 2Mbit/sec.
• The 802.11b standard specifies the additional
  rates of 5.5Mbit/sec and 11Mbit/sec.

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Interoperability

• Wi-Fi branding has been created to guarantee
  interoperability.
• Any product with the Wi-Fi logo is guaranteed to
  work with other Wi-Fi products. The Wireless
  Ethernet Compatibility Alliance (WECA) was formed
  in 1999 to certify the compatibility of Wi-Fi
  products, and to promote Wi-Fi as the global
  wireless LAN standard.

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Bluetooth

• Bluetooth is the name for a wireless technology
  that allows a wide range of devices to
  communicate with each other.
• A universal short range radio link replaces the
  need for cables to connect different types of
  device.
• For example, mobile phones, laptops, keyboards,
  PDAs, fax machines, projectors, and most other
  peripherals could be connected with Bluetooth.

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Bluetooth

• Bluetooth uses a rapid acknowledgement and
  frequency hopping scheme to ensure a robust link.
  
• Bluetooth radios also work in the 2.4GHz band.
• The maximum data rate is 1Mbit/sec.

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Security

• Wireless LANs are shared media and are therefore
  (theoretically) susceptible to packet sniffing.
• There are also a number of wireless LAN specific
  security issues. For example, if you have
  wireless LAN coverage in your car park, it is as
  if you had installed Ethernet ports in your car
  park as far as an attacker attempting to gain
  access to your internal network is concerned.

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Security

• There are a number of security features that
  typically appear in access points
• SSIDs
• MAC address filtering
• WEP (Wired Equivalent Protection)
• The Cisco Aironet 350 series also offers a range
  of new security features based on the EAP
  (Extensible Authentication Protocol) and IEEE
  802.1x standards.

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Security

• All of these features have weaknesses, ranging
  from poor security in the case of SSIDs to poor
  scalability in the case of MAC address filtering.
• WEP also suffers from a number of recently
  published weaknesses.

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Security

• WEP stands for Wired Equivalent Privacy
• It is intended to provide a level of protection
  that is normally found in a wired network.
• Wired networks are normally protected by physical
  security mechanisms such as controlled access to
  a building.

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Security

• Wireless networks, on the other hand, can have
  coverage areas outside the building, e.g. in the
  car park.
• Since wireless LANs are not necessarily protected
  by physical security, it was decided to include
  WEP encryption into the 802.11 standard to
  provide an equivalent level of protection.

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Security

• WEP is not intended as a complete security
  solution, just as physical security is not a
  complete security solution in the case of a wired
  network.
• It needs to be augmented with additional measures
  such as access control, end to end encryption,
  password control, authentication, VPNs and
  firewalls.

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Wireless LAN Components

• A number of different wireless LAN components are
  available each of which serve different purposes.
• PC cards, PCI cards, access points, multifunction
  bridges, workgroup bridges, antennae.

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Wireless LAN Components

• PC and PCI cards are used to provide an
  individual laptop, desktop or PDA computer
  respectively with access to the wireless network.
• PC and PCI cards can be purchased with either
  fixed or detachable antennae.
• Currently compact flash wireless LAN NICs are not
  available, but are in development.

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Wireless LAN Components

• Access points are the wireless equivalent of an
  Ethernet hub.
• Acts as a bridge between the wired network and
  the wireless network, transferring information
  from one to the other.
• Different vendors access points can support
  different numbers of (theoretical) simultaneous
  users. In practice the limitation on the number
  of concurrent users arises from contention for
  the medium.

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Wireless LAN Components (Access Points)
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Wireless LAN Components

• The coverage of an access point depends heavily
  on the environment in which the access points are
  to be deployed.
• Factors such as thick walls, steel
  reinforcements, microwave ovens, filing cabinets
  can all have an effect.
• Multiple access points can be deployed in a
  cellular architecture for coverage of arbitrarily
  large areas.

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Wireless LAN Components

• Ethernet bridges are typically used in
  building-to-building line of sight applications.
• Distances of up to 25 miles are possible but this
  distance is limited in Ireland due to maximum
  radiated power restrictions imposed by the ESTI.

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Wireless LAN Components (Bridge)
Point to point link
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Wireless LAN Components (Bridge)
Point to Multi-point link
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Wireless LAN Components (Bridge)
Overcoming obstacles
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Wireless LAN Components

• Workgroup bridges are used to connect devices to
  a wireless network for which it would not be
  possible to provide a direct connection.
• Issues such as lack of device driver support or
  no NIC available would lead to a requirement for
  a workgroup bridge.

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Wireless LAN Components (Workgroup Bridge)
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Wireless LAN Components

• One of the key benefits of wireless LAN
  components is that they can all be disconnected
  and reused at a new location with great ease.
• This allows total investment protection in
  network infrastructure in the case of an office
  relocation.

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Cellular Architecture

• Within the 2.4GHz band used for wireless
  networking, there are 3 non-overlapping sub-bands
  available.
• If two access points using the same sub-band (or
  overlapping sub-bands) are within range of each
  other, they will contend for access to the
  medium.
• These sub-bands can be used in a cellular
  architecture to provide coverage over an
  arbitrarily large area.

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Cellular Architecture
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Roaming

• The area covered by a single access point is
  known as the coverage cell of that access point.
• Roaming is the ability to move seamlessly from
  one coverage cell to another.
• In order for roaming to work, there must be
  overlapping coverage cells.

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Roaming
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Rate Shifting

• As the distance from the nearest access point
  increases, the signal from the access point will
  weaken.
• Under these circumstances, the network throughput
  may in fact be increased by decreasing the data
  rate. This is because a slower data transmission
  rate will lead to fewer retries.
• Access points and NICs can usually rate shift
  between 11, 5.5, 2 and 1 Mbps as required.

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When to use wireless LANs

• Wireless LANs are not ideal for every situation,
  and they are not necessarily a substitute for a
  wired network.
• Rather, they should be used to overlay and/or
  supplement your current wired network.
• Wireless LANs allow users the choice between the
  speed of a wired network and the convenience of a
  wireless network.
• Wireless LANs are also ideal in situations where
  running cable is either infeasible or impossible.

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When to use wireless LANs

• When you have a highly portable or mobile
  workforce. Portable means users who change
  location regularly whereas mobile means users who
  are moving as they work. E.g. people using PDAs
  to manage stock in a warehouse or shop.
• Wireless LANs are also ideal in situations where
  there are multiple buildings where laying cable
  between the buildings is not an option, for
  example buildings either side of a public road.

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When to use wireless LANs

• Outlying buildings represent another situation
  where wireless LANs are extremely helpful.
• Within a building, wireless LANs are extremely
  useful in cases such as warehouses, hangars, shop
  floors, open plan offices, and general offices
  where the convenience of wireless connectivity is
  seen as helpful.
• Offices in listed buildings, leased temporary
  space, or offices with extremely thick walls are
  other scenarios where wireless LANs can be used.

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Characteristics of wireless LAN communication

• In the case of both intra- and inter-building
  wireless LANs, the data rate is 11Mbps.
• This figure depends on distance from the access
  point or bridge.
• Greater distances are possible at lower data
  rates.
• In the case of non-point-to-point links, the
  medium is shared between all of the nodes who
  wish to access the network.

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Characteristics of wireless LAN communication

• This shared nature applies equally in the case of
  inter-building links.
• If two access points with overlapping coverage
  cells are using the same frequency band they will
  contend with each other for access to the
  bandwidth.
• This situation can be avoided by using a well
  designed cellular architecture.

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Characteristics of wireless LAN communication

• Wireless LANs use CSMA/CA (carrier sense,
  multiple access with collision avoidance) as
  opposed to CSMA/CD (carrier sense, multiple
  access with collision detection) which is used by
  Ethernet.
• There is a protocol overhead associated with
  CSMA/CA but this is offset by the fact that no
  bandwidth is wasted on collisions.

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Characteristics of wireless LAN communication

• On a congested shared Ethernet segment, the
  expected throughput is about 30 whereas on a
  congested shared wireless segment, the throughput
  can be as much as 50.
• This figure will vary from vendor to vendor.

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Vendors providing wireless solutions

• Enterasys
• Lucent
• 3COM
• Cisco Systems
• Colubris
• SpectraLink
• BreezeCOM
• Intersil

• WRQ
• NDC
• Elan
• Symbol Systems
• Intermic

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Management

• As with most network components, there are four
  different ways to manage the wireless LAN access
  points and bridges via a console port, telnet ,
  a built in web server and SNMP.
• Some access points support propagation of
  configuration and firmware upgrades through the
  network.
• Centralised administration, e.g. centralised
  authentication source (RADIUS), central MAC
  address filtering database, etc. is missing from
  most product lines. (except Aironet 350 series)

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Integrated voice and video over wireless LAN

• Voice over IP will work over a wireless network,
  if your existing network infrastructure supports
  VoIP functionality.
• SpectraLink systems produce a 802.11 wireless
  integrated telephone and data system.