Wireless and Mobile Networks

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Wireless and Mobile Networks
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Wireless Rules

• 802.11 wireless used radio frequencies that are
  unlicensed.
• 802.11 is power limited to comply with Federal
  Communication Commission (FCC) rules part 15.
• 802.11 may suffer interference from cordless
  phones and microwave ovens (also unlicensed)

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Signal Strength/Multipath/Fading

• Radio signals at ultra high frequencies are
  primarily line of sight.
• Radio signal intensity decreases with the square
  of the distance (path loss)
• Radio signal strength decreases inversely
  proportional to frequency.
• Radio signals bounce off of most surfaces
• Multipath is caused when more than 1 path is used
  from sender to receiver.

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Wireless degradation

• As the signal strength at the receiver decreases,
  the data rate decreases.
• As more users attach to the AP the bandwidth is
  shared dividing the available rate among the
  users.

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Hidden Terminal and Multipath Problem
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Workarounds

• Antennas
• Concentrate the radio energy in the area needed.
• Yagi antennas
• Dish antennas
• Install antennas as high as possible
• Feedlines
• Place antenna as close as possible to the access
  point.
• Repeater
• Remote access point, receives signals and
  retransmits them.

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Effects of Antenna Design
Antenna
Gain Antenna
Access Point
Access Point
Directional Antenna
Access Point
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Standards

• 802.11a up to 54 Mbps 2.4 Ghz
• 802.11g up to 54 Mbps 5.1 GHz
• 802.11b up to 5-11 Mbps 2.4 Ghz
• CDMA 56Kbps-384Kbps Varies
• 802.11 with standard antennas may only go up to
  300 feet (no obstacles)

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802.11 Architecture

• Base Stations/Access Points
• Infrastructure
• Becomes part of an existing network
• Ad Hoc
• No outside world access or connection

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802.11 Channelization

• Radio Frequencies are separated into channels
• 802.11 uses 11 channels.
• Wireless devices scan the assigned channels to
  find activity.
• Access Points transmit beacon frames to advertise
  their availability.

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Frequency Chart for 802.11b
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802.15 and Bluetooth

• Very short range (10 meters)
• Low Power (1 milliwatt power)
• Low data rates (up to 720Kbps)
• Uses spread spectrum frequency hopping over 79
  channels.
• Ad Hoc network structure in a master/slave
  organization.

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Mobility

• Moving from one AP to another while maintaining
  the same IP address.
• Simple approach is a flat network with an open
  subnet mask (255.255.0.0).
• Wireless nodes have limited range (300 feet) so
  you may change nodes 18 times while traveling one
  mile.

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Mobile routing

• Indirect
• Routes back to home network then to foreign
  network
• Direct
• Routing changes provide a path directly to the
  mobile node.

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Outline

• What is the problem at the routing layer when
  Internet hosts move?!
• Can the problem be solved?
• What is the standard solution? mobile IP
• What are the problems with the solution?
• Other approaches?

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Internet hosts Mobility

• Wireless networking allows Internet users to
  become mobile
• As users move, they have to be handed over from
  one coverage area to another (since the coverage
  areas of access points are finite)
• Ongoing connections need to be maintained as the
  user moves

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Problems?

• What are the problems?
• The IP address associated with a mobile host is
  network dependent!
• When user connects to another network, IP address
  needs to change
• Packets belonging to ongoing connections somehow
  need to be delivered to the mobile host

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Problems (Contd.)?

• What are the options?
• Make IP address host specific instead of network
  specific obvious pitfalls?
• Change IP address of host and start using the new
  IP address in the subsequent packets belonging to
  the connections

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Intuitive Solution

• Take up the analogy of you moving from one
  apartment to another
• What do you do?
• Leave a forwarding address with your old
  post-office!
• The old post-office forwards mails to your new
  post-office, which then forwards them to you

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Mobile IP Basics

• Same as the post-office analogy
• Two other entities home agent (old
  post-office), foreign agent (new post-office)
• Mobile host registers with home agent the new
  location
• Home agent captures packets meant for mobile
  host, and forwards it to the foreign agent, which
  then delivers it to the mobile host

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Reverse path?

• Same as in the post-office analogy
• Packets originating from the mobile host go
  directly to the static corresponding host

• Hence the name
• triangular routing

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Mobile IP Entities

• Mobile host
• Corresponding host
• Home address
• Care-of address
• Home agent
• Foreign agent

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Mobile IP in detail

• Combination of 3 separable mechanisms
• Discovering the care-of address
• Registering the care-of address
• Tunneling to the care-of address

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Discovering the care-of address

• Discovery process built on top of an existing
  standard protocol router advertisement (RFC
  1256)
• Router advertisements extended to carry available
  care-of addresses called agent advertisements
• Foreign agents (and home agents) send agent
  advertisements periodically
• A mobile host can choose not to wait for an
  advertisement, and issue a solicitation message

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Agent advertisements

• Foreign agents send advertisements to advertise
  available care-of addresses
• Home agents send advertisements to make
  themselves known
• Mobile hosts can issue agent solicitations to
  actively seek information
• If mobile host has not heard from a foreign agent
  its current care-of address belongs to, it seeks
  for another care-of address

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Registering the Care-of Address

• Once mobile host receives care-of address, it
  registers it with the home agent
• A registration request is first sent to the home
  agent (through the foreign agent)
• Home agent then approves the request and sends a
  registration reply back to the mobile host
• Security?

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Registration Authentication

• Mobile IP requires the home agent and mobile
  host to share a security association
• MD5 with 128-bit keys to create digital
  signatures for registration requests to be used
  (registration message header used for creating
  signature)
• Any problems? replay attacks
• Solved by using an unique message identifier
  (timestamp or pseudorandom number)

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Illustration
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Foreign Agent Security?

• No foreign agent authentication required
• Foreign agent can potentially discard data once
  registration happens
• However, the problem is same as in
  unauthenticated route advertisements (RFC 1256)
  in the wireline context

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Home agent discovery

• If the mobile host is unable to communicate with
  the home agent, a home agent discovery message is
  used
• The message is sent as a broadcast to the home
  agents in the home network

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Tunneling to the Care-of address

• When home agent receives packets addressed to
  mobile host, it forwards packets to the care-of
  address
• How does it forward it? - encapsulation
• The default encapsulation mechanism that must
  be supported by all mobility agents using mobile
  IP is IP-within-IP (RFC 2003)
• Using IP-within-IP, home agent inserts a new IP
  header in front of the IP header of any datagram

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Tunneling (contd.)

• Destination address set to the care-of address
• Source address set to the home agents address
• Tunnel header uses 4 for higher protocol id
  this ensures that IP after stripping out the
  first header, processes the packet again
• Tunnel header of 55 used if IP minimal
  encapsulation used (RFC 2004)

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Illustration
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Recap

• Host mobility and Internet addresses
• Post-office analogy
• Home agent, foreign agent, care-of address, home
  address
• Registration and Tunneling
• IPv6 and Mobility support

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Cellular Internet access

• Text messaging
• Web Browsing
• Cell phones use FDM (frequency division
  multiplex) and TDM (time division multiplex) to
  increase utilization of scarce radio frequencies.
• MSC (mobile switching center)
• PSTN (public switched telephone network)

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Wireless Networks
Remember Homework 3 on the Web Site