Transcript Wireless and Mobile Networks

Wireless and Mobile Networks
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)
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.
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.
Hidden Terminal and Multipath Problem
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.
Effects of Antenna Design
Antenna
Gain Antenna
Access Point
Access Point
Directional Antenna
Access Point
Standards
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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)
802.11 Architecture
• Base Stations/Access Points
– Infrastructure
• Becomes part of an existing network
– Ad Hoc
• No “outside world” access or connection
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.
Frequency Chart for 802.11b
802.15 and Bluetooth
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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.
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.
Mobile routing
• Indirect
– Routes back to home network then to foreign
network
• Direct
– Routing changes provide a path directly to the
mobile node.
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?
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 …
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
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
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
Mobile IP Basics
• Same as the post-office analogy
• Two other entities – home agent (old postoffice), 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
Reverse path?
• Same as in the post-office analogy
• Packets originating from the mobile host
go directly to the static corresponding host
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HA
SH
• Hence the name
triangular routing
MH
MH
FA
Mobile IP Entities
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Mobile host
Corresponding host
Home address
Care-of address
Home agent
Foreign agent
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
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
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
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?
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)
Illustration
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
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
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
Tunneling (contd.)
• Destination address set to the care-of address
• Source address set to the home agent’s
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)
Illustration
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 …
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)
Wireless Networks
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