Wireless and Mobile NetworksWireless and Mobile Networks

Wireless RulesWireless 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/FadingSignal 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 degradationWireless 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 ProblemHidden Terminal and Multipath Problem

Workarounds 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 DesignEffects of Antenna Design

Access Point

Antenna Gain Antenna

Access Point

Access Point

Directional Antenna

StandardsStandards

• 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 Architecture802.11 Architecture

• Base Stations/Access Points– Infrastructure

• Becomes part of an existing network

– Ad Hoc• No “outside world” access or connection

802.11 Channelization802.11 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 802.11bFrequency Chart for 802.11b

802.15 and Bluetooth802.15 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.

MobilityMobility

• 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 routingMobile routing

• Indirect– Routes back to home network then to foreign

network

• Direct– Routing changes provide a path directly to the

mobile node.

Outline 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 & MobilityInternet 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?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.)?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 SolutionIntuitive 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 BasicsMobile 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

Reverse path?Reverse path?

• Same as in the post-office analogy

• Packets originating from the mobile host go directly to the static corresponding host …

HA

SH MH

FA

MH• Hence the nametriangular routing

Mobile IP EntitiesMobile IP Entities

• Mobile host

• Corresponding host

• Home address

• Care-of address

• Home agent

• Foreign agent

Mobile IP in detail …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 addressDiscovering 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 advertisementsAgent 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 AddressRegistering 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 AuthenticationRegistration 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)

IllustrationIllustration

Foreign Agent Security?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 discoveryHome 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 Tunneling to the Care-of addressaddress

• 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.)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)

IllustrationIllustration

RecapRecap

• 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 accessCellular 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 NetworksWireless Networks

Remember Homework 3 on the Web Site