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Goals of Today’s Lecture
• Wireless links: unique channel characteristics
– High, time-varying bit-error rate
– Broadcast where some nodes can’t hear each other
• Mobile hosts: addressing and routing challenges
– Keeping track of the host’s changing attachment point
– Maintaining a data transfer as the host moves
– Two specific technologies
– Wireless: 802.11 wireless LAN (aka “WiFi”)
– Mobility: Mobile IP
– Wireless Links and Wireless Networks
Wireless Links: High Bit Error Rate
• Decreasing signal strength
– Disperses as it travels greater distance
– Attenuates as it passes through matter
Wireless Links: High Bit Error Rate
• Interference from other sources
– Radio sources in same frequency band
– E.g., 2.4 GHz wireless phone interferes with 802.11b wireless LAN
– Electromagnetic noise (e.g., microwave oven)
Wireless Links: High Bit Error Rate
• Multi-path propagation
– Electromagnetic waves reflect off objects
– Taking many paths of different lengths
– Causing blurring of signal at the receiver
Dealing With Bit Errors
• Wireless vs. wired links
– Wired: most loss is due to congestion
– Wireless: higher, time-varying bit-error ate
• Dealing with high bit-error rates
– Sender could increase transmission power
 Requires more energy (bad for battery-powered hosts)
 Creates more interference with other senders
– Stronger error detection and recovery
 More powerful error detection codes
 Link-layer retransmission of corrupted frames
Wireless Links: Broadcast Limitations
• Wired broadcast links
– E.g., Ethernet bridging, in wired LANs
– All nodes receive transmissions from all other nodes
• Wireless broadcast: hidden terminal problem
Wireless Links: Broadcast Limitations
• Wired broadcast links
– E.g., Ethernet bridging, in wired LANs
– All nodes receive transmissions from all other nodes
Wireless broadcast: fading over distance
• Example Wireless Link Technologies
• Data networks
– Indoor (10-30 meters)
 802.11n: 200 Mbps
 802.11a and g: 54 Mbps
 802.11b: 5-11 Mbps
 802.15.1: 1 Mbps
– Outdoor (50 meters to 20 kmeters)
 802.11 and g point-to-point: 54 Mbps
 WiMax: 5-11 Mbps
• Cellular networks, outdoors
– 3G enhanced: 4 Mbps
– 3G: 384 Kbps
– 2G: 56 Kbps
Wireless Network: Wireless Link
• Wireless Network: Wireless Hosts
• Wireless Network: Base Station
• Wireless Network: Infrastructure
• Scenario #1: Infrastructure Mode
• Scenario #2: Ad Hoc Networks
Infrastructure vs. Ad Hoc
• Infrastructure mode
– Wireless hosts are associated with a base station
– Traditional services provided by the connected network
– E.g., address assignment, routing, and DNS resolution
• Ad hoc networks
– Wireless hosts have no infrastructure to connect to
– Hosts themselves must provide network services
– Similar in spirit to the difference between
– Client-server communication
– Peer-to-peer communication
Different Types of Wireless Networks
• WiFi: 802.11 Wireless LANs
• 802.11 LAN Architecture
Channels and Association
• Multiple channels at different frequencies
– Network administrator chooses frequency for AP
– Interference if channel is same as neighboring AP
• Access points send periodic beacon frames
– Containing AP’s name (SSID) and MAC address
– Host scans channels, listening for beacon frames
– Host selects an access point to associate with
Mobility Within the Same Subnet
• H1 remains in same IP subnet
– IP address of the host can remain same
– Ongoing data transfers can continue uninterrupted
• H1 recognizes the need to change
– H1 detects a weakening signal
– Starts scanning for stronger one
• Changes APs with same SSID
– H1 disassociates from one
– And associates with other
• Switch learns new location
– Self-learning mechanism
CSMA: Carrier Sense, Multiple Access
• Multiple access: channel is shared medium
– Station: wireless host or access point
– Multiple stations may want to transmit at same time
• Carrier sense: sense channel before sending
– Station doesn’t send when channel is busy
– To prevent collisions with ongoing transfers
– But, detecting ongoing transfers isn’t always possible
CA: Collision Avoidance, Not Detection
• Collision detection in wired Ethernet
– Station listens while transmitting
– Detects collision with other transmission
– Aborts transmission and tries sending again
• Problem #1: cannot detect all collisions
– Hidden terminal problem
– Fading
• Problem #2: listening while sending
– Strength of received signal is much smaller
– Expensive to build hardware that detects collisions
• So, 802.11 does not do collision detection
Medium Access Control in 802.11
• Collision avoidance, not detection
– Once a station starts transmitting, send in its entirety
– More aggressive collision-avoidance techniques
– E.g., waiting a little after sensing an idle channel
– To reduce likelihood two stations transmit at once
• Link-layer acknowledgment and retransmission
– CRC to detect errors
– Receiving station sends an acknowledgment
– Sending station retransmits if no ACK is received
– Giving up after a few failed transmissions
Host Mobility
• Varying Degrees of User Mobility
• Moves only within same access network
– Single access point: mobility is irrelevant
– Multiple access points: only link-link layer changes
– Either way, users is not mobile at the network layer
• Shuts down between changes access networks
– Host gets new IP address at the new access network
– No need to support any ongoing transfers
– Applications have become good at supporting this
• Maintains connections while changing networks
– Surfing the ‘net while driving in a car or flying a plane
– Need to ensure traffic continues to reach the host
Maintaining Ongoing Transfers
• Seamless transmission to a mobile host
• E.g., Keeping Track of Mobile Friends
• Sending a letter to a friend who moves often
– How do you know where to reach him?
Option #1: have him update you
– Friend contacts you on each move
– So you can mail him directly
Option #2: ask his parents when needed
– Parents serve as “permanent address”
– They can forward your letter to him
– Or, they can update you