Wireless'and'Mobile'Networks'Reading:'Sec7ons'2.8'and'4.2.5''COS'461:'Computer'Networks'Spring'2009'(MW'1:30‐2:50'in'COS'105)'Mike'Freedman'Teaching'Assistants:'WyaO'Lloyd'and'Jeff'Terrace'hOp://www.cs.princeton.edu/courses/archive/spring09/cos461/'1!Goals'of'Today’s'Lecture'• Wireless'links:'unique'channel'characteris7cs'– High,'7me‐varying'bit‐error'rate'– Broadcast'where'some'nodes'can’t'hear'each'other'• Mobile'hosts:'addressing'and'rou7ng'challenges'– Keeping'track'of'host’s'changing'aOachment'point'– Maintaining'a'data'transfer'as'the'host'moves'• Some'specific'examples'– Wireless:'802.11'wireless'LAN'(aka'“WiFi”)'– Mobility:'Boeing'Connexion'and'Mobile'IP'2!( Many slides adapted from Jim Kuroseʼs lectures at UMass-Amherst )#Widespread'Deployment'• Worldwide'cellular'subscribers'– 1993:''34'million'– 2005:''more'than'2'billion'– 2009:''more'than'4'billion'' ' ' '>'landline'subscribers'• Wireless'local'area'networks'– Wireless'adapters'built'in'to'most'laptops,'and'even'PDAs'– More'than'220,000'known'WiFi'loca7ons'in'134'countries'– Probably'many,'many'more'(e.g.,'home'networks,'corporate'networks,'…)'3!Wireless'Links'and'Wireless'Networks'4!Wireless'Links:'High'Bit'Error'Rate'• Decreasing'signal'strength'– Disperses'as'it'travels'greater'distance'– AOenuates'as'it'passes'through'maOer'5!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'– Electromagne7c'noise'(e.g.,'microwave'oven)'6!Wireless'Links:'High'Bit'Error'Rate'• Mul7‐path'propaga7on'– Electromagne7c'waves'reflect'off'objects'– Taking'many'paths'of'different'lengths'– Causing'blurring'of'signal'at'the'receiver'7!receiver!transmitter!Dealing'With'Bit'Errors'• Wireless'vs.'wired'links'– Wired:'most'loss'is'due'to'conges7on'– Wireless:'higher,'7me‐varying'bit‐error'ate'• Dealing'with'high'bit‐error'rates'– Sender'could'increase'transmission'power'• Requires'more'energy'(bad'for'baOery‐powered'hosts)'• Creates'more'interference'with'other'senders'– Stronger'error'detec7on'and'recovery'• More'powerful'error'detec7on'codes'• Link‐layer'retransmission'of'corrupted'frames'8!Wireless'Links:'Broadcast'Limita7ons'• Wired'broadcast'links'– E.g.,'Ethernet'bridging,'in'wired'LANs'– All'nodes'receive'transmissions'from'all'other'nodes'• Wireless'broadcast:'hidden'terminal'problem'9!A B C • !!A!and!B!hear!each!other!• !!B!and!C!hear!each!other!• !!But,!A!and!C!do!not!So,!A!and!C!are!unaware!of!their!interference!at!B!Wireless'Links:'Broadcast'Limita7ons'• Wired'broadcast'links'– E.g.,'Ethernet'bridging,'in'wired'LANs'– All'nodes'receive'transmissions'from'all'other'nodes'• Wireless'broadcast:'fading'over'distance'10!A B C A’s signal strength space C’s signal strength • !!A!and!B!hear!each!other!• !!B!and!C!hear!each!other!• !!But,!A!and!C!do!not!So,!A!and!C!are!unaware!of!their!interference!at!B!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)'• 802.11a'and'g'point‐to‐point:'54'Mbps'• WiMax:'5‐11'Mbps'• Cellular'networks,'outdoors'– 3G'enhanced:'4'Mbps'– 3G:'384'Kbps'– 2G:'56'Kbps'11!Wireless'Network:'Wireless'Link'12!network infrastructure Wireless link • Typically used to connect mobile(s) to base station • Also used as backbone link • Multiple access protocol coordinates link accessWireless'Network:'Wireless'Hosts'13!network infrastructure Wireless host • Laptop, PDA, IP phone • Run applications • May be stationary (non-mobile) or mobileWireless'Network:'Base'Sta7on'14!network infrastructure Base station • Typically connected to wired network • Relay responsible for sending packets between wired network and wireless host(s) in its “area” • E.g., cell towers, 802.11 access pointsWireless'Network:'Infrastructure'15!network infrastructure Network infrastructure • Larger network with which a wireless host wants to communicate • Typically a wired network • Provides traditional network services • May not always existScenario'#1:'Infrastructure'Mode'16!network infrastructure Infrastructure mode • Base station connects mobiles into wired network • Network provides services (addressing, routing, DNS) • Handoff: mobile changes base station providing connection to wired network17!Ad hoc mode • No base stations • Nodes can only transmit to other nodes within link coverage • Nodes self-organize and route among themselves Scenario'#2:'Ad‐Hoc'Networks'Infrastructure'vs.'Ad'Hoc'• Infrastructure'mode'– Wireless'hosts'are'associated'with'a'base'sta7on'– Tradi7onal'services'provided'by'the'connected'network'– E.g.,'address'assignment,'rou7ng,'and'DNS'resolu7on'• 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'communica7on'– Peer‐to‐peer'communica7on'18!Different'Types'of'Wireless'Networks'Infrastructure-based Infrastructure-less Single-hop Base station connected to larger wired network (e.g., WiFi wireless LAN, and cellular telephony networks) No wired network; one node coordinates the transmissions of the others (e.g., Bluetooth, and ad hoc 802.11) Multi-hop Base station exists, but some nodes must relay through other nodes (e.g., wireless sensor networks, and wireless mesh networks) No base station exists, and some nodes must relay through others (e.g., mobile ad hoc networks, like vehicular ad hoc networks) 19!WiFi:'802.11'Wireless'LANs'20!802.11'LAN'Architecture'21!• Access Point (AP) – Base station that communicates with the wireless hosts • Basic Service Set (BSS) – Coverage of one AP – AP acts as the master – Identified by an “network name” known as an SSID BSS 1 BSS 2 Internet hub, switch or router AP AP SSID: Service Set Identifier!Channels'and'Associa7on'•
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