•115-441: Computer NetworkingLecture 23: Mobile and Wireless NetworkingLecture 23: 11-18-02 2Wireless Challenges• Force us to rethink many assumptions• Need to share airwaves rather than wire• Don’t know what hosts are involved• Host may not be using same link technology• Mobility• Other characteristics of wireless• Noisy à lots of losses• Slow• Interaction of multiple transmitters at receiver• Collisions, capture, interference• Multipath interferenceLecture 23: 11-18-02 3Overview• Link layer challenges• Internet mobility• TCP Over Noisy LinksLecture 23: 11-18-02 4Cellular Reuse• Transmissions decay over distance• Spectrum can be reused in different areas• Different “LANs”• Decay is 1/R2in free space, 1/R4in some situations•2Lecture 23: 11-18-02 5IEEE 802.11 Wireless LAN• 802.11b• 2.4-5 GHz unlicensed radio spectrum• up to 11 Mbps• direct sequence spread spectrum (DSSS) in physical layer• all hosts use same chipping code• widely deployed, using base stations• 802.11a• 5-6 GHz range• up to 54 Mbps• 802.11g• 2.4-5 GHz range• up to 54 Mbps• All use CSMA/CA for multiple access• All have base-station and ad-hoc network versionsLecture 23: 11-18-02 6IEEE 802.11 Wireless LAN• Wireless host communicates with a base station• Base station = access point (AP)• Basic Service Set (BSS) (a.k.a. “cell”) contains:• Wireless hosts• Access point (AP): base station• BSS’s combined to form distribution system (DS)Lecture 23: 11-18-02 7• Ad hoc network: IEEE 802.11 stations can dynamically form network without AP• Applications:• Laptops meeting in conference room, car• Interconnection of “personal” devicesAd Hoc NetworksLecture 23: 11-18-02 8CSMA/CD Does Not Work• Collision detection problems• Relevant contention at the receiver, not sender• Hidden terminal• Exposed terminal• Hard to build a radio that can transmit and receive at same timeABCABCDHidden Exposed•3Lecture 23: 11-18-02 9Hidden Terminal Effect• Hidden terminals: A, C cannot hear each other• Obstacles, signal attenuation• Collisions at B • Collision if 2 or more nodes transmit at same time• CSMA makes sense:• Get all the bandwidth if you’re the only one transmitting• Shouldn’t cause a collision if you sense another transmission• Collision detection doesn’t work• CSMA/CA: CSMA with Collision AvoidanceLecture 23: 11-18-02 10IEEE 802.11 MAC Protocol: CSMA/CA802.11 CSMA: sender- If sense channel idle for DISF(Distribute Inter Frame Space)then transmit entire frame(no collision detection)- If sense channel busythenbinary backoff802.11 CSMA receiver:- If received OKreturn ACK after SIFS(Short IFS)(ACK is needed due tohidden terminal problem)Lecture 23: 11-18-02 11IEEE 802.11 MAC Protocol802.11 CSMA Protocol: others• NAV: Network Allocation Vector• 802.11 frame has transmission time field• Others (hearing data) defer access for NAV time unitsLecture 23: 11-18-02 12Collision Avoidance Mechanisms• Problem:• Two nodes, hidden from each other, transmit complete frames to base station• Wasted bandwidth for long duration !• Solution: • Small reservation packets• Nodes track reservation interval with internal “network allocation vector” (NAV)•4Lecture 23: 11-18-02 13Collision Avoidance: RTS-CTS Exchange• Explicit channel reservation• Sender: send short RTS: request to send• Receiver: reply with short CTS: clear to send• CTS reserves channel for sender, notifying (possibly hidden) stations• RTS and CTS short:• collisions less likely, of shorter duration• end result similar to collision detection• Avoid hidden station collisionsLecture 23: 11-18-02 14Overview• Link layer challenges• Internet mobility• TCP Over Noisy LinksLecture 23: 11-18-02 15Routing to Mobile Nodes• Obvious solution: have mobile nodes advertise route to mobile address/32• Should work!!!• Why is this bad?• Consider forwarding tables on backbone routers• Would have an entry for each mobile host• Not very scalableLecture 23: 11-18-02 16How to Handle Mobile Nodes?(Addressing)• Dynamic Host Configuration (DHCP)• Host gets new IP address in new locations• Problems• Host does not have constant name/address à how do others contact host• What happens to active transport connections?•5Lecture 23: 11-18-02 17How to Handle Mobile Nodes?(Naming)• Naming• Use DHCP and update name-address mapping whenever host changes address• Fixes contact problem but not broken transport connectionsLecture 23: 11-18-02 18How to Handle Mobile Nodes? (Transport) • TCP currently uses 4 tuple to describe connection• <Src Addr, Src port, Dst addr, Dst port> • Modify TCP to allow peer’s address to be changed during connection• Security issues• Can someone easily hijack connection?• Difficult deployment à both ends must support mobilityLecture 23: 11-18-02 19How to Handle Mobile Nodes?(Link Layer)• Link layer mobility• Learning bridges can handle mobility à this is how it is handled at CMU• Encapsulated PPP (PPTP) à Have mobile host act like he is connected to original LAN• Works for IP AND other network protocolsLecture 23: 11-18-02 20How to Handle Mobile Nodes?(Routing 1)• Multicast• Solves similar problem à how to route packets to different sets of hosts at different times• Can’t we just reuse same solutions?• Don’t really have solution for multicast either!•6Lecture 23: 11-18-02 21How to Handle Mobile Nodes?(Routing 2)• Allow mobile node to keep same address and name• How do we deliver IP packets when the endpoint moves?• Can’t just have nodes advertise route to their address• What about packets from the mobile host?• Routing not a problem• What source address on packet?• Key design considerations• Scale• Incremental deploymentLecture 23: 11-18-02 22Basic Solution to Mobile Routing • Same as other problems in Computer Science• Add a level of indirection• Keep some part of the network informed about current location• Need technique to route packets through this location (interception)• Need to forward packets from this location to mobile host (delivery)Lecture 23: 11-18-02 23Interception• Somewhere along normal forwarding path• At source• Any router along path• Router to home network• Machine on home network (masquerading as mobile host)• Clever tricks to force packet to particular destination• “Mobile subnet” –
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