Topics in Wireless and Mobile NetworkingWired Carrier Sense Multiple Access (CSMA)Wired Collision Detect (CD)Wireless CSMAIssue 1: Wireless Collision DetectIssue 2: The Hidden Terminal ProblemIssue 3: The Exposed Terminal ProblemPartial Solution: Virtual Carrier SenseVirtual Carrier Sense – 2Virtual Carrier Sense - 3IEEE 802.11 Usage Model802.11 Carrier SensingUse of RTS and CTSMultirate Support in 802.11802.11 “Standards”Check Your UnderstandingMobility Support in the Network LayerWhy Not Change Addresses?Location RegistryIETF Mobile IP RequirementsThe Mobile NodeThe Home AgentThe Foreign AgentThe Care-of AddressAgent Discovery ProtocolRegistrationRegistration Through a Foreign AgentTunnelingThe Need for AuthenicationCheck Your UnderstandingCheck Your UnderstandingMobile IP and Cellular TelephonyIS-41 Protocol (GSM)1Topics in Wireless and Mobile NetworkingDavid A. MaltzCarnegie Mellon [email protected] 11/3/0423Wired Carrier Sense Multiple Access (CSMA)How to share a common channel?• Listen for carrier before transmitting• Carrier is just energy from another transmission• While you hear carrier, wait before transmitting4Wired Collision Detect (CD)• Listen while transmitting• If what you hear isn’t what you’re sending, then collision:– Abort transmission of current packet– Try again after a random delay– Each collision for same packet doubles average delay5Wireless CSMACSMA can be used in wireless, but has problems• wired network: signal strength at sender and receiver are essentially the same• wireless network: inverse square law (or worse) applies (Precv= Pxmit/Dk, k > 2)CSMA does not give the right information in wireless:• Carrier sense detects signals at the transmitter• But collisions occur at the receiverDistancePower6Issue 1: Wireless Collision DetectWireless can’t do collision detect like EthernetCan’t effectively listen while you send:• In some systems, the hardware isn’t flexible enough:– Transmit and receive are on different frequencies– Transceiver might be half-duplex• In any case, all you could hear is yourself any way:– The inverse square law– Your own signal strength at your own antenna is much stronger than anybody else’s signalPowerDistance7Issue 2: The Hidden Terminal ProblemConsider the following situation:• A is sending to B•C is out of range of A’s transmissions to B• C wants to send (to anybody)CSMA doesn’t work well for wireless here:• C can’t know to wait since it can’t hear carrier from A• B can hear both A and C, thus collision at B• A is “hidden” to C8Issue 3: The Exposed Terminal ProblemConsider the following situation:• B is sending to A•C is in range of B’s transmissions to A• C wants to send to anybody but BCSMA doesn’t work well for wireless here either:• C thinks it should wait since it can hear carrier from B• If A is out of range of C, then C waits needlessly• C is “exposed” to B9Partial Solution: Virtual Carrier SensePacket types:• Request-to-Send (RTS): Sender sends to receiver before sending a data packet• Clear-to-Send (CTS): Receiver replies if ready for data packet to be sent• Acknowledgment (ACK): receiver sends if data is received successfullyAll packets contain:• Address of the sender of the intended data packet• Address of the receiver of the intended data packet• Duration of the remainder of the transmission10Virtual Carrier Sense – 211Virtual Carrier Sense - 3• Hidden terminal problem is avoided:C waits to send since it hears B’s CTS• Exposed terminal problem is avoided:C does not wait to send since it does not hear A’s CTSDoes (and cannot) not prevent all collisions!12IEEE 802.11 Usage ModelHost computer sees an “ethernet interface”• Just like a wired LAN• Uses 48-bit 802.3 MAC addresses• All hosts “in range” of each other see common shared channel• Supports ARP, broadcast, LAN multicast• Can directly communicate with neighbors13802.11 Carrier Sensing802.11 uses both physical and virtual carrier sensing:• Physical carrier sense provided by PHY• Virtual carrier sense provided by MACVirtual carrier sensing:• Maintained by station through Network Allocation Vector (NAV)• NAV records prediction of future traffic on medium• Counter that counts down busy time at uniform rate• Set based on Duration field in received packets (e.g., RTS, CTS)• When nonzero, virtual carrier sense thinks medium is busyCarrier sense mechanism combines both mechanisms:• Medium considered busy whenever either indicates carrier• Medium also considered busy whenever our own transmitter is on14Use of RTS and CTSOther data senders must wait until entire RTS/CTS/Data/ACK finishedRTS/CTS only used for data packets larger than some threshold --- You can tune this!Multirate Support in 802.11PLCP hdr MAC PDU DataPLCP preamble144 bits 48 bitsTo enable sharing the media among many nodes:• All control information must be transmitted at rate understood by all stations • After control information, transceivers change to rate agreed on by sender and receiver• Preamble and header sent at lowest coding rate– 1 Mbps in .11b/g– 6 Mbps in .11a 1516802.11 “Standards”802.11b• In theory: 1,2,5.5,11 Mbps• Reality: 5-6 Mbps802.11a• In theory: 54 Mbps• Reality: 20-24 Mbps802.11g• Specification: 54Mbps, Claims: 108 Mbps• Reality: 20-70Mbps 1 in 4 new devices fails compliance testing –they’re probably marketed anywayhttp://news.com.com/2100-7351-5139499.html17Check Your Understanding18Mobility Support in the Network Layer19Why Not Change Addresses?• Must notify all hosts with open connections– Used to identify endpoints of connections– Used within some transport protocols• Must also notify all hosts using connectionless protocols• Cannot change hosts with “well known”addresses• Name server must be updated:– Caching of addresses used for scalabilty– Too expensive to update quickly20Location Registry21IETF Mobile IP Requirements22The Mobile Node23The Home Agent24The Foreign Agent25The Care-of Address26Agent Discovery Protocol27Registration28Registration Through a Foreign Agent29TunnelingThis use of encapsulation known as tunnelingPath from encapsulator to decapsulator known as tunnelIntermediate routers need not know about Mobile IP!30The Need for Authenication31Check Your UnderstandingWhen a Mobile Node sends packets, what source IP address should it use?
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