15-441: Computer NetworkingWireless NetworkingOutline• Wireless Challenges• 802.11 Overview• Link Layer• Ad-hoc Networks15-44104-24-082Assumptions made in Internet• Host are (mostly) stationary• Address assignment, routing• Links in the network are fairly homogeneous• Transport protocols, applications• Hosts are fairly powerful• End to end principle: push functionality to end points• Security is an end host issue• No security inside the network (architecturally)15-44104-24-083Mobility• Many clients today are mobile• Mobility inside a subnet is supported• E.g. moving across APs that are part of a single EBSS• Mobility across subnets is harder because the IP address is used as address and identifier• Identifier: who you are• Address: where you can be found• Keep IP address: network gets confused• Delivers packets to wrong “old” subnet• New IP address: host gets confused• Transport protocols, applications, etc.15-44104-24-084Link Heterogeneity• Original links were basically telephone lines• Today: huge diversity• Optical fiber (wavelengths) … copper … wireless• Need to share airwaves rather than wire• Don’t know what hosts are involved• Host may not be using same link technology• Wireless is generally slow• Error characteristics: higher on wireless - attenuation, interference, multipath• Latency: absolute delay and variance in delay• Mobility adds to diversity• E.g. hand off can cause delays and sudden changes in available bandwidth15-44104-24-085Device Capabilities• Originally: mainframes and personal computers• Today: sensors … supercomputers• Note: almost any networked device today is more capable than early computers!• But our requirements and expectations have increased• Anything at or above “PC class” is very capable• Laptop: view as mobile PC• PDA: view as 5 year old PC (kind of)• Cell phone: not even close to a PC• Sensors: often run as private networks15-44104-24-086Security• Access to the network enables all kinds of attacks• Argues for pushing security inside the network• Firewalls are a very ad hoc way of doing this• Wireless creates unique challenges• Do not need physical connection to sniff or send• WEP, 802.1x, etc.• But wireless security needs to be linked into system wide security15-44104-24-087Overview• Wireless Challenges• 802.11 Overview• Link Layer Challenges• Ad-hoc Networks15-44104-24-088IEEE 802.11 Wireless LAN• 802.11b• 2.4-2.5 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• 2.4-2.5 GHz range• up to 54 Mbps• All use CSMA/CA for multiple access• All have base-station and ad-hoc network versions15-44104-24-089IEEE 802.11 Wireless LAN• IEEE 802.11a• PHY Standard : 8 channels : up to 54 Mbps : some deployment• IEEE 802.11b• PHY Standard : 3 channels : up to 11 Mbps : widely deployed.• IEEE 802.11d• MAC Standard : support for multiple regulatory domains (countries)• IEEE 802.11e• MAC Standard : QoS support : supported by many vendors• IEEE 802.11f• Inter-Access Point Protocol : deployed• IEEE 802.11g• PHY Standard: 3 channels : OFDM and PBCC : widely deployed (as b/g)• IEEE 802.11h• Suppl. MAC Standard: spectrum managed 802.11a (TPC, DFS): standard• IEEE 802.11i• Suppl. MAC Standard: Alternative WEP : standard• IEEE 802.11n• MAC Standard: MIMO : standardization expected late 200815-44104-24-0810IEEE 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)15-44104-24-0811• Ad hoc network: IEEE 802.11 stations can dynamically form network without AP• Applications:• Vehicles exchange information (VANET)• Laptops meeting in conference room, car• Interconnection of “personal” devicesAd Hoc Networks15-44104-24-0812Overview• Wireless Challenges• 802.11 Overview• Link Layer Challenges• Ad-hoc Networks15-44104-24-0813CSMA/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 Exposed15-44104-24-0814IEEE 802.11 MAC Protocol: CSMA/CA802.11 CSMA: sender- If sense channel idle for DISF(Distributed Inter Frame Space)then transmit entire frame(no collision detection)- If sense channel busythen binary backoff802.11 CSMA receiver:- If received OKreturn ACK after SIFS(Short IFS)(ACK is needed due tolack of collision detection)15-44104-24-0816IEEE 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 units15-44104-24-0817Collision 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)15-44104-24-0818Collision 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 collisions• Not widely used/implemented• Consider typical traffic patterns15-44104-24-0819Overview• Wireless Challenges• 802.11 Overview• Link Layer Challenges• Ad-hoc Networks15-44104-24-0820Ad Hoc Networks• All the challenges of wireless, plus some of:• No fixed infrastructure• Mobility (on short time scales)• Chaotically decentralized (:-)• Multi-hop!• Nodes are both traffic sources/sinks and forwarders• The big challenge: Routing15-44104-24-0821Ad Hoc Routing• Find multi-hop paths through network• Adapt to new routes and movement / environment changes• Deal with interference and power issues• Scale well with # of nodes• Localize effects of link changes15-44104-24-0822Traditional Routing
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