Overview : Computer Networking. IEEE MAC Protocol: CSMA/CA Internet mobility TCP over noisy links

Transcription

1 Overview : Computer Networking Lecture 24: Wireless Eric Anerson Fall Internet mobility TCP over noisy links Link layer challenges an WiFi Cellular 2 IEEE MAC Protocol: CSMA/CA But CSMA Does Not (Always) Work CSMA: sener - If sense channel ile for DIFS (Distribute Inter Frame Space) then transmit entire frame (no collision etection) - If sense channel busy then binary backoff CSMA receiver: - If receive OK return ACK after SIFS (Short IFS) - ACK is neee ue to lack of collision etection - SIFS < DIFS: priority 3 Carrier Sense problems Relevant contention at the receiver, not sener Hien terminal Expose terminal Hien A B C Expose A B C D 4 1

2 Collision Avoiance Mechanisms Problem: Two noes, hien from each other, transmit complete frames to base station Waste banwith for long uration! Solution: Small reservation packets Noes track reservation interval with internal network allocation vector (NAV) Collision Avoiance: RTS-CTS Exchange Explicit channel reservation Sener: sen short RTS: request to sen Receiver: reply with short CTS: clear to sen CTS reserves channel for sener, notifying (possibly hien) stations RTS an CTS short: collisions less likely, of shorter uration en result similar to collision etection Avoi hien station collisions Not wiely use Overhea is too high Not a serious problem in typical eployments 6 8 IEEE MAC Protocol How About Expose Terminal? RTS/CTS implemente using NAV: Network Allocation Vector Also use with ata packets frame has transmission time fiel Others (hearing ata) efer access for NAV time units Can increase the carriersense threshol Signal nees to be stronger before noe efers Coul this create other problems? Expose terminals are ifficult to eal with Even har to etect them! Expose A B C D

3 Some IEEE Stanars IEEE Family IEEE a PHY Stanar : 8 channels : up to 54 Mbps : some eployment IEEE b PHY Stanar : 3 channels : up to 11 Mbps : wiely eploye. IEEE MAC Stanar : support for multiple regulatory omains (countries) IEEE e MAC Stanar : QoS support : supporte by many venors IEEE f Inter-Access Point Protocol : eploye IEEE g PHY Stanar: 3 channels : OFDM an PBCC : wiely eploye (as b/g) IEEE h Suppl. MAC Stanar: spectrum manage a (TPC, DFS): stanar IEEE i Suppl. MAC Stanar: Alternative WEP : stanar IEEE n MAC Stanar: MIMO : stanarization expecte late 2008 Protocol Release Data Freq. Rate (typical) Rate (max) Legacy GHz 1 Mbps 2Mbps? a GHz 25 Mbps 54 Mbps b GHz 6.5 Mbps 11 Mbps g GHz 25 Mbps 54 Mbps n /5 GHz 200 Mbps 600 Mbps Range (inoor) ~30 m ~30 m ~30 m ~50 m b Channels Going Faster: g In the UK an most of EU: 13 channels, 5MHz apart, GHz In the US: only 11 channels Each channel is 22MHz Significant overlap Non-overlapping channels are 1, 6 an 11 1, 2, 5.5 an 11 Mbps rates using DSSS technology g basically extens of b Use the same technology DSSS for ol rates (1,2, 5.5, 11) Uses OFDM technology for new rates (6 Mbs an up) Using OFDM makes it easier to buil a/g cars Since a uses OFDM But it creates an interoperability problem since b cars cannot interpret OFDM signals Solutions: sen CTS using CCK before OFDM packets in hybri environments, or use (optional) hybri packet format CCK Preamble Heaer CCK OFDM CCK Payloa CCK OFDM OFDM 3

4 802.11a Discussion Uses OFDM in the 5.2 an 5.7 GHz bans What are the benefits of a compare with b? Greater banwith (up to 54Mb) 54, 48, 36, 24, 18, 12, 9 an 6 Mbs Less potential interference (5GHz) More non-overlapping channels Less contention ue to competition But oes not provie interoperability with b, as g oes Beyon CSMA: Scheule Access What s wrong with ranom access? Power Save Moe Power Save Moe Core iea: Client sleeps, AP buffers ata until client wakes up. Beacons inclue traffic information map (TIM) (awake) clients request their packets What about broacast / multicast?

5 Prioritize Access Backoff-base contention: Whoever goes first, wins DIFS -> AIFS =SIFS + AIFSN[AC]*st CWmax, CWmin Overview Internet mobility TCP over noisy links Link layer challenges an WiFi Cellular Cellular versus WiFi Implications WiFi Spectrum Cellular License WiFi Unlicense Spectrum WiFi Unlicense Implication No control open, iverse access Service moel MAC services Provisione for pay Fixe banwith SLAs Unprovisione free no SLA Best effort no SLAs Service moel MAC services Unprovisione free Best effort no SLAs No guarantees maximize throughput, fairness??? 5

6 Implications Cellular Overview Spectrum Service moel MAC services Cellular License Provisione for pay Fixe banwith SLAs Implication Provier has control over interference Can an must charge + make commitments TDMA, FDMA, CDMA; access control Cellular esign Frequency Reuse Capacity an Interference Elements of a cellular network How oes a mobile phone take place? Paging Hanoff Frequency Allocation Traffic Engineering The Avent of Cellular Networks Cellular Network Design Options Mobile raio telephone system was base on: High power transmitter/receivers Coul support about 25 channels in a raius of 80 Km To increase network capacity: Multiple low-power transmitters (100W or less) Small transmission raius -> area split in cells Each cell with its own frequencies an base station Aj t ll iff t f i Simplest layout Ajacent antennas not equiistant how o you hanle users at the ege of the cell? Ieal layout But we know signals travel whatever way they fell like 2 6

7 The Hexagonal Pattern Call progression A hexagon pattern can provie equiistant access to neighboring cell towers = 3R In practice, variations from ieal ue to topological reasons Signal propagation Tower placement R (a) Monitor for strongest signal (b) Request for connection Call progression Call progression (c) Paging () Call accepte (e) Ongoing call (f) Hanoff 7

8 Hanoff between 2 cells Base station A Base station B How to Increase Capacity? Aing new channels Frequency borrowing Sectoring antennas Microcells Antennas on top of builings, even lamp posts Form micro cells with reuce power Goo for city streets, roas an insie builings Cell splitting Cell sectoring Cell size ~ Km, Minimum ~ 1.5Km Requires careful power control an possibly more frequent hanoffs for mobile stations A raius reuction by a factor of reuces the coverage area an increases the require number of base stations by a factor of 2 Cell ivie into wege shape sectors 3-6 sectors per cell, each with own channel set Subset of cell s channel, use of irectional antennas 8

9 Cellular Stanars 1G systems: analog voice Not unlike a wire voice line (without the wire) 2G systems: igital voice Many stanars Example: GSM - FDMA/TDMA, most wiely eploye, 200 countries, a billion people 2.5G systems: voice an ata channels Example: GPRS - evolve from GSM, packet-switche, 170 kbps (30-70 in practice) Cellular Stanars 3G: voice (circuit-switche) an ata (packetswitche) Several stanars Uses Coe Division Multiple Access (CDMA) UMTS 4G: 10 Mbps an up, seamless mobility between ifferent cellular technologies LTE the ominating technology Packet switche 9