2. Advanced Medium Access Control Protocols

1. 2. Advanced Medium Access ControlProtocols • 2.1 Traditional CSMA/CD • 2.2 The 802.11 MAC Protocol for Wireless LANs

2. What is a LAN? • Definition • A LAN (Local Area Network) is a network for the bit-serial transmission of data between independent peer stations. • It is usually under the legal control of a single user (single enterprise) and is usually limited to the user pemises.

3. Characteristics of a Local Area Network • High transmissionspeed(10 - 1000 MBit/s) • Easy, inexpensiveconnectionofstations • Noneedtotake Telekom rulesandregulationsintoaccount • Different typesofdevicescanbeconnectedeasily: • personal computers • high-end Unix workstations • departmentservers/fileservers/mailservers • mainframes • printersandotherperipheraldevices • Interconnection to Wide-Area Networks (WANs) ispossible via routers (layer 3).

4. …. Layer 3 Network Layer LLC Logical Link Control Layer 2b Layer 2 LAN Standard IEEE 802 MAC Layer 2a Media Access Control Physical Layer Layer 1 physical medium LANs in the ISO Reference Model

5. Medium Access Control • Problem: • broadcast medium • independent stations • => send collisions will occur • Solution: Medium Access Control • Two allocation principles: • collision detectionLet collisions take place, detect them, repeat the transmission. • collision avoidanceUse a circulating token to control the access to the medium.

6. 2.1 Traditional CSMA/CD (Ethernet) • CSMA/CD = CarrierSensingwithMultiple Access andCollisionDetection • Standard: IEEE 802.3 und ISO IS 8802/3: MAC andPhysical Layer for CSMA/CD • Topology: Bus • Bi-directionaldataflow • Bus interruption => networkfailure

7. CSMA/CD Basic Assumptions and Principles • Assumptions • All stationscanheareachother on the medium. • The frametransmission time ismuchlongerthanthemaximumpropaga-tiondelaybetweenthestations. In otherwords, thesenderis still sendingwhenthemostdistantstation in the LAN segmentbeginstohearthe trans-mission. • Principles • CarrierSensing, Multiple Access (CSMA) • (also called "listen before talk") • A stationthatwantsto send sensesthe medium: • Ifthe medium isoccupied, sendingispostponed. • Ifthe medium isfree, sendingbeginsimmediately.

8. CSMA/CD Retry Strategies • What will a station do whenitfindsthe medium busy? • non-persistentThe stationwaits a randomexponentialbackoff time intervalandthenstarts a newtransmissionattempt. • 1-persistentThe stationwaitsforthe end ofthetransmissonandthenstartsitsowntransmissionimmediately after the end oftheothertransmission (trans-missionprobability = 1) • p-persistent (0 < p < 1)The stationwaitsforthe end ofthetransmissonandthensendsitsdataimmediatelywith a probabilityporwaits a randomexponentialbackoff time intervalwithprobability 1-p. • Not that non-persistent is not the same a p-persistent withp = 0! • IEEE/ISO CSMA/CD (the Ethernet) is 1-persistent.

9. Random Exponential Backoff Time • A random exponential backoff time means that a station picks a value k from a list of exponential ones (k = 0,1,2,4,8,…,2m-1) at random where m = min(n,10) and n is the number of unsuccessful attempts, and waits k*512 bit times. The reason for this is that a station does not know with how many stations it is competing; it should wait longer if it has tried several times al-ready.

10. Channel Utilization with Different Retry Strategies

11. Collisions • If two or more stations begin to send at the same time, a collision occurs. • Note: There is an increased danger of collisions when another transmission is ending: All stations who’s wish to send arose during the other transmission are “synchronized” to the end of this other transmission. • Therefore, stations involved in a collision will wait a random time interval be-fore re-trying to send.

12. CSMA/CD: Protocol (1) multiple access

13. CSMA/CD: Protocol (2) collision!

14. Info P r e a m b l e S D D A S A F C S CSMA/CD – Frame Format Preamble = 7 Bytes, 7 x ‘10101010’ SD = Starting Delimiter (1 Byte, contains ‘10101011’) DA = Destination Address (6 Bytes) SA = Source Address (6 Bytes) Info = n bytes of data FCS = Frame Check Sequence (32-bit CRC)

15. 48 -bit format I/G U/L 46 bit address 16 - bit - Format I/G 15 bit address I/G = 0 individual address I/G = 1 group address U/L = 0 globally administered address U/L = 1 locally administered address CSMA/CD – Address Field Format • The address field format according to IEEE 802

16. Cable Types for the Ethernet

17. Hubs and LAN Switching • Hubs • In theearly Ethernet yearsthe Ethernet cableswereactuallyinstalled in a physicalbustopology (yellowcableorThin-Wire Ethernet withcommercial, prefabricatedcoaxialcablesand T-connectors). The bustopologyturned out tobeimpracticalforlocatingfaults, connectingnewstations, etc. • Today Ethernet hubswith a star-shapedwiringarecommon, in particular in cableconducts in offices (“category 5 cable"). • Note thatthe medium accesscontrolprotocol in a hub is still CSMA/CD. However, collisionsnowtakeplace in the hub ratherthan on thecoaxcable. coaxcable bus cat 5 cable hub

18. Ethernet LAN with a Hub and Two Coaxial Cables 10BaseT Hub Repeater

19. LAN Switching • Throughput in an Ethernet LAN can be further increased by LAN switching. The hub is replaced by a frame-switching node (switch) which forwards the individual frames between the star-topology cables. The format of the Ethernet frames remains the same, thus the end systems (stations) do not notice the difference. • When doing LAN switching, the hub is replaced by a frame switch. It eva-luates the MAC destination address and forwards the frame on the appro-priate link. Unlike the hub, the LAN switch must internally operate much faster than the link rate, and it must contain buffers so that many frames arriving at the same time can be forwarded without collisions.

20. 2.2 802.11 MAC for Wireless LANs • Design goals • A wireless local area network with an 802 MAC layer • Should fit under the same LLC layer (802.2, Logical Link Control). Thus it should be compatible with all higher layers, in particular IP. • Support for stationary wireless stations and mobile wireless stations (”roaming”) • Can operate with an infrastructure (access points) or without an infra-structure (in ad-hoc mode). Many transparencies of this chapter were originally provided by professors Martin Mauve and Hannes Hartenstein. Their support is gratefully acknowledged.

21. STA1 STA3 BSS1 STA2 STA5 BSS2 STA4 IEEE 802.11 - System Architecture (1) • Independent Basic Service Set (IBSS) • Set of mobile stations that are able to communicate directly with each other, without connection to a wired network. • The stations do not have “forwarding” functionality. The IBSS only permits a one-hop communication (1-hop ad-hoc mode).

22. 802.11 LAN 802.x LAN STA1 BSS1 Portal Access Point Distribution System Access Point ESS BSS2 STA2 STA3 802.11 LAN IEEE 802.11 - System Architecture (2) • IEEE 802.11 defines access point, distribution system, portal and distribution ser-vices.

23. Access Point BSS STA1 STA2 802.11 LAN IEEE 802.11 - System Architecture (3) • Infrastructure Basic Service Set • Communication is always between a mobile station and an access point. • Extended communication possibilities, but frames between mobile stations are sent over the air twice.

24. Distribution System Access Point BSS2 STA2 STA3 802.11 LAN IEEE 802.11 - System Architecture (4) • Extended Service Set • Set of Infrastructure BSSs that are connected by a distribution system. 802.11 LAN STA1 BSS1 Access Point ESS

25. Media Access Control (MAC) in 802.11 • Traffic Classes • Asynchronous data service (standard) • Exchange of data packets on a “best effort” basis • Supports broadcast and multicast • Available in both modes, infrastructure mode and ad-hoc mode • Service with QoS guarantees (optional) • Implemented by the so-called Point Coordination Function (PCF) • Only available in infrastructure mode

26. A B C Why are New MAC Algorithms Needed? • In wireless communication new problems arise because the physical layer operates differently from cable-based physical layers. For example, unlike in an Ethernet segment, not every station can hear every other station dir-ectly. In particular: from “A hears B” and “B hears C” we cannot always conclude that “A hears C”! A cannot sense if C is sending, and vice versa.

27. A C The Problem of the “Hidden Terminal” • A and C both want to send to B. • A senses the medium and notices that it is free. A begins to send. • At the same time, C senses the medium and sees that it is free (the signal of A does not reach C). C also begins to send. • The signals collide in B, B receives none of the two messages correctly. • Neither A nor C can determine the collision since they do not hear each other. B

28. CSMA/CD does not work • Another reason why CSMA/CD will not work is that wireless cards cannot send and receive at the same time. • We conclude: CSMA/CD will not work in wireless networks.

29. IEEE 802.11 MAC (1) • DFWMAC: Distributed Foundation Wireless Medium Access Control • Three variants are standardized: • DFWMAC-DCF CSMA/CA (mandatory in all WLANs) • Distributed Coordination Function – Carrier Sensing Multiple Access with Collision Avoidance • “listen before talk“ • Reduction of collisions when “busy” by a “random backoff timer“ • Acknowledgment of receipt through an ACK (in unicast mode only) • The problem of the “hidden terminal” is not solved! • DFWMAC-DCF with RTS/CTS (optional) • Basic function like DFWMAC-DCF CSMA/CA • In addition, avoidance of the hidden terminal problem by a Request-to-Send / Clear-to-Send mechanism (RTS/CTS).

30. IEEE 802.11 MAC (2) • DFWMAC-PCF (optional) • PCF = Point Coordination Function • collision-free, centralized access control procedure for services with time guarantees • Basic idea: the access point polls the stations in a round-robin fashion

31. Priorities by Three Waiting Periods (1) IFS = Inter-Frame Spacing • SIFS (Short Inter-Frame Spacing) • Highest priority, for ACK, CTS, answer to polling • PIFS (PCF IFS) • Intermediate priority, for time-guaranteed services by the PCF (Point Coordination Function). The access point waits the PIFS time before using the medium for control purposes (in particular for polling). • DIFS (DCF IFS) • Lowest priority, for asynchronous (best effort) data services

32. DIFS DIFS PIFS SIFS Medium occupied Competition Next frame t Priorities by Three Waiting Periods (2)

33. DFWMAC-DCF CSMA/CA: Concept • “Listen-before-talk”: stationsreadyto send sense the medium (carrier sensing). • Ifthe medium isfreeforthedurationof an appropriate Inter-Frame Spacing (IFS), thestationsendsimmediately. • Ifthe medium isbusy, thestationwaits a DIFS andthenwaitsfor an additional random back-off time in order toreducetheprobabilityof a collision. • Ifthe medium isaccessedbyanotherstationduringthe back-off time, andtheotherstationcanbeheardbeforetheownbackofftimer rings, theown back-off timerisstoppeduntilthenext trans-missionattempt. Itisnotreset; thusthebackoff time will beshorter after everyunsuccess-fulattempt. Thisincreasesthepriorityofstationsthathavewaitedbe-fore. • Note: Collisionsare still possible, although not very probable.

34. DIFS data sender SIFS Ack receiver DIFS data other stations t waiting period Competitionperiod Acknowledgements (Unicast Case) • Problem:Collision detection to recognize a successful transmission on the sender side (like in the Ethernet) does not work! Therefore explicit acknow-ledgments are needed. If the sender does not receive an acknowledgment, a retransmission takes place in the MAC layer! SIFS < slot time

35. Example DIFS DIFS DIFS DIFS boe bor boe bor boe busy Station1 boe busy Station2 busy Station3 boe busy boe bor Station4 boe bor boe busy boe bor Station5 t arrival of a packet at the MAC layer boe back-off time passed medium busy (frame, ack etc.) busy bor remaining back-off time

36. DIFS RTS data sender SIFS SIFS SIFS CTS ACK receiver DIFS NAV (RTS) data other stations NAV (CTS) t waiting period competition DFWMAC-DCF with RTS/CTS • Optional. Onlyavailableforunicasttransmission. Usually not advisableforshortpackets. Basically a methodbywhichthereceiverinforms all stations in hisradiorangetoshutupfor an upcomingtransmission. Solvesthehidden terminal problem. • Extension oftheaccessprocedureby a Network AllocationVector NAVforthereservationoffutureslotsforthe same pair oftransmitterandreceiver. RTS = Request To Send, CTS = Clear To Send.

37. DFWMAC-PCF (with Point Coordination Function) • The point coordinator is always an access point. PCF is not available in ad-hoc mode. • PCF is an extension (additional function); PCF and DCF can be used at the same time. • There is now a “Contention-Free Period” (CFP), in which the PCF coordi-nates the access to the medium without competition with other stations. • PCF uses PIFS (the intermediate IFS) in order to control the medium. • The NAV is used to communicate the CFP to the stations (to make the reservation). • The assignment of guaranteed sending periods to the stations is achieved by polling. A guaranteed sending period per round leads to a guaranteed minimum bandwidth and a guaranteed maximum delay to access the me-dium.

38. t2 t3 t4 t0 t1 super frame PIFS SIFS D3 D4 CFend medium busy point coordinator PIFS SIFS SIFS D1 D2 SIFS point coordinator U4 SIFS SIFS wireless stations U1 U2 wireless stations NAV ofstations NAV Competition-free period t NAV ofstations NAV competition PCF Example with Polling

39. 2 2 6 6 6 2 6 0-2312 4 Frame Control Duration ID Address 1 Address 2 Address 3 Sequence Control Address 4 Data CRC Version, type, fragmenting, security... IEEE 802.11 - Frame Structure • Frame Types • Control Frame, Management Frame, Data Frame • Sequence numbers (Sequence Control) • important for detecting duplicate packets after lost ACKs • Four Addresses • Receiver, transmitter (physical), BSS identifier, transmitter (logical) • Other • Transmission duration, data, checksum (CRC) Not all fields exist in all frames

40. Addressing: Four Addresses in the Frame Twobits in the Frame Controlfielddecidehowthefouraddressesareinterpreted. The addressesusuallyare 48-Bit MAC addresses. DS: Distribution System AP: Access Point DA: Destination Address SA: Source Address BSSID: Basic Service Set Identifier RA: Receiver Address TA: Transmitter Address

41. Roaming (better: Hand-over or Hand-off) • Whathappenswhen a stationmovesandtheradio link getsweakoris lost? • Scanning • Scanning oftheenvironment (listen forbeaconsof APs or send a probe intothe medium andwaitfor an answer). • Reassociation Request • Containsinformationaboutthepast AP andasksfor a newassociation. • Reassociation Response • Success: i.e., an AP answers, thestationisphysicallyconnectedagain • Failure: continuetoscan • AP acceptsReassociation Request • Registration ofthenewstationwiththedistributionsystem • The old AP isinformedbythedistributionsystemtoreleasetheconnec-tiontothisstation.

42. 1 Mm 300 Hz 10 km 30 kHz 100 m 3 MHz 1 m 300 MHz 10 mm 30 GHz 100 m 3 THz 1 m 300 THz VLF LF MF HF VHF UHF SHF EHF Infrared UV PHY: Why 2.4 GHz? • Unlicensed ISM band (Industrial, Scientific, Medical) • Advantage: usage free of charge, no permission required • Problem: non-exclusive use (e.g., garage door openers and microwave ovens work at 2.4 GHz) = > a very noisy frequency band • Solution: use spread-spectrum technology. Mandatory in the US. • Frequency ranges and wavelengths:

43. Frequency Hopping Spread Spectrum (1) • Several channels from the entire frequency band are used in parallel. Sen-ders and receivers rapidly jump between the channels in an agreed se-quence. In this way, narrow-band noise has less effect.

44. t d t 1 0 1 user data 1 0 t f d f 3 slow hopping f 2 (3 bits/hop) f 1 t t d f f 3 fast hopping f 2 (3 hops/bit) f 1 t Frequency Hopping Spread Spectrum (2)

45. Frequencies in the 2.4 GHz Band

46. LAN Bridges A brigdeconnectstwoormore LANs thatmayuse different MACs (e.g., an Ethernet LAN with a Wireless LAN). Itcanbeconsideredas a layer-2 gate-way. A bridgehas a frame filter.Itforwardsonlythoseframestotheneighbor LAN whosedestinationaddressis not in thelocalsegment. A bridgeoperates on layer 2 exclusively (on MAC frames). Protocolsofthehigherlayersare not takenintoconsideration. In particular, a LAN bridgeisquite different from an IP router. Bridges areusuallyself-learning, i.e., theylearnfromtheincomingframeswhich MAC addressesarelocated in whichattached LANs.

47. bridge protocol, bridge management LLC LLC MAC relay MAC- MAC- instance instance A B Layer 1 Layer 1 ConnectionA Connection B Network A Network B Protocol Layers of a LAN Bridge

48. Example of a Bridge between Three LANs bridge Sales Production Research and Development

49. Entries in the Table of a Self-Learning Bridge • Example

50. Spanning Trees • In order to make the interconnection of LANs more reliable, there are often several interconnection points (bridges) between them. In order to avoid the cycling of frames in such a network, the bridges must agree on a spanning tree for the graph. This is done with a spanning tree protocol. bridge1 Sales Production Research and Development bridge2