CS5530 Mobile/Wireless Systems Medium Access Control (MAC)

Transcription

1 CS5530 Mbile/Wireless Systems Medium Access Cntrl (MAC) Yanyan Zhuang Department f Cmputer Science CS5530 UC. Clrad Springs Ref. CN5E, NT@UW, WUSTL

2 Outline Access links Access cntrl Channel allcatin Static allcatin Dynamic allcatin Carrier sense prtcls CS Ref. CN5E, NT@UW, WUSTL

3 Access Links Pint-t-pint links Easy crdinatin Bradcast links Mre than tw statins transmit at the same time: cllisin Challenge: determine wh gets t use the channel when there is cmpetitin CS Ref. CN5E, NT@UW, WUSTL

4 Access Cntrl Access cntrl Ensure nly ne statin transmits Ensure n cllisin ccur MAC (Medium Access Cntrl) The prtcls used t determine wh ges next n a bradcast channel Abve physical layer, belw IP layer CS Ref. CN5E, NT@UW, WUSTL

5 Channel Allcatin Prblem Hw t allcate a single bradcast channel amng cmpeting users Static Channel Allcatin FDM (Frequency Divisin Multiplexing) } Divides spectrum int frequency bands, with each user having exclusive pssessin f sme band t send their signal CS Ref. CN5E, NT@UW, WUSTL

6 Channel Allcatin Prblem Hw t allcate a single bradcast channel amng cmpeting users Static Channel Allcatin FDM (Frequency Divisin Multiplexing) TDM (Time Divisin Multiplexing) } Users take turns (rund-rbin), each ne peridically getting entire bandwidth fr a little burst f time CS Ref. CN5E, NT@UW, WUSTL

7 Channel Allcatin Prblem Hw t allcate a single bradcast channel amng cmpeting users Static Channel Allcatin FDM (Frequency Divisin Multiplexing) TDM (Time Divisin Multiplexing) Advantage/disadvantage } A small and cnstant # f users, each has a steady stream f traffic, static allcatin is simple and efficient } When # f senders is large and varying r the traffic is bursty: inefficient CS Ref. CN5E, NT@UW, WUSTL

8 Channel Allcatin Prblem Hw t allcate a single bradcast channel amng cmpeting users Static Channel Allcatin Dynamic Channel Allcatin N base statin/central entity invlved, purely distributed CS Ref. CN5E, NT@UW, WUSTL

9 Dynamic Channel Allcatin Alha Seminal cmputer netwrk cnnecting Hawaiian islands in late 1960s } When shuld ndes send? } A new prtcl was devised by Nrm Abramsn Hawaii CS Ref. CN5E, NT@UW, WUSTL

10 Dynamic Channel Allcatin Alha Seminal cmputer netwrk cnnecting Hawaiian islands in late 1960s } When shuld ndes send? Hawaii } A new prtcl was devised by Nrm Abramsn Simple idea } Nde just sends when it has traffic (n carrier sensing) } If there was a cllisin (n ACK received) then wait a randm time (called backff time) and resend CS Ref. CN5E, NT@UW, WUSTL

11 Dynamic Channel Allcatin Alha Pure Alha } Statins transmit at any time } Fr analysis, assume same length } Vulnerable time: 2T CS Ref. CN5E, NT@UW, WUSTL

12 Dynamic Channel Allcatin Alha Sltted Alha } Statins transmit nly in a slt } Vulnerable time: T CS Ref. CN5E, NT@UW, WUSTL

13 Dynamic Channel Allcatin Alha s lw efficiency Statins transmit at will, withut knwing what thers are ding à many cllisins (W)LANs Pssible fr statins t detect what thers are ding, and thus adapt their behavir accrdingly Carrier Sense Multiple Access (CSMA) Prtcls Prtcls in which statins listen fr a carrier (i.e., a transmissin) and act accrdingly CS Ref. CN5E, NT@UW, WUSTL

14 Carrier Sense Prtcls ALOHA inspired Bb Metcalfe t invent Ethernet fr LANs in 1973 Ndes share 10 Mbps caxial cable Hugely ppular in 1980s, 1990s : 2009 IEEE CS Ref. CN5E, NT@UW, WUSTL

15 Carrier Sense Prtcls CSMA imprves ALOHA by listening befre send (Dh!) Can d easily with wires, nt wireless 1-persistent CSMA A statin has data t send: listens t the channel } If channel is idle, statin sends data } If channel is busy, statin waits until idle, then transmits a frame } If a cllisin ccurs, statin waits a randm time and starts all ver CS Ref. CN5E, NT@UW, WUSTL

16 Carrier Sense Prtcls Still pssible t listen and hear nthing when anther nde is sending because f delay Just after a statin begins sending, anther statin becmes ready t send and sense the channel If the first statin s signal has nt yet reached the secnd ne, the latter will sense an idle channel and begin sending, resulting in a cllisin X CSMA is a gd defense against cllisins nly when delay is small CS Ref. CN5E, NT@UW, WUSTL

17 Carrier Sense Prtcls p-persistent CSMA: applies t sltted channels If channel is idle, it transmits with a prbability p } With a prbability q = 1 p, it defers until the next slt If the statin senses that the channel is busy, it waits until the next slt } If next slt is als idle, it either transmits r defers again, with prbabilities p and q CS Ref. CN5E, NT@UW, WUSTL

18 CSMA/CD (with Cllisin Detectin) Reduce the cst f cllisins by detecting them and abrting the rest f the frame time Again, we can d this with wires Abrt! X X X X X X X X Abrt! Cllisin detectin is an analg prcess } Hardware must listen t channel while it is transmitting } If the signal it reads back is different frm the signal it put ut, then a cllisin is ccurring } Implicatin: a received signal must nt be tiny cmpared t the transmitted signal Difficult fr wireless, as received signals may be 1,000,000 times weaker than transmitted signals CS Ref. CN5E, NT@UW, WUSTL

19 CSMA/CD (with Cllisin Detectin) Hw des a statin detect a cllissin? Minimum packet length Tt: transmissin time, time t transmit a packet ut (decided by length and link bandwidth) Tp: prpagatin time, time t transmit a packet t anther statin (decided by the distance between statins, and bandwidth) CS Ref. CN5E, NT@UW, WUSTL

20 CSMA/CD (with Cllisin Detectin) Minimum packet length Assume Tp = 1 hur Case 1 A and B transmit simultaneusly } 10am statins A and B starts transmitting } 10:30am cllisin happens } 11am A and B receive cllisin signal Case 2 A transmits befre B } 10am A starts transmitting } 10:59:59am B starts transmitting } 11am cllisin happens } 12pm A receives cllisin signal In wrst case, the time t detect a cllisin is 2*Tp CS Ref. CN5E, NT@UW, WUSTL

21 CSMA/CD (with Cllisin Detectin) Minimum packet length In wrst case, the time t detect a cllisin is 2*Tp Detecting cllisin } Receive cllisin signal while transmitting } Tt >= 2*Tp, where Tt = Length (bits) /B (bits per sec) Example: Tp = 1msec, B = 1Mbps, what s the minimum packet length fr CSMA/CD? } Tt = Length/B >= 2*Tp. Length >= 2*Tp*B = 2*10^(-3)*10^6 = 2000 bits } What if a packet is shrter than 2000 bits? Padding CSMA/CD used fr Ethernet CS Ref. CN5E, NT@UW, WUSTL

22 CSMA/CD (with Cllisin Detectin) Shuld I resend right away? Binary Expnential Backff (BEB): cleverly estimates prbability 1st cllisin, wait 0 r 1 frame times (0 2^1-1) 2nd cllisin, wait frm 0 t 3 times (0 2^2-1) 3rd cllisin, wait frm 0 t 7 times (0 2^3-1) BEB dubles interval fr each successive cllisin Quickly gets large enugh t wrk Very efficient in practice CS Ref. CN5E, NT@UW, WUSTL

23 Classic Ethernet, r IEEE Mst ppular LAN f the 1980s, 1990s Multiple access with 1-persistent CSMA/CD with BEB CS Ref. CN5E, NT@UW, WUSTL

24 Wireless LAN Prtcls Different frm Ethernet A wireless system cannt detect a cllisin while it is ccurring } The received signal at a statin may be a millin times weaker than transmitted signal A statin n a wireless LAN may nt be able t transmit frames t, r receive frames frm all ther statins because f the limited range } In wired LANs, when ne statin sends a frame, all ther statins receive it (with sme delay) A naive apprach Try CSMA: just listen fr ther transmissins and nly transmit if n ne else is ding s What s the prblem? CS Ref. CN5E, NT@UW, WUSTL

25 Wireless LAN Prtcls Remember hidden terminals and expsed terminals? 1. Ndes may have different areas f cverage desn t fit Carrier Sense 2. Ndes can t hear while sending can t Cllisin Detect CS Ref. CN5E, NT@UW, WUSTL

26 MACA (Multiple Access with Cllisin Avidance) MACA uses a shrt handshake instead f CSMA (Karn, 1990) uses a refinement f MACA Prtcl rules: 1. A sender nde transmits a RTS (Request-T-Send, with frame length) 2. The receiver replies with a CTS (Clear-T-Send, with frame length) 3. Sender transmits the frame while ther ndes hearing the CTS stay silent q Cllisins n the RTS/CTS are still pssible, but less likely 26 Ref. CN5E, NT@UW, WUSTL

27 MACA Hidden Terminals (1) AàB with hidden terminal C 1. A sends RTS, t B A RTS B C D 27 Ref. CN5E, NT@UW, WUSTL

28 MACA Hidden Terminals (2) AàB with hidden terminal C 2. B sends CTS, t A, and C t A RTS CTS B CTS Alert! C D 28 Ref. CN5E, NT@UW, WUSTL

29 MACA Hidden Terminals (3) AàB with hidden terminal C 3. A sends frame while C defers Frame Quiet Ref. CN5E, NT@UW, WUSTL

30 MACA Expsed Terminals (1) BàA, CàD as expsed terminals B and C send RTS t A and D A RTS B C RTS D 30 Ref. CN5E, NT@UW, WUSTL

31 MACA Expsed Terminals (2) BàA, CàD as expsed terminals A and D send CTS t B and C All OK All OK A RTS CTS B C RTS CTS D 31 Ref. CN5E, NT@UW, WUSTL

32 MACA Expsed Terminals (3) BàA, CàD as expsed terminals B sends frame t A while C sends frame t D A Frame B C Frame D 32 Ref. CN5E, NT@UW, WUSTL

33 802.11, r WiFi Very ppular wireless LAN started in the 1990s Clients get cnnectivity frm a (wired) AP (Access Pint) It s a multi-access prblem J Varius flavrs have been develped ver time Faster, mre features Access Pint T Netwrk Client 33 Ref. CN5E, NT@UW, WUSTL

34 802.11, r WiFi MAC sublayer prtcl different frm that f Ethernet Radis are nearly always half duplex: cannt transmit and listen fr cllisin at the same time Transmissin ranges f different statins may be different } With a wire, the system is engineered s that all statins can hear each ther tries t avid cllisins with a prtcl called CSMA/CA (CSMA with Cllisin Avidance) Cnceptually similar t Ethernet s CSMA/CD, with channel sensing befre sending and expnential back ff after cllisins 34 Ref. CN5E, NT@UW, WUSTL

35 CSMA/CA A statin that has a frame t send starts with a randm backff It des nt wait fr a cllisin The number f slts t backff is chsen in the range 0 t, say, 15 Waits until the channel is idle, and cunts dwn idle slts Idle: by sensing that there is n signal fr a shrt perid f time Cunt dwn pauses when frames are sent by ther statins Sends its frame when the cunter reaches 0 If frame gets thrugh, destinatin sends a shrt ack Lack f an ack is inferred as an errr, whether a cllisin r therwise The sender dubles the backff perid and tries again } Cntinues with expnential backff until (1) the frame has been successfully transmitted, r (2) the maximum # f retransmissins has been reached 35 Ref. CN5E, NT@UW, WUSTL

36 CSMA/CA Statin A is the first t send a frame t D While A is sending, statins B and C becme ready t send They see that the channel is busy and wait fr it t becme idle Shrtly after A receives an acknwledgement, the channel ges idle Rather than sending a frame right away and clliding, B and C bth perfrm a backff Send? Send? Time 36 Ref. CN5E, NT@UW, WUSTL

37 CSMA/CA While A is sending, statins B and C becme ready t send C picks a shrt backff, and thus sends first B pauses its cuntdwn while it senses that C is using the channel, and resumes after C has received an acknwledgement B then cmpletes its backff and sends its frame Send? Send? Time 37 Ref. CN5E, NT@UW, WUSTL

38 CSMA/CA Difference cmpared t CSMA/CD Starting backffs early helps t avid cllisins } This avidance is wrthwhile because cllisins are expensive, as the entire frame is transmitted even if ne ccurs Acknwledgements are used t infer cllisins because cllisins cannt be detected 38 Ref. CN5E, NT@UW, WUSTL

39 Services Assciatin service Mbile statins cnnect t APs Typically used after a statin mves in radi range f an AP Upn arrival, statin learns the identity and capabilities f the AP (frm beacn frames r by directly asking AP) } Capabilities include data rates, security arrangements, pwer-saving capabilities, quality f service supprt, etc. Statin sends a request t assciate with AP. The AP may accept r reject the request. 39 Ref. CN5E, NT@UW, WUSTL

40 Services Re-assciatin Lets a statin change its preferred AP Useful fr mbile statins mving frm ne AP t anther AP in the same extended LAN Disassciate Either statin r AP may disassciate breaking the relatinship A statin uses this befre shutting dwn r leaving the netwrk AP may use it befre ging dwn fr maintenance 40 Ref. CN5E, NT@UW, WUSTL

41 Services Authenticatin service Statins must authenticate befre sending frames via AP Authenticatin is handled in different ways depending n the chice f security scheme } If the netwrk is pen, anyne is allwed t use it. } Otherwise, credentials are needed t authenticate 41 Ref. CN5E, NT@UW, WUSTL

42 Services Authenticatin service Statins must authenticate befre sending frames via AP Authenticatin is handled in different ways depending n the chice f security scheme } If the netwrk is pen, anyne is allwed t use it. } Otherwise, credentials are needed t authenticate WPA2 (WiFi Prtected Access 2): AP talks t an authenticatin server that has a username and passwrd database WEP (Wired Equivalent Privacy): Use is discuraged because f design flaws that make WEP easy t cmprmise 42 Ref. CN5E, NT@UW, WUSTL

43 Services Distributin service Once frames reach the AP, distributin service determines hw t rute them } If destinatin is lcal t AP, frames sent ut directly ver the air } Otherwise, frwarded ver the wired netwrk Integratin service Handles translatin needed fr a frame t be sent utside LAN, r t arrive frm utside LAN } The cmmn case: cnnecting the wireless LAN t the Internet 43 Ref. CN5E, NT@UW, WUSTL

44 Services Privacy service Fr infrmatin sent ver a wireless LAN t be kept cnfidential, it must be encrypted Manages details f encryptin and decryptin } The encryptin algrithm fr WPA2 is based n AES (Advanced Encryptin Standard) QS traffic scheduling service Handles traffic with different pririties } Give vice and vide traffic preferential treatment cmpared t besteffrt and backgrund traffic 44 Ref. CN5E, NT@UW, WUSTL

45 802.11/WiFi Summary CSMA/CA Cannt detect cllisin à passively avid instead Varius WiFi services Assciatin, authenticatin, privacy, etc. Andrid 45 Ref. CN5E, WUSTL

46 Bluetth In 1994, the L. M. Ericssn cmpany Cnnecting mbile phnes t ther devices (e.g., laptps) withut cables With fur ther cmpanies (IBM, Intel, Nkia, and Tshiba), frmed a SIG (Special Interest Grup, i.e., cnsrtium) in 1998 t develp a wireless standard } Intercnnecting cmputing and cmmunicatin devices and accessries using shrt-range, lw-pwer, inexpensive wireless radis } Named after Harald Blaatand (Bluetth) II ( ), a Viking king wh unified Denmark and Nrway 46 Ref. CN5E, NT@UW, WUSTL

47 Bluetth A brief histry Bluetth 1.0 released in July 1999 Higher data rates were added t Bluetth 2.0 in release in 2009: device pairing in cmbinatin with release in December 2009 specified lw-pwer peratin 5 release June 2016: IT technlgy, 4x range, 2x speed, 8x increase in bradcasting capacity f lw energy transmissins Bluetth prtcl Let devices find and cnnect t each ther, an act called pairing, and securely transfer data 47 Ref. CN5E, NT@UW, WUSTL

48 Bluetth Architecture Basic unit f a Bluetth system is a picnet Cnsists f a master nde and up t seven active slave ndes } Older tech: within a distance f 10 meters 48 Ref. CN5E, NT@UW, WUSTL

49 Bluetth Architecture Multiple picnets can c-exist & cnnected via a bridge nde in multiple picnets An intercnnected cllectin f picnets is called a scatternet Tw picnets can be cnnected t frm a scatternet. 49 Ref. CN5E, NT@UW, WUSTL

50 Bluetth Architecture A picnet is essentially a centralized TDM Master cntrlling the clck and determining which device gets t cmmunicate in which time slt All cmmunicatin is between the master and a slave; n direct slave-slave cmmunicatin In a picnet, there can be up t 255 parked ndes Devices switched t a lw-pwer state t reduce battery drain Cannt d anything except respnd t an activatin r beacn signal frm master 50 Ref. CN5E, NT@UW, WUSTL

51 Bluetth Applicatins Bluetth specifies particular applicatins and prvides different prtcl stacks Applicatins are called prfiles Audi and vide Intercm prfile allws tw telephnes t cnnect as walkie-talkies Headset and hands-free prfiles prvide vice cmmunicatin between a headset and its base statin } Used fr hands-free telephny while driving a car 51 Ref. CN5E, NT@UW, WUSTL

52 Bluetth Applicatins Human interface device prfile } Fr cnnecting keybards and mice t cmputers } Let a mbile phne r ther cmputer receive images frm a camera r send images t a printer } Use a mbile phne as a remte cntrl fr a (Bluetth-enabled) TV Generic access prfile } On which all f the ther prfiles are built, prvides a way t establish and maintain secure links between master and slaves 52 Ref. CN5E, NT@UW, WUSTL

53 Bluetth MAC Layers Has a link cntrl layer, similar t MAC layer Master defines a series f 625-μsec time slts Master s transmissins starting in even slts, slaves transmissins in the dd nes (TDM) Paylad f frame can be encrypted fr cnfidentiality with a key chsen when the master and slave cnnect 53 Ref. CN5E, NT@UW, WUSTL

54 Bluetth MAC Layers Link manager prtcl Pairing prcedure: t make sure devices are allwed t cmmunicate } Secure simple pairing enables users t cnfirm that bth devices display the same passkey (device generated), r bserve a passkey n ne device and type it int the secnd device 54 Ref. CN5E, NT@UW, WUSTL

55 Bluetth MAC Layers Link manager prtcl Pairing prcedure: t make sure devices are allwed t cmmunicate Link manager prtcl sets up lgical channels, called links } SCO (Synchrnus Cnnectin Oriented) link: used fr real-time data, such as telephne cnnectins, allcates a fixed slt in each directin } ACL (Asynchrnus Cnnectin-Less) link: used fr packet-switched data (ccur at irregular intervals), delivered n a best-effrt basis N guarantees 55 Ref. CN5E, NT@UW, WUSTL

56 Bluetth Summary Shrt range, lw pwer Picnet, scatternet Essentially TDM Applicatin prfiles Pairing, links 56 Ref. CN5E, WUSTL

57 RFID (Radi Frequency IDentificatin) NFC (Near Field Cmmunicatin) and UHF (Ultra High Frequency) EPC (Electrnic Prduct Cde): fcus f this sectin Aut-ID Center at the Massachusetts Institute f Technlgy in 1999 Replacement fr barcdes } Carry a larger amunt f infrmatin } Electrnically readable up t 10 m, even when it is nt visible 57 Ref. CN5E, NT@UW, WUSTL

58 RFID Architecture Tw cmpnents: tags and readers Tags are small, inexpensive devices } Have a unique 96-bit EPC identifier and a small amunt f memry that can be read and written by reader } Memry might be used t recrd the lcatin histry f an item, e.g., as it mves thrugh the supply chain } Have n battery and must gather pwer frm transmissins f a nearby reader antenna integrated circuit 58 Ref. CN5E, NT@UW, WUSTL

59 Architecture Tw key cmpnents: tags and readers Readers are like access pints in WiFi netwrks much mre pwerful than tags } Have pwer surces, multiple antennas } In charge f when tags send and receive messages } Must slve multiple access issue caused by tags within reading range } Inventry tags: t discver identifiers f nearby tags 59 Ref. CN5E, NT@UW, WUSTL

60 Architecture Reader always transmits T send bits t tags Tags Harvest signal t get pwer RFID architecture. T send data, a tag changes whether it is reflecting the signal frm the reader r absrbing it: backscatter } A lw-energy way fr tags t create a weak signal that shws up at the reader 60 Ref. CN5E, NT@UW, WUSTL

61 Multiple Access T inventry nearby tags Reader needs t receive data frm each tag that gives its ID A multiple access prblem } Reader bradcasts a query t ask tags fr IDs } Tags that replied right away wuld cllide like in an Ethernet 61 Ref. CN5E, NT@UW, WUSTL

62 Multiple Access Slt 0 Reader sends a Query message QRepeat advances t next slt 62 Ref. CN5E, NT@UW, WUSTL

63 Multiple Access Slt 0 Reader sends a Query message QRepeat advances t next slt.slt 2 Tags pick slt 2 t reply Tags d nt send IDs when they first reply } Sends a shrt 16-bit randm number in an RN16 message } If there is n cllisin, the reader sends an ACK message } The tag has acquired the slt and sends its identifier 63 Ref. CN5E, NT@UW, WUSTL

64 Multiple Access Message exchange IDs are lng à cllisins n ID messages are expensive } A shrt exchange t test if tag can safely use the slt t send ID Once ID has been transmitted, tag temprarily stps respnding t Query messages } All remaining tags can be identified 64 Ref. CN5E, NT@UW, WUSTL