2 Polling. Politecnico di Milano Facoltà di Ingegneria dell Informazione

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1 Politecnico di Milano Facoltà di Ingegneria dell Informazione Polling Multiple Access in Wireless Networks: Models and echnologies Prof. Antonio Capone

2 Assumptions and notation o In the following we drop the assumption of global coordination and analyze distributed mechanisms o Let us assume that arrival times in the M local queues are described by a Poisson process with rate λ/m (λ global rate) o he system status is described by vector n = ( n n,..., ) 1, o Where n i is the number of packets in queue i o he system evolution is described by the process N(t) n M A. Capone: Wireless Networks

3 Polling o Polling schemes are scheduled access schemes where stations access the channel according to a cyclic order o he polling message, or token, is the grant for access the channel o he token can be distributed by a central station (roll-call polling) or passed from station to station (hub polling or token system) o Let us assume that packet transmission time is and that token passing time is h, both constant o Polling schemes differentiate based on the service policy (exhaustive, gated, limited) A. Capone: Wireless Networks 3

4 Exhaustive Polling o With exhaustive polling, stations when receive the token transmit all packets in the queue before releasing it o Let us analyze the behavior of this system o he probability that the channel is transmitting a packet at a random time t is give by ρ = λ A. Capone: Wireless Networks 4

5 Exhaustive Polling o he average waiting time E[W] in the queue can be calculated considering three components E [ W] = W + W + W 1 3 Arrival in queue 8 transmission A. Capone: Wireless Networks 5

6 Exhaustive Polling W E N 1 = [ c ] o E[Nc] is the average number of packets transmitted before considered packet o Using Little s result is can be expressed as: E[ N ] = λe[ W c ] λ a o herefore: W = λ E[ W] = ρe[ W] 1 N A. Capone: Wireless Networks 6

7 Exhaustive Polling W W 3 = ρ + (1 ρ) = M 1 h h o he total average waiting time is given by: E[ W ] = ρe[ W ] + ρ + (1 ρ) h + ( M 1) h A. Capone: Wireless Networks 7

8 Exhaustive Polling o Solving by E[W] we get: ρ M ρ E[ W] = + (1 ρ) (1 ρ) h Waiting time of a single queue (M/D/1) Additional waiting time due to token passing time o Note that: ρ max =1 A. Capone: Wireless Networks 8

9 Exhaustive Polling o he average token cycle time is given by the transmission time of all packets that arrive during a cycle plus the token passing time [ ] = λ [ ] E C [ ] E C E C Mh = 1 ρ + Mh A. Capone: Wireless Networks 9

10 Gated Polling o With gated polling, stations when receive the token can transmit all packets that are in queue at the time when the token arrives o he expression of the average waiting time is similar to previous case with an additional term o his is the additional cycle the packet has to wait when it arrives when the token is already at the station ρ W = Mh = 4 M hρ A. Capone: Wireless Networks 10

11 Gated Polling o herefore we get: E[ W ] = ρ (1 ρ) + M + ρ (1 ρ) h o Again ρ max =1 A. Capone: Wireless Networks 11

12 Limited Polling o With limited polling, stations when receive the token can transmit only up to k packets o he special case of k=1 is called Round-Robin o Here we have one more additional term which are the additional cycles the packet has to wait, one per each packet in the queue at the arrival moment E[ N ] W = c Mh = λe[ W ] h 5 M A. Capone: Wireless Networks 1

13 Limited Polling o herefore we get: E[ W ] = (1 ρ h ρ + ) + M + ρ h + (1 ρ ) h o Now we have: ρ max = + h A. Capone: Wireless Networks 13

14 Polling in real networks o here are several examples where polling is used for regulating access to a channel in wireless technologies n WiFi (Point Coordination Function PCF or HCF Hybrid Coordination Function) n Bluetooth o he main difference with simple schemes we considered so far is that the station sequence can be dynamically changed A. Capone: Wireless Networks 14

15 Bluetooth vs Bluetooth is an industrial specification for WPANs. he WG adapted the industrial specifications of Bluetooth for the levels 1 and : Ericsson internal project 4. 98: Bluetooth SIG created (Ericsson, IBM, Intel, oshiba, Nokia) 5. 99: new members join the SIG (3Com, Lucent echnologies, Microsoft, Motorola) A. Capone: Wireless Networks 15

16 Bluetooth M Danish King of medieval, Harald Blaatand II, aka Bluetooth ( ) He unified Denmark and Sweden o Radio technology o Low cost o Small range (10-0 m) o Low complexity o Small size o ISM.4 GHz band o Created by an industrial consortium o Only the first two levels have then been standardized by IEEE A. Capone: Wireless Networks 16

17 Application scenarios o Headset A. Capone: Wireless Networks 17

18 Application scenarios o Data synchronization A. Capone: Wireless Networks 18

19 Application scenarios Adsl, fiber, etc. GPRS, UMS, etc. o Access point A. Capone: Wireless Networks 19

20 Physical layer o ISM band at.4 GHz o 79 (3 in France and Japan) channels spaced of 1 MHz ( MHz) o Modulation G-FSK (1 Mb/s) o Device classes Class Power (mw) Power (dbm) Range (approx) Class mw 0 dbm ~ 100 m Class,5 mw 4 dbm ~ 10 m Class 3 1 mw 0 dbm ~ 1 m A. Capone: Wireless Networks 0

21 Physical layer o Frequency Hopping (FH) o 1600 hops/s (65 µs per hop) o he FH sequence is pseudo random and determined by the clock and the address of the master device that regulates the access to the channel o he other devices are slaves and follows the sequence f k defined by the master A. Capone: Wireless Networks 1

22 Physical layer f k f k+1 f k+ f k+3 master slave 65 µs o he numbering of the slots is defined by the clock of the master o he sequence is given by the master ID and a generation algorithm A. Capone: Wireless Networks

23 Physical layer f k f k+3 f k+4 f k+5 f k+6 master 3-slot packet slave 65 µs o It is possible to transmit packet with duration of 1, 3 or 5 intervals A. Capone: Wireless Networks 3

24 Physical layer f k f k+5 f k+6 master 5-slot packet slave 65 µs o It is possible to transmit packet with duration of 1, 3 or 5 intervals A. Capone: Wireless Networks 4

25 Piconet o he simplest network architecture defined in Bluetooth is called piconet o he piconet is an ad hoc network composed of or more devices o A device acts as master and the other as slaves o Communication can take place only between master and slave and not directly between slaves o Up to 7 slaves can be active in a piconet o he others can be in n Stand-by (not part of the piconet) n Parked (part of the piconet but not active, up to a maximum of 56 devices) A. Capone: Wireless Networks 5

26 Piconet S S S SB P SB M P S S SB S S P o Addresses n MAC address of 48 bits n AMA (Active Member Address) 3 bits n PMA (Parked Member Address) 8 bits A. Capone: Wireless Networks 6

27 ypes of connections o Bluetooth considers two types of connections o SCO (Synchronous Connection Oriented) n Fixed rate bi-directional connection (circuit) n FEC for improving quality n Rate of 64 Kbit/s o ACL (Asynchronous ConnectionLess) n Packet switched connection shared between master and active slaves based on a polling access scheme n Several options for packet formats and physical layer codes (1, 3, 5 slots) n Rate up to Kbit/s symmetric (using 5-slot packets in both directions) and 73./57.6 Kbit/s asymmetric (using 5-slot packets in one direction and 1-slot packets in the other) A. Capone: Wireless Networks 7

28 Multiple Access Master Slave 1 Slave Slave 3 SCO (Synchronous Connection Oriented) ACL (Asynchronous ConnectionLess) A. Capone: Wireless Networks 8

29 Polling in Bluetooth Master S S S SB Slave 1 Slave P S SB S M S P P S SB Slave 3 SCO (Synchronous Connection Oriented) ACL (Asynchronous ConnectionLess) A. Capone: Wireless Networks 9

30 Polling in Bluetooth o Some key characteristics of Bluetooth multiple access mechanism make the direct application of previously derived formulas not possible: n n n n Queues are not visited in a sequential order (master queue is always visited in odd slots) oken passing time is always one slot, but the slot is used for data transmission if the queue is not empty Exhaustive service makes no sense for Bluetooth since after each packet transmission by the master/ slave at least a slot is used by the slave/master Bluetooth makes use of packets with different lengths (1, 3, or 5 slots) o We derive expressions for the waiting time in two special cases A. Capone: Wireless Networks 30

31 Polling in Bluetooth o Let us assume the master has one separate queue per slave and all queues are visited according to a fixed sequence o Arrival in the queues are independent Poisson processes 1 m m: number of B devices m-1: slaves M=(m-1): total queues γ: arrival rate in each queue λ: total arrival rate : slot duration 1 m Master queues Slaves queues A. Capone: Wireless Networks 31

32 Polling in Bluetooth o Case 1) n 1-limited service (round-robin) n 1-slot packets only o We observe that n Cycle length is fixed and equal to (m-1) slots n System is equivalent to a DMA with (m-1) slots per frame n here are several equivalent ways of calculating the waiting time n We use the same approach adopted for the general polling schemes A. Capone: Wireless Networks 3

33 Polling in Bluetooth o Case 1) W W W W W [ ] E W [ ] = γe W ( m 1) = = ( m 1) = 0 = 0 = γe W [ ][ ( m 1) 1] (m-1) ( m 1) ( m 1) = = 1γ ( m 1) γ + γ 1 ( m 1) γ A. Capone: Wireless Networks 33

34 Polling in Bluetooth o Case ) n 1-limited service (round-robin) n 1, 3, and 5-slots packets o We observe that: n he system is equivalent to a polling system with: o oken passing time equal to 1 slot o Service time equal to packet length minus one slot A. Capone: Wireless Networks 34

35 Polling in Bluetooth o Case ) n 1-limited service (round-robin) n 1, 3, and 5-slots packets A. Capone: Wireless Networks 35

36 Polling in Bluetooth o Case ) n Notation: p p p L : packet lenght L X X X : prob.of : prob.of : prob.of = p + 3p X X 1-slot packets 3-slot packets 5-slot packets + 5p : servicedurantion in the equiv.system 1 = p 3 = 4 p E[ z] = + 16 p A. Capone: Wireless Networks p 5 5 5

37 Polling in Bluetooth o Case ) n Waiting time: A. Capone: Wireless Networks 37 ( ) = + + = = + + = = + + = = = L m L m m X L m m L m L m m L X L m L m L L m z E L L L L M z E L L E W γ γ γ γ γ γ γ γ ρ ρ ρ ρ ρ ρ ρ ρ 1) ( 1 1) ( 1) ( 1) ( 1) ( 1 1) ( 1) ( 1 1) ( 1) ( 1) ( 1) ( 1) ( 1 1) ( 1) ( 1 1 1) ( ] [ 1 1 1) 1 ( 1 ] [ 1) 1 ( 1 ] [