2 – Polling - Intranet DEIBhome.deib.polimi.it/capone/wn/liu/2-Polling.pdf · 2 – Polling Multiple Access in Wireless Networks: Models and Technologies ... i is the number of

Politecnico di Milano Facoltà di Ingegneria dell’Informazione

2 – Polling

Multiple Access in Wireless Networks: Models and Technologies Prof. Antonio Capone

A. Capone: Wireless Networks 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  The system status is described by vector

o  Where ni is the number of packets in queue i o  The system evolution is described by the

process N(t)

( )Mnnn ,...,, 21=n


Polling o  Polling schemes are scheduled access

schemes where stations access the channel according to a cyclic order

o  The polling message, or token, is the grant for access the channel

o  The 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 T and that token passing time is h, both constant

o  Polling schemes differentiate based on the service policy (exhaustive, gated, limited)


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  The probability that the channel is transmitting a packet at a random time t is give by

Tλρ =


Exhaustive Polling o  The average waiting time E[W] in the queue

can be calculated considering three components

Arrival in queue 8

transmission

321][ WWWWE ++=


Exhaustive Polling

o  E[Nc] is the average number of packets transmitted before considered packet

o  Using Little’s result is can be expressed as:

o  Therefore:

TNEW c ][1 =

][][ WENE c λ=

N

λa

T

][][1 WEWTEW ρλ ==


Exhaustive Polling

o  The total average waiting time is given by:

hMW

hTW

21

2)1(

2

3

2

−=

−+= ρρ

hMhTWEWE2)1(

2)1(

2][][ −

+−++= ρρρ


Exhaustive Polling

o  Solving by E[W] we get:

hMTWE)1(2)1(2

][ρρ

ρρ

−

−+

−=

Waiting time of a single queue (M/D/1)

Additional waiting time due to token passing time

o  Note that:

1max =ρ


Exhaustive Polling

o  The average token cycle time is given by the transmission time of all packets that arrive during a cycle plus the token passing time

[ ] [ ]

[ ]ρ

λ

−=

+=

1MhCE

MhTCECE


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  The expression of the average waiting time is similar to previous case with an additional term

o  This is the additional cycle the packet has to wait when it arrives when the token is already at the station

ρρ hMhM

W ==4


Gated Polling

o  Therefore we get:

o Again

hMTWE)1(2)1(2

][ρρ

ρρ

−

++

−=

1max =ρ


Limited Polling

o  With limited polling, stations when receive the token can transmit only up to k packets

o  The 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

hWEMhMNEW c ][][

5 λ==


Limited Polling

o  Therefore we get:

o Now we have:

h

TTh

MT

TThWE

)1(2)1(2][

+−

++

+−

=ρ

ρ

ρ

ρ

hTT+

=maxρ


Polling in real networks

o  There 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  The main difference with simple schemes we considered so far is that the station sequence can be dynamically changed


Bluetooth vs. 802.15.1

1.  Bluetooth is an industrial specification for WPANs

2.  The WG 802.15.1 adapted the industrial specifications of Bluetooth for the levels 1 and 2

3.  ’96-’97: Ericsson internal project 4.  ’98: Bluetooth SIG created (Ericsson, IBM,

Intel, Toshiba, Nokia) 5.  ’99: new members join the SIG (3Com,

Lucent Technologies, Microsoft, Motorola)


BluetoothTM

o  Radio technology o  Low cost o  Small range (10-20 m) o  Low complexity o  Small size o  ISM 2.4 GHz band o  Created by an industrial

consortium o  Only the first two levels

have then been standardized by IEEE 802.15.1

■  Danish King of medieval, Harald Blaatand II, aka Bluetooth (940-981)

■  He unified Denmark and Sweden


Application scenarios

o Headset



o Data synchronization



o Access point

Adsl, fiber, etc.

GPRS, UMTS, etc.


Physical layer

o  ISM band at 2.4 GHz o  79 (23 in France and Japan) channels

spaced of 1 MHz (2402-2480 MHz) o Modulation G-FSK (1 Mb/s) o Device classes

Class Power (mW)

Power (dBm)

Range (approx)

Class 1 100 mW 20 dBm ~ 100 m

Class 2 2,5 mW 4 dBm ~ 10 m

Class 3 1 mW 0 dBm ~ 1 m


Physical layer

o  Frequency Hopping (FH) o  1600 hops/s (625 µs per hop) o  The 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  The other devices are ‘slaves’ and follows the sequence fk defined by the master


Physical layer

o  The numbering of the slots is defined by the clock of the master

o  The sequence is given by the master ID and a generation algorithm

master

slave

fk fk+1 fk+2 fk+3

625 µs


Physical layer

o  It is possible to transmit packet with duration of 1, 3 or 5 intervals

master

slave

fk fk+3

625 µs

fk+4 fk+5 fk+6

3-slot packet


Physical layer

o  It is possible to transmit packet with duration of 1, 3 or 5 intervals

master

slave

fk

625 µs

fk+5 fk+6

5-slot packet


Piconet o  The simplest network architecture defined in

Bluetooth is called piconet o  The piconet is an ad hoc network composed

of 2 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  The 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 256 devices)


Piconet

o  Addresses n  MAC address of 48 bits n  AMA (Active Member Address) 3 bits n  PMA (Parked Member Address) 8 bits

M

S

P SB

S

S

S S

S

S

P

P

SB

SB


Types 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 433.9 Kbit/s symmetric (using 5-slot packets in both directions) and 723.2/57.6 Kbit/s asymmetric (using 5-slot packets in one direction and 1-slot packets in the other)


Multiple Access Master

Slave 1

Slave 2

Slave 3

SCO (Synchronous Connection Oriented)

ACL (Asynchronous ConnectionLess)


Polling in Bluetooth

Master

Slave 1

Slave 2

Slave 3 SCO (Synchronous Connection Oriented) ACL (Asynchronous ConnectionLess)

M

S

P SB

S

S

S S S

S

P

P

SB

SB

Polling in Bluetooth o  Some key characteristics of Bluetooth multiple

access mechanism make the direct application of previously derived formulas not possible: n  Queues are not visited in a sequential order (master

queue is always visited in odd slots) n  Token passing time is always one slot, but the slot is

used for data transmission if the queue is not empty n  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

n  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


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

m: number of BT devices m-1: slaves M=2(m-1): total queues γ: arrival rate in each queue λ: total arrival rate T: slot duration


…

… Master queues

Slaves’ queues

1

1

2

2

m

m

…

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 2(m-1)

slots n  System is equivalent to a TDMA with 2(m-1)

slots per frame n  There 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 32



…

[ ]

[ ][ ]

[ ]Tm

TmTTmT

TmWE

mWTEWWW

TmTmW

WTEW

γγγγ

γ

γ

)1(21)1(

)1(21)1(1)1(2

00

)1(2)1(2

5

4

3

2

1

−−

−=

+−−−

−=

−−=

=

=

−=−

=

=2(m-1)T


n  1-limited service (round-robin) n  1, 3, and 5-slots packets

o  We observe that: n  The system is equivalent to a polling system

with: o  Token passing time equal to 1 slot o  Service time equal to packet length minus one

slot



n  1-limited service (round-robin) n  1, 3, and 5-slots packets



n  Notation:

A. Capone: Wireless Networks 36 XXzE

ppXppX

XpppL

Lppp

2][

164

42system equiv. in thedurantion service:

53lenghtpacket :

packetsslot -5 of prob. :packetsslot -3 of prob. :packetsslot -1 of prob. :

253

253

531

5

3

1

=

+=

+=

++=


n  Waiting time:


( )

=−−

−−+−+

−−

−=

=−−

−−+−+

−−−

−−=

=

⎟⎠

⎞⎜⎝

⎛−

−

+−+

−−

=

=

⎟⎠

⎞⎜⎝

⎛ +−−

++

+−−

=

TLTm

TLmmXLm

m

TLTm

TLmmLX

LTmTLm

T

LL

mzE

LL

T

LTTL

MzE

LTTLWE

γγ

γγ

γγ

γγ

ρ

ρ

ρ

ρ

ρ

ρ

ρ

ρ

)1(21)1()1()1(

)1(21)1(

)1(212)1()1(2)1(2

)1(2)1(21)1()1(2

112

)1(2][

11

)11(12][)11(1

][

2

2

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2 – Polling - Intranet DEIBhome.deib.polimi.it/capone/wn/liu/2-Polling.pdf · 2 – Polling Multiple Access in Wireless Networks: Models and Technologies ... i is the number of