Scherer Bluetooth

7/31/2019 Scherer Bluetooth

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Scatternet Formation in

Bluetooth

CSC 457

Bill Scherer

November 8, 2001


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Outline

Introduction

Overview of Bluetooth

Scatternet Formation Protocols


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What is Bluetooth?

What is Bluetooth?

Ad Hoc wireless networking

Specification and protocol suite

Initiated by Ericsson in 1994


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Why Should I Care About It?

Up and coming

In billions of devices by 2005 (Business Week, 18September 2000)

Cool

Cordless desktop

Briefcase e-mail

Wire-free headphones

Cheap

As little as 29 incremental

80K transistors


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Next Up: Overview

Introduction




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Physical Layer: Media

2.4 GHz Band (license-free)

Slotted Bandwidth

79 hop frequencies (23 in Japan, France, Spain)

1 MHz each

625sec hop intervals (1600 hops/sec)

10/100 Meter range

Up to 500 kbits/sec bandwidth


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Frequency Hopping CDMA

Hop Pattern

Permutation of the available hop frequencies

Clock

Current offset within the hop pattern

Referred to as "Channels"


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Organization of Bluetooth

Networks Piconets

Master/Slave

Shared channel

Scatternets

Grouped Piconets

Bridges

Shared Slaves

S

MB

S

SS

MS

S

S


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Next Up: Scatternet Formation

Introduction



S

M B

S

S

S

MS

S

S


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Scatternet Formation

How do we go from (A) to (B)?

?

??

?

?

?

??

?

?

(A) (B)

S

MB

S

S

S

MS

S

S


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Establishing a Connection

0) Slave: must be in Page Scan mode

1) Master: enter Page mode

2) Slave: Slave response to page

3) Master: Master response to slave

4) Slave, Master are now connected

M S

1M S

2M S

3M S

4


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Scatternet Topologies

Roughly possible topologies for n

nodes

6 topologies for 3 nodes:

n(n-1)

22

M S

M

S M

S

M M

S

S S

M

S M

M

M S

S


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Good Topology Properties

Fully connected

Masters belong to exactly one Piconet

Bridges connect only two Piconets

Avoid overload on the bridge node

Minimal number of Piconets forming

minimal diameter Scatternet

Reduce cost of routing


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BTCP (Bluetooth Connection Protocol)

Bluetooth Connection Protocol

Based on Leader Election

Identifying one node to be in charge

Two phase protocol

1) Elect a leader

2) Assign roles


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Leader Election

All nodes start with VOTES = 1.

Look for other nodes (send/listen on special

discovery channel) When two nodes meet, higher VOTES

wins, gets all votes and MAC addresses

from loser. Loser enters Page Scan mode

Election ends when no more nodes found


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Role Assignment

Winner of election picks "sub-masters" and

bridges for minimum possible Piconets

Winner forms temporary Piconet with sub-

masters, gives them assignment, list of

slaves

Sub-masters page in slaves


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LMS

Law, Mehta, Siu from MIT

Randomized, distributed

Multiple rounds, but no separate phases

Every node starts out as a leader

Also assumes all nodes can see each other


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During a Round of LMS

Each leader flips a coin to see whether itgoes into Scan or Seek mode

Scan mode: Listen for another node (discovery channel)

If contacted, go into Page Scan mode

Seek Mode Look for slave on discovery channel

Connect via Page


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Retirement

Once two leaders connect, one must retire

Invariants for partial Scatternets:

Each leader either has no slave, or has at least

one unshared slave

Each leader has fewer than kslaves in its

Piconet Five cases needed to preserve invariants


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Case 1

One leader has no slaves

Join other Piconet and retire (if room)

Take a slave, other leader retires (otherwise)

S

M

SL

S

S

S

M

BL

S

Sretired


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Case 2

The two leaders have < k- 1 slaves between

them

S

M

S

S

M

SS

S

M

S

S

S

SS

retired


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Case 3

At least k- 1 slaves between the leaders

fill up and retire one of them

retiredS

M

S

S B

S

M

S

S

*

S

M

S

S B

S

M

S

S

*


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Cases 4, 5

Special cases to make the algorithm work

Refer to paper if you want the full details

http://perth.mit.edu/~ching/pubs/

PerformanceOfScatternet.pdf

Important thing is that even in these cases,

one of the leaders retires


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A Bit of Theory

Time Complexity: BTCP

(n/k) for n nodes, kslaves per Piconet

Due to centralized nature

Time Complexity: LMS

O(log n)

~1/2 the leaders retire each round


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Transport Layer: Services

SCO (Synchronous Connection Oriented)

Fixed 64 kbit/sec symmetrical link

2 slots at a time (one each direction)

ACL (Asynchronous Connectionless)

432.6 kbit/sec symmetrical link

721.0/57.6 kbit/sec asymmetrical link 5 slots at a time

Choice: 1 ACL, 3 SCOs, or one of each


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FHCDMA Advantages

Resistance to interference

Can still get through on other parts

Resistance to multipath effects

Reflection, like an echo

Multiple access for co-located devices

Multiple simultaneous hop patterns

Graceful bandwidth degradation


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Connection States

Active

Sending/Receiving normally

Sniff Typically slaves only

Low-power mode

Not listening on every receive slot Hold (SCO communications only)

Park (not participating)

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