IEEE 802.15

1

• IEEE 802.15 Survey of Scatternet Formation
  Protocols and Algorithms

Vivek Kumar Joy Ghosh University at Buffalo
2
Why Bluetooth ?

• Operates in license free ISM band centered around
  2.45GHz (available worldwide)
• Interference immunity (FH) ,Crosstalk avoidance
  (TDD) also gt single chip
• Low power (30µA in sleep ,300µA standby ,8-30mA
  transmitting) Cell phone power (50 100 mA ?)
• Channel access contention free ( 802.11 ?)
• Saves contention overhead (feasibility with
  625bits packet ?)
• No line of sight requirement (IrDA ?) ,low
  cost,high data rate ( 1Mbps, practically 721kbps
  ?) BT2.0 10Mbps
• gt Bluetooth very viable for wire replacement,
  short range ad-hoc n/w ,and now aiming to be
  access point technology to wide area voice/data
  n/w (competes with 802.11)
• We must think bigger than piconets !!!(? Routing
  )

3
Overview

• What do we know already?
• Bluetooth basics
• Connection establishment
• Concept of an ad-hoc piconet
• Today Concept of a scatternet
• Set of independent piconets -gt scatternet?
• Protocols
• Tree Scatternet Formation (TSF)
• BlueStar-BlueConstellation
• BlueMesh
• BTSpin (proposed by us)

4
Scatternets

• Internetworking multiple piconets
• How to make two or more independent and
  non-synchronized piconets communicate with each
  other?
• Bluetooth alludes the possibility, but does not
  say how
• Realisation possible with bridges slave relay
  (shared slave) or master relay

5
Problem ?

• How do a collection of isolated devices form a
  scatternet?i.e how do we go from A to B
• Each device is not aware of the others
• ? The scatternet formation protocol must be
  distributed
• Devices are in the communication range of each
  other
• ? Potentially, any two devices could be connected
  directly
• Potentially large number of topologies for n
  nodes ,which one is best ?

A
B
6
Criteria for efficient Scatternet Formation

• Number of Piconets
• Scatternet should have minimum number of Piconets
• Reduces the inefficient intra-piconet
  communication
• Reduces co-channel interference
• Number of Roles per node
• The number of roles per node should be minimized
• Gateway nodes (Bridges) incur additional cost of
  switching, scheduling and re-synchronization
  between piconets
• Size of Piconets
• Piconets should be as full as possible (7 slaves
  in each piconet)
• Optimal system capacity utilization
• More than 7 not desirable ? park / unpark

7
Criteria for efficient Scatternet Formation

• Average Shortest Path (ASP)
• Average number of hops needed for 2 devices to
  communicate
• Lower ASP ? better routing efficiency
• Message Overhead
• Minimize control packet overhead
• Bandwidth is scarce
• Very small packets (625 bits) 40 bytes for TCP
  headers!!
• Convergence
• Algorithm should converge quickly to possible
  optimal solution
• Low power devices spend power in setting up
  scatternet (Energy!)
• Applicable to short term ad hoc networks (Time!)

8
Classification of Scatternet Protocols
Centralized Distributed
Single-hop Static BTCP, Marsan_MinMax, 1-Factors BlueTrees, BlueRings, Lin et al.
Single-hop Dynamic None LMS, TSF
Multi-hop Static Marsan_ILP BlueMesh, BlueStar_Basagni
Multi-hop Dynamic None Three phase-Bluestar, BTSpin
9
Tree Scatternet Formation Protocol (TSF)

• Proposed by G. Tan et. al. MIT Technical Report,
  Oct 2001
• Scatternet Topology is at any time a collection
  of one or more rooted trees
• Trees are autonomously attempting to merge. Goal
  only one tree.
• Why Trees gt no cycles, simple routing (unique
  path), minimal of links
• Vulnerable to single point faults importance for
  fast self-healing
• Possible bottleneck at roots while routing.
• Three kinds of nodes roots, non-roots and free
  nodes
• Each node knows its type via state diagrams and
  IACs
• Each node in the network runs algorithm
  transitioning between two states
• FORM social task, to reduce number of
  scatternet components
• COMM state for data communication within
  component

10
FORM STATE.

• FORM state
• Every node tries to make a link to another node
  belonging to a different tree/component
• Two substates FORMINQ and FORMINQSCAN
• All nodes spend time in this state
• Roots spend more time in it than others
• How long shall we be social? T_form random
  (ETinq, D)
• Too long gt waste of time if environment does not
  change
• Too short gt not able to meet properly
• Randomized to avoid periodic synchronization
  effects
• E Tinq is the time taken to complete an INQUIRY
  process.
• T_form must be at least as long to ensure that a
  handshake can take place.
• D decides the size of random interval

11
The FORM state.
Assume each node knows it type
Link formation combination Entries with 0 are
Invalid

• Every node tries to join with other components
• Opportunity distributed formation
• Non-root connects to Free only if Free is not
  within its Roots range
• Danger how to save the invariant to have only
  trees? ? Use the knowledge of the nodes type!

12
The FORM state avoid the invariant

• Non-root/Root and Non-root/Non-Root connections
  could create cycles
• Links of root nodes are saved for merging with
  other trees later (no Root/Free Connections)
• Non-root nodes help free nodes to join scatternet
• Rule Free node becomes the slave (with
  probability 0.5 a role switch procedure takes
  place)
• Therefore a node can never be in more than two
  piconets
• Only root nodes attempt to heal partitions join
  another tree as a slave
• Healing function gt roots spend more time in FORM
  state
• When root joins root ,the master becomes the new
  root of the merged tree

13
The FORM state.

• Improvement The coordinator
• We dont want a root to waste time and energy to
  do INQUIRY, root should do communication only
• Thus a root designates a node in the component to
  be coordinator
• Elects a child randomly
• Maybe the child does not want to be coordinator
  gt elects one of its children and so on
• A leaf must accept the job (no bottleneck anyway)

14
The FORM state.

• Coordinators establish link, then send PAGE and
  PAGE SCAN info to their roots and break the link
• Roots can make the link quickly
• New root selects new coordinator randomly
• Idea change coordinator also periodically,
  thats robust if a coordinator breaks and
  distributes the energy-intensive task on many
  nodes
• Problem not only the coordinators but also the
  roots have to be in radio range

15
The COMM State

• Communication-within-component state
• How long shall we work in our component?
• T_comm fcomm random(R,D)
• fcomm for free nodes fcomm0
• for other nodes suggestion as a function of age
  (how long ago it joined the scatternet younger
  nodes should spend more time in trying to form
  bigger scatternet, older ones more in data
  communication) and number of children in the
  current tree (the more children the more a node
  is expected to be involved in communication)

16
Healing

• Dynamic environment
• Nodes can leave anytime
• Two ways a connectivity can be lost
• Master loses connection to slave
• Slave loses connection to master
• When a master detects loss of child gt master
  must only check if it has become a free node
• No control messages, works completely locally
• When a slave loses its parent gt slave updates
  its node type and sets age to zero
• Leaf node becomes free node
• Internal node becomes root node

17
Problem

• Arrival en masse
• Results in many components
• TSF allows only roots to merge
• But Bluetooth allows max. 7 connections per
  device
• Consequence roots may not be able to merge
  together because theyre full, so components
  remain seperated

18
Solution

• When a root node is about to reach max. number of
  children, it designates a child to become the
  root
• Then the two nodes switch roles as master and
  slave

19
Routing (1)

• Root topology
• No loops possible
• Unique paths
• Therefore simple routing
• Idea nodes have unique addresses based upon
  their position in the tree
• Mapping from IP to these adresses using ARP (a
  node returns its scatternet address in response
  to ARP query)
• With this identifier, packet forwarding protocol
  works by simply having each node look at the
  destination and forward it along one of its links
• No per packet overhead like source routing, no
  extra memory needed like distance vector

20
Routing (2)

• A partial realization
• Each node has an address dependent on its
  position and a portion of address space that it
  can distribute to its children
• Each node knows it childrens addresses
• Forwarding can be done by longest prefix match
  if no childs address is applicable the packet is
  pushed upwards
• But what happens when nodes leave? How do we
  merge trees?

21
Generated Topologies
22
Statistics Formation delay
23
Effects of increasing inter-piconet scheduling
latency
n/w size50
24
TSF Summary

• Tree structure
• Incremental and efficient
• Devices need not to be in radio range
• Simple routing
• Comments? Questions?
• Further reading
• http//nms.lcs.mit.edu/projects/blueware/body.htm

25
BlueStar BlueConstellation

• A distributed approach to forming mesh like
  scatternets

26
Phases of the Protocol

• Phase I
• Topology discovery
• Phase II
• BlueStar (Piconet) formation
• Phase III
• BlueConstellation (Scatternet) formation

27
I. Topology discovery

• Each node has unique ID and weight
• Symmetric 1-hop neighbor information
• Requires pair of neighbors to be in opposite mode
• Inquirer sends ID pkt. containing GAC
• Inquired
• On receiving ID pkt, backs off randomly
• If ID pkt persists, sends FHS pkt. (ID BT
  clock)
• Temporary piconet for symmetric knowledge
• Phase time needs to be determined well
• Nodes may not discover all neighbors!

28
II. BlueStar formation

• init nodes
• Nodes with biggest weight in their neighborhood
• Ties broken with unique ID
• init nodes become MASTERS of neighbors
• Non-init nodes wait to hear from bigger neighbors
  to decide their role
• Become SLAVE to first big MASTER to page
• If all big neighbors are SLAVES, become MASTER
• Communicate role to all smaller neighbors
• If role ! MASTER, learn roles of smaller
  neighbors
• For smaller SLAVE neighbor, note its MASTER ID

29
BlueStar formation protocols

• initOperations()
• if (for each neighbor u myWeight gt uWeight)
• myRole MASTER
• go to PAGE mode
• PAGE(v, MASTER, v) to all smaller neighbors
• exit

30
BlueStar formation protocols (contd.)

• ReceivedPage (deviceID u, string role, deviceID
  t)
• record that u has paged record role(u)
• if (role(u) SLAVE) ? master(u) t
• if (myWeight lt uWeight)
• if (role(u) MASTER)
• if (myRole none) ? myMaster u
  myRole SLAVE
• else ? inform u about myMaster ID
• if (some bigger neighbor is yet to page)
  ? wait for following page
• else
• if (all bigger neighbors are SLAVES)
• myRole MASTER PAGE (v, MASTER, v) to
  all neighbors
• else
• PAGE (v, SLAVE, myMaster) to each
  neighbor switch to PAGE SCAN
• else if (all neighbors have not paged) ?
  wait for next page

31
Example Network Phase I (topology)
Init Node Non-Init Node
32
Example Network Phase II (piconets)
Master Slave
33
III. BlueConstellation formation

• mNeighbors
• Neighboring masters that are atmost 3 hops away
• 2 hops 1 slave in between 2 masters
• 3 hops 2 slaves in between 2 masters
• iMasters
• Biggest weight amongst all mNeighbors
• Scatternet formation
• Select gateways to connect masters to all its
  mNeighbors

34
BlueConstellation formation protocol

• mInitOperations()
• if (for each mNeighbor u myWeight gt
  uWeight)
• myRole iMaster
• instruct gateway slaves to 3 hop
  mNeighbors to page slave of mNeighbor
• instruct gateway slaves to 2 hop
  mNeighbors to page mNeighbor
• page slaves of 2 hop mNeighbors
  identified as gateways
• else
• instruct all gateway slaves to bigger
  mNeighbors to go to page scan
• if (slaves of bigger mNeighbors
  identified as gateways are neighbors)
• go to page scan
• while (links to bigger mNeighbors are
  not up) ? WAIT
• instruct gateway slaves to smaller
  mNeighbors to go to page mode
• if (slaves of smaller mNeighbors
  identified as gateways are nearby)
• go to page mode

35
Example Network Phase III (scatternets)
6
3
23
28
9
34
15
12
5
7
14
42
1
4
45
8
10
35
32
51
19
Master Slave Slave-Bridge Master-Br
idge
36
Conclusion

• Advantages
• Assign weights to specify suitability of a Master
• Robust connected mesh (multiple paths)
• No leader required to initiate process
• Multihop scatternet
• Disadvantages
• Many temporary piconets formed incurs delay
• Unable to account for joining / leaving nodes
• May have more than 7 slaves in a piconet Park!

37
BlueMesh

• Degree constrained multihop scatternet

38
What is new?

• If there are more than 7 neighbors, choose a
  maximum of 7 slaves which cover all other
  neighbors
• No need for park / unpark
• No extra hardware required (unlike some other
  solutions provided which require GPS in each
  device to perform degree reduction techniques on
  network topology graph)

39
Phases of the Protocol

• Phase I
• Topology discovery
• Phase II
• Scatternet formation

40
I. Topology discovery

• Each node has unique ID and weight
• Weight signifies nodes suitability as master
• Symmetric 1-hop neighbor information
• Done similar to last discussed protocol
• Symmetric 2-hop neighbor information
• Nodes with biggest weight in neighborhood (init
  nodes) pages its smaller neighbors and exchange
  1-hop neighbor information
• After being paged by all bigger weight neighbors,
  non-init nodes start paging and exchanging
  similar information with its smaller neighbors

41
II. Scatternet formation

• BlueMesh formed in local successive iterations
• Each iteration has two parts
• Role Selection
• Gateway Selection
• After each iteration
• Masters, Gateway Slaves and non-Gateway Slaves
  exit execution of the iterative process
• Intermediate gateways proceed to form extra
  piconets needed to connect 3-hop Masters

42
Iterative Process Role Selection

• Definitions
• N(v) set of vs 1-hop neighbors
• S(v) set of at most 7 slaves selected by master
  v
• C(v) set of vs
• Bigger slave neighbors
• Smaller neighbors
• M(v) set of vs masters

43
Iterative Process Role Selection (contd.)

• All init nodes execute the following process
• Master (v)
• 1 myRole ? master
• 2 PageMode
• 3 ComputeS(v)
• 4 for each u in S(v)
• 5 do Page (u, v, master, true, NIL)
• 6 for each u in C(v) S(v)
• 7 do Page (u, v, master, false, NIL)
• 8 exit

44
Iterative Process Role Selection (contd.)

• ComputeS(v)
• 1 S(v) ? NULL
• 2 U ? C(v)
• 3 while U ! NULL
• 4 do x ? bigger node in U
• 5 S(v) ? S(v) U x
• 6 U ? U N(x)
• 7 S(v) ? S(v) U Get (7 - S(v), C(v) S(v))
• Get (n, B) selects n nodes from set B
• Criteria for selection is user specified

45
Iterative Process Role Selection (contd.)

• NonInit(v)
• 1 PageScanMode
• 2 for each bigger neighbor w
• 3 do Wait Page (v, w, r, j, NIL)
• 4 if ((r master) (j true))
• 5 myRole ? slave Join (w) M(v) ? M(v)
  U w
• 6 if (myRole slave)
• 7 PageMode
• 8 for each w in C(v)
• 9 do Page (w, v, slave, false, NIL)
• 10 PageScanMode
• 11 for each w in C(v)
• 12 do Wait Page (v, w, r, j, NIL) from
  smaller w
• 13 Wait Page (v, w, slave, false, M(w))
  from bigger w
• 14 PageMode
• 15 for each w in C(v)
• 16 do Page (w, v, slave, false, M(v))
• 17 PageScanMode
• 18 for each smaller slave w

46
Iterative Process Gateway Selection

• Slaves tell Masters
• Neighbor roles
• Neighbors list of Masters
• Whether any neighboring Master selected them as
  slave
• Masters decide
• Which slaves shall act as gateways to which
  piconets
• Which slaves need to be intermediate gateways
• Intermediate nodes carry on the iterative process

47
BlueMesh Simulation

• Parameters
• 200 nodes
• Transmission radius of 10 m
• Node distribution random and uniform
• Square region with size varying with number of
  nodes to always give connected networks

48
BlueMesh Simulation Results I
49
BlueMesh Simulation Results II
50
Conclusion

• Advantages
• No more than 7 slaves per piconet
• On an average of 2.3 roles per node
• Route lengths not significantly longer than
  shortest path in the network
• Disadvantages
• Multiple phases might incur unacceptable delay
• Still not suitable for nodes leaving/joining
• Simulations assumed network connectivity

51
Some other work (other than Trees Mesh)

• BlueRing
• Forms a ring like network
• No routing required ? pass it on the ring
• Overcomes single and multi point failures
• Handles nodes joining / leaving
• Disadvantages
• Centralized algorithm ? leader election
• Assumes one hop network
• Changing direction to fix single point failures
  increase network traffic unnecessarily

52
BTSpin our proposal

• Distributed and Adaptive Single phase Scatternet
  formation

53
Main features

• Distributed
• No leader required
• No distribution of unique weights
• Single phase ? no topology discovery
• Spin technique
• Grows piconet omni directionally
• Handles nodes joining
• Allocates substantial time for intra-piconet
  traffic
• Backup Gateways
• Handles nodes leaving / single point failures
• Creates a multi-hop multi-path meshed network

54
Spin Technique

• Used to maintain balance between Scatternet
  formation and intra-piconet data communication.
• Spin INQ phase followed by INQ SCAN phase.
• Spin node obtains parameters like size of
  Piconet, Master ID
• Master puts Spin slave in hold mode.
• Master itself also spins
• Slave-Slave bridges are not scheduled for spin.

55
Definition of 2-hop 3-hop connection
(i) A B are two hop connected (ii) A B are
three hop connected
56
Free node meets with a Slave node
57
Slave node meets with a Slave node
58
Slave node meets with a Master node
59
Master node meets with a Master node
60
Master node meets a M-S Bridge node
61
Backup Gateway

• We observed other multi-phased, meshed based
  approaches have large number of temporary
  piconets
• BTSpin keeps a backup gateway table
• When two connected nodes disconnect without
  forming a bridge ,the information about Master ID
  ,FHS and type of node is recorded
• Used later to directly page potential bridge
  nodes without INQ INQ SCAN phase

62
Node Failures

• Slave node do nothing
• Master node If not Bridge it becomes free node,
  Bridges continue their role in the other piconet.
• Bridge node if Slave-Slave bridge use backup
  information.

63
Scatternet formation delay
64
Total number of Piconets
65
Average number of roles
66
References

• G. Tan, A. Miu, J. Guttag, H. Balakrishnan,
  Forming Scatternets from Bluetooth Personal Area
  Networks, in Technical Report MIT-LCS-TR-826,
  October 2001
• Ching Law and Kai-Yeung Siu. A Bluetooth
  scatternet formation algorithm. In Proceedings of
  the IEEE Symposium on Ad Hoc Wireless Networks
  2001, San Antonio, Texas, USA, November 2001.
• Ching Law, Amar K. Mehta, and Kai-Yeung Siu.
  Performance of a new Bluetooth scatternet
  formation protocol. In Proceedings of the ACM
  Symposium on Mobile Ad Hoc Networking and
  Computing 2001, Long Beach, California, USA,
  October 2001.
• Ching Law, Amar K. Mehta, and Kai-Yeung Siu. A
  new bluetooth scatternet formation protocol. ACM
  Mobile Networks and Applications Journal, 2002.
• S. Basagni and C. Petrioli, A scatternet
  formation protocol for ad hoc networks of
  Bluetooth devices, in Proceedings of the IEEE
  Semiannual Vehicular Technology Conference, VTC
  Spring 2002, Birmingham, AL, May 69 2002.
• C. Petrioli and S. Basagni, Degree-Constrained
  Multihop Scatternet Formation for Bluetooth
  Networks, in Proceedings of IEEE Globecom, 2002,
  Taipei, Taiwan, R.O.C., November 2002
• Ting-Yu Lin, Yu-Chee Tseng, Formation, Routing,
  and Maintenance Protocols for the BlueRing
  Scatternet of Bluetooths, 36th Annual Hawaii
  International Conference on System Sciences
  (HICSS 2003)

67
Bluetooth Implementation Practical Considerations

• Use two IAC types
• Repetition IAC are the ID packets a master sends
  in INQ state
• Two types GIAC and LIAC
• Nodes can filter different IAC types
• Roots only transmit and receive LIAC, other nodes
  only GIAC
• This can still lead to a bad connection because
  the FHS packets can not be distinguished. We have
  to tear down this bad connection when we discover
  that the types are incompatible.

Back
68
Connection Establishment
Back
69
Review-Piconets

• Set of Bluetooth devices (each has clock and
  unique bluetooth device address BDA)
• 1 master, up to 7 slaves (why ?)
• Communication only via master
• Time Divison Duplex (TDD)
• Polling master allocates time slots for slaves
• Pseudo-random frequency hopping scheme to
  minimize interference e.g. with other piconets
• Pseudo 32 frequencies chosen out of 79 allowed
  ones

Back