A Routing Vector Method (RVM) for Routing Bluetooth Scatternets

1. A Routing Vector Method (RVM) for Routing Bluetooth Scatternets Pravin Bhagwat IBM.Thomas J. Watson Research Center ,Yorktown Heights,NY

2. Introduction for Bluetooth • Operates in the globally available 2.45GHz ISM band • Use a frequency-hop / time-division-duplex (FD / TDD) scheme • The channel is divided into 625-msec intervals—slots

3. Introduction (master-slave) • The master transmission starts in even-numbered slots • The slave transmission starts in odd-numbered slots • A slave is allowed to transmit in a given slot if the master has addressed it in the preceding slot • No direct MAC layer communication between slaves

4. Introduction (cont.) • A group of interconnected piconets is referred to as a scatternet • A node can participate in two or more piconets on a time sharing basis • A unit can receive packets from one piconet and relay to another piconet • Protocols for inter-piconet communication have not been defined

5. Problems for inter-piconet commucation • For each node: • Increase buffering requirement • Higher store and forward delay • Delay and buffering requirement will be reduced if the forwarding is supported at the Bluetooth slot level

6. Scatternets vs Wireless LANs • Two bluetooth nodes cannot hear each other unless they form a master-slave pair • In the wireless LAN, any two nodes within proximity can hear each other’s transmission

7. Scatternets • Differs from classical ad hoc networks in terms of: • Applications • Traffic characteristics • Mobility patterns • Scaling requirements • It will be quasi-static, short-lived and small

8. Purposes • Describe the protocols for route discovery and packet forwarding • Illustrates 3 main design compromises: • Minimization of soft-state • Protocol simplicity • Bandwidth conservation

9. Model Elements • Every Bluetooth unit has a globally unique 48 bit Bluetooth address • A unit can be a slave in two or more piconets and a master in another piconets • A unit belongs to more than one piconet is referred to as a relay

10. Model Elements (cont.) • Slaves in a piconet are assigned 3 bit MAC addresses—MacAddr • The 000 address is reserved for broadcast within the piconet • 2 different link types are defined: • Synchronous connection (SCO) • Asynchronous connection (ACL)

11. Packet format • A packet can cover 1 or 3 or 5 slots • 72-bit access code • 54-bit header • 0-2745 bit payload • One of the fields of the layer 2 header is the 3-bit MacAddr • Use layer 3 control infromation

12. Bluetooth packet format

13. Intra-piconet communication • Forwarding unicast packets by the master from one slave unit to another • Use layer 3 header in the layer 2 payload of the packet— • Forwarding flag(FF) • Destination MacAddr(DA)

14. Bluetooth packet

15. Intra-piconet communication • FF = 0 means that the payload of the packet is destined for the master and needs not be forwarded • FF = 1 means that the payload of the packet is destined for another slave in the same piconet and DA contains the MacAddr of the destination unit

16. Intra-piconet communication • FF = 1 and DA contains 000: the contents of the packet are intended to be broadcast in the piconet • The master knows the Bluetooth address of every slave in its piconet and the corresponding MacAddr • The slave can learn about the 3-bit MacAddr of another slave by querying the master

17. Routing Protocols • Table based routing protocols : • Lightweight Mobile Routing Protocol • Destination-sequenced distance vector Protocol • Murthy-Garcia-Luna-Aceves Protocol • Source based routing protocols : • PARIS • Ring Network Bridges • DSR

18. Source Routing • Source routing vector method leads to a large overhead • But preferable in scatternet systems • No more protocols to construct the table entries and to delete those entries • Easy to find out the duplicate packet when master broadcast

19. Routing Vector Method • Piconet are represented by Local Identification numbers(LocID) • The sequence of LocIDs are carried in the packet header • The overhead is reduced from 48+3=51 bits per hop to 3+3=6 bits per hop

20. Inter-piconet unicast communication • A relay assigns a 3-bit local identifier—LocID to each directly connected piconet • LocID = 000 is reserved to identify the relay itself • BF = 0 : inter-piconet unicast packet • BF = 1 : inter-piconet broadcast packet

21. Routing of unicast packets • BF=0 : the RVF(routing vector field) contains a sequence of alternating id’s LocID,MacAddr,LocId,MacAddr,etc. • LocID is a 3-bit local identifier • MacAddr is a 3-bit MAC address • The source node learns the route via the Route Discovery protocol

22. Routing of unicast packets • When a relay receives such a packet, it sends the packet to the master of the piconet corresponding to the first LocID • The master forward this packet to the unit whose MAC address is given by the first MacAddr • Before sending, it chops off the first pair(LocID,MacAddr)

23. Example • A scatternet is shown as fig3 • 7 piconets : M1~M7 • A packet sent by unit A destined for E • The route is A-B-C-D-E • RVF field : 3,4,2,7,5,6,0 (table 1)

24. Example scatternet

25. Table 1

26. Inter-piconet broadcast • If a unit has already seen this packet,it discard it • If it has not, the unit forwards the packet to all neighbors • Broadcast packets carry BF=1

27. Inter-piconet broadcast • When a relay receives an inter-piconet broadcast packet, it looks in the RVF • If its own Bluetooth address is already in RVFdiscard the packet • If notadds its own Bluetooth address to RVF and send the payload to all masters it is connected to, except the one it has received the packet from

28. The sequence number method • Use master memory to reduce the traffic of broadcast packets • The procedure is similar to the PI protocol • Broadcast packet include the source Bluetooth id(48bit) and a sequence number

29. Route Discovery • Discovery of the first route • The source initiates a broadcast of SEARCH packets • SEARCH packets are Layer 3 control packets that are broadcast in the entire scatternet

30. Table 2

31. Route Discovery(cont.) • The payload of the SEARCH packets accumulates the list of pairs that represent the route from the destination to the source as follows • When the destination unit receives the first SEARCH packet, it returns a unicast REPLY to the source,using the path in the SEARCH packet

32. Conclusion • An environment consisting of a collection of devices communicating with each other over a Bluetooth scatternet • Nodes may join or leave anytime • Non-PC class devices to be the first class participants of the network

33. Conclusion (cont.) • Keep state in the packet in order to minimize state in intermediate nodes • Although a number of ad hoc routing schemes already exist, a unique set of design compromises need to be made for adapting known methods over Bluetooth scatternets

34. Discovery of the second route • Using SEARCH2 message • Route built is similarly to the first • The relays on the first route delay the SEARCH2 packet transmission for a certain period • The relays that are not on the first selected route are given chance to be on the route