A Routing Vector Method (RVM) for Routing Bluetooth

Scatternets

Pravin BhagwatIBM.Thomas J. Watson

Research Center ,Yorktown Heights,NY

Introduction for Bluetooth

Operates in the globally available 2.45GHz ISM bandUse a frequency-hop / time-division-duplex (FD / TDD) schemeThe channel is divided into 625-msec intervals—slots

Introduction (master-slave)

The master transmission starts in even-numbered slotsThe slave transmission starts in odd-numbered slotsA slave is allowed to transmit in a given slot if the master has addressed it in the preceding slotNo direct MAC layer communication between slaves

Introduction (cont.)

A group of interconnected piconets is referred to as a scatternetA node can participate in two or more piconets on a time sharing basisA unit can receive packets from one piconet and relay to another piconetProtocols for inter-piconet communication have not been defined

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

Scatternets vs Wireless LANs

Two bluetooth nodes cannot hear each other unless they form a master-slave pairIn the wireless LAN, any two nodes within proximity can hear each other’s transmission

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

Purposes

Describe the protocols for route discovery and packet forwardingIllustrates 3 main design compromises: Minimization of soft-state Protocol simplicity Bandwidth conservation

Model Elements

Every Bluetooth unit has a globally unique 48 bit Bluetooth addressA unit can be a slave in two or more piconets and a master in another piconetsA unit belongs to more than one piconet is referred to as a relay

Model Elements (cont.)

Slaves in a piconet are assigned 3 bit MAC addresses—MacAddrThe 000 address is reserved for broadcast within the piconet2 different link types are defined: Synchronous connection (SCO) Asynchronous connection (ACL)

Packet format

A packet can cover 1 or 3 or 5 slots72-bit access code54-bit header0-2745 bit payloadOne of the fields of the layer 2 header is the 3-bit MacAddrUse layer 3 control infromation

Bluetooth packet format

Intra-piconet communication

Forwarding unicast packets by the master from one slave unit to anotherUse layer 3 header in the layer 2 payload of the packet— Forwarding flag(FF) Destination MacAddr(DA)

Bluetooth packet

Intra-piconet communication

FF = 0 means that the payload of the packet is destined for the master and needs not be forwardedFF = 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

Intra-piconet communication

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

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

Source Routing

Source routing vector method leads to a large overheadBut preferable in scatternet systemsNo more protocols to construct the table entries and to delete those entriesEasy to find out the duplicate packet when master broadcast

Routing Vector Method

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

Inter-piconet unicast communication

A relay assigns a 3-bit local identifier—LocID to each directly connected piconetLocID = 000 is reserved to identify the relay itselfBF = 0 : inter-piconet unicast packetBF = 1 : inter-piconet broadcast packet

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 identifierMacAddr is a 3-bit MAC addressThe source node learns the route via the Route Discovery protocol

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 LocIDThe master forward this packet to the unit whose MAC address is given by the first MacAddrBefore sending, it chops off the first pair(LocID,MacAddr)

Example

A scatternet is shown as fig37 piconets : M1~M7A packet sent by unit A destined for EThe route is A-B-C-D-ERVF field : 3,4,2,7,5,6,0 (table 1)

Example scatternet

Table 1

Inter-piconet broadcast

If a unit has already seen this packet,it discard itIf it has not, the unit forwards the packet to all neighborsBroadcast packets carry BF=1

Inter-piconet broadcast

When a relay receives an inter-piconet broadcast packet, it looks in the RVFIf its own Bluetooth address is already in RVFdiscard the packetIf notadds 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

The sequence number method

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

Route Discovery

Discovery of the first routeThe source initiates a broadcast of SEARCH packetsSEARCH packets are Layer 3 control packets that are broadcast in the entire scatternet

Table 2

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 followsWhen the destination unit receives the first SEARCH packet, it returns a unicast REPLY to the source,using the path in the SEARCH packet

Conclusion

An environment consisting of a collection of devices communicating with each other over a Bluetooth scatternetNodes may join or leave anytimeNon-PC class devices to be the first class participants of the network

Conclusion (cont.)

Keep state in the packet in order to minimize state in intermediate nodesAlthough 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

Discovery of the second route

Using SEARCH2 messageRoute built is similarly to the firstThe relays on the first route delay the SEARCH2 packet transmission for a certain periodThe relays that are not on the first selected route are given chance to be on the route