Wireless Home Networks based on a Hierarchical Bluetooth Scatternet Architecture

Wireless Home Networks based on a Hierarchical Bluetooth Scatternet Architecture W. Lilakiatsakun'. 2, A. Seneviratne' I School of Electrical Engineering and Telecommunication University of New South Wales, Sydney, Australia. 2 Department of Computer Engineering Mahanakorn University of Technology 51 Cheum-Sampam Rd, Nong Chok, Bangkok 10530, Thailand. w.lila@ee.unsw.edu.au. a.seneviratne@unsw.edu.au Abstract Wireless home networking is being addressed by numerous groups such as the HomeRF working group, and the Bluetooth consortium. Of these, Bluetooth technology promises a wider acceptance in term of home networking in the near future. The potential of easily embedding Bluetooth technology in communication devices, computing devices and home appliances make it a more attractive alternative to SWAP[I]. Although the layer up to the data link layer has been standardized by the Bluetooth consortium, how complete home networking solutions can be realized, is still not clear. This paper addresses this aspect, and proposes a way to establish complete home network deploying Bluetooth technology of 721 Kbps provided by Bluetooth is adequate for most data exchanges in a home environment. Finally, voice can also be supported at 64 Kbits per second. Even though, the operating frequency band is at 2.4GHz, the unlicensed band Industrial Scientific-Medical(ISM), Bluetooth still provides means to cope with several unpredictable sources of interference such as cordless phones and microwave ovens. Frequency-hop spread spectrum techniques are applied with a high hopping rate and short packet lengths (1600 hop/s for single slot packets) for noise immunity. Thus Bluetooth technology is quite suitable for a home wireless-networking environment. 1. Introduction In the future, houses will be fully instrumented, which will enable the occupants obtain and process information from the instrument for control, security and management purposes. For example, the status of the heating system, whether it is turned on or off or, the status of the home security system should be able to be monitored remotely. Furthermore, it will provide the infrastructure for connecting the conventional computers and peripheral devices that will be common in households. This scenario is schematically shown in Fig. 1 However, the success of such a system depends significantly on ability of the instruments to be networked at low cost. It is anticipated that Bluetooth will provide this networking capability for a number of reasons. Firstly, Bluetooth devices are small, provides wireless connectivity, and are low cost. Secondly, due to its powerful communication ability, it facilitates easy information exchange. Thirdly, it enables the creation, and modification of network in real time. This enables the network to be set up easily. In addition, the bandwidth Figure 1: Home scenario Despite all its advantages, at present, the Bluetooth specification only covers up to the link layer. Higher layer protocols which are required for intra and inter-piconet communication have not been defined. This severely limits the flexibility of the Bluetooth networks that can be realized. However, if a protocol can be specified for fully supporting intra and inter-piconet communication, the realization of home wireless networks will become 1531-2216101 $10.00 0 2001 IEEE 48 1

significantly easier. In this paper we present such a protocol and illustrates its viability. 2. Piconets and Scatternets Bluetooth units that are within the range of each other can set up ad hoc connections. Devices can share a channel and form a piconet. One device becomes a master regulating traffic between the other devices attached to that piconet. The devices other than the master are regarded as slaves. This master-slave configuration introduces a number of restrictions. Furthermore, slaves can not establish a connection themselves, only the master can make connections. Each piconet generates its own hop sequence using the master identity and clock. Two types of links have been defined which allows the support of voice and data services. Synchronous connection-oriented (SCO) links aimed at supporting voice services reserve two consecutive slots at fixed intervals. Asynchronous connectionless (ACL) links aimed at supporting data services use a master unit directed polling scheme. This is schematically shown in Figure 2. Maser Slave1 Slave2 Figure 2: SCO and ACL links in a piconet with one master and two slaves As mentioned earlier, units that share the same area and are within range of one another can establish ad hoc connections between themselves. This is used for the formation of a scatternet, a group of piconets. Intercommunication between piconets in a scatternet is provided on a time division multiplex basis. That is, a unit can sequentially participate in different piconets, however can only be active in one piconet at a time This is shown in Figure 3. Figure 3: An example of a scatternet 3. Bluetooth Home Network Architecture Since Bluetooth has been designed as short-range radio interfacing, normally a range of 10 meters, and a maximum of eight active devices can be attached to a piconet. The home networks however need a much larger coverage than 10 meters and also may be required to support more than 8 active devices. Thus, a scatternet, which is d.esigned for a home network, need to overcome these restrictions of standard Bluetooth scatternets. Bluetooth provides flexibility in establishing an ad hoc network. 'Thus the formation of scatternet can be quite complex. This leads to numerous problems. Firstly, in term of communication, routing protocol between piconets becomes complicated. Secondly, the number of hops and communication delay become unpredictable. To ovwcome these drawbacks we propose the method specifying the formation of scatternet in a particular fashion. The proposal is to use of a two level piconet hierarchy when forming a scatternet. The level 1 piconets are used for communication among themselves. To distinguish them from standard piconets we refer to them as personal area networks (PANs). The level 2 piconet is primarily used for providing interconnectivity (routing) of PANs. To distinguish this we refer to it as a routing area network (RAN). A slave in each PAN is also a slave of its RAN. These slaves act as gateways, between the PANs and their respective RANs and are called the gateway slaves. In PANs devices that act as the master and gateway slave are fixed. The other nodes can be mobile devices. The master of the RAN acts as a router and have to be fixed devices as well. This two level Bluetooth home network architecture is depicted in Figure 4. 482

The first field, COM (Communication Type) selects the type of communication. The types of communication supported are shown in Table 1. Figure 4: Bluetooth Home Network Architecture The Network is initially created as a RAN. The master node of the RAN will elect its slaves and assigns them an address. This address becomes the Piconet Addresses (PA) of that node. The slave member nodes of the RAN will be in a parked state, and will wait for a page from the master of PAN it belongs to. In addition to the above, each slave of a PAN is given a number as Device Address (DA). The combination of PA and DA are used in routing protocol in order to uniquely indicate a source and destination node in a packet header. 4 Routing Protocol Routing protocol for Bluetooth scatternets has already been proposed [2]. This protocol is suitable for short lived and small sized network. However, as explained earlier, home networks will have a longer life-time and will be larger in size. Therefore, the protocol proposed in [2] cannot be used.. To overcome this we proposed the use of new protocol. The operation of the protocols is described in section 4.2. 4.1 Protocol Header Format The header format of the proposal protocol is shown as Figure 5. It consists of 2 byte layer 3 header with 8 fields Table 1 : Communication in proposed architecture l+l Meaning Reserved PAN-PAN Not used The second field is the M (Master Flag). This defines whether the packet is of a master node within the destination network. If this bit is set, the master of a PAN in destination network will treat received packet as its own. The third field is the SW(Switch Flag). This is used for switching between PANS and RANs. When Switch Flag is set, it is switched from PAN to a RAN or vice versa, i.e. - any slave which receives the packet, will automatically pass it to either the RAN or PAN its connected. If it is not set, the packet belongs to the receiving slave. The fourth field, the Destination Piconet Address (DPA), used for indicating which piconet is to be routed. The fifth field, the Destination Device Address (DDA) indicates the address of the destination node within given piconet defined by DPA. In case of value is zero, means PAN broadcasting. The sixth field, the Source Piconet Address (SPA) indicating to the received devices which piconet transmitted the packet. The seventh field, the Source Device Address (SDA) indicates the originating device within given piconet defined by SPA. SPA-SDA and DPA-DDA, are used to specify a unique source and destination node. Finally, the payload is the information from higher layer such as application functions, control information and so on. 4.2 Protocol Operation Figure 5: Packet Header Format Two modes of communications are supported in our proposed architecture: intra-piconet referred as PAN communication, and inter-piconet referred as PAN-PAN communication. Both unicast and broadcast communication will be supported. Similarly to other protocols, a destination device address (DDA) zero is 483

used for broadcasts, and non zeros DDA's are used for unicast communications. 4.2.1 PAN or Intra-Piconet communication All nodes in a piconet, can communicate with each other. However, all traffic is driven by the master node,. Thus, the master-slave communication requires only one hop. Whereas slave-slave communication will requires two hops, none from the slave-to the master, and another from the master to the slave. The this mode of PAN communication is shown in Figure 6. maximum hops for a PAN-PAN communication is 6 hops as shown in Figure 7. In Figure 7, a notation of Slave#m/n, is used, where m is the address of piconet, assigned by the master RAN node, and n is the address of device assigned by the master PAN node. The term of X in tables is the unspecified meaning or don't care term. \ PAN#4/ PANM Figure 6: An example of PAN communication Table 2: the packet header of PAN communication Figure 7: An example of PAN-PAN communication Table 3: The packet header of PAN-PAN communication Pih shvcni t ~~ata Mater to Slaven2 COM M SW DPA DDA SPA SDA 11 n I I 2 I I o I) n I 2 I I I Path 1 COM 1 M I SW I DPA I DDA I SPA I SDA I Let us assume that slave #1 in Figure 6 wants to send packets to slave#:! which is within the same piconet. First, slave#l creates packet header as shown in the first row of Table 2. It will first put its own piconet and device address in the SPA and SDA respectively. COM is defined as 0 to indicate intra PAN communication. DPA is same as SPA, and 1 implies that the source and destination node are in same piconet. DDA, 2, informs the master the identity of the destination node. M is set to zero to indicate that it is slave-slave communication, with the master acting as a switching node. When a packet is transmitted by the master, the SW flag is assigned by it as follows. If the COM field is 0 that means the destination node is within its piconet, the SW flag will be set to zero. Otherwise it will be set to 1. Receiving slave nodes, will only look at the SW flag. If it is zero, slave will treat packets as its own. 4.2.2 PAN-PAN or Inter-piconet communication The traffic between piconets is regulated by a RAN master node as well as by a PAN master node. The 0 0 0 3 3 I I Assume that slave#l of PAN#l wants to send the packets to slave#3 of PAN#3. The operation of the protocol along each hop is summarized in Table 3. The first row of Table 1. shows a COM field 2, as it's a PAN- PAN communication packet. M is zero because the to the destination is not the master node. DDA and DPA are set to 3 as they indicate destination node and the destination piconet. ]?inally, SDA and SPA are set to 1, as they indicate source node and originating piconet. When the packet is received by the master node of PAN1 it first examines COM field. As COM field is set to 2, it will recognize that the received packet is not of its piconet. Therefore:, it will decrease COM field by one and alter SW to 1, and forward this packet to gateway slave. The gateway slave will check whether SW is set to 0. If it is not zero, slave node will relay the packet to the to the 484

master of the other piconet it is a member of, ie. the RAN. When the packet arrives at the master node of RAN, it again checks the COM field. In the case when it is non-zero, the master node decreases the COM, and forwards the packet to a slave node defined by the DPA. The slave processes the packet in a similar manner when it was received from the RAN master node. Finally, when the packet reaches the master node of the destination piconet, it will perform as PAN communication. As can be seen from Table 3, the master nodes only need a forwarding function, and the ability to compare and modify two fields, COM and SW. The COM field will be decreased by one after the packet header is processed by master nodes. SW flag will be assigned by master node to be one whenever the hop will have to change from PAN to RAN or vice versa. When the last hop is reached, this flag will have to be 0. That means the next node is the destination and no more packet forwarding is required. The gateway slave nodes simply provide switching functionality and the functionality necessary to decide whether to forward the packet or not. The forwarding decision is done using the SW field. When packets arrive, SW flag is checked. If this flag is set, the gateway slave has to switch itself to join with another piconet and send the packet to the new master, otherwise the packet is owned by receiving slaves. [2] P.Bhagwat, A.Segal1, A routing Vector Method (RVM) for Routing in Bluetooth Scatternets, Mobile Multimedia Communications, 1999. (MoMuC 99), 1999 IEEE International Workshop on 1999, pp.375-379 [3] M.Kalia, S.Garg, R.Shorey, Efficient Policies for Increasing Capacity in Blutooth: An Indoor Pico-Cellular Wireless System, Vehicular Technology Conference Proceedings, V01.2,2000, pp.907-91 1 5 Conclusion We have proposed method of extending the coverage of Bluetooth networks for the home networks which require wider coverage,. and the support of a larger number of nodes. This overcomes the problems of pic0 and sctternet architecture that have been proposed for Bluetooth networks, using a hierarchical network architecture. The only limitation of our architecture is that the master and salve gateway nodes, need to be fixed. We do not believe that will be a serious limitation as all devices in a home network need not be mobile. We believe this approach should be able support a greater number of nodes: at least 64 active nodes. More than nodes can be added by means of parkedunparked state seamlessly [3]. Furthermore, we can provide deterministic delays using this approach. We are in the process of simulating the operation of the proposed architecture, and the early results indicate that it provides an efficient way of providing larger coverage for Bluetooth based networks efficiently. 6 References [I J http:nwww.hornerf.or&ech 485