## 3 Asst  
### Basic Concept  
This section presents the basic design of ASST that is based on the following assumptions: (1) a vast field is covered by a large number of homogeneous sensor nodes that communicate with each other through short-range radios. (2) Each sensor node is aware of its own location and uses geographic routing such as GPSR to accomplish long distance delivery. (3) Service information means the service description that describes the characteristic of the service, as shown in Figure 2. (4) The storage space on the sensor node is sufficiently large to save the service information. Recently, sensor nodes with a very large memory space, such as RISE (RIVerside Sensor) with 1 GB of flash memory, have been developed so that sensor nodes can store a large amount of service information and occupy very little space [17]. Services have descriptions that include a service name and a service provider ID. Figure 2A shows an example of the service description of a printer. The service description can differ from applications to application and mainly depends on the types of services. Users wishing to use services provided by a wireless sensor network need to obtain the service description. The simple way to obtain the service description is by query flooding the entire network, as shown in Figure 2B. In the figure, a user with a laptop wants to print documents. Query packets with the service name that the user wants to use are broadcasted to all the nodes in the network until they reach the service provider matching the requirement in the query packet. The user connects to the printer server on the basis of the service  
Figure 2: Example of service description and service location in a wireless network. description reply sent by the service provider. The flooding is easy and simple, but it may require a long time to discover the required service provider. Furthermore, the flooding could cause a considerable amount of energy consumption on the sensor nodes as a result of broadcast storming. Therefore, efficient service location protocols that provide service information quickly and with low network load are required in a wireless sensor network. ASST is based on GHT and extends the DHT. Service descriptions that consist of a service name, a service provider ID, _etc._, are stored in a distributed manner using an algorithm based on DHT. The specific service description is stored in repository nodes that correspond to the hash value of the service name. Geographic routing delivers the information for the service descriptions to and from the repository nodes. When a service provider wants to advertise its service description information on the network, the target point Q(T\({}_{\rm x}\),T\({}_{\rm y}\)) hash key corresponding to the service name is first obtained using a well-known hash function such as MD5 and SHA. Once the target point Q(T\({}_{\rm x}\), T\({}_{\rm y}\)) hash key is obtained, the service provider finds the repository nodes surrounding the hash value and stores the service description in those nodes. For example, in Figure 3, a node provides a printer service and wants to advertise its service to the network. First, the node finds the repository nodes for the printer by calculating the hash key for the printer service. If the printer's hash key is (11, 28), nodes around (11, 28) become the repository nodes for the printer. A user wanting to use a printer finds the repository nodes for the printer by calculating the hash key of the printer service, same as the service provider. Figure 3: Example of ASST. ASST provides a general architecture for service discovery. Both the store and the query operations can be viewed as a general insertion and lookup, and the key can be looked-up in the hash table. Different applications can have different insertion and lookup characteristics. Therefore, different policies should be applied that are based on the characteristics of application.