Related works

For the purpose of enhancing the efficiency of NP service, some researches tended to enhance the efficiency of NP services by improving the response time of NPDB queries. [11][11] proposed to enhance the efficiency of NPDB queries by improving searching algorithm. However, bandwidth is scarce resource of telecommunications networks, which is occupied during the process of call setup.

When data search delay exceeds the threshold, the efficiency of NP call process degrade rapidly that will result in the congestion of transmission lines. Enhancing the efficiency of NPDB queries is not sufficient. Unless service providers mitigate the heavy traffic load or establish more telecommunications lines to tolerate the traffic load, the enhancement is limited.

In order to alleviate the heavy workload of operator networks to enhance the efficiency of NP service, many studies proposed that implementing caches in telecommunications system can effectively alleviate the amount of query messages and improve the efficiency of data access [16][4][5][20]. Kim and Yong stated the factors affecting the cache hit ratio in mobile computing environment including the distribution of queried target objects and the query pattern [16]. Telecommunication

networks are designed as computation and intelligent centric, implementing caches in operator networks is easy to maintain and benefit environment-dependent decision making [22]. Refer to the hierarchy of telecommunications network (Fig. 2-1), central switches serve so many users and the dialed numbers a central switch receives is scattered. Every NP call requires ported number translation. The size of a cache should be large to accommodate sufficient data (double of the size of the numbers dialed from users) to achieve acceptable hit ratio. Jain et al. proposed a hashing scheme in [11] to improve cache hit ratio. But keeping the large amount of portable users in FMC environment will require a lot of memory size. The cache size and the probability of collision will increase as the amount of portable users increases. Chan and Leong addressed that clients should take a more active role in maintaining cached items. In [21], [16] and [4], authors suggested distributing spatial replicas of databases to different sites and proposed caching schemes on mobile handsets (MS) with respect to environment properties to provide efficient data access to users. However, mobile users move in and out of several service regions, the limited cache size on a MS will be hard to keep ample environment information. In order to guarantee the validity of cached data, MS need to update cached data frequently according to temporal and spatial properties. The requests of cache updates from a large amount of MS will cumulate the traffic of the operator network.

The study of Carpenter et al discovered that 99% of the calls are set to the numbers had been called in a week, and assumed that an individual customer’s calling behavior exhibits a strong locality of reference [5]. Assuming intelligent peripherals are available, they proposed to maintain a profile and a cache of a user ’s frequently accessed data in the user ’s terminal. The work of [6][7] presented the chance to solve NP problem by enhanced user terminals. The results showed that shift knowledge to intelligent peripherals with better computation power and storage capacity can minimize information passing and effectively alleviate traffic load of core network.

Therefore, performing ported number translation in user terminals will alleviate the traffic load of NPDB queries and mitigate the workload of NP call process in operator networks. But it is expensive and tardy in updating user terminals comprehensively.

And the update of the routing information in local caches will occupy telecommunications lines, which consumes extra communication bandwidth and crowd out arrival calls.

While a comprehensive solution to solve NP problem is not available, another solution is to find some mechanism which can effectively reduce the amount of requests needed for ported number translations. If the amount of requests which need for ported number transla tion is small, the switch system will have enough computation capability to handle the received call request. From the result of [5],

number translation should be performed in a network entity with the properties of evident dialed number locality; consequently, the quantity of routing information needed to be maintained is modest. In addition, the network entity must possess ability of storage and computation to keep valid routing information of ported numbers and to provide the service of ported number translation.

In the fixed- lined telecommunications environment, most organizations establish PBX to save telecommunications cost and benefit intra-organization communications.

That is, most of the calls (both NP and non-NP calls) in business hours are relayed by PBX to the public telecommunications network. PBX is a network entity with computation power and storage that can perform the function of ported number translations. In business hours, a major part of the calls are generated from organizations. Applying the knowledge of number translation and keeping the routing information of ported numbers in organization-based telecommunication networks will be an effective approach to enhance the efficiency of NP service. In mobile telecommunication systems before 2.5G, intelligent terminals and data transmission channels are not available. Ported number translations can not be performed in user terminals. An organization-based network which possesses the property of dialed number locality can be utilized to provide the service of ported number translation.

However, mobile PBXs are not generally available. A network entity to act as a mobile PBX in the mobile telecommunication network is required. In 3G mobile telecommunication systems, user terminals are smart and powerful to perform computation and storage tasks. Because the dialed numbers of an individual user often presents strong locality, keeping the routing information of a user’s frequently dialed ported numbers in the terminal will benefit the efficiency of NP service.

On the other hand, a mechanism to synchronize the distributed routing information with that in NPDB is necessary to guarantee that every dialed ported number can be translated to the right destination address. Dispatching routing information from NPDB to organization-based network and 3G mobile terminals can be transmitted by IP-based networks without consuming telecommunication transmission resource. When ported numbers are resolved in the early stage of NP call process, the translated routing information must be recognizable to the public switching network. A method for local telecommunications networks and 3G user terminals to notify the switching core networks that the dialed numbers were translated to effective addresses is also needed.

Based on the above idea, the following three mechanisms which can tell the central switching system that the call-origination request needs for ported number translation or not are proposed:

n PBX with ported number translation capability

n An organization-based mobile PBX system with ported number translation capability

n The dual mode mobile phone with ported number translation capability

The details of these three mechanisms are described in the following three chapters.