Wireless Local Loop (WLL) sometimes is also called radio in the loop (RITL), fixed-radio access (FRA), or fixed-wireless access. It is a system that connects subscribers to the public switched telephone network (PSTN) using wireless signals as a substitute for copper for all or part of the connection between the subscriber and the switch. This includes cordless access systems, proprietary fixed radio access, and fixed cellular systems. Figure 1 shows a standard Wireless Local Loop architecture. 

Figure 1. Standard WLL Architecture.

In this architecture, the communication links from segment 1 to segment 4 could all be wireless. Wireless Local Loop (WLL) is a new communications access method that uses radio waves for transmission of information between customers and service provider sites, rather than traditional fixed methods such as copper or fiber optic delivery. WLL will enable many customers who are not connected to our fiber network to receive a multitude of voice, data and Internet services. Benefits of using Wireless Local Loop includes:

• Multiple services can be delivered over WLL, ensuring higher savings through bundled solutions.
• WLL provides the basis for Intranets, Extranets and Wide Area Networks.
• For customers who are off-net, WLL provides faster provisioning and service upgrade time for communication services.
• For customers who are currently connected to the fixed network, added resilience and/or overflow could be offered through a wireless connection.
• Wireless connections will also provide a more cost-effective connectivity option for customers who are some distance from the core network.
• WLL is designed to provide bursty data services such as Frame Relay and IP.
• WLL provides "always-on" connectivity at a flat rate, without additional dial-up charges.
• Ideal for teleworking or Remote Access to Corporate LANs

2.System Architecture

The wireless local loop system in this project is dedicated to users living in the remote area, where fixed network is not easy to install and cost is very high. In this system, only the communication between the user terminal and base station is carried by wireless. Other communications are on wired network, because the distance between the base station and PSTN switch is not very long. The WLL system architecture in this project is illustrated in Figure 2.
Figure 2. WLL System Deployment Diagram

In this system, every Base Station can serve up to 100 user terminals, which are dispersed in different location with in the coverage cell of a base station. The air interface between the user terminals and base station is carried by CDMA radio signal. Base stations are deployed in different locations around the local center office. The base station controller and PSTN switch are deployed in the center office. Base stations and base station controller are connected by cables. The interface between base station and base station controller is usually called Abis Interface, which is implemented by HDLC protocol at the link layer in this system. Base station is connected to the PSTN switch by cable, the interface between base station controller and the switch is usually called A Interface, which are implemented by Chinese Signaling System 1, Signaling System 7, and V5.1/5.2 Signaling system. Base station controller is also connected to the Operation and Maintenance Center through cable. The interface between Base Station Controller and OAM Center is usually called the Q₃ Interface, which is implemented by RS-232 protocol in this system.

3.1 Software System Architecture
Figure 3. BSC Software Architecture

3.2 Software Development Environment

In this system software are developed on Motorola MC68302 system, which includes MC68302 CPU, MC68302 C Compiler, MC68302 linker, debugger and XRAY Simulator. The operating system is MC68302 Event Driven eXecutive (EDX) system, which is an embedded, multi task, realtime operating system. For software design, SDL (Specification Description Language) and Message Sequence Chart (MSC) are used to describe the protocol and interaction. The Finite State Machine (FSM) is used for protocol optimization and verification. Detailed telecommunications software design must follow the requirement of Protocol Engineering.

The CSS1 module is responsible of translating the wireless signal from the base station into the Chinese Signaling System 1, which is a variant of signaling system 1. The CSS1 module is comprised of five modules: Call Processing module, Multiple Frequency Channel (MFC) signaling module, MFC driver module, Trunk signaling module and Trunk driver module. Figure 4 shows the software module diagram of the CSS1 module.

Figure 4. CSS1 module diagram

Figure 5. MSC of FST Caller Initialization

Figure 6. MSC of FST Callee Initialization

Figure 7. MSC of FST Caller Release

Figure 8. MSC of FST Callee Release

Figure 9. SDL of a state transition

5.PC Controller Design

Figure 10. Component of OAM Center

6.Conclusion

In this project, we designed and implemented a wireless local loop system based on Motorola MC68302 system. My work in this project includes the following parts:
• Design and implement the CSS1 module, which is a solution for A Interface. 
• Design and implement the PC OAM center for the Base Station Controller. The OAM center is responsible for initializing the base station controller and base stations, allocating the trunks, initializing the subscriber information and system maintenance, etc.

 

 
 
 
 
 
 
 

During the design and implementation phases, some valuable experience is obtained:

• Using the formal protocol design tools, such as SDL, MSC, and FSM is very important for telecommunication software design. By now, such formal tools are integrated into the Ration Rose tool kits.
• Because the final software has to be "burnt" into the RAM of the BSC cases, then load and executed by CPU, it is important to keep the software small, robust, and cost-efficient. After coding and testing, the source code has to be optimized, sometimes even at assembling language level. This is different from developing software on the PC, in which easy to read and understand is a critical requirement.