In 1999, Mitsubishi Electric Corp. introduced a large-scale, business-use PHS network system that has enabled the staff to use PHS anywhere in their offices and factories throughout the country. Using the PHS terminal distributed to each staff member, they can make calls without knowing the whereabouts of the receivers on those premises.
As of September 1999, the system was made accessible by some 2,500 employees at the headquarters and a branch office in the Tokyo area, plus some 1,500 at an office and a factory in the Osaka area. The company plans to expand the system in the near future to involve some 18,000 employees at its 40 offices and factories.

Basic system configuration

This system encompasses indoor PBXs, each installed in an office or factory and connected to a business-use PHS CS (Cell Station). All PHS calls are forwarded to main lines, leased lines and then PHS PSs (Personal Stations) or regular telephones via those CSs (see Fig. 1).
To achieve smooth inter-office roaming, the company has adopted 1.5 Mbit/s digital leased lines. When a PHS PS is moved from the coverage of its “home” PBX to that of another “visitor” PBX, the PS’s location information is transmitted via D channel on the leased lines.

Inter-office roaming

The PHS PSs belonging to the same office/factory can be reached by dialing their respective extension number. On the other hand, PHS PSs belonging to other offices/factories can be reached by dialing the designated number for the office/factory plus their extension number (see Fig. 2-A). The employees follow the same dialing procedures even when a target PHS PS has been moved to a “visitor” PBX’s coverage (see Fig. 2-B).
Due to the system design, incoming calls via PSTNs, including those from customers, are forwarded to the PHS PSs through dial-in connections. For the convenience of the sales staff who are often outside the office, the company has set up public PHS subscriptions for their PSs. Incoming calls are automatically transferred to their PSs when they are away from the office via two public PHS networks at the current time.

Remarkable results

About 90% of the employees who have accessed their private PHS network said they appreciated the system. Its improved functionalities have proven much more satisfactory compared with the previous system, which did not offer connections with wired telephones or inter-office roaming.
Among the advantages of introducing this PHS network system are:

• the ability of staff to contact each other without knowing the called parties’ whereabouts;
  
• lowered possibility of important calls from customers being missed, in particular by the sales staff;
  
• lessened need to transfer calls, since the PHS PSs are reachable as long as they are within the “home” or “visitor” PBX coverage;
  
• phone calls can be made when visiting an office or factory away from their home office, as long as they are within the coverage of the CS on the premises; and,
  
• list of phone numbers that has been input by the staff on their PSs can be used on a business trip.