Technical method for synchronization within an asynchronous network

Let me explain in further detail. The major difference between a line exchange network like ISDN and packet exchange network such as LAN and IP networks is that the former is based on synchronous communication and the latter is based on asynchronous communication.

In other words, a line exchange network consists of a highly accurate clock at the center and the network operates according to that clock. On the contrary, a special system is needed to synchronize devices on a LAN operating on independent clocks.

The concept of synchronization within an asynchronous network is nothing new. However, there are various issues when attempting to operate such a system. Slowing down synchronization will increase stability but will cause slow start-up. Quick start-up up to synchronization means instability. Control during the excessive state (such as immediately after turning on the power or disconnecting a system) is especially difficult. Although I cannot explain the details, various efforts went in to completing this technology.

One of the techniques used to solve this problem was to define a number of these excessive states and then define each operation. The other characteristic was to install special hardware in the VoIP-LSI to separately assign tasks with the software in order to optimally operate the algorithm. In such way, we created a system to gain synchronization within an asynchronous network. This technology allowed Oki Electric to guarantee communication with no loss on an IP-based broadband network.

Issues of PHS accommodation

PHS was the most difficult among all devices to accommodate. PHS uses a method called TDMA (Time Division Multi Access)-TDD (Time Division Duplex). The system allows multiple persons to use the same frequency band by means of time sharing, rather than one caller dominating an entire frequency band. Other than PHS, this method is also used by cellular phones.

TDMA allows everyone to share a frequency, which is a wireless resource, by accurately dividing the frequency according to a schedule. Time sharing requires accurate time management. Since the system requires accurate time management for it to work, calls cannot be made at times or is disconnected by interference of the adjacent slot when the time does not match. In other words, matching of time is also required in addition to synchronization.

Can LU really accommodate a PHS terminal? From the viewpoint of a developer, this was indeed an extremely difficult and technical task. However, those who sold PBX only felt natural that PHS terminals would be accommodated like the others. This gap in thinking made me suffer a great deal.

PHS will mostly likely operate without fine control in a small-scale network or under little load. However, fine adjustments are required to operate PHS at a level equivalent to a conventional PBX without any problems. I had a hard time convincing this even to those within the company, and I often felt frustrated by this fact. Development of VoIP actually consisted of extremely deep and technical elements even in the most unnoticeable parts. As a developer, it would make me extremely happy if people understood even a little what went into the development of such technologies.

The process of how we overcame the difficulty is explained in the next episode.