1. Field of the Invention
This invention relates to a packet data transmission system in radio communication systems centering on telephones such as a digital cellular system and a digital cordless telephone system.
2. Description of the Related Art
In a conventional radio communication system consisting of a plurality of base stations, a switching system, and a plurality of mobile stations existing in a radio zone formed by the base stations, to transmit and receive telephone and data signals between the base and mobile stations, a TCH is assigned between the mobile and base stations using a CCH, then the TCH is used to execute one-to-one two-way data communication between the mobile and base stations.
For example, as described in Japanese Patent Unexamined No. Hei 6-77886, when receiving a data communication request from a mobile station, the base station and switching system select one or a plurality of TCHs in accordance with the channel use state and if the requested TCH can be selected, assigns the TCH to the mobile station, which then executes data communication on the assigned TCH.
On the other hand, to transmit and receive data between a plurality of mobile stations and a base station, in the radio communication system a common channel is provided for accommodating a plurality of mobile stations and data is transmitted and received by making random access in the common channel in addition to the above-mentioned method of assigning a TCH between mobile and base stations by the line switching for providing a one-to-one two-way data communication line. The random access method is used for a CCH in the line switching.
For example, as described in Japanese Patent Unexamined No. Hei 5-95358, in channel contention control in the random access system, an up link signal from a mobile station to a base station and a down link signal are put into slots on a common channel, the slots of the up and down link signals are synchronized with each other, and transmission disable/enable information for controlling mobile station transmission is suffixed to each of the slots making up the down link signal.
In each base station, the up link channel is monitored in each slot and in the slot detecting an up link signal from a mobile station, the transmission disable/enable information suffixed to the slot of down link signal is changed to transmission disable for disabling other mobile stations from originating calls in the next slot for avoiding signal collision.
When receiving the transmission disable/enable information indicating transmission disable from the base station, the mobile station where up link information to the base station occurs continues to monitor the transmission disable/enable information until the information is changed to transmission enable. When the information is changed to transmission enable, the mobile station judges transmission start or standby in the next slot according to the transmission probability.
In the above-mentioned data communication by the line switching, one mobile station occupies one TCH and high-speed data communication can be conducted, but a connection delay occurs because of the TCH assignment process. In the data communication by the random access system, the connection delay is small, but if a plurality of mobile stations access one TCH, high traffic is handled and the throughput lowers. To use a plurality of TCHS, management among the TCHs is required.
In the invention, radio physical channels for transmitting and receiving user information are called communication channels (TCHS) and those for transmitting and receiving control information used for radio channel establishment and call connection are called control channels (CCHs). However, to use all or some of TCHs to transmit and receive the control information, all or some of the TCHs are logically called TCH attendant control channels (ACHs). Further, when a TCH is used to transmit and receive a telephone signal, it is logically called a voice channel (VCH) and when a TCH is used for data communication using random access, it is called a random access channel (RACH).
That is, the VCH and RACH represent different channels in function, but are TCHs as radio physical channels.