Mobile communication system with access function to computer network

In a mobile communication system, when a mobile station (PS1 to PSj) makes a request for connection to a service provider (SP), a base station (BS1 to BSn) acquires an IP address "IP1" assigned for the base stations and assigns to the requesting mobile station an IP address "IP2" for the mobile stations that has been acquired in advance in place of the IP address "IP1". When data transfer is started, the base station changes the source address in data sent by the mobile station from "IP2" to "IP1" and then transfers the resulting data to the service provider. On the other hand, the base station changes the destination address of data sent by the service provider from "IP1" for the base stations to "IP2" for the mobile stations for subsequent transfer to the mobile station.

BACKGROUND OF THE INVENTION 
The present invention relates to a cellular mobile communication system 
such as a mobile telephone system or a cordless telephone system, and more 
specifically to a system having a function of making access to a computer 
network such as Internet. 
With the recent spread of mobile communication systems, such as cellular 
telephone and cordless telephone systems, etc., attention has been paid to 
so-called mobile computing which allows radio terminals used in these 
systems to make access to Internet. 
In mobile computing using the dialup IP connection scheme that is the most 
common one, the public network is first called by a radio terminal through 
a radio base station and the server of a service provider is then called 
via the public network. The server of the service provider assigns an IP 
address to the calling radio terminal via the public network and the radio 
base station. After that, the use of the IP address thus acquired permits 
the radio terminal to make access to Internet over the radio base station, 
the public network, and the server of the service provider. 
To make access to Internet via the service provider in this manner, each 
user is individually requested beforehand to register himself or herself 
with the service provider. On the other hand, some of companies, 
department stores and hotels have been making an attempt to register as an 
organization with the service provider and to allow each staff or customer 
to make access to Internet through the use of their respective 
privatemobile communications systems. 
However, the conventional mobile communications systems merely connect the 
calling user terminal and the service provider server by a communications 
link including a radio channel only. For this reason, even with the use of 
the private mobile communications system registered as an organization 
with the service provider, each user cannot make access to Internet unless 
he or she registers himself or herself with the service provider. 
BRIEF SUMMARY OF THE INVENTION 
It is a first object of the present invention to provide a mobile 
communication system which allows users to make access to a computer 
network, such as Internet, with no need for each of the users to register 
with a service provider. 
To attain the first object, in the mobile communication system of the 
present invention, a first address is acquired from the provider when a 
mobile station makes a request for connection to the computer network and 
the requesting mobile station is assigned a second address acquired in 
advance in place of the first address. When the mobile station sends data 
that makes the second address its source address, the source address is 
changed from the second address to the first address and the data 
subjected to address replacement is transferred to the provider. On the 
other hand, when data containing the first address as a destination 
address is sent from the provider, the destination address is changed from 
the first address to the second address and the data subjected to address 
replacement is transferred to the mobile station. 
The address assignment and replacement functions may be incorporated in 
each of base stations, or an switching or control unit within a 
communication network. 
According to the present invention, data transfers between mobile stations 
and base stations or the communication network are made by the user of the 
second address uniquely assigned to the mobile stations by the mobile 
communication system, and data transfers between the base stations and the 
communication network or computer network are made by the use of the first 
address assigned to the system by the provider. Thus, it becomes possible 
for even a mobile station that is not registered with the provider to make 
access to the computer network, such as Internet, via the provider by 
utilizing the mobile communication system that is registered as an 
organization with the provider. 
A second object of the present invention is to provide a mobile 
communication system which permits service information the communication 
system provides to be easily obtained without making access to a computer 
network such as Internet. 
To attain the second object, the mobile communication system a storage unit 
for storing service information prepared in advance and a service 
information transfer function. When a signal to make a request for 
retrieval of the service information is sent from a mobile station, the 
service information is read from the storage unit, then sent to the 
requesting mobile station. 
To be specific, a plurality of base stations form their respective radio 
zones and, for each of the radio zones, local service information peculiar 
to the area is created and stored in the storage unit. When a request for 
local service information is sent from a mobile station, the radio zone 
where it is situated is determined and local service information peculiar 
to that radio zone is selectively read from the storage unit, then sent to 
the requesting mobile station. 
Thus, the mobile communication system of the present invention allows a 
user at each mobile station to obtain local service information, such as 
an area map, information about noted places, etc., that has been created 
uniquely by the mobile communication system with no need to make access to 
the computer network such as Internet. 
A third object of the present invention is to provide a mobile 
communication system which permits a location to be accessed to be changed 
easily even in a state where a radio link remains held. 
To attain the third object, the mobile communication system of the present 
invention is provided with a decision function of, when data is sent from 
a mobile station, deciding whether a destination address contained in the 
data is a third address for the mobile communication system acquired in 
advance or a fourth address other than the third address. When the 
decision is the third address, local service information is read from the 
storage unit according to the data contents, then sent to the sending 
mobile station. On the other hand, when the decision is the fourth 
address, the first address is acquired from the provider and the source 
address contained in the data from the mobile station is changed from the 
second address to the first address. The resulting data is sent to the 
provider. 
According to the present invention, therefore, when a mobile station sends 
data to access the computer network while accessing local service 
information of the mobile communication system, the location to be access 
is automatically changed to the provider with the radio channel between 
the mobile station and the base station held. For this reason, there is no 
need for the mobile station to disconnect the radio channel for subsequent 
retransmission and make another request for connection to the provider. 
Additional object and advantages of the invention will be set forth in the 
description which follows, and in part will be obvious from the 
description, or may be learned by practice of the invention. The object 
and advantages of the invention may be realized and obtained by means of 
the instrumentalities and combinations particularly pointed out in the 
appended claims.

DETAILED DESCRIPTION OF THE INVENTION 
(First Embodiment) 
Referring now to FIG. 1, there is illustrated a first embodiment of a 
mobile communications system of the present invention, which is equipped 
with multiple base stations BS1 to BSn that are distributed throughout a 
service area. The base stations form their respective radio zones Zl to 
Zn. The diameter of the radio zones is set to 100 to 500 meters for a 
cordless phone system and to several kilometers for a cellular phone 
system. In the radio zones Zl to Zn, each of mobile stations PS1 to PSj is 
connected to a corresponding one of the base stations BS1 to BSn by a 
radio channel. 
To connect the mobile stations PS1 to PSj and the base stations BS1 to BSn 
by radio channels, the four-channel multiplex multi-carrier TDMA (Time 
Division Multiple Access)--TDD (Time Division Duplex) technique is used by 
way of example. In this technique, as shown in FIG. 4, a transmit signal 
frame sent on each of radio frequencies f1 to fn is made up of four time 
slots UL1 to UL4 for uplink from mobile station PS to base station and 
four time slots DL1 to DL4 for downlink from base station BS to mobile 
station PS. Four pairs of time slots are made by combining one of the 
uplink time slots UL1 to UL4 with one of the downlink time slots DL1 to 
DL4 and each pair of time slots is assigned to a mobile station as a 
two-way radio channel, thereby allowing radio communication between the 
mobile station and a base station. That is, a single radio frequency 
provides four two-way radio channels. 
The Personal Handyphone System (PHS) that is now used in Japan employs 37 
radio frequencies (f1 to f37), f12 and f18 being used for control purposes 
and the other frequencies being used for communication. Of the radio 
frequencies for communication, f1 to f10 are used for direct communication 
between mobile stations and the remaining frequencies are used for 
communication with a public network via base stations and communications 
between base stations installed in homes and mobile stations. 
Each of the base stations BS1 to BSn is connected by a corresponding one of 
subscriber lines SL1 to SLN to a public network PNW, which, in turn, is 
connected to a service provider SP by a private line PL. The service 
provider SP establishes connections between the base stations BS1 to BSn 
and Internet INW on demand. The public network consists of an integrated 
service digital network (ISDN). 
In this embodiment, the mobile communication system is registered as an 
organization with the service provider SP. Owing to this registration, the 
mobile communication system is assigned a fixed IP address "IP2" in 
advance by the service provider SP. The IP address "IP2" is assigned to 
each of the mobile stations BS1 to BSn when it issues a request for 
connection to Internet INW. 
As shown in FIG. 2, each of the base stations BS1 to BSn is composed of a 
radio unit 1 equipped with an antenna, a modem 2, a TDMA unit 3, a line 
interface unit 4, a controller 5, and a memory 6. 
A radio frequency signal sent over a radio channel by a mobile station is 
received by the antenna 11 and then entered into a receiver 13 via a 
switch 12 for switching between sending and receiving. In the receiver 13, 
the received radio frequency signal is mixed with a local oscillation 
signal generated by a frequency synthesizer 14 for conversion into an 
intermediate frequency signal. The frequency of the local oscillation 
signal generated by the synthesizer 14 is specified by the controller 5. 
The radio unit 1 is equipped with a received electric field strength 
detector (RSSI-DET) 16 for detecting the strength of the radio frequency 
signal sent from the mobile station. The detected value is presented to 
the controller 5. 
The intermediate frequency signal output from the receiver 13 is applied to 
a demodulator 21 of the modem 2. In the demodulator 21, the intermediate 
frequency signal is subjected to digital demodulation, whereby a digital 
communication signal is recovered. 
In a TDMA decoder 31 of the TDMA unit 3, the digital communication signal 
is separated with each time slot and the resulting digital communication 
signals are input to the line interface unit 4. 
The line interface unit 4 has a codec 41 and an interface (I/F) 42. The 
digital communication signals are subjected in the codec 41 to error 
correction and voice decoding for conversion to baseband digital 
communication signals and then sent from the interface 42 through the 
corresponding subscriber line to the public network PNW. 
On the other hand, an incoming digital communication signal sent from the 
public network PNW over the corresponding subscriber line is input via the 
interface 42 to the codec 41 where it is subjected to voice encoding and 
error correction and then input to a TDMA encoder 32 of the TDMA unit 3. 
In the TDMA encoder 32, voice data output from the codec 41 are time-slot 
multiplexed with voice data associated with other channels. Transmit data 
output from the TDMA encoder 32 is input to a modulator 22 which converts 
it to a digital modulated signal. A transmitter 15 converts the digital 
modulated signal to a radio frequency signal by mixing it with a local 
oscillation signal generated by the synthesizer 15 and amplifies the radio 
frequency signal up to a predetermined transmission power level. The radio 
frequency signal from the transmitter 15 is transmitted to the mobile 
stations via the switch 12. 
The memory 6 consists of, for example, a semiconductor memory, such as ROM, 
RAM, or the like, or a hard disk, in which control programs for the 
controller 5, various pieces of control data, a protocol for making access 
to the service provider SP, and the IP address "IP2" assigned in advance 
by the service provider SP are stored. 
The controller 5, equipped with a microcomputer as its main control unit, 
has a provider access control function 51, an IP address assignment 
control function 52, and an IP address replacement control function 53 in 
addition to normal applications such as a radio link control function and 
so on. 
The provider access control function 51 responds a request for connection 
to the service provider SP issued by a mobile station SP to establish a 
communication link with the service provider in accordance with a 
protocol, such as TCP/IP (Transmission Control Protocol/Internet 
Protocol), and then obtains an IP address "IP1" for the base station from 
the service provider SP. 
The IP address assignment control function 52 assigns to that mobile 
station SP which issued a request for connection to the service provider 
SP the IP address "IP2" for mobile stations that has been acquired in 
advance in place of the IP address "IP1" just acquired from the service 
provider. 
When the base station receives data sent from the mobile station PS toward 
the service provider SP, the IP address replacement function 53 replaces 
the IP address "IP2" indicating the sending end and appended to that data 
with the IP address "IP1" for base stations. Also, it changes the IP 
address which indicates the location to which the data is to be directed 
and which is appended to the data sent from the service provider SP from 
the "IP1" for base stations to the "IP2" for mobile stations. 
Next, the operation of the mobile communication system thus arranged will 
be described taking a request for connection to the service provider SP 
sent from the mobile station SP1 to the base station BS1 by way of 
example. FIG. 6 shows a sequence illustrating the operation. 
First, when a dialup connection operation is performed in the mobile 
station SP1, a radio communication link is set up between the mobile 
station SP1 and the base station BS1. When, in this state, the mobile 
station PS1 sends a request to establish a link with the service provider 
SP, the base station BS1 sends data necessary to establish a link to the 
server of the service provider SP to thereby make a request to establish a 
link. In response to this, the server sends an acknowledge signal to the 
base station BS1 after making sure that the received data satisfies 
predetermined conditions. Upon receipt of the acknowledge signal, the base 
station BS1 notifies the connection requesting mobile station PS1 that the 
link has been established. 
Upon receipt of that notification, the mobile station PS1 next issues to 
the base station BS1 a request for Internet access (request for an 
address). The server then assigns to the base station BS1 the IP address 
"IP1" for access to Internet. Upon receipt of the IP address "IP1", the 
base station BS1 holds it for itself and instead assigns to the mobile 
station BS1 the IP address "IP2" for mobile stations that has been 
provided in advance by the service provider SP. Thus, the mobile station 
BS1 becomes enabled for access to Internet. The base station BS waits for 
transmit data from the mobile station PS1. 
Assume here that the mobile station PS1 sends data in which IPx is set as 
the destination address to make access to a certain server on the network 
and IP2 is set as the source address as shown in FIG. 4. When receiving 
the transmit data over a radio communication link, the base station BS1 
replaces the IP address "IP2" indicating the location that sent the 
transmit data with the IP address "IP1" assigned to the base station by 
the service provider SP. The base station then sends the transmit data 
subjected to address replacement to the server of the service provider SP 
over the public network PNW as shown in FIG. 4. 
On the other hand, assume that the server of the service provider SP sends 
to the base station BS1 data in which IPx is set as the source address and 
IP1 is set as the destination address. Upon receipt of the data over the 
public network PNW, the base station BS1 changes the address indicating 
the location to which the data is sent from the IP1 for base stations to 
the IP2 for mobile stations. The resulting data is sent to the mobile 
station over the radio channel. 
After that, the base station BS1 makes data transfers between the mobile 
station PS1 and the service provider SP while making IP address 
replacement in the same manner as above. 
As described above, with the mobile communication system of the present 
embodiment, when the mobile station PS makes a request for connection to 
the service provider SP, the base station BS acquires the IP address "IP1" 
for base stations from the service provider SP and assigns to the 
requesting mobile station PS the IP address "IP2" for mobile stations that 
has been acquired in advance in place of the IP address "IP1". When the 
data transfer is started, the base station changes the source address in 
data sent from the mobile station PS from IP2 to IP1 and then sends the 
data to the service provider SP. On the other hand, the base station BS 
changes the destination address in data sent from the service provider SP 
from IP1 to IP2 and then sends the data to the mobile station PS. 
Therefore, any mobile station that is not individually registered with the 
service provider is allowed to make access to a computer network, such as 
Internet, via the service provider, provided that access is made through a 
mobile communication system that has been registered as an organization 
with the service provider. 
A second embodiment of the present invention will be described next. FIG. 7 
is a circuit block diagram of a radio base station BS in the second 
embodiment. In this figure, like reference numerals are used to denote 
corresponding parts to those in FIG. 2 and detailed descriptions thereof 
are omitted. 
A memory 60 has a service information storage section 61, which, for each 
radio zone, stores information peculiar to that area. The service 
information includes, for example, an area map and information about noted 
stores and places within the area. 
A controller 50 provides a service information access control function 51 
and a location-to-be-accessed decision function 55 in addition to the 
provider access control function 51, the IP address assignment control 
function 52, and the IP address replacement control function 53. 
Upon receipt of data bound for the base station from a mobile station, the 
service information access control function 54 makes access to the service 
information storage section 61 within the memory 60 on the basis of the 
data contents to thereby read service information requested by the mobile 
station. The service information is then sent to the mobile station. 
Each time data is received from a mobile station, the 
location-to-be-accessed decision function 55 makes a decision as to 
whether the data is bound for the base station or the service provider SP, 
on the basis of the destination address appended to that data. If the 
decision is that the data is bound for the base station, then the function 
55 activates the service information access control function 54 to make 
transfer of service information. If, on the other hand, the result is that 
the data is bound for the service provider, then the function 55 activates 
the provider access control function 51 to thereby request the service 
provider to make access to Internet. 
The operation of the system thus arranged will be described below. 
The base stations BS1 to BSj acquire an IP address "IP3" for the base 
stations and an IP address "IP2" to be assigned to the mobile stations PS1 
to PSj in advance from the service provider SP and holds them. 
Assume here that the mobile station PS3 shown in FIG. 1 performs a dial 
operation to make a request for connection to the base station BS2 in 
order to obtain service information for the area where it is situated. 
Then, a request for establishment of a link is issued by the mobile 
station PS3 as shown in FIG. 8. In response to this, the base station BS2 
assigns a link channel to the mobile station PS3. Thus, the connection 
between the mobile station PS3 and the base station BS2 is established by 
the radio communication link. 
The mobile station PS3 next sends to the base station a request for 
Internet access. In response to this, the base station BS2 assigns the IP 
address "IP2" acquired in advance from the service provider SP to the 
mobile station PS3. After that, the mobile station PS3 sends data for 
making a request for service information transfer with the IP address 
"IP2" as the source address and the IP address "IP3" as the destination 
address. FIG. 9 shows the format of such data. 
Upon receipt of data from the mobile station PS3, the base station BS2 
first makes a decision as to whether the data is bound for itself or the 
service provider SP on the basis of the destination address in the data. 
If the destination of the data is the base station, it makes access to the 
service information storage section 61 within the memory 60 to selectively 
read service information corresponding to the received data contents. The 
service information is sent to the mobile station PS3 with the "IP2" 
assigned to the mobile station PS3 as the destination address and the 
"IP3" as the source address. 
Then, the mobile station PS3 can obtain from the base station local service 
information for the area where it is situated. 
Assume now that desired area information is not contained in the service 
information obtained from the base station. In such a case, with the 
conventional system, the mobile station PS3 would disconnect the radio 
communication link by terminating access to the service information 
storage section 61, then resend a request for connection to the service 
provider SP. This would require time and labor to acquire new information 
from a desired server on Internet. 
The second embodiment takes the following countermeasures against the 
problem. FIG. 10 shows a sequence according to the second embodiment. The 
mobile station PS3 sends data the destination of which is a desired server 
on Internet without disconnecting the radio communication link. FIG. 10 
shows an example of sending data with "IPx" appended as the destination 
address. 
When recognizing that the destination address of the data received from the 
mobile station PS3 is IPx that is not assigned to the base stations, the 
base station BS2 sends a request for establishment of a link to the 
service provider SP as shown in FIG. 10 and, upon receipt of an 
acknowledge signal, sends a request for Internet access. Upon receipt of 
an acknowledge from the service provider SP for that request, the base 
station BS2 recognizes the IP address "IP1" sent together with the 
acknowledge as its own address. 
Next, the base station BS2 transfers the data received previously from the 
mobile station PS3 and addressed to Internet to the service provider SP 
with the source address changed from "IP2" for mobile stations to its own 
address "IP1". Upon receipt of data from the service provider SP, the base 
station changes the destination address of the data from its own address 
"IPL" to "IP2" for mobile stations and then sends the data to the mobile 
station PS3. 
Each time data for a server on Internet is received from the mobile station 
PS3 and each time data for itself is transferred from the service provider 
SP, the base station BS2 makes source address and destination address 
replacement. 
Accordingly, even when making access to the service information storage 
section 61 in the base station BS2, the user at the mobile station PS3 can 
make access to a server on Internet without disconnecting the radio 
communication link by simply changing (replacing) the destination address 
of data. 
On the other hand, assume that, when data transfer is made between the 
mobile station PS3 and the server on Internet, the mobile station PS3 
sends data for the base station BS2 in order to access the service 
information storage section 61 in the base station BS2 again. Then, the 
base station BS2 recognizes that the destination of the data is changed to 
itself and notifies the service provider SP that the link is closed. The 
base station makes access to the service information storage section 61 in 
the memory 60 according to the data contents, reads the corresponding 
service information, and sends it to the mobile station PS3. 
Accordingly, even when making access to a server on Internet, the user at 
the mobile station PS3 can make access to the service information storage 
section 61 without disconnecting the radio communication link by simply 
changing the destination address of data. 
Thus, according to the second embodiment, each time data is received from a 
mobile station, a decision is made as to whether the destination of the 
data is either a base station or a service provider and the location to be 
accessed is changed according to the result of the decision. This allows 
users at mobile stations to switch between access to a base station and 
access to a server on Internet at their discretion without disconnecting a 
radio communication link to the base station by simply rewriting the 
destination address of the data. 
The first and second embodiments are arranged such that IP address 
assignment control for the base stations PS1 to PSj and address 
replacement for data transferred between the mobile stations and the 
service provider SP are made by each of the base stations. These functions 
may be performed by a mobile switching unit installed in the public 
network PNW. 
By so doing, access by all the mobile stations within the system to 
Internet can be controlled collectively by the mobile switching unit. This 
eliminates the base station requirement of having the various control 
functions described above, significantly reducing the control burden 
imposed on the controller of each base station. In general, a large number 
of base stations are installed in mobile communication systems of a 
cellular type. For this reason, the installation cost of the whole system 
can be significantly reduced by incorporating the control functions into 
the mobile switching unit. 
In addition, the service information storage section 61, the service 
information access control function and the location-to-be-accessed 
decision function may be incorporated into the mobile switching unit, not 
into the base stations. 
The embodiments were described taking an application of the invention to a 
public mobile communication system by way of example. Of course, the 
present invention may also be applied to private mobile communication 
systems employed by companies. 
Additional advantages and modifications will readily occur to those skilled 
in the art. Therefore, the invention in its broader aspects is not limited 
to the specific details and representative embodiments shown and described 
herein. Accordingly, various modifications may be made without departing 
from the spirit or scope of the general inventive concept as defined by 
the appended claims and their equivalent.