Patent Application
20100067677
1. Field of the Invention
The present invention relates generally to public switched telephone networks. Particularly, the present invention relates to filtering out unwanted phone solicitations.
2. Description of the Related Art
A major annoyance with owning a telephone is receiving unwanted phone calls from sources such as telemarketers, salespeople, solicitors, etc.
Certain devices allow the user to screen the calls before answering. Caller-ID allows the user to at least see the name and phone number of the caller before deciding to answer the incoming call. However, certain callers may choose to block their information from being sent to the party being called (e.g., dialing *67 before dialing the number). Therefore, the user has no way of determining whether to answer the call.
Some devices maintain a list of phone numbers that the user wishes to block or allow. If the incoming number matches the number on the list, the device blocks or allows the phone number to ring the user's telephone. However, the caller-ID information sent over the telephone line is not available until after the first ring. Therefore, the user's telephone will ring at least once before the device can take any action. Therefore, even if the device blocks the illegitimate call, the user telephone will ring at least once each time an illegitimate call is received. Furthermore, maintaining the list requires constant user intervention, which may not be convenient for the user.
A device (call silencer) for filtering unsolicited phone calls can be communicably connected between the incoming phone loop line and the user's telephone. The call silencer can monitor, detect, and allow or disallow incoming calls. The call silencer circuit draws power form the incoming telephone loop line and does not require additional power supply. Once in operation, the call silencer circuit can silence or allow incoming phone calls without requiring programming, data input, or intervention from the user. The call silencer monitors incoming call patterns and user behavior to decide which of the incoming calls are allowed to ring the user's telephone.
The call silencer circuit comprises a central processing unit (CPU) with communicably connected non-volatile memory, such as flash memory, and random access memory (RAM). The software program and a permitted call list can be stored in the flash memory so that if the call silencer is disconnected from the telephone line, no data is lost. The call silencer also includes a dual-tone multi-frequency decoder for detecting and decoding outgoing phone numbers. Outgoing numbers can be added to the permitted call list.
A ring detector detects an incoming call and communicates an incoming call message to the CPU. The CPU activates a Ring blocking circuit so that the first ring is blocked from ringing the user's telephone. At the end of the first ring, a caller-ID decoder circuit determines the caller-ID information of the incoming call. If the incoming call has a toll-free number, the call is disconnected via a hang-up circuit. If the phone number is not a toll-free number, the CPU stores the number in the buffer and disconnects the phone call. If the same caller calls again within a predetermined time, the incoming call is allowed to ring the user's telephone. Also, if the incoming call has a phone number that matches the permitted call list, the incoming call is allowed to ring the user's telephone.
An off-hook detector circuit detects the ending of a phone call by the user. If the duration of the ended phone call is above a predetermined time, the corresponding incoming phone number is stored in the permitted call list. The CPU stores the updated permitted call list into flash memory to protect the data against power loss.
The present invention has other advantages and features which will be more readily apparent from the following detailed description of the invention and the appended claims, when taken in conjunction with the accompanying drawings, in which:
a-3e illustrate flowcharts for the operation of the call silencer in accordance with one or more embodiments of the invention.
The Flash Memory 203 can store the software and a database that includes a permitted call list. The software includes programs for the operation of the call silencer 101. The permitted call list can include the phone numbers that the call silencer 101 permits to ring the user's telephone. Note that there may be more phone numbers that need to be blocked than the phone numbers that need to be permitted. Therefore it can be more efficient to store a permitted call list as opposed to storing a blocked call list. The Flash Memory 203 serves as a non-volatile memory, the contents of which are not erased upon loss of power supply. The software and the permitted call list can be shadowed into the RAM 205 for execution and access. The transfer of code to the RAM 205 can be performed upon power on. The permit call list may be brought into the RAM 205 upon power on as well. Alternatively, the software program may control the flow of the permitted call list to and from the flash memory 203 during program execution. For example, the program may store the permitted call list back into the flash memory 203 upon changes in the content of the permitted call list stored in the RAM 205. This makes sure that the flash memory 203 has the most updated version of the permitted call list, and upon unexpected power loss, the updated permitted call list can be safely stored in non-volatile memory.
The DTMF detector and decoder 207 detects and decodes dual tone multi-frequency signals originating from the user's telephone. The user typically uses the keypad on his/her telephone instrument initiate a call to another user. Each digit pressed by the user results in a dual tone multi-frequency code being transmitted to the central telephone office. For example, pressing the digit ‘5’ produces two tones at frequencies 770 Hz and 1336 Hz. The DTMF detector and decoder 207 detects and then decodes the outgoing DTMF signals into a series of digits. These digits represent a phone number that the user has dialed. The DTMF decoder 207 can communicate the phone number to the CPU 201, which in turn can store the phone number in a permitted caller's List. The permitted caller list is stored in the Flash Memory 203, and can be updated with each new phone number detected by the CPU 201.
The Ring detector circuit 209 can identify an incoming call's first ring. The ring signal is typically a 20 Hz signal with at least 70V (rms) amplitude. The Ring detector circuit senses for the ring signal and notifies the CPU 201 on the occurrence of the first ring. The CPU 201 then activates the Ring Blocking circuit 211 so that the ring signal is blocked from reaching the user's telephone. The Ring Blocking circuit 211 may comprise a filter that has a stop band designed to be located at the ring signal frequency (e.g., 20 Hz). This filter may be turned on and off by the CPU 201 to block the ring signal from reaching the user's telephone.
The user telephone can be in one of two states (1) Off-hook and (2) On-hook. The Off-hook state signifies that the user has lifted the handset, or pressed the talk button on the telephone. The telephone is in Off-hook state either when the user is initiating a phone call or when the user has answered a received phone call. When the user rests the handset on the cradle or turns off the talk button on the handset, then the telephone is in On-hook state. The user telephone is connected to the central telephone office by a local loop circuit. In the On-hook state, the local loop circuit is open, and therefore no current flows in the local loop circuit. While in the Off-hook state, the local loop circuit is closed resulting in a current flow. The Off-hook detector circuit 213 can detect whether the user telephone is in the Off-hook state or the On-hook state. The status of the user telephone is communicated to the CPU 201.
If the CPU 201 determines that the incoming call is from an unwanted caller, the CPU 201 sends a signal to the Hang-up circuit 215 to answer the call and immediately hang-up on the unwanted caller.
The Caller-ID decoder circuit 217 decodes the called ID information received form the central telephone office. The caller ID information may include the phone number of the caller, the name of the caller, the city and the state the call is being made from, etc. The caller ID information is typically transmitted by the central telephone office after the first ring and before the second ring. The information can be transmitted in the form of a bit stream. For example, the bit stream may be transmitted at 1200 baud with 7 data bits and 1 stop bit. The information may be in “short form,” which includes the data/time of the call and the telephone number of the caller. The information may be transmitted in “long form,” which may additionally include the name and address of the calling phone. The digital information transmitted may be encoded in ASCII characters. The Caller-ID decoder circuit 217 decodes the information within the short-form or long-form transmission and determines the phone number of the caller. This phone number is then communicated to the CPU 201.
The call silencer circuit CS draws power from the incoming telephone loop line for its operation. No additional power source is required. Therefore no additional power cables or power outlets are required for the operation of the call silencer. In addition, because the critical data is stored in non-volatile memory, even if the call silencer is disconnected from the telephone line, no data is lost. The user may simply re-connect the call silencer to the telephone line for the call silencer to resume operation.
a-3e disclose exemplary flowcharts depicting the operation of the call silencer. The program waits in step NEXT_CALL (step 301) for an incoming call or an outgoing call. The user can initiate a call by placing the telephone in the off-hook state, and then dialing the desired number. Dialing a phone number generates DTMF tones, which carry the phone number in signal form to the central telephone office. The DTMF detector and decoder (
When the user is receiving a call, the central telephone office sends a ring signal to the user telephone. The Ring detector circuit (
When the Ring detector circuit (
The caller-ID decoder circuit (
b shows a flowchart that performs validation of the number of the incoming call. The CPU 201 loads the permitted call list from the flash memory 203 (step 327). The CPU 201 compares the phone number of the incoming call with the phone numbers stored in the recent call buffer (RCB) (step 329). Note that in step 329, the CPU 201 may not compare all of the digits of the incoming phone number with the numbers stored in the RCB. For example, the CPU 201 may compare only 5 digits of a 10 digit number to the corresponding 5 digits of the 10 digit numbers stored in the RCB.
Comparing only subset of the total number of digits of the phone number reduces the time required to make a comparison. For example, if it takes time t to compare one digit of the incoming phone number to the corresponding digit of all the numbers stored in the RCB, and the number of digits in a phone number is 10, then the total time required to compare all the digits to the corresponding digits of all the numbers is 10t. However if only m digits of the incoming number are compared, then the time taken to complete the comparison is mt. The downside to comparing only a subset of digits is collision. A collision occurs when the m selected digits of the incoming number match the corresponding digits of a number in the RCB while at least one of the remaining (uncompared) digits is different. A collision may also occur when the m selected digits of the incoming number match the corresponding digits of more than one number stored in the RCB. In either case, the collision results in an incorrect comparison result. The probability of a collision is dependent on the number of digits compared and the probability distribution of each of the digits of the incoming numbers. If it is assumed that the probability distribution is uniform, then the probability of a collision is equal to 1/10m. Therefore, more the number of digits compared (larger the value of m), smaller the probability of the occurrence of a collision. So there is trade-off between faster comparison time by comparing only a subset of digits of the incoming phone number, and the increasing probability of an incorrect comparison due to collision. In practice, however, the probability distribution of each digit of the incoming numbers may not be uniform. For example, most of the user's received calls may be from the same local area-resulting in the first three digits (the area code) of those calls being same. Therefore the actual probability of collision may also depend upon the selected digits' positions within the phone number. The call-silencer may choose less than half the number of digits of the phone number for comparison.
Referring back to
c shows a flowchart for silencing the incoming call after the determination that the incoming phone number is not legitimate, and therefore cannot be permitted to ring the user's telephone. The program activates the hang-up circuit (
Therefore, phone calls from illegitimate phone numbers can be silenced by the call silencer 101. A number may be determined to be illegitimate if the phone number is a toll free number or if the phone call comes in without caller-ID information or if the number does not match any entry in the RCB or the permitted call list or if the phone number matches an entry in the RCB but the elapsed time from the last time a call was received from the same phone number is greater than time T1.
The call silencer allows a call to reach the user's telephone only when either the caller's phone number is in the permitted call list in the database, or if the phone number is already stored in the RCB and the elapsed time from the last time a call was received from the same phone number is less than or equal to T1 (
c shows a flowchart relating to the sub-process HANG_UP. The program checks whether the phone is in the off-hook state (step 349). If the user is currently carrying on a conversation over the phone with a caller, the phone will be in the off-state. Once the user hangs-up, i.e., the phone is no longer in the off-hook state, the program determines the time duration of the last call (step 351). If the time duration of the last call is less than T2, the program moves to step NEXT_CALL (step 301). But if the time duration of the last call is greater or equal to T2, then the program determines whether the number matches any entries either in the RCB or in the permit call list in the database (step 353). If the number matches an entry in the permit call list or is found in the RCB, then that entry is moved to the top of the permit call list (step 355). Note that if the permit call list has reached its upper limit, the entry at the bottom of the permit call list is deleted. Alternatively, an entry from anywhere within the permit call list may be deleted. The deletion may be based on attributes such as the time stamp of the phone number, the number of calls received from the same phone number, etc. If the number does not match any entry in the RCB (step 357), then the number is stored in the RCB (step 359). Because the number found in the RCB is already stored in the database in step 355, the number is deleted from the RCB to free up buffer space (step 361). The updated permit call list is then copied to the flash memory (
As described above the call silencer 101 can automatically learn the phone numbers based on the call behavior of the user. No user intervention is necessary. Because the data is stored in non-volatile memory, the user has the ability to disconnect and re-connect the call silencer 110 at will without being concerned about losing desired operation.
