8.6
Call wait caller ID (CWCID) or calling identity delivery on call waiting (CIDCW) includes all features of Type-1 caller ID plus the capability of call wait caller ID display in the off-hook state. CIDCW uses only MDMF, and this feature is also called Type-2 caller ID or off-hook Caller ID. CIDCW can happen when a call is already in progress. Ring is not used with CIDCW as alert. The channel seizure signal is not used with CIDCW. With CIDCW, the identification of the calling party can be seen without keeping the current call on hold or any additional operations. In this section, the FSK-based Type-2 caller ID method is described.
8.6.1
Call Wait ID Flow in PSTN
In this section, call wait caller ID functioning in PSTN is explained. A and B are the end telephones connected to the PSTN central office as indicated in Fig. 8.7(a). With the help of the central office, A and B will establish the call and continue the conversation. In the example, A receives another call from C through the central office. The basic sequence from the established call between A and B are described as follows:
• The voice call continues between A and B.
• C is dialing A, and the central office senses that A is busy.
• If subscriber A does not have the call wait service, the central office sends a busy tone to C and User-A cannot speak to User-C.
Figure 8.7. Call wait ID. (a) Functional representation of phones in call wait ID. (b) Call wait ID signal sequence.
• If User-A has subscribed for call wait service, the central office can play a call wait short beep tone to A by muting speech from B to A path. This tone is called the subscriber-alerting signal (SAS) or call waiting tone. The central office will keep playing a special ring-back tone to C if User-C has activated the call waiting/call forward service; otherwise, a normal ring-back tone is played to C.
• After hearing the call wait beep, User-A can switch to C by using flash-hook (or any other programmed digit). After flash-hook, User-B will be in wait mode, and at User-B, a music tone is played.
• After completing the conversation with C, A can switch back to B, or A can keep switching between B and C.
• If A has subscribed for call wait ID service, the central office will send CAS to phone-A immediately after the first call wait tone is played. The CPE refers to TE or the phone here. If User-A has a call-wait-ID-capable phone, it will keep the microphone path on mute and send an ACK (acknowledge) signal back to the central office. This ACK signal is either a DTMF “A” or “D.” The DTMF digit “D” is the most common ACK signal.
• If the central office receives ACK from phone-A, it sends an FSK caller ID signal to phone-A. In the absence of ACK, the central office can terminate the call wait ID operation. ACK will not be delivered, if the phone is not supporting call wait ID service.
• Phone-A displays a call wait ID. On the sensing end of the FSK signal, phone-A comes out of microphone mute and voice can continue. If the
central office does not send FSK before timeout, phone-A will un-mute on its own.
• After the call wait beep, phone-A can always establish a call with phone-C (using flash-hook) without waiting for call wait ID or at any time during this phase.
• The central office will be sending SAS, CAS, and FSK signals in place of speech during call wait ID. Speech is interrupted while playing these signals.
8.6.2
Call Wait ID Signals and Tones
Type-2 caller ID includes all the features of Type-1 caller ID for FSK generation plus the capability of CIDCW. The sequence of events for off-hook data transmission is the same for both Telcordia and ETSI [ETSI ETS 300 659-2 (2001)]. Refer to the recommendations for timing and timing deviations [ETSI ETS 300 659-2 (2001)], [GR-30-CORE (1998)]. The basic sequence of events at the SPCS of the central office is shown in Fig. 8.7(b). Typical call wait ID timing parameters of Fig. 8.7(b) are given below.
Tsas—SAS tone duration, 300 ± 50 ms T1—Allowed time between SAS and CAS, 0 to 150 ms Tcas—Duration of alerting signal duration, 75 to 85 ms Tack—Duration of ACK detection, 55 to 65 ms
T2—Duration between ACK and the FSK data transmission, 50 to 200 ms T3—Time to unmute the voice path, 40 to 120 ms Tout—ACK timeout, 155 to 165 ms
The signals and timing referred in Fig. 8.7(b) is explained to some more details here.
SAS: The subscriber-alerting signal is a single frequency of 440 Hz that is applied for approximately 300ms. This tone is heard when a call is in progress and call waiting beeps to indicate a second call. The SAS tone (also called a call waiting tone) is used to indicate the user on the second call. This tone is not required for the TE to receive the CID information.
CAS: A CPE alerting signal alerts the TE that it has CID information to send followed by the SAS tone. The CAS is also referred to as the DT-AS dual-tone-alerting signal with a dual-tone signal combination of 2130 Hz and 2750 Hz for 80 ms. The power levels are of -15dBm ± 2dB per tone and accept up to -32 dBm per tone. Once the TE hears the CAS, it mutes the handset of the telephone and returns an ACK signal to the central office. There are slight deviations in specifications among
Telcordia and ETSI [GR-30-CORE (1998), ETSI ETS 300 659-1 (2001)] for the CAS signal.
ACK: The ACK signal has a nominal tone duration of 60 ms and is either a DTMF digit “A” or “D.” The digit “D” is the most common ACK signal, and it consists of the frequencies 941 Hz and 1633 Hz. Digit “A” consists of the frequencies 697 Hz and 1633 Hz. On receiving the ACK signal, the central office sends the CID information. The DTMF tone details are given in topic 7. There are slight deviations in specifications between the ETSI and Telcordia. The digit specifications and timing also vary by specific country requirements. For ETSI [ETSI ETS 300 659-2 (2001)], it is a network option to consider DTMF digits “A,” “B,” or “C” as a valid alternative ACK at the SPCS side. As per Telcordia, the SPCS allows the use of digit “A” as a valid alternative ACK. In either case, the DTMF tones must comply with the relevant specifications for DTMF transmission and reception.
FSK Transmission: FSK data have to be prepared in proper format for CIDCW operation. CIDCW uses only MDMF [GR-30-CORE (1998)]. The message type indicates only MDMF format for CIDCW. The code value for MDMF message type is of binary 10000000 (decimal 128). No channel seizure precedes the data. The CID information is sent serially at a rate of 1200 bps using continuous-phase binary FSK for modulation. The two frequencies used to represent the binary states are 1200 Hz for the mark (logic-1) and 2200 Hz for the space (logic-0) as shown in Table 8.1. The transmission characteristics are similar to on-hook FSK caller ID signals.
Mark signal difference between CID and CIDCW: The CID and CIDCW differ in channel seizure signal and mark signal. In CID, 300 bits of channel seizure and 180 bits of mark signal are used. In CIDCW, only 80 bits of mark signal are used and the channel seizure signal is not applicable. Call wait ID still uses the checksum similar to CID.
Talk – off and timeout: CIDCW is applicable during active voice conversation. Talk-off is one of the problems from the voice conversation experienced with CIDCW. Talk- off occurs when the CPE falsely detects a CAS. The TE may interpret the speech signal as CAS arrived from the central office. This happens as a rare event. When talk-off occurs, both parties can hear a brief interruption in the conversation and the party on the far end can hear the ACK signal that is sent to the central office by the TE. When the handset is muted by a talk-off operation, the TE may timeout and unmute the handset after waiting a set period for an FSK signal. A TE detector behaves differently with different people because of the variations in each person ‘s speech patterns. Speech can also cause a disturbance to CAS signal detection. This is called talk-down [URL (pic)]. Talk-off occurs when CAS is falsely detected by the TE. Talk-down occurs when TE is not detecting the valid CAS tone.
8.6.3
Call Wait ID Functioning in VoIP
This section considers a VoIP system playing the call wait ID on an FXS interface and phone. The call wait ID specific to the FXO interface is given in the next section. CID and CIDCW generations are the basic supplementary features supported in VoIP deployments. In VoIP deployment, the VoIP adapter or end-interfacing infrastructure is responsible for framing caller ID message format using MDMF, FSK generation, tone generation, DTMF, SAS, CAS generation, and ACK detection to support supplementary features such as caller ID, call wait tones, and call wait ID. The signaling protocol in the VoIP adapter controls the call flow, and the processor resides on a gateway to generate various signals required for CID display. The call wait ID and caller ID feature can be controlled for activation and deactivation in the VoIP adapter itself. The caller can restrict the caller ID supplementary service for a particular call or for a set of calls by sending the CLIR message to the proxy server in SIP.
If End User- A has subscribed to the call wait ID/caller ID service and caller ID service is restricted from VoIP adapter-A, then adapter-A sends the CLIR message in a signaling packet in negotiation to VoIP adapter- B. The adapter-B can restrict the caller ID message by keeping the display number as “P” or “O” in message framing similar to on-hook caller ID. In this case, User-B gets optional or private on phone display. If A has enabled the call wait ID service, VoIP adapter-A enables the CLIP message in a signaling packet along with the display name of the calling party. The VoIP adapter decodes the caller ID number from negotiation fields, and caller ID data are generated in the VoIP adapter to display on phone-B.
8.6.4
Implementation Care in Call Wait Caller ID in VoIP
This section describes the care in handling of CIDCW in VoIP calls. Basic caller ID sends the voice band signal to the phone. In CIDCW, real–ime acknowledgments are present and these acknowledgments are time sensitive. The following points have to be taken care while handling CIDCW. Actual implementations may consider several microlevel details beyond these listed points.
• It is necessary to minimize the total interruption of the voice path during the CIDCW transmission. The adapter should not block the voice transmission for more than 1.2s [GR-30-CORE (1998)].
• FSK data transmission should not begin if the processor is still receiving any part of the ACK signal to prevent collisions. It is achieved by avoiding any false detection of ACK or by ensuring the ACK is received for maximum detection time.
• The voice packets may be decoded during call wait ID, but they should not be played on near-end port.
• Application should not transmit the CID data during or after flash-hook.
The following requirements describe how the system should handle flash or call disconnect during the calling number display on call wait.
• False detection of flash-hook may cause interrupt in CIDCW display.
• The adapter should stop FSK data transmission as soon as it detects a flash or disconnect on the customers line.
• It is possible that the user can disconnect the current on-going call while receiving the calling number on call wait. In such cases, a power ring has to applied to the called party line after disconnect and the processor shall continue data transmission until the end of the release period.
DTMF ACK comes from phone to the VoIP system. While handling of the DTMF receiver during call wait ID, the DTMF module has to be enabled to detect the ACK from the phone. It may be possible that the gateway can send this ACK signal either in in-band or out-of-band packets to the other end that may create disturbance in voice call. So it is suggested in requirements of call wait ID, gateway should suppress and ignore the ACK digit during the call wait caller ID phase. It is also recommended that the DTMF receiver should be disabled from the line within the interval following the detection of the ACK and before reestablishment of the voice path in order to avoid any false detection of digits during the data transmission.
