Channel associated signaling

With channel associated signaling (CAS), signaling information is carried in the voice channel or in a channel that is permanently tied to the voice channel. A number of different channel associated signaling protocols are used. They are generally classed as being either T1 (used in Canada, Japan, China (Hong Kong S.A.R.), the U.S.A., and other countries or regions) or E1 (used in Europe, Latin America, and other countries).

In general, each protocol has its own set of telephony capabilities. Your choice is dependent on what subset of protocols your switch or PABX supports, and which protocols provide the functionality that your applications require. The lists in T1 channel associated signaling protocols and E1 channel associated signaling protocols are meant only as a rough guide, because a given type of switch might support additional functionality, or only a subset of the functionality of the protocol.

T1 channel associated signaling protocols

Blueworx Voice Response supports the following T1 signaling protocols:

FXS (loop start and ground start)
E&M (immediate start, delay start, and wink start)
DID (immediate start, delay start, and wink start)
SAS (loop start)

The functions provided by these protocols are summarized in Table 1.

FXS:

Blueworx Voice Response is connected to a switch, using the two-way foreign exchange subscriber (FXS) protocol that is described in ANSI TIA/EIA-464-B (loop start or ground start operation); outgoing address signaling that uses DTMF tones, dial pulses, or MFR1 is supported.

FXS is a station (line-side) protocol that supports call transfer. Answer supervision is not provided and far-end disconnection is indicated only with ground start.

ESF framing connection that uses 4-bit ABCD (ESF) robbed-bit framing format is supported on DTTA trunks.

E&M and DID:

Blueworx Voice Response is connected to a switch, using the two-way E&M tie line (trunk-side) protocol described in ANSI TIA/EIA-464-B. Incoming and outgoing address signaling using DTMF tones, dial pulses, or MFR1 is supported for immediate, delay, and wink address register start types. Customers who attach to the PTT in Taiwan can use incoming MFR1 tones to get number information during call setup using modified MFR1.

ESF framing connection using 4-bit ABCD (ESF) robbed-bit framing format is supported on DTTA trunks.

E&M supports both answer supervision and far-end disconnect.

DID is the same as the E&M tie-line protocol, except that it does not support making outbound calls.

SAS:

Blueworx Voice Response is connected to a switch, using the two-way special access subscriber (SAS) protocol described in ANSI TIA/EIA-464-B (loop start operation only); outgoing address signaling using DTMF tones, dial pulses, or MFR1 is supported.

SAS is a station (line-side) protocol that supports call transfer. Answer supervision is not provided and far-end disconnection is not indicated.

Table 1. Functions provided by T1 CAS protocols.
For each T1 CAS protocol, this table shows if it is a trunk-side or a line-side protocol, whether or not PSTN, PABX, and channel bank connectivity are supported, and also whether or not answer detection, call transfer, far-end disconnect, ANI, and DID or DNIS are supported.
Protocol	Type	Connectivity			Function
In the Type column, Trunk means ‘trunk-side protocol’ and Line means ‘line-side protocol’.		PSTN	PABX	Channel bank	Answer detection	Call transfer	Far-end disconnect	ANI	DID or DNIS
E&M ¹	Trunk	Yes	Yes	Yes ²	Yes	No ³	Yes	Yes	Yes
FXS Loop Start ¹	Line	Yes	Yes	Yes ²	No	Yes	No ⁴	No ⁵	No ⁵
SAS Loop Start	Line	Yes	Yes	Yes	No	Yes	No ⁴	No ⁵	No ⁵
FXS Ground Start ¹	Line	Yes	Yes	Yes ²	No	Yes	Yes	No ⁵	No ⁵
Note: For both 2-bit AB (SF) format and 4-bit ABCD (ESF) format, as defined in TIA/EIA 464-B. A channel bank for 4-bit (ESF) format CAS signaling must support extended superframe (ESF). Yes, if the switch offers a release link trunk. Yes, if a disconnect clear signal is provided. Some PABX and ACD systems send number identification by sending DTMF digits before, or after the call is answered.

E1 channel associated signaling protocols

Blueworx Voice Response supports the following CEPT (E1) signaling protocols:

E&M (immediate start, delay start, and wink start)
EL7
FXS Loop Start
Italy
R2
R2MFC
RE
SL
TS003
U.K. Callstream¹
U.K. Exchange
U.K. Tie/DDI

The functions provided by these protocols are described below and then summarized in Table 2.

E&M:

This is the same as T1 E&M protocol (see E&M and DID).

This protocol is a tie line (trunk-side) protocol that supports incoming and outgoing address signaling, answer supervision, and indication of far-end disconnection. Call transfer is not available unless the switch offers a release link trunk when using the E&M protocol.

EL7:

Blueworx Voice Response is connected to the Ericsson MD110 PABX via a 2048 kilobits digital link. This proprietary protocol was developed by Ericsson specifically as an interface to Blueworx Voice Response and is based on the RE protocol, enhanced to provide answer supervision and far-end disconnect.

This protocol is a station (line-side) protocol that supports call transfer and outgoing address signaling.

Italy:

Blueworx Voice Response is connected to a digital central office switch in Italy via the two-way Exchange Line/DID protocol described in Comitato Elettrotechnico Italiano (CEI) standard 103.7. This protocol supports incoming and outgoing calls using DTMF or dial pulses for address signaling.

This protocol is a central office trunk (trunk-side) protocol that supports outgoing address signaling and recall (which can be used for call transfer if offered by the switch), and receives answer supervision and indication of far-end disconnection.

R2:

Blueworx Voice Response is connected to a switch via the two-way R2 digital line signaling protocol that is described in CCITT Q.421. Incoming and outgoing address signaling that uses DTMF tones, dial pulses, or MFR1 is supported.

This protocol is a tie line (trunk-side) protocol that supports incoming and outgoing address signaling, answer supervision, and indication of far-end disconnection.

R2MFC:

R2MFC is supported only in countries that use the E1 standard, and although the fundamentals of the protocol are standardized by the ITU, exact details vary widely between countries. Blueworx Voice Response currently only supports the Korean variant of the protocol, and you should contact your IBM representative if you wish IBM to consider adding additional R2MFC variants.

R2MFC is based on the R2 protocol (described above), but the address signaling information is transferred using a special set of multi-frequency tones. There is one set of tones for the incoming information and another for the outgoing information . When used together these comprise a “handshaking” protocol for the transmission of information between the two R2MFC network elements, enabling information to be transferred more quickly and reliably than by equivalent methods such as dial pulses or DTMF

On an incoming R2MFC call, the called number (DNIS) is present in SV185 when the incoming call state table is invoked. Outbound calls can be made using the MakeCall state table action. To specify a called number only, set the Phone Number parameter of the MakeCall action to the value of the called number, and then set the Format parameter to a string of # characters that is equal to the number of digits in the called number. To request R2MFC to additionally transmit the calling number, append the calling number to the called number in Phone Number (separated by a C character), and then set Format to a string of # characters that is equal to the total number of characters (including the C) in Phone Number. For example:

To set called number 12345, set Phone Number to 12345 and Format to #####.
To set called number 12345 and calling number 67890, set Phone Number to 12345C67890 and Format to ###########

RE:

Blueworx Voice Response is connected to a channel bank via the remote extension (RE) protocol, which is similar to T1 FXS loop start (see FXS).

This protocol is a station (line-side) protocol that supports call transfer and outgoing address signaling, but does not receive answer supervision or indication of far-end disconnection.

The signaling bit patterns for RE are the same as those for U.K. Tie/DDI. However, incoming seizure is determined by recognizing the ringing pattern as defined by system parameters.

SL:

Blueworx Voice Response is connected to a digital central office switch in France via the two-way signaling protocol that is described in ST/PAA/TPA/1064; this protocol is similar to T1 FXS (see FXS) in that it supports both incoming and outgoing calls but only outgoing address signaling using DTMF, MFR1, or dial pulses.

This protocol is a station (line-side) protocol that supports call transfer, answer supervision, and indication of far-end disconnection.

TS003:

Blueworx Voice Response is connected to the Telstra network in Australia via the TS003 protocol. Customers who are connected directly to the PTT (that is, with no PABX) can use this.

U.K.Callstream:

Blueworx Voice Response is connected to the British Telecom Callstream service in the United Kingdom. Incoming address signaling is dial pulses with immediate start operation. Call transfer is supported.

U.K.Exchange:

Blueworx Voice Response is connected to British Telecom or Mercury switched networks in the United Kingdom via the Exchange Line protocol described in British Approvals Board for Telecommunications standard OTR001 and Mercury SS5502.

U.K.Tie/DDI:

Blueworx Voice Response is connected to British Telecom or Mercury switched networks in the United Kingdom via the Tie Line/Direct Dialing Inward protocol (commonly known as SSDC5) that is described in British Approvals Board for Telecommunications standard OTR001 and Mercury SS5502. Tie line operation supports incoming and outgoing calls using delay start or immediate start address signaling with DTMF or dial pulses. DDI operation supports incoming calls only, by using the same types of address signaling.

This protocol can also be used whenever inverted (with respect to ANSI TIA/EIA-464-B) E&M is required.

This protocol is a tie-line (trunk-side) protocol that supports incoming and outgoing address signaling, answer supervision, and indication of far-end disconnection.

Table 2. Functions provided by E1 CAS protocols.
For each E1 CAS protocol, this table shows if it is a trunk-side or a line-side protocol, whether or not PSTN, PABX, and channel bank connectivity are supported, and also whether or not answer detection, call transfer, far-end disconnect, ANI, and DID or DNIS are supported.
Protocol	Type	Connectivity			Function
In the Type column, Trunk means ‘trunk-side protocol’ and Line means ‘line-side protocol’.		PSTN	PABX	Channel bank	Answer detection	Call transfer	Far-end disconnect	ANI	DID or DNIS
E&M ¹	Trunk	Yes	Yes	Yes	Yes	No ²	Yes	Yes	Yes
FXS Loop Start	Line	Yes	Yes	Yes	No	Yes	No ³	No ⁴	No ⁴
EL7/CAS	Line	No	Yes ⁵	No	Yes	Yes	Yes	No ⁶	No ⁶
Italy	Trunk	Yes	No	No	Yes	No	Yes	No	Yes
R2 ⁷	Trunk	Yes	Yes	Yes	Yes	No	Yes	Yes	Yes
R2MFC ^{8, 9}	Trunk	Yes	No	No	Yes	No	Yes	Yes	Yes
RE	Line	No	No	Yes	No	Yes	No	No	No
SL ¹⁰	Line	Yes	No	No	Yes	No	Yes	No	No
TS003 ¹¹	Trunk	Yes	Yes	Yes	Yes	No	Yes	Yes	Yes
UK Callstream	Trunk	Yes	No	No	Yes	No¹²	Yes	No	No
UK Exchange	Trunk	Yes	No	No	Yes	No	Yes	No	No
UK Tie/DDI	Trunk	Yes	No	No ¹³	Yes	No ¹²	Yes	Yes	Yes
In the Type column, Trunk means “trunk-side protocol” and Line means “line-side protocol” Note: For connection to Siemens Hicom 300 switch. Yes, if the switch offers a release link trunk or an ACL exchange data link is available. Yes, if a disconnect clear signal is provided. Some PABX and ACD systems send number identification by sending DTMF digits before or after the call is answered. Unique protocol for Ericsson MD110. Yes, if a VMS exchange data link is available on the MD110. R2 digital line signaling as specified by ITU-T Q.421. Korean only R2MFC uses R2 for line signaling Subscriber Loop, used in France. Multifrequency compelled (MFC) is not supported. Yes, to support ”inverted“ E&M. “Mid-call diversion” is supported.

Disadvantages of channel associated signaling

Although channel associated signaling is widely available in almost every country, several problems occur when signaling protocols ofthis type are used to connect voice response units. Such problems are:

Because only two bits or four bits of signaling information are available for each channel (in each direction), the channel-associated signaling bits can be used only to pass very basic information between the voice response unit and the switch (such as ring and on-hook). With channel associated signaling, incoming call information such as ANI, DNIS, or DID, or outgoing information such as the dialed number, must be sent in-band as a sequence of tones (MF or DTMF) in the voice channel. These tones might be heard by the caller, and can cause annoyance.
DTMF or MF tones cannot be sent faster than about 10 tones per second. This means that for long numbers, the caller might have to wait while call information is being transferred between the switch and the voice response unit.
No error checking occurs on MF or DTMF tones, so for line errors incorrect information might not be detected.
There is a wide variation in channel-associated line signaling protocols across the world.
Beyond the line signaling protocols, a voice response unit must to support a variety of different tone sequencing schemes (DTMF, R1 MF, R2 MFC, and so on).
Some channel associated signaling connections do not provide positive answer detection (that is, a called subscriber has picked up the phone) or hang-up detection. This means that the voice response unit may have to assume that a call has been answered when it detects voice energy on the line. Similarly, it may have to assume that a caller has hung up when no voice energy has been detected for some time.