Videotelephony Teleservice for ISDN
TAGS: F.721videotelephony teleservice for isdn

Videotelephony Teleservice for ISDN

The videotelephony teleservice for ISDN is a symmetrical, bi-directional, real-time, audiovisual teleservice in which speech and moving pictures are interchanged by means of one or two B-channels, using 64 kbit/s circuit-mode connections in the ISDN. The picture information transmitted is sufficient for adequate representation of fluid movements of a person displayed in head and shoulders view.

Description

1 General description

The videotelephony teleservice is defined as a fully standardized ISDN teleservice following the principles are given in Recommendation I.210. Two cases can be identified for the videotelephony teleservice:

  • Case I: Videotelephone based on using one circuit-mode 64 kbit/s connection.
  • Case II: Videotelephone based on using two circuit-mode 64 kbit/s connections.

For the case I the 64 kbit/s connection carries both speech and video information; for case II the first connection either carries speech or both speech and some video information and the second connection carries video information.

The basic videotelephony teleservice is characterized by the transmission of moving pictures displayed

continuously in colour, simultaneously with the speech of the persons involved in the call (generally two in the case of a point-to-point connection).

The speech quality of this teleservice must be at least as good as that applicable to the telephony teleservice in the 64 kbit/s ISDN based on bandwidths of 3.1 kHz or 7 kHz respectively.

The videotelephony teleservice shall allow the communication between:

  • two users (e.g. terminals) in a point-to-point configuration via the ISDN over one or two circuit mode 64 kbit/s connection(s);
  • three or more users in a multipoint configuration as invoked by some supplementary services.

Videotelephone terminals must be capable of supporting the telephony teleservice.

An essential feature of the service is that, besides videotelephony, it also provides to the user the possibility to communicate with other ISDN telephone or videotelephone terminals by using only the speech communication facility.

It shall be possible to use videotelephone terminals to communicate with 3.1 kHz telephone terminals connected to the public switched telephone network (PSTN).

2 Specific terminology

Fall-back: Procedure performed either by the network or by the calling videotelephone terminals to establish calls to 3.1 kHz telephone terminals.

  • Call 1: The first call invoked in the videotelephony teleservice. It identifies the first 64 kbit/s connection between the subscribers. The call is invoked for all the service cases.
  • Call 2: The second call invoked in the videotelephony teleservice. It identifies the second 64 kbit/s connection between the two subscribers. The call is invoked for case II only (2 ´ 64 kbit/s).
  • Retention timer: This timer specifies the amount of time that the network retains the call information of the original call upon encountering busy or being released. This timer is a network provider option. The value of this timer is greater than 15 seconds.
  • Videotelephone terminal: A terminal that supports the videotelephony teleservice.
  • 1 kHz telephone terminal: A terminal that supports the telephony 3.1 kHz teleservice.
  • 7 kHz telephone terminal: A terminal that supports the telephony 7 kHz teleservice.

Intercommunication and interworking considerations

1 Intercommunication/interworking with other terminals

Intercommunication with 3.1 kHz and 7 kHz ISDN terminals or interworking with PSTN shall be offered.

1.1 General principles

The videotelephony service shall include voice encoding according to Recommendation G.711. It may include additional speech encoding as an optional feature. The following fundamental requirements shall be fulfilled:

  • The user of a videotelephone terminal shall be able to establish calls to 3.1 kHz and 7 kHz telephone terminals (if the 7 kHz capability is supported) connected to the ISDN and to telephone terminals connected to the PSTN. Optionally, it should be able to reach other ISDN audiovisual terminals.
  • A videotelephone terminal shall be able to accept calls from 3.1 kHz and 7 kHz (if 7 kHz capability is supported) telephone terminals connected to the ISDN and from 3.1 kHz telephone terminals connected to the PSTN. Optionally, it should be able to accept calls from other ISDN audiovisual terminals.

As an option, videotelephone terminals may be pre-programmed to receive incoming videotelephone calls only. This latter functions may be requested by users possessing, e.g. both a videotelephone terminal and a 3.1 kHz telephone terminal connected to the same access arrangement.

1.2 Fall back procedure

1.2.1 Fall back in the destination network

Fall-back to 3.1 kHz telephony shall be an inherent feature of videotelephony teleservice and shall be provided as a default procedure.

The user shall be offered the possibility of indicating whether be requires interworking/fall-back to the 3.1 kHz telephony teleservice. A request for the videotelephony teleservice without fall-back (if indicated by the calling terminal) shall be possible. The following procedure shall apply:

  • If the calling user has indicated that fall-back is allowed, the network may offer the call to the called user at all videotelephone and 3.1 kHz telephone terminals if possible. The called user can accept the call either as a videotelephone or a 3.1 kHz telephone call at any terminal where the call is offered.

Note – The called terminals may recognize the fall-back situation and indicate it to the user.

  • The calling user shall be informed of the resultant telecommunication service, i.e. the videotelephony or 3.1 kHz telephony teleservice.
  • If no terminal accepts the call, this shall be indicated to the calling user.
  • If a busy condition is met at the terminals, supplementary services, e.g. completion of calls to the busy subscriber shall apply.

Note – Echo cancellation will be disabled for videotelephone calls. If fall-back occurs, there is no current signalling mechanism for re-enabling the echo cancellers.

  • When fall-back is not implemented by the network (possible short-term situation), fall-back may be performed end-to-end by the calling videotelephone terminal by originating a 3.1 kHz telephone call.

1.2.2 Fall-back when the ISDN does not offer the videotelephone teleservice

If the calling user has indicated that fall-back is allowed but the destination network does not support the

videotelephone capabilities the calling user shall receive an indication that fall-back has occurred and an indication of the resultant telecommunication service. The called user shall be offered the incoming call as a telephony 3.1 kHz call.

2 Interworking with private ISDNs

If the called user is on private ISDN, the fall-back procedures will be performed by the private ISDN. The result of call presentation (videotelephony or 3.1 kHz telephony) within the private ISDN shall be indicated to the public ISDN.

Attributes/values

  • Application of the attribute method

Depending on the case that applies the videotelephony teleservice description is based on invocation of one or two calls: call 1 and call 2 described according to the attribute method.

Low layer attributes

  • Call 1

Transfer mode: circuit.

Transfer rate: 64 kbit/s.

Transfer capability: 7 kHz.

Note – Before 7 kHz audio bearer service is available, as an interim solution videotelephone should use “unrestricted digital information” as the transfer capability when calling other videotelephones.

Structure: 8 kHz integrity.

Establishment of communication: demand.

Symmetry: bidirectional symmetric.

Configuration of communication: point-to-point, multipoint.

Note that in the case where fall-back to the 3.1 kHz telephony teleservice occurs, values of 3.1 kHz telephony teleservice bearer capability apply. Also if optionally a 3.1 kHz telephone call is established first, attributes of 3.1 kHz telephony teleservice bearer capability apply (telephone call instead of call 1).

  • Call 2

Transfer mode: circuit.

Transfer rate: 64 kbit/s.

Transfer capability: unrestricted digital information.

Structure: 8 kHz integrity.

Establishment of communication: demand.

Symmetry: bidirectional symmetric.

Configuration of communication: point-to-point, multipoint.

Access attributes

  • Call 1

Acces channel and rate: D(16) or D(64) for signalling, B(64) for user information.

Signalling access protocol, layer 1: Recommendations I.430/I.431.

Signalling access protocol, layer 2: Recommendation Q.921.

Signalling access protocol, layer 3: Recommendation Q.931.

Information access protocol, layer 1: Recommendations H.221, G.711, G.722 (option), H.242,

                                                                    H.261, AV.254.

Information access protocol, layer 2.

Information access protocol, layer 3.

Note that in the case where fall-back to the 3.1 kHz telephony teleservice occurs or if optionally a telephone call is established first, attribute 9.4 has the following values: Recommendations I.430/I.431, G.711.

  • Call 2

Acces channel and rate: D(16/64) for signalling, B(64) for user information.

Signalling access protocol, layer 1: Recommendations I.430/I.431.

Signalling access protocol, layer 2: Recommendation Q.921.

Signalling access protocol, layer 3: Recommendation Q.931.

Information access protocol, layer 1: Recommendations H.221, H.242, H.261, AV.254.

Information access protocol, layer 2.

Information access protocol, layer 3.

High layer attributes

  • Call 1

Type of user information: Speech (telephony), video, data, audiovisual (information).

Layer 4 protocol functions: Recommendation H.221.

Layer 5 protocol functions: Recommendation H.242.

Layer 6 protocol functions: Recommendations G.722 (option), G.711, H.261.

Layer 7 protocol functions.

Note that in the case where fall-back to 3.1 kHz telephony teleservice occurs or, if optionally a 3.1 kHz telephone call is established first, the value of attribute 10) is “speech” and the value of attribute 13) is “Recommendation G.711”.

  • Call 2

Type of user information: video.

Layer 4 protocol functions: Recommendation H.221.

Layer 5 protocol functions: Recommendation H.242.

Layer 6 protocol functions: Recommendation H.261.

Layer 7 protocol functions.

 

Sanmati
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