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- Annex L
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- (to Recommendation Q.931)
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- Low layer information coding principles
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- L.1 Purpose
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- This annex describes principles that shall be used when the calling
- user specifies information during call setup regarding low layer
- capabilities required in the network and by the destination terminal.
-
- Note - In this context and throughout this annex the term "called user" is the
- end point entity which is explicitly addressed. This may be an addressed
- interworking unit (IWU) (see I.500-Series Recommendations [51] and X.31 [14] case A).
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- L.2 Principles
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- L.2.1 Definitions of types of information
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- There are three different types of information that the calling ISDN user
- may specify during call setup to identify low layer capabilities needed in the
- network and by the destination terminal:
-
- a) type I information is information about the calling terminal
- which is only used at the destination end to allow a
- decision regarding terminal compatibility. An example
- would be modem type. This information is encoded in
- octets 5 to 7 of the Low layer capability information
- element;
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- b) type II information is the selection of bearer service from the
- choices of bearer services offered by the network to
- which the calling user is connected. This type of
- information is present even if no interworking occurs. An
- example is unrestricted digital information (UDI).
- This information is coded in:
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- i) octets 3 and 4 (including octets 4a and 4b if necessary) of
- the Bearer capability information element when
- the transfer mode required by the calling user is
- circuit mode,
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- ii) octets 3, 4, 6 and 7 (including 4a and 4b if necessary) of
- the Bearer capability information element when
- the transfer mode required by the calling user is
- packet mode;
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- c) type III information is information about the terminal or
- intended call which is used to decide destination
- terminal compatibility and possibly to facilitate
- interworking with other ISDNs or other dedicated networks. An
- example is A-law encoding. This information is encoded
- in octet 5 of the Bearer capability information
- element.
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- L.2.2 Examination by network
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- Type I information is user-to-user (i.e. not examined by network) while
- both types II and III should be available for examination by the destination
- user and the network. The Low layer capability information element is an information
- element which is not examined by the network while the Bearer capability
- information element is an information element which is examined by the user and the
- network.
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- L.2.3 Location of type I information
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- Type I information (i.e. terminal information only significant to the
- called user) shall, when used, be included in the Low layer compatibility
- information element.
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- L.2.4 Location of types II and III information
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- Type II (i.e. bearer selection) information shall be included in the
- Bearer capability information element. Type III information, when used, is included
- in the Bearer capability information element. The network may use and modify the
- information (e.g. to provide interworking). The rationale for the user including
- some terminal related information in the type III information (interworking
- related) is shown by the following example.
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- Normally with UDI, the rate adaption technique chosen is related to the
- terminal. The specification of a particular rate adaption scheme with a UDI bearer
- service could allow a compatibility decision by the destination terminal in a
- purely ISDN situation. However, it could also conceivably be used to allow
- interworking with a PSTN, assuming that the appropriate functions (i.e. data extraction,
- modem pool) are available at the interworking unit.
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- If the rate adaption information is carried in the Low layer
- compatibility information element, and not in the Bearer capability information element, then
- interworking by the network providing the bearer capability would not be
- possible. However, if the rate adaption information is carried in the Bearer capability
- information element, interworking would be possible.
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- Hence, there is some terminal related information which may be considered
- interworking related. The consequence for the calling user of not including such
- terminal related information in the Bearer capability information element is that
- the call may not be completed if an interworking situation is encountered.
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- L.2.5 Relationship between Bearer capability and Low layer compatibility
- information elements
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- There shall be no contradiction of information between the Low layer
- compatibility and the Bearer capability at the originating side. However, as some
- Bearer capability code points may be modified during the transport of the call, this
- principle implies that there should be minimal duplication of information between
- Bearer capability information element and Low layer compatibility information
- element.
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- Note - If as a result of duplication, a contradiction occurs between the Bearer
- capability information element and the Low layer compatibility information
- element at the terminating side, the receiving entity shall ignore the conflicting
- information in the Low layer compatibility information element.
-
- The following example, dealing with the specification of the encoding
- scheme used by the terminal for the speech or 3.1 kHz audio bearer services, shows
- the consequences of duplication.
- It is expected that some ISDNs will support only A-law and some only µ-law, with conversion provided by the µ-law network. (See Recommendation G.711.)
- If the encoding scheme is specified in both the Bearer capability information
- element and the Low layer compatibility information element, interworking between
- two ISDNs might require a change of the user information layer 1 protocol in the
- Bearer capability information element (e.g. from A-law to µ-law), while the
- encoding scheme specified in the Low layer compatibility information element would
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- presumably be forwarded to the destination unchanged. Since, to determine
- compatibility, the destination terminal examines both the Bearer capability information
- element and the Low layer compatibility information element, it would receive
- conflicting information regarding the encoding scheme used.
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- L.3 Information classification
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- The following are the examples of classifying low layer information
- currently identified. This information is provided to facilitate understanding of the
- characteristics of types II and III information.
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- L.3.1 Examples for speech and 3.1 kHz audio bearer services
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- a) Type II information (common to all applications using these bearer
- services):
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- - information transfer capability = speech or 3.1 kHz audio;
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- - information transfer mode = circuit;
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- - information transfer rate = 64 kbit/s;
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- - user information layer 1 protocol = A/µ law.
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- b) Type III information for interworking with CSPDN (3.1 kHz audio
- applications are assumed) - Figure L-1/Q.931:
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- - user information layer 1 protocol = rate adaption + user rate
- (Note);
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- Note - Only those profiles conforming to CCITT standardized rate
- adaption are allowed when only the above information
- is provided.
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- c) Type III information for interworking with PSTN:
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- i) voice applications: Figure L-2/Q.931:
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- - user information layer 1 protocol = A/µ law;
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- ii) voice band data applications: Figure L-3/Q.931:
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- - user information layer 1 protocol = A/µ law.
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- L.3.2 Examples for 64 kbit/s UDI circuit mode bearer service
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- a) Type II information (common):
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- - information transfer capability = unrestricted digital
- information;
- - information transfer mode = circuit;
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- - information transfer rate = 64 kbit/s.
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- b) Type III information for interworking with PSPDN (packet applications):
- Figure L-4/Q.931:
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- - no type III information is required.
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- c) Type III information for interworking with PSTN:
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- i) voice applications: Figure L-5/Q.931 - no type III information is
- required;
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- ii) rate-adapted data applications: Figure L-6/Q.931 - no type III
- information is required.
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- d) Type III information for interworking with PSTN with end-to-end digital
- connectivity (data applications) Figure L-7/Q.931:
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- - user information layer 1 protocol = rate adaption + user rate
- (Note).
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- Note - The profile described in I.463 [52] is allowed.
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- L.3.3 Examples for ISDN virtual-circuit bearer service
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- a) Type II information (common):
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- - information transfer capability = unrestricted digital
- information;
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- - information transfer mode = packet;
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- - information transfer rate = ------;
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- - user information layer 1 protocol = rate adaption + user rate
- (Note 1);
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- - user information layer 2 protocol = LAPB (Note 2);
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- - user information layer 3 protocol = X.25 [5] packet layer
- protocol (Note 2).
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- Note 1 - This parameter is included only when user packet
- information flow is rate adapted. Only those profiles
- conforming to X.31 are allowed when only the above
- information is provided for layer 1 protocol.
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- Note 2 - Only those profiles conforming to X.31 are used.
- See Figures L-8, L-9 and L-10/Q.931.
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- b) Type III information for interworking with PSPDN, CSPDN, PSTN:
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- - no type III information is necessary.
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- Note 1 - Is user rate sufficient to specify the type of modem at IWF?
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- FIGURE L-1/Q.931
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- BC = 3.1 kHz audio, voice band data ■ CSPDN
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- FIGURE L-2/Q.931
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- BC = 3.1 kHz audio, voice ■ PSTN
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- FIGURE L-3/Q.931
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- BC = 3.1 kHz audio, voice band data ■ PSTN
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- FIGURE L-4/Q.931
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- BC = 64 kbit/s UDI, packet application ■ PSPDN
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- FIGURE L-5/Q.931
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- BC = 64 kbit/s UDI, voice ■ PSTN
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- FIGURE L-6/Q.931
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- BC = 64 kbit/s UDI, rate adapted data ■ PSTN
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- FIGURE L-7/Q.931
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- BC = 64 kbit/s UDI ■ existing digital network
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- FIGURE L-8/Q.931
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- BC = ISDN virtual circuit (VC) ■ PSPDN
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- FIGURE L-9/Q.931
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- BC = ISDN virtual circuit (VC) ■ CSPDN
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- FIGURE L-10/Q.931
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- BC = ISDN virtual circuit (VC) ■ PSTN
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- L.4 Scenarios outside the scope of ISDN standardization
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- L.4.1 Examples for speech and 3.1 kHz audio bearer services
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- a) Type II information (common):
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- - information transfer capability = speech or 3.1 kHz audio;
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- - information transfer mode = circuit;
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- - information transfer rate = 64 kbit/s;
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- - user information layer 1 protocol = A/µ law.
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- b) Type III information for interworking with PSTN - voice band data
- applications - modem type conversion occurs: Figure L-11/Q.931:
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- - user information layer 1 protocol = rate adaption + user rate +
- other attributes (if required).
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- L.4.2 Examples for 64 kbit/s UDI circuit mode bearer services
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- a) Type II information (common):
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- - information transfer capability = unrestricted digital
- information;
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- - information transfer mode = circuit;
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- - information transfer rate = 64 kbit/s.
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- b) Type III information for interworking with PSTN - voice band data
- applications - Figure L-12/Q.931:
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- - no type III information is required.
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- Note - This scenario seems to be a part of PSTN services.
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- FIGURE L-11/Q.931
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- BC = 3.1 kHz audio, voice band data ■ PSTN
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- Note - This scenario seems to be a combination of interworking with PSTN and a
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- FIGURE L-12/Q.931
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- BC = 64 kbit/s UDI, voice-band data ■ PSTN
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- Annex M
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- (to Recommendation Q.931)
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- Low layer compatibility negotiation
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- This annex describes an additional low layer compatibility checking procedure
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- If, for any reason, the network is unable to convey this information
- element, it shall act as described in section 5.8.7.1 (unrecognized information
- element). Users are advised not to include in the Low layer compatibility information
- element sent from the called user to the calling user, attributes which would
- have the same value as the ones contained in the Low layer compatibility information
- element received from the calling party.
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- M.4 Low layer compatibility negotiation options
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- The Low layer compatibility information element contains a negotiation
- indicator which may have one of the following values:
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- a) low layer compatibility negotiation not allowed (default): then
- the called user shall not invoke negotiation;
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- b) out-band low layer compatibility negotiation allowed: the called
- user may then invoke low layer compatibility
- negotiation, as needed, according to section 3 above;
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- c) in-band negotiation allowed: the called user may then invoke low
- layer compatibility negotiation using the supported
- in-band negotiation, according to service or
- application requirements;
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- d) either in-band or out-band negotiation allowed: the called user
- may invoke one or the other low layer compatibility
- negotiation procedures according to its requirements.
- If the call is end-to- end ISDN, and the out-band low
- layer compatibility negotiation is supported by both
- parties, then this method of negotiation is preferred.
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- Annex N
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- (to Recommendation Q.931)
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- Procedures for establishment of bearer connection
- prior to call acceptance
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- N.1 General
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- Annex O
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- (to Recommendation Q.931)
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- Optional procedures for bearer service change
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- The procedure for bearer service change may not be provided on all
- networks. On those networks that support it, a user may use this procedure
- after making a suitable subscription-time arrangement.
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- Note 1 - The definition of necessary mechanisms (if any) within
- Signalling System No. 7 to support this procedure, including any undesirable
- charging implications, is for further study.
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- When a bearer service requested in an originator's SETUP message cannot
- be provided by the network, the network would reject the call or, under some
- circumstances, the network may change the bearer service and provide bearer
- service change notification. These procedures are currently applicable only to a
- change from 64 kbits/unrestricted to 64 kbit/s restricted, and from 64 kbit/s
- restricted to 64 kbit/s restricted with rate adaption.
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- Note 2 - During an interim period some networks may only support restricted
- 64 kbit/s digital information transfer capability, i.e. information transfer
- capability solely restricted by the requirement that the all-zero octet is not
- allowed. For interworking the values given in Appendix 1 of Recommendation I.340
- should apply. The interworking functions have to be provided in the network
- restricted capability. The ISDN with 64 kbit/s transfer capabilities will not be
- offered by this interworking, other than by conveying the appropriate signalling
- message to or from the ISDN terminal.
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- Note 3 - The possibility of changing from 3.1 kHz audio to speech is for further
- study.
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- Up to three Bearer capability information elements may be present in the
- SETUP message from the originating user, corresponding to the allowed bearer service
- modifications given above. The Bearer capability information element shall be
- immediately preceded by the Repeat indicator information element with the meaning field
- specifying "Prioritized list for selecting one possibility". Hence, the order of
- Bearer capability information elements would indicate order of bearer service
- preference.
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- If the SETUP message contains Bearer capability information elements not
- agreeing with any of the permissible ordered combinations listed above, the network
- will reject the call attempt.
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- After sending a CALL PROCEEDING message, when the originating network or
- terminating premises equipment determines that the preferred bearer service cannot be
- provided, it sends a NOTIFY message toward the call orginator. The NOTIFY message
- contains a Notification indicator information element with a coding which indicates
- to the originating party the change in bearer service and also contains a Bearer
- capability information element specifying the attributes of the new bearer service.
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- Receipt of the NOTIFY message is not acknowledged. The call originator may
- allow the call to continue or may initiate call clearing in accordance with
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