home
***
CD-ROM
|
disk
|
FTP
|
other
***
search
/
Internet Standards
/
CD2.mdf
/
ccitt
/
1992
/
q
/
q931l.asc
< prev
next >
Wrap
Text File
|
1991-12-31
|
24KB
|
743 lines
Annex L
(to Recommendation Q.931)
Low layer information coding principles
L.1 Purpose
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).
L.2 Principles
L.2.1 Definitions of types of information
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;
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:
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,
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;
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.
L.2.2 Examination by network
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.
L.2.3 Location of type I information
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.
L.2.4 Location of types II and III information
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.
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.
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.
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.
L.2.5 Relationship between Bearer capability and Low layer compatibility
information elements
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.
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
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.
L.3 Information classification
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.
L.3.1 Examples for speech and 3.1 kHz audio bearer services
a) Type II information (common to all applications using these bearer
services):
- information transfer capability = speech or 3.1 kHz audio;
- information transfer mode = circuit;
- information transfer rate = 64 kbit/s;
- user information layer 1 protocol = A/µ law.
b) Type III information for interworking with CSPDN (3.1 kHz audio
applications are assumed) - Figure L-1/Q.931:
- user information layer 1 protocol = rate adaption + user rate
(Note);
Note - Only those profiles conforming to CCITT standardized rate
adaption are allowed when only the above information
is provided.
c) Type III information for interworking with PSTN:
i) voice applications: Figure L-2/Q.931:
- user information layer 1 protocol = A/µ law;
ii) voice band data applications: Figure L-3/Q.931:
- user information layer 1 protocol = A/µ law.
L.3.2 Examples for 64 kbit/s UDI circuit mode bearer service
a) Type II information (common):
- information transfer capability = unrestricted digital
information;
- information transfer mode = circuit;
- information transfer rate = 64 kbit/s.
b) Type III information for interworking with PSPDN (packet applications):
Figure L-4/Q.931:
- no type III information is required.
c) Type III information for interworking with PSTN:
i) voice applications: Figure L-5/Q.931 - no type III information is
required;
ii) rate-adapted data applications: Figure L-6/Q.931 - no type III
information is required.
d) Type III information for interworking with PSTN with end-to-end digital
connectivity (data applications) Figure L-7/Q.931:
- user information layer 1 protocol = rate adaption + user rate
(Note).
Note - The profile described in I.463 [52] is allowed.
L.3.3 Examples for ISDN virtual-circuit bearer service
a) Type II information (common):
- information transfer capability = unrestricted digital
information;
- information transfer mode = packet;
- information transfer rate = ------;
- user information layer 1 protocol = rate adaption + user rate
(Note 1);
- user information layer 2 protocol = LAPB (Note 2);
- user information layer 3 protocol = X.25 [5] packet layer
protocol (Note 2).
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.
Note 2 - Only those profiles conforming to X.31 are used.
See Figures L-8, L-9 and L-10/Q.931.
b) Type III information for interworking with PSPDN, CSPDN, PSTN:
- no type III information is necessary.
Note 1 - Is user rate sufficient to specify the type of modem at IWF?
FIGURE L-1/Q.931
BC = 3.1 kHz audio, voice band data ■ CSPDN
FIGURE L-2/Q.931
BC = 3.1 kHz audio, voice ■ PSTN
FIGURE L-3/Q.931
BC = 3.1 kHz audio, voice band data ■ PSTN
FIGURE L-4/Q.931
BC = 64 kbit/s UDI, packet application ■ PSPDN
FIGURE L-5/Q.931
BC = 64 kbit/s UDI, voice ■ PSTN
FIGURE L-6/Q.931
BC = 64 kbit/s UDI, rate adapted data ■ PSTN
FIGURE L-7/Q.931
BC = 64 kbit/s UDI ■ existing digital network
FIGURE L-8/Q.931
BC = ISDN virtual circuit (VC) ■ PSPDN
FIGURE L-9/Q.931
BC = ISDN virtual circuit (VC) ■ CSPDN
FIGURE L-10/Q.931
BC = ISDN virtual circuit (VC) ■ PSTN
L.4 Scenarios outside the scope of ISDN standardization
L.4.1 Examples for speech and 3.1 kHz audio bearer services
a) Type II information (common):
- information transfer capability = speech or 3.1 kHz audio;
- information transfer mode = circuit;
- information transfer rate = 64 kbit/s;
- user information layer 1 protocol = A/µ law.
b) Type III information for interworking with PSTN - voice band data
applications - modem type conversion occurs: Figure L-11/Q.931:
- user information layer 1 protocol = rate adaption + user rate +
other attributes (if required).
L.4.2 Examples for 64 kbit/s UDI circuit mode bearer services
a) Type II information (common):
- information transfer capability = unrestricted digital
information;
- information transfer mode = circuit;
- information transfer rate = 64 kbit/s.
b) Type III information for interworking with PSTN - voice band data
applications - Figure L-12/Q.931:
- no type III information is required.
Note - This scenario seems to be a part of PSTN services.
FIGURE L-11/Q.931
BC = 3.1 kHz audio, voice band data ■ PSTN
Note - This scenario seems to be a combination of interworking with PSTN and a
part of PSTN services.
FIGURE L-12/Q.931
BC = 64 kbit/s UDI, voice-band data ■ PSTN
Annex M
(to Recommendation Q.931)
Low layer compatibility negotiation
This annex describes an additional low layer compatibility checking procedure
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.
M.4 Low layer compatibility negotiation options
The Low layer compatibility information element contains a negotiation
indicator which may have one of the following values:
a) low layer compatibility negotiation not allowed (default): then
the called user shall not invoke negotiation;
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;
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;
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.
Annex N
(to Recommendation Q.931)
Procedures for establishment of bearer connection
prior to call acceptance
N.1 General
Annex O
(to Recommendation Q.931)
Optional procedures for bearer service change
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.
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.
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.
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.
Note 3 - The possibility of changing from 3.1 kHz audio to speech is for further
study.
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.
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.
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.
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