Note 1 - For broadcast data link terminal equipment which are explicitly
addressed using sub-addressing or DDI, the point-to-point column in the above table
shall be used.
Note 2 - When a terminal equipment on a broadcast data link is incompatible, an
option of "ignore or reject" is permitted, see 5.2.2.
Note 3 - Some terminal equipment on this interface may understand the High layer
compatibility or Low layer compatibility information element and would reject the
call if incompatible.
B.4 Interworking with existing networks
Limitations in network or distant user signalling (e.g. in the case of an
incoming call from a PSTN or a call from an analogue terminal) may restrict the
information available to the called user in the incoming SETUP message. A called user
should accept limited compatibility checking (e.g., without the
High layer compatibility information element) if a call is routed from an existing
network which does not support High layer compatibility information element
transfer.
In cases where the network cannot provide all incoming call information, or
where the network is not aware of the existence or absence of some service
information (such as compatibility information), the incoming SETUP message includes a
Progress indicator information element, containing progress indicator No. 1 "Call is
not end-to-end ISDN, further call progress information may be available in-band" or
No. 3 "Origination address is non-ISDN" (see Annex I).
The terminal equipment receiving a SETUP with a progress indicator
information element shall modify its compatibility checking, the terminal equipment should
regard the compatibility as successful if it is compatible with the included
information, which as a minimum, will be the bearer capability information element. A
terminal equipment expecting information in addition to the Bearer capability
information element in a full ISDN environment need not reject the call if such information
is absent but a Progress indicator information element is included.
Annex C
(to Recommendation Q.931)
Transit network selection
This annex describes the processing of the transit network selection
information element.
C.1 Selection not supported
Some networks may not support transit network selection. In this case, when
a Transit network selection information element is received, that information
element is processed according to the rules for unimplemented non- mandatory information
elements (see 5.8.7.1).
C.2 Selection supported
When transit network selection is supported, the user identifies the
selected transit network(s) in the SETUP message. One Transit network selection
information element is used to convey a single network identification.
The user may specify more than one transit network. Each identification is
placed in a separate information element. The call would then be routed through the
specified transit networks in the order listed in the SETUP. For example, a user
lists networks A and B, in that order, in two Transit network selection information
elements within a SETUP message. The call is first routed to network A (either
directly or indirectly), and then to network B (either directly or indirectly), before
being delivered.
As the call is delivered to each selected network, the corresponding
transit selection may be stripped from the call establishment signalling, in accordance
with the relevant internetwork signalling arrangement. The Transit network selection
information element(s) is/are not delivered to the destination user.
No more than four Transit network selection information elements may be
used in a single SETUP message.
When a network cannot route the call because the route is busy, the network
shall initiate call clearing in accordance with 5.3 with cause No. 34 "no
circuit/channel available".
If a network does not recognize the specified transit network, the network
shall initiate call clearing in accordance with 5.3, with cause No. 2 "no route
to specified transit network". The diagnostic field shall contain a copy of the
contents of the Transit network selection information element identifying the
unreachable network.
A network may screen all remaining transit network selection information
elements to:
a) avoid routing loops, or
b) ensure an appropriate business relationship exists between
selected networks, or
c) ensure compliance with national and local regulations.
If the transit network selection is of an incorrect format, or fails to
meet criteria a), b) or c), the network shall initiate call clearing in
accordance with 5.3, with cause No. 91 "invalid transit network selection".
When a user includes the Transit network selection information element,
pre-subscribed default Transit network selection information (if any) is
overridden.
Annex D
(to Recommendation Q.931)
Extensions for symmetric call operation
D.1 Additional message handling
In symmetric applications, the SETUP message will contain a
Channel Identification information element indicating a particular B Channel to
be used for the call. A point-to-point data link shall be used to carry the SETUP
message.
The procedure described in 5 for the user side should normally be
followed. Where additional procedures are required, they are detailed below.
D.1.1 B Channel selection - symmetric interface
Only B Channels controlled by the same D Channel will be the subject of
the selection procedure. The selection procedure is as follows:
a) The SETUP message will indicate one of the following:
1) channel is indicated, no acceptable alternative, or
2) channel is indicated, any alternative is acceptable.
b) In cases 1) and 2), if the indicated channel is acceptable and
available, the recipient of the SETUP message reserves it for
the call. In case 2), if the recipient of the SETUP message cannot
grant the indicated channel, it reserves any other available B
Channel associated with the D Channel.
c) If the SETUP message included all information required to
establish the call, the recipient of SETUP message indicates the
selected B Channel in a CALL PROCEEDING message transferred across the
interface and enters the Incoming Call Proceeding state.
d) If the SETUP message did not include all the information required
to establish the call, B Channel is indicated in a SETUP
ACKNOWLEDGE message sent across the interface. The additional call
establishment information, if any, is sent in one or more INFORMATION messages
transferred across the interface in the same direction as the SETUP
message. When all call establishment information is received, a CALL
PROCEEDING, ALERTING, or CONNECT message, as appropriate, is
transferred across the interface.
e) In case 1) if the indicated B Channel is not available, or in case
2) if no B Channel is available, a RELEASE COMPLETE message with a
cause value of No. 44 "requested circuit/channel not available" or
No. 34 "no circuit/channel available" respectively is returned to the
initiator of the call. The sender of this message remains in the
Null state.
f) If the channel indicated in the CALL PROCEEDING or SETUP
ACKNOWLEDGE message is unacceptable to the initiator of the call, it clears
the call in accordance with 5.3.
D.1.2 Call confirmation
Upon receipt of a SETUP message, the equipment enters the Call Present
state. Valid responses to the SETUP message are a SETUP ACKNOWLEDGE, an ALERTING,
a CALL PROCEEDING, a CONNECT, or a RELEASE COMPLETE message.
If the indicated channel is acceptable to the initiator of the call, the
initiator shall attach to the indicated B Channel.
D.1.3 Clearing by the called user employing user-provided
tones/announcements
In addition to the procedures described in 5.3.3, if the bearer
capability is either audio or speech, the called user or private network may apply in-band tones/announcements in the clearing phase. When in-band
tones/announcements are provided, the DISCONNECT message contains progress indicator No. 8 "in-band information or appropriate pattern is now available" and the called user
or private network proceeds similarly as stipulated in
5.3.4.1 for the network.
D.1.4 Active indication
Upon receipt of a CONNECT message, the initiator of the call shall
respond with a CONNECT ACKNOWLEDGE message and enter the Active State.
D.2 Timers for call establishment
User end points implement the network side timers T301, T303 and T310
along with the corresponding network side procedures for actions taken upon
expiration of these timers. See Table 9-2/Q.931 for the call establishment user- side
timers and procedures.
D.3 Call collisions
In symmetric arrangements, call collisions can occur when both sides
simultaneously transfer a SETUP message indicating the same channel. In the absence
of administrative procedures for assignment of channels to each side of the
interface, the following procedure is employed.
First, one side of the interface will be designated the "network" and
the other side of the interface will be designated the "user". Second, for the
three possible scenarios where the same channel is indicated by combinations of
preferred and exclusive from the user and network sides, the following procedure is
used:
a) "network" preferred, "user" preferred:
the "network" preferred channel is awarded and an alternate channel
is indicated in the first response to the "user" SETUP message;
b) "network" exclusive, "user" exclusive:
the "network" exclusive channel is awarded and the "user" SETUP
message is cleared with a RELEASE COMPLETE message with cause
No. 34 "no circuit/channel available";
c) "network" preferred, "user" exclusive; or "network" exclusive,
"user" preferred:
the side of the interface with an exclusive indicator in a SETUP
message is awarded the channel and an alternate channel is
indicated in the first response to the side using a preferred indicator in
the SETUP message.
Channel identification is allowed in both directions for ALERTING and
CONNECT.
Annex E
(to Recommendation Q.931)
Network specific facility selection
This annex describes the processing of the Network-specific facilities
information element. The purpose of this information element is to indicate
which network facilities are being invoked.
E.1 Default provider
When the length of the network identification field is set to zero in
the Network-specific facilities information element, then the services
identified in this information element are to be provided by the network side of the
interface receiving the information element (default provider). If the Network-specific facilities information element is recognized but the network
facilities are not understood, then this information element is processed according to
rules for non-mandatory information element content error (see
5.8.7.1).
E.2 Routing not supported
Some networks may not support the routing to the remote network of the
contents of the Network-specific facilities information element. In this case,
when a Network-specific facilities information element is received, that
information element is processed according to the rules for unimplemented non-
mandatory information elements (see 5.8.7.1).
E.3 Routing supported
When Network-specific facility information element routing is
supported, the user identifies the network provider in this information element in the
Q.931 SETUP message. One Network-specific facility information element is used
to identify a network provider.
The user may specify more than one network provider by repeating the
Network-specific facilities information element. Each identification is placed in
a separate information element. The information is routed to the indicated
network provider as long as the call is also handled by the network provider (see
Annex C, Transit network selection). For example, if the user lists network
providers A and B in separate Network-specific facilities information elements in a
call control message, there must be corresponding Transit network selection
information elements in the SETUP message identifying those networks (or default
call routing via A and B that was established prior to call establishment).
As the signalling messages containing Network-specific facilities
information elements are delivered to the indicated remote network, they may be
stripped from the signalling messages, in accordance with the relevant
internetworking signalling arrangement. The Network-specific facilities information
elements may be delivered to the identified user.
No more than four Network-specific facilities information elements may
be used in a SETUP message. When the information element is repeated, the order
of presentation of the elements in a message is not significant. Further, there
does not have to be a one-to-one correspondence between Network-specific
facilities information elements and Transit network selection information elements.
If a network cannot pass the information to the indicated network
provider, either due to:
- the network indicated is not part of the call path, or
- no mechanism exists for passing the information to identified
network.
The network shall initiate call clearing in accordance with 5.3, with
cause No. 2 "no route to specified transit network". The diagnostic field may
optionally contain a copy of the first 5 octets of the network-specific facilities
information element.
When the user includes the Network-specific facilities information
element in the SETUP message, pre-subscribed default service treatment (if any) is
overridden.
Annex F
(to Recommendation Q.931)
D Channel backup procedures
F. Foreword
The procedure defined in this annex can be used when non-associated
signalling is applied to multiple primary rate access arrangements. This feature can
be provided on a subscription basis and is network dependent.
F.1 General
In associated signalling, the D Channel signalling entity can only assign
calls to channels on the interface containing the D Channel. When the
D Channel signalling entity can assign calls to channels on more than one
interface (including the one containing the D Channel), this is called non- associated
signalling. Figure F-1/Q.931 is an example of associated signalling used on each
of the three interfaces between a user (e.g., a PABX) and a network. Replacing
associated signalling with non-associated signalling on these interfaces results in
the example shown in Figure F-2/Q.931.
When non-associated signalling is employed, the reliability of the
signalling performance for the ISDN interfaces controlled by the D Channel may be
unacceptable. To improve the reliability, a D Channel backup procedure employing a
standby D Channel is necessary. The next section describes the backup procedure
which is optional for end-points that use non-associated signalling.
