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Chapter 5
AG ISDN B Channel Assignment
- 5.1 Introduction
- 5.2 Default Channel Assignment
- 5.3 Exclusive Mode
- 5.4 Non-Exclusive Mode (Preferred Mode)
- 5.4.1 Switching Considerations For Disconnect Handling
- 5.5 Assigning Incoming Calls to TCP Instances
5.1 Introduction
- When a TCP instance is launched, a B channel is assigned to it. This channel is called the default channel. When placing a call, the TCP will always request the use of the default channel from the network. It directs the network to respond in one of two ways, or modes:
- Non-Exclusive Mode - (also called Preferred Mode) If a TCP's default B
channel is not available on the network, the network directs the TCP to use
another channel. In this case, the application must perform switching, to
connect the new B channel with the DSP resource associated with the TCP.
- This chapter describes how a TCP instance is assigned a default channel. It also describes Exclusive Mode and Non-Exclusive Mode, and explains how to set the mode for each TCP instance.
Note: In the US, ISDN systems almost always place and receive calls in Exclusive Mode. In Europe, both modes are used: many central office (CO) switches require the TE equipment side to use Non-Exclusive Mode.
- For details on AG Quad board switch models, see the AG Quad Installation and Developer's Manual. For details on the AG-T1 or AG-E1 switch model, see the AG-T1 and AG-E1 Installation and Developer's Manual. For general information on switching, see Getting Started With MVIP Switching.
5.2 Default Channel Assignment
- A default channel is assigned to each TCP instance in either of the following ways:
- The default channel can be specified by setting the ADI.ISD.networkstream
and ADI.ISD.networkslot parameters for that TCP instance. For example,
MVIP-95 0:0, 1:01 on an AG Quad T (Trunk A) corresponds to B channel 1.
(Parameters are documented in Appendix B).
5.3 Exclusive Mode
- In Exclusive Mode, a TCP instance effectively "owns" its default channel. To place an outbound call, the TCP asks the network for the default channel. If the channel is not available, the network clears the call. When the TCP senses this, CT Access sends ADIEVN_ CALL_DISCONNECTED to the application.
- For example, suppose the application attempts to establish an outbound call using a TCP which is set up to use default B channel 5 in Exclusive Mode (corresponding to MVIP-95 0:4, 1:4). The TCP asks the network for B channel 5. If the channel is available, the network signals the TCP to continue with the call on that channel. Otherwise, the network clears the call.
- In Exclusive Mode, any incoming call will be routed to a TCP instance only if the call is on its default channel. All calls on that channel will be routed to the TCP.
- To set a TCP instance to Exclusive Mode, set the ADI.ISD.exclusive parameter to 1.
- You can also use the ADI.ISD.direction parameter to specify whether the channel is incoming-only, outgoing-only, or bidirectional. Incoming calls will never be offered to an outgoing-only TCP channel. A call placed on an incoming-only channel will be rejected.
Note: The setting of the ADI.ISD.direction parameter should match the configuration of the connected equipment (e.g., the network switch). For example, if the application attempts to place a call on a channel that is configured on the connected equipment to be incoming-only, then the outbound call will always be rejected.
5.4 Non-Exclusive Mode (Preferred Mode)
- When a TCP instance places a call in Non-Exclusive Mode, it requests the use of its default channel from the network. The network may allow the use of the default channel, or direct TCP to use a different channel. If a different channel is assigned, the application must perform MVIP switching to route the call to the DSP resource associated with the TCP instance.
- To do so, the application must:
- Determine which B channel is chosen by the network for the TCP (i.e. the default channel or another channel). If no channel is available, the call is rejected by the network.
When the application receives the ADIEVN_PLACING_CALL event, the stream and timeslot corresponding to the channel are available in the stream and timeslot fields in the ADI_CALL_STATUS structure. The application can then call adiGetCallStatus to retrieve this information.
- Connect the stream and timeslot for that channel with the stream and timeslot assigned to the CTA context on which the call is placed.
To perform this connection, the application can use the CT Access switching service.
The switching should be performed as soon as possible after ADIEVN_PLACING_CALL is received, to insure that call progress analysis succeeds.
For an example of an application that performs this switching, see the isdnchan sample program documented in Chapter 6.
- For example, a TCP attempts to establish an outbound call on its default B channel. In this case, the default channel is 5, corresponding to MVIP-95 0:4,1:4. The network indicates that channel 5 is not available, but channel 9 is available (corresponding to MVIP-95 0:8,1:8). Since the TCP is running in Non-Exclusive Mode, the call is established on channel 9. The application must now connect the TCP instance's DSP resource (MVIP-95 16:4, 17:4) with the B channel to establish the voice path (for example: 0:8 => 17:4, 16:4 => 1:8)2.
- Incoming calls from any B channel may be routed to the TCP instance unless the B channel on which the call is located is assigned to another TCP running in Exclusive Mode. For details, see Section 5.4.1, Switching Considerations For Disconnect Handling.
- To set a TCP to Non-Exclusive Mode, set the ADI.ISD.exclusive parameter to 0 in your application.
5.4.1 Switching Considerations For Disconnect Handling
- (Non-Exclusive Mode only) When a call is disconnected, the application must disconnect only the trunk-to-DSP side of the full duplex switching connection that it made when the call was established. The DSP-to-trunk connection must not be disconnected because it may interfere with a new call arriving on the B channel that is handled by another thread or process.
- Disconnecting the trunk-to-DSP connection ensures that if a new outgoing call is placed on the CTA context, any call progress analysis functions will receive only silence until the application makes a new connection.
5.5 Assigning Incoming Calls to TCP Instances
- When a call arrives on a B channel, a TCP instance is assigned to the channel as follows:
- If a TCP instance is currently assigned to the channel in Exclusive Mode (and is not set up as outgoing-only), the call is routed to the TCP.
If the TCP outgoing-only, the call is rejected.
- If a TCP is currently assigned to the channel in Non-Exclusive Mode (i.e., the channel is the TCP's default channel), the call is routed to the TCP if it is not already handling another call (and is not set up as outgoing-only).
- Otherwise, the call is passed to any free TCP that is in Non-Exclusive Mode and is not set up as outgoing-only.
- If no TCP is available, the call is rejected.
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1
In MVIP-90 terms, this is 16:0.
2
In MVIP-90 terms, this connection is 16:8 <=> 18:4.
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