The AG ISDN product allows you to create powerful applications that interact with ISDN services in a variety of ways, using the NMS CT Access or AG Access application programming interfaces (APIs) and one or more AG boards.
This chapter:
Integrated Services Digital Network (ISDN) is a continually evolving international standard for networking a wide range of services, including voice and non-voice services. The network is completely digital, from end-to-end: voice information is digitized and sent in digital form. Signaling information is sent separately from voice information, using a method called common channel signaling (CCS).
ISDN is transmitted over standard T1 and E1 carriers. T1 and E1 are typically four-wire digital transmission links. T1 is used mainly in the United States, Canada, Hong Kong and Japan. E1 is used in Europe and Australia.
Data on a T1 or E1 trunk is transmitted in channels. Each channel carries information digitized at 64000 bits per second (bps). For primary rate ISDN, T1 carries 24 channels. E1 carries 32 channels.
With primary-rate ISDN, the channels are used as follows:
ISDN communications can be described at many levels, from the way bits are transferred from machine to machine to the sets of messages computers pass to one another. A scheme for communication at a certain level is called a protocol.
In the late 1970's, the International Organization for Standardization (ISO) established the Open System Interconnect (OSI) model for communication. ISDN is based on this model. In OSI, seven separate levels, or layers, of communication are defined. The first three layers, called the chained layers, are the lowest levels. The chained layers are:
Layers higher than these are end-to-end layers. They describe how information is exchanged and delivered end-to-end. They also define process-to-process communication, and describe application-independent user services, user interfaces and applications, etc.
Figure 1: OSI Protocol Layering Model
The functionality provided by a layer includes the services and functions of all of the layers below it. A Service Access Point (SAP) is the point at which a layer provides services to the layer directly above it. With each SAP is associated a unique Service Access Point Identifier (SAPI).
Cooperation between entities on the same layer is governed by a peer-to-peer protocol specific to the layer and the entity. To exchange information between two or more layer entities, a connection must be established between the layer entities using the protocol of the layer directly below. Connections are provided by a layer between two or more SAPs.
Data message units are conveyed between peer-to-peer entities at the lowest layer by means of a physical connection. Layer (n+1) requests services from layer n via primitives. These primitives allow the logical exchange of information and control between two adjacent layers.
Figure 2: Message Primitives Exchanged Between Layers
Four types of primitives are exchanged between adjacent layers:
A layer issues this primitive type to request a service from the layer directly below it.
A layer providing a service issues this primitive type to notify the layer above it of any specific activity that is related to the service. An INDICATION that a layer receives may be the result of an activity performed by the layer directly below it that is related to a REQUEST given by a peer entity.
A layer issues this primitive type to acknowledge the receipt of an INDICATION from a lower layer.
A layer providing a requested service issues this primitive type to confirm that the activity has been completed.
ISDN equipment is classified into a number of categories by international and United States domestic standards1. Among those categories are the following:
The interface between each category is called a point:
Figure 3 illustrates the component categories and associated reference points:
Figure 3: ISDN Service Points and Equipment Classification
Alliance Generation (AG) ISDN protocol software allows you to write CT Access or AG Access applications that communicate with T1 or E1 trunks to perform voice processing functions and call control using ISDN Common Channel Signaling protocols.
AG ISDN software is designed to use one or more AG Quad, AG-T1, or AG-E1 boards as the physical interface to trunk lines. In addition to line interfaces, these boards also feature powerful on-board digital signal processing (DSP) resources that can handle much of the call control and voice processing overhead. For more information, see Sections 2.3.1 and 2.3.2.
AG ISDN allows access to ISDN services in three ways:
This AG ISDN configuration is called the NCC configuration. For more information about this access method, see the AG ISDN for Natural Call Control Developer's Manual.
This AG ISDN configuration is called the ACU configuration. This access method is discussed in this manual.
This AG ISDN configuration is called the LAPD configuration. This access method is briefly discussed in this manual.
You specify the configuration to use when initializing the ISDN protocol stack, as described in Section 4.4.1.
The AG ISDN ACU configuration allows access to Q.931 (layer 3) call control, using the AG ISDN Messaging API. Switch- and country-invariant D channel messages are exchanged with the application at this interface. The application uses the API to communicate directly with the ACU on the board, sending message sequences and handling network responses. The ACU is an entity within the protocol stack which receives commands from the API and communicates with the lower ISDN layers.
Access at this level allows the developer direct control over D channel messages and greater control over the contents of these messages.
The protocol stack runs in ACU configuration, as shown in Figure 4. In this configuration, the protocol stack implements all ISDN layer 2 and layer 3 functionality. The ISDN messaging API commands the ACU, which in turn communicates with the lower ISDN layers.
Note that events coming from the ACU are placed in the same event queue as other AG Access/CT Access events, allowing the user to access ISDN events in the same way that other events are accessed.
Figure 4: AG ISDN Application Architecture (ACU Configuration)
Access at the data link layer is useful if an application must support a private data link protocol, or if the user wishes to create a complete Q.931 protocol at the application level. At this level, the messages sent and received by the application constitute LAPD frames.
The protocol stack runs in LAPD configuration, shown in Figure 5. In this configuration, the protocol stack implements ISDN layer 2 functionality. No ACU is present. Instead, the application uses the ISDN library to send LAPD frames directly to the data link layer (layer 2).
Note that events coming from the data link layer are placed in the same event queue as other CT Access or AG Access events, allowing the user to access ISDN events in the same way that other events are accessed.
Figure 5: AG ISDN Application Architecture (LAPD Configuration)
As depicted with the solid arrows at the upper left of Figure 6, the AG ISDN product supports access across the S and the T reference points. At the ACU SAP, access to S/T is transparent.
In addition, the user can configure an ISDN protocol stack to emulate the network, using the partner_equip field passed to isdnStartProtocol. See Chapter 7 for more details on the parameters available to configure the ISDN protocol stack.
Figure 6: Reference Points Supported by AG ISDN
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1 The International Telecommunications Union (ITU) document I.411 defines ISDN user-network interface reference configurations. In the USA, the definition is provided by a number of documents, including AT&T TR-41449 ISDN Primary Rate Interface Specification.