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Overview

Calls with or without Bearer Path

The Call Agent feature enables the CSP to connect VoIP calls with or without bearer paths through the CSP. In the current release, only SIP software uses the Call Agent functionality. The CSP can accept requests from SIP clients for a specific host-based voice service and then establish a SIP signaling connection for the requested service.

Modes

The Call Agent feature supports calls in the following two modes, on a call-by-call basis:

bearer mode

bearer-free mode

In bearer mode, the RTP stream passes through the VDAC-ONE or IP Network Interface Series 2 card. Bearer-free mode avoids "hairpinning" the call because the RTP stream does not have to pass through the CSP via the VDAC-ONE or IP Network Interface Series 2 card.

In addition, you can have a call channel transition between the two modes during a call. This dual switching scenario is especially applicable to pre-paid applications. See Dual Switching Scenario.

In both modes, call control exists on the CSP Matrix Series 3 Card.

Figure 6-1 Bearer and Bearer-Free Mode

Benefits of Call Agent

The benefits of the Call Agent mode include the following:

Lower CODEC costs

Lower port costs

Higher Quality of Service (fewer IP-TDM-IP transitions and lower delay)

Increased scaling beyond CSP physical channel capacity

Ability for host applications to use media resources separate from the CSP.

Reduced bandwidth requirements, because no bearer packets go through the CSP.

Virtual and Physical Span/Channels

With the Call Agent feature, there is a clear separation between virtual and physical span/channels.

The virtual span/channels provide the context for call processing only. The physical span/channels support bearer switching and call processing.

The Call Agent feature is optimized to offer maximum Busy Hour Call Attempt (BHCA) throughput when the total number of virtual span/channel resources are divided into smaller resource groups within the internal router database.

Example: If there are 10,000 virtual span/channels provisioned in the CSP, you should configure the internal router database with ten (or more) resource groups with 1024 (or less)
span/channels to get maximum BHCA rate.

Host-to-CSP API

The Route Control message and PPL Download message support a criteria type named Virtual VoIP. During host-initiated routing, the host uses the Route Control message to hunt for a virtual VoIP channel and returns a virtual span/channel to the host.

Call Agent Switching

Call Agent switching is characterized by the absence of bearer resources, and therefore the absence of physical switching service, within the CSP. In Call Agent switching, Layer 4 (L4) and host applications are exposed to more network signaling data than in traditional switching, where only Layer 3 Plus (L3P) is exposed to this data. Call Agent switching propagates B-side media parameters to the A-side by sending a new PPL Event Indication message to host applications.

RFC 2833 Multi-Unicast

Some host applications that use bearer-free mode still need access to DTMF events. RFC 2833 meets this requirement.

The CSP can receive DTMF digit events from an endpoint and pass them to a host application. The CSP does this by requesting the RTP terminal to multi-unicast the RFC 2833 telephony event stream to the CSP and remote endpoint. The CSP does the following:

1. Terminates this stream on the CSP Matrix Series 3 Card.

2. Interprets the RFC 2833 stream.

3. Processes the events as requested by the host.

4. Passes the digit strings up to the host in the Call Processing Event (0x002E) message.

At this point the host can act on the event.

The CSP also supports the Subscribe and Notify method. Refer to SUBSCRIBE and NOTIFY Method for DTMF Detection.

API Messages

The following API Messages support the Call Agent feature. They are documented in the API Reference.

Connect With Data (0x0005)

PPL Event Indication (0x0043)

PPL Table Download (0x00D6)

Route Control (0x00E8)

VoIP Protocol Configure (0x00EE)

VoIP Protocol Query (0x00EF)

DSP Service Request (0x00BD)

DSP Service Cancel (0x00BE)

Virtual Card Configure (0x00E0) (supports virtual VDAC card for Call Agent)

Call Processing Event (0x002E)

TLVs

The following TLVs support the Call Agent feature. They are documented in Tag Length Value Blocks chapter of the API Reference.

0x0065 Physical VoIP Channel

0x0071 Virtual VoIP Channel

0x0115 Call Agent Mode

0x0116 Channel Service TLV

0x0119 Media Offer Stage

0x01ED RFC 2833 Jitter Buffer Size

0x01EE Invalid Packet Alarm Threshold

0x01EF Initial UDP Port Number

0x01F0 Number of UDP Ports

0x01F1 Dynamic Payload Type

0x01 Add Virtual Card (supports virtual VDAC card for call agent)

0x02 Remove Virtual Card (supports virtual VDAC card for call agent)

0x011A Call Agent Physical Channel ID

0x2954 SIP Request URI User Name

0x2A00 Media Remote End Point Information

0x2A01 Media Per Stream Information

0x2A02 Media Per Codec Information

0x2A03 Media Type

0x2A07 Media Port

0x2A08 Media Payload Type

0x2A09 Media Payload Description

0x2A0A Media Payload Size

0x2A0B Media Clock Rate

0x2A0E Media Connection Address

0x2A13 Media Flow Direction

Atomic Functions

Atomic Functions numbers 225-234 support the Call Agent feature. They are documented in Layer 4 Atomic Functions in the Developers Guide: Programmable Protocol Language.

Voice over Internet Protocol. The delivery of voice information in digital form in discrete packets rather than in the traditional circuit-committed protocols of the public-switched telephone network (PSTN).
Session-Initiation Protocol. A signaling protocol used for establishing sessions in an IP network. A session could be a simple, two-way telephone call, or it could be a collaborative multi-media conference session.
Real-Time Transport Protocol. Supports transport of real-time data like interactive voice and video over packet-switched networks.
Internet Protocol. Part of the TCP/IP family of protocols used to route messages and tracks the addresses of nodes.
Coder/Decoder. A CODEC converts voice signals from their analog form to digital signal acceptable to modern digital transmission systems.
See Time Division Multiplexing.
Range of channels provided by T1 or E1 transmission service. A physical span is a group of channels taking up a single physical wire. A logical span is a group of channels taking up less than a single logical wire, or carried over more than one physical wire.
A computer, attached to the Excel platform through an RS-232 or Ethernet link, from which you run the telecommunications application that controls the Excel platform.
See Programmable Protocol Language.
Call processing layer of the CSP.
Protocol layer of the CSP.
A condition that, when met, drives a state machine from one state to another. A condition that drives a Component out of its current state by invoking an associated primitive.
In PPL, a condition that, when met, drives a state machine (PPL Component) from one state to another, by invoking an associated Primitive. In PPL files, an Event is marked by an arrow and text.
Digital Signal Processor
Tag-Length-Value block. A standardized block that allows data to inserted in a standard way into API messages.
The point in the Excel platform where a DS0 connects to a network. Also known as a "socket."
PPL. Excel feature allowing support of custom protocols, designed through a graphical interface tool. Protocol is defined by associating a State Event Table with a Primitive Table. See State Machine.