The Cisco HSI adds an H.323 interface to the Cisco Public Switched Telephone Network (PSTN) Gateway (PGW 2200). This interface allows calls to be established between the PSTN and an H.323 network (see Figure 1-1).

The Cisco HSI provides the following services:

Translation of signaling protocols for establishing, controlling, and releasing calls
Administration of network parameters and protocol capabilities
System and call-related statistics
Fault reporting
Overload management
Event logging
Simple Network Management Protocol (SNMP) interface

The Cisco HSI does not operate in an active/standby configuration and, therefore, does not provide the same level of redundancy as the PGW 2200 which is configured as active/standby. We therefore recommend that you use enough HSI nodes to support the number of simultaneous calls plus one. This ensures (Trunk Group Caveats dependant) that, if one HSI fails, the calls would be supported adequately by the remaining active HSI's.

Figure 1-1: Cisco HSI System Overview

PGW 2200

The PGW 2200 consists of the hardware and software that perform the signaling and call control tasks (such as digit analysis, routing, and circuit selection) and seamlessly switch calls from the PSTN through to the IP network.

IP Network

The purpose of the Cisco HSI is to enable the PGW 2200 to interoperate with the H.323 network.

Cisco HSI System Description

The Cisco HSI system has two subsystems (see Figure 1-2):

Operations, Administration, and Maintenance (OAM) subsystem
Call control subsystem

Figure 1-2: Cisco HSI Subsystems

OAM Subsystem

The OAM subsystem provides the following services:

Man-Machine Language (MML) interface that enables the user to retrieve operational parameters and modify configuration values through direct input or through batch files
SNMP interface that allows statistics and alarm retrieval
Process management to provide automatic restart of the Cisco HSI application process and control over the running of the process
Statistics, events, call trace, and alarm output to files
Alarm events output to the MML interface
Overload control

Call Control Subsystem

The call control subsystem provides the following services:

Manages the Reliable User Data Protocol (RUDP) and H.323 stacks
Implements Enhanced ISDN User Part (E-ISUP)
Manages H.323 call control
Performs the conversion of calls between H.323 and E-ISUP call control messages
Provides call management and overload reduction actions

RUDP

RUDP transports the E-ISUP messages between the PGW 2200 and the Cisco HSI.

RUDP is a Cisco proprietary, connection-oriented, packet-based transport protocol.

RADVision H.323

The Cisco HSI uses the RADVision H.323 stack. The system uses the H.225 (Q.931 and registration, admission, and status [RAS] protocol) and H.245 protocols to implement the H.323 endpoint signaling function.

RADVision H.323 enables the creation of real-time voice H.323 calls over IP networks.

E-ISUP

E-ISUP is a proprietary Cisco protocol based on ISUP. E-ISUP is used for inter-PGW 2200 call control. E-ISUP uses a subset of ISUP messages. The main differences between ISUP and E-ISUP are as follows:

E-ISUP is for the control of packet voice connection. It does not have circuit management messages such as circuit reset and blocking.
E-ISUP is transported over RUDP in an IP network.
E-ISUP provides the ability to transport Session Description Protocol (SDP) information (such as endpoint IP address and codec specifications) for call endpoints.

The Cisco HSI provides a conversion between the E-ISUP call control protocol originating from the PGW 2200 and the H.323 call control protocol originating from the IP network (see Figure 1-1).

Operational Environment

This section provides operational environment requirements for the Cisco HSI.

Hardware Requirements

The Cisco HSI is targeted to run on a Sun Netra T1 100/105 platform with an 18-GB hard disk drive, a Sun Netra 120 platform with a 36-GB hard disk drive, or a Sun Fire V120 platform with a 36-GB hard disk drive. These platforms run on the Sun Solaris 8 operating system.

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