Packet Voice Network Overview

Packet Voice Network Overview

A call agent such as the PGW2200 provides call control capability for the next-generation network. It controls how narrowband TDM voice traffic is consolidated over the packet infrastructure and ways in which you can apply services to those calls. You can use the virtual switch controller (VSC) in a variety of applications to provide call control functions. Examples of applications that are enabled on the packet voice network architecture include the following:

  • Packet voice Interexchange Carrier (IXC) tandem applications

  • Packet voice Local Exchange Carrier (LEC) Class 4 relief applications

  • Endpoint client multimedia applications

  • Corporate voice on-net and off-net services

  • VoIP local/end office applications on cable infrastructure

Figure depicts a generic packet voice application and illustrates various architectural components and how they fit and interact with each other.

Packet Voice Network Architecture

Network Elements

This section reviews each network element that is identified in Figure. These elements include the following:

Call Agent: PGW2200

The Cisco PGW2200 is a carrier-class call agent that performs the signaling and call control tasks (such as digit analysis, routing, circuit selection, and more) within the PSTN gateway infrastructure. Taking advantage of a vast SS7 protocol library and supporting industry-standard control protocols, including MGCP, H.323, and SIP, the Cisco PGW2200 gives service providers the capability to seamlessly route voice and data calls between the PSTN and packet networks.

The Cisco PGW2200 consists of the following three required elements:

  • Cisco media gateway controller (MGC) software, running on Sun Microsystems general computing platforms

  • Cisco Signaling Link Terminals (SLT)

  • LAN switch for IP interconnectivity of Cisco PGW2200 elements

The following are the optional elements:

  • H.323 Signaling Interface (HSI) adjunct processor

  • Management products, including the Cisco MGC Node Manager, Cisco Voice Services

    Provisioning Tool (VSPT), and Cisco Billing and Measurement Server (BAMS)

The following PSTN gateway applications are enabled by the Cisco PGW2200:

  • VoIP transit

  • Primary Rate Interface (PRI) Grooming and TDM Offload

  • SIP PSTN gateway

  • H.323 PSTN gateway

At a high level, the PGW2200 provides the following core capabilities:

  • Call-signal processing, including ISDN Level 3 (Q.931), SS7 Level 4 (ISDN User Part [ISUP]), H.323, Multi-Frequency/channel-associated signaling (MF/CAS), and call signaling toward devices located at residential gateways connected through cable or digital subscriber line (DSL) Customer Premise Equipment (CPE). It also includes the capability to translate between different signaling types on different call legs.

  • Address resolution, call routing, resource management, connection control, and call detail record (CDR) generation.

  • Service access functions for accessing services that execute on external server platforms (such as SCP or Service node).

  • Management interfaces using Simple Network Management Protocol (SNMP) for faults, performance, and configuration. Used as a web-based configuration tool and element management system.

Media Gateway

The MG performs the following high-level functions:

  • Physical T1/E1 TDM facility termination from the PSTN or private branch exchanges (PBX)

  • Echo cancellation into the circuit-switched network

  • Balance of the jitter buffers

  • Voice activity detection (VAD), such as silence suppression and comfort noise regeneration

  • Voice compression using International Telecommunication Union (ITU) recommendations such as G.711, G.723.1, and G.729

  • Tone generation, which generates dial, busy, ring-back, and congestion tones

  • Dual-tone multifrequency (DTMF) transport, which enables use of touch tones for voice-mail applications with codecs that support DTMF detection/transport

  • µ-law and a-law transcoding when required

  • Quality of service (QoS) support

Service Control Point

The SCP provides the execution environment for service logic. It is responsible for processing transaction requests and returning a response. A typical transaction request in the voice world is a number translation.

Examples of this service include 800 (toll-free) service and Local Number Portability (LNP). A toll-free application that is running on the SCP, for example, has a sophisticated logic that enables the end user to control how incoming calls are routed. You can base toll-free call routing on dialed number, time of day, day of week, geographic point of origination, and even on how busy a terminating automatic call distribution (ACD) might be at a given moment. Customers or the service provider (SP) can own the SCP.

Cable Headend

The Universal Broadband Router is an integrated cable modem termination system (CMTS) and Cisco 7200 series router that uses radio frequency (RF) line cards.

The Universal Broadband Router provides a single integrated solution with CMTS functionality, the capability to terminate the Data-over-Cable Service Interface Specifications (DOCSIS) protocol, and the capability to perform all the required data routing functions. Instantiation of this component also includes a digital subscriber line access multiplexer (DSLAM).

Residential Gateway

The residential gateway is a voice/data CPE device that provides from two to four ports of plain old telephone service (POTS) capability. The device runs the DOCSIS protocol to provide packet data and telephony services over the hybrid fiber-coaxial (HFC) cable to the CMTS. Another example of this component is a DSL modem.

H.323/SIP Endpoint/Client

The H.323/SIP client represents a broad range of voice/multimedia applications that are hosted natively on the IP network and are run by the endpoints that support SIP or H.323 as their VoIP protocol. These endpoints might have multimedia capabilities besides voice.

Network Interfaces

The four main network interfaces for the PGW2200 call agent are signaling termination, inter-call agent signaling, connection control, and services control, as illustrated in Figure.

Network Interfaces

Each PGW2200 network interface is discussed in the following sections.

Signaling Termination

The signaling termination capability enables PGW2200 to mediate between many signaling variants, such as SS7, PRI, CAS, and H.323, to name a few.

SS7 Links

Several mechanisms are available to terminate SS7 signaling traffic on the PGW2200:

  • Nonassociated signaling (A-links) These are terminated directly on the PGW2200 using either a V.35 or T1/E1 physical interface. Optionally, to increase reliability characteristics, you can configure a set of Signaling Link Terminals (SLT) to handle the lower layers of SS7. The SLTs are implemented using Cisco 2600 series routers fronting Sun servers that host the PGW2200 application.

  • Fully associated signaling (F-links) These carry bearer traffic and are terminated on the packet gateway. The packet gateway is responsible for executing Message Transfer Parts (MTPs) 1 and 2, encapsulating MTP Layer 3 (MTP L3) protocol data units, and sending them to the PGW2200 for MTP L3 and ISDN User Part (ISUP) processing. The transport between the packet gateway and the PGW2200 is carried out using Reliable User Data Protocol (RUDP), a thin reliability layer on top of User Datagram Protocol (UDP).

PRI Links

The PRI links carry a D channel and terminate directly on the voice gateway. The voice gateway peripherals execute Level 1 (L1) and Level 2 (L2)the lower layers of the PRI interface (Q.921). Layer 3 (L3; Q.931) is encapsulated in the RUDP packet and sent to the PGW2200 for call processing.

CAS Links

CAS links terminate directly on the voice gateway. The gateway periphery handles low-level CAS protocols, such as line and address signaling. You use a CAS application programming interface (API) to backhaul the call-processing events over IP to the PGW2200 for call handling.


The PGW2200 handles the precall-level Registration, Admissions, and Status (RAS) requests in addition to call-level Q.931 requests that originate from the H.323 clients. This signaling termination follows delivery procedures described in the H.323 standard. In other words, the PGW2200 has H.225 RAS/Q.931 capabilities, but it does not have H.323 gatekeeper functionality.

Inter-PGW2200 Signaling

The PGW2200-to-PGW2200 protocol scales the network by distributing control over multiple PGW2200 platforms. A modified ISUP protocol called Enhanced ISUP (E-ISUP) exchanges call-control information between the PGW2200s over an IP network using RUDP. MTP information is not required, so it is not transported.

The E-ISUP messages also carry Session Description Protocol (SDP) elements in ISUP generic digits information elements, which the PGW2200 uses to specify connection attributes in MGCP.


The industry is moving toward using SIP or a variant of SIP, known as SIP-T, for an inter-MGC communication protocol.

Connection Control: MGCP

You can establish end-to-end voice connections in the packet network by using MGCP, an open mechanism to set up connections in IP networks. MGCP is a TCP/UDP-based transaction protocol that permits manipulation of the connections represented by physical or logical endpoints. The connections are described using attributes such as IP addresses, codecs, and so on. MGCP manages call setup requests and connections from phones that are connected to gateways, such as cable or DSL modems.

Services Control

Access to service can follow two paths:

  • IN (AIN/INAP/convergence sublayer-1 [CS-1]) platforms such as SCPs interface initially over standards-based AIN/INAP interfaces that are transported over the SS7 network, with future migration to IP-based transport.

  • Service node services (such as calling cards and voice mail) initially connect over TDM PRI interfaces. In the future, the service node platforms will transition to IP networks to avoid unnecessary TDM/IP interworking.

Figure clarifies the interworking with PGW2200 in the network.

Call Agent Transformation

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