March 25, 2011, 10:35 p.m.
posted by who
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:
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
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:
The following PSTN gateway applications are enabled by the Cisco PGW2200:
At a high level, the PGW2200 provides the following core capabilities:
The MG performs the following high-level functions:
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.
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).
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.
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.
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.
Each PGW2200 network interface is discussed in the following sections.
The signaling termination capability enables PGW2200 to mediate between many signaling variants, such as SS7, PRI, CAS, and H.323, to name a few.
Several mechanisms are available to terminate SS7 signaling traffic on the PGW2200:
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 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.
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.
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.
Access to service can follow two paths:
Figure clarifies the interworking with PGW2200 in the network.
Call Agent Transformation