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IP Trunks

The SP market is converging to IP, but the PSTN is still the prevalent infrastructure and will be utilized for a while. However, more SPs use the IP network where possible. They achieve this by placing a trunking VoIP switch, which provides Class IV or Tandem switch functionality, to most commonly address two needs. First, it serves as a long-haul SIP trunking switch to carry traffic between SPs of different regions. Second, it acts as a PSTN bypass and an inter-SP trunk interconnect to offload long-distance traffic. The architecture presented in Figure 3-5 touches on various technologies, but the discussion in this section is focused on signaling protocol–related metrics.

Figure 3-5

Figure 3-5 IP Trunk Deployment Architecture

Some of the key components of the IP trunk architecture are as follows:

  • The Cisco PSTN Gateway (PGW): The Cisco PGW 2200 is a carrier-class call agent that performs the signaling and call-control tasks (such as digit analysis, routing, circuit selection, and more) between the PSTN and the IP infrastructure. PGW is also called a trunking switch, and it performs Class IV–type functionality.
  • Analog Telephone Adapter (ATA): The Cisco ATA 186 is a handset-to-Ethernet adapter that turns traditional telephone devices into IP devices, which enables the analog phones to be connected to an IP network. Customers can take advantage of the many new IP telephony applications by connecting their analog devices to Cisco ATAs.
  • SIP Proxy Server: The Cisco SIP Proxy Server is a call control engine that enables SPs to build scalable, reliable VoIP networks today. Based on the SIP, the Cisco SIP Proxy provides a full array of call-routing capabilities to maximize network performance in both small- and large-packet voice networks.
  • Cisco Access Servers: The Cisco access gateway provides universal port data, voice, and fax services on any port at any time. It is used as a common gateway for terminating IP trunks that carry VoIP and other types of traffic. It can be a collection point for signaling and media metrics. Cisco MGX 8850 and AS5400 are examples of the access gateways and are depicted in Figure 3-5.

IP phones also communicate through SIP trunk and SIP proxy servers. Figure 3-5 highlights a deployment of a Cisco PGW VoIP trunking switch in a residential broadband network. It touches aspects of PSTN and IP architecture connectivity. In this particular architecture, the PGW sits at edge of the IP network and deals with offloading the VoIP traffic to the PSTN. The traffic is routed through SIP proxies onto the trunking gateway. Figure 3-5 represents a mixture of networks with various integration boundaries. This shows the SIP connectivity from various sources: the business access, PSTN incoming and outgoing calls, and residential access. All the services provided by these networks need to be tracked. They can be tracked by signaling and media metrics and can help in sizing and service-level assurance (SLA) for the integration points. The SIP, MGCP, and SS7 protocol-related metrics are some of the key metrics that need to be tracked in the IP trunk deployment architecture.

The other major use of IP trunks is across international boundaries, where H.323 networks are prevalent, as seen in Figure 3-6. Figure 3-6 also shows trunk connectivity between the two PGW gateways that are respectively part of large, complex networks. Note the various integration points and the diverse protocol networks. To manage the VoIP service, the capacity and the SLA across these network integration points also become complex. The signaling and other key metrics can help in tracking, trending, and isolating service issues and better plan for capacity and manage SLA.

Figure 3-6

Figure 3-6 IP Trunk Deployment Across International Boundaries

In cases in which it is economical to route the traffic over to IP, providers offload the long-distance traffic to another provider rather than using the PSTN. This offloading is provided by a switch that is performing Class IV or toll-switch functionality. You can see in Figure 3-6 that country B connects to country A through an SIP trunk. That way, it can reach H3.323 networks. The PGW keeps track of all CDRs and is extensively used to apply policies for routing traffic through it to optimize cost. In general, services provided by this critical switch need to be tracked. The call and protocol metrics provided by the switch are crucial for running the VoIP network in an efficient way. Thus, the network management capabilities that facilitate this collection and dashboarding become the key to running the VoIP network.

Figure 3-6 shows SIP, MGCP, and H.323 protocols being used for signaling communication. The corresponding traffic counters represent the KPIs needed to effectively monitor the network. The counter collection points are the respective switching, aggregation, and endpoints. Chapter 7 covers these KPIs in detail.

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