Small/Medium Business Applications (Voice over T1/E1/PRI/CAS)
The SP uses small/medium business applications to provide IP-based telephony to SMB customers, typically when only a low number of devices are needed at the customer premises (usually this number is less than 50).
SPs often use an Integrated Access Device (IAD) to provide voice and data service to the customer premises. The IADs are connected to a Local Exchange Carrier (LEC) leased Primary Rate Interface (PRI)/Channel Associated Signaling (CAS) line, which is aggregated through a bigger transport connection like DS3 to the VoIP SP. The CMS residing at the SP is used to provision the IADs.
MGCP is the signaling protocol used to communicate to the IADs, and both signaling and the data ride the same leased line. Figure 3-4 reflects such a topology. The SP also routes PSTN/SS7-bound calls originating at the customer premises through the CMS that's managing the IAD/MGCP link. The trunking gateway and the SS7 gateway (which can be a Cisco Internet Transfer Point [ITP]) handle the PSTN-bound bearer and signaling traffic, respectively. The CMS acts as a central switching point and is thus an ideal place for collecting key performance indicators (KPI) because it acts as central switching component. The MGCP-based communication counters for announcement servers, trunking gateway and IADs, along with SIP-based communication counters for voicemail server and PSTN-related SS7 signaling (SIGTRAN) protocol counters make up the KPIs for the Small Business model.
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Figure 3-4 Small/Medium Business Deployment Architecture
Integrated Access Device (IAD) is also one of the key elements for the small-business network. It can be a collection point for MGCP-related metrics. An IAD is a device used to multiplex and demultiplex traffic in the customer's premises. The IAD is used primarily to route traffic and signaling over to a single T1 line or to an ISDN PRI trunk. This is also called a voice gateway that utilizes E1 lines in the case of non-U.S. markets.