POTS access to DSLAM
One of the simpler approaches to delivering a VoIP service is to deploy it in stages. Voice services that coexist with DSL connections have used a frequency splitter at the customer premises to separate the lower-frequency voice band from the DSL signals. A VoIP architecture does not need to be IP end-to-end. Instead, a voice switch at the central office (CO) or exchange can provide the interworking between the POTS and IP voice signaling. This avoids the extra complexity of having to deploy SIP-capable CPE. Many DSLAM vendors are integrating voice switch capability into their products, which enables the local loop at the CO to be terminated on the DSLAM for both voice and data services. This eliminates the need for external splitters and voice switch hardware if a single provider has exclusive use of the copper to the customer.
In terms of the service to the customer, the features are almost the same as POTS or ISDN, which have been used for many years. On the provider side, the voice switch in the CO takes any analog calls from the customer and sends them down a VoIP trunk to one of the soft switches. These soft switches perform call routing and billing. A sample end-to-end architecture is described in the next section. This is a good stepping-stone before a full-blown end-to-end VoIP architecture. Such an architecture might also be used in environments where a competitor wants to supply both a voice and DSL service to the customer, rather than allow the incumbent to have the voice-band service.
Designing and deploying triple-play services
How to provide business VoIP services
Providing business VoIP services through a digital subscriber line access multiplexer
How to design and implement a business VoIP network
Reproduced from chapter two of the book Broadband Network Architectures: Designing and Deploying Triple-Play Services by Chris Hellberg, Dylan Greene and Truman Boyes. Copyright 2007, Cisco Systems, Inc. Reproduced by permission of Pearson Education, Inc., 800 East 96th Street, Indianapolis, IN 46240. Written permission from Pearson Education, Inc. is required for all other uses.