This chapter presented a major design challenge for VoIP: achieving high availability. Specifically, our VoIP availability design goal is the "five nines," 99.999 percent of availability, which equates to approximately five minutes of downtime per year.
Most companies perform a phased migration to VoIP, as opposed to a forklift upgrade. Interconnecting existing PBX/key system units over the IP WAN is often the first step in a phased migration, and it offers toll bypass cost savings.
We then considered various analog and digital interfaces available for our routers and what could connect to those interfaces. Analog phones, for example, can connect to FXS interfaces. The telephone wall jack in our home can connect to an FXO interface. Also, PBXs in our company might connect to remote PBXs using E&M interfaces. We can leverage our company's existing investment in those interfaces by connecting the PBX's E&M interfaces to E&M interfaces on our routers.
Whereas FXS, FXO, and E&M are examples of analog interfaces, digital interfaces include T1, E1, and ISDN. Recall that a T1 circuit has 24 channels, and we can use all 24 channels to send voice traffic if we use CAS, which is sometimes called "robbed-bit" signaling. However, the CCS option uses one of the 24 channels just for signaling.
An E1 interface has 32 channels. However, we only use 30 of those channels for voice. The first channel is used for framing and synchronization. The seventeenth channel is used for signaling, and it's interesting that the seventeenth channel is used for signaling in both the CAS and CCS modes.
ISDN comes in two flavors, BRI and PRI. The BRI flavor (which includes two 64-kbps B-channels) might be appropriate for a SOHO environment, whereas PRI (which includes 23 64-kbps B-channels on a T1-based PRI interface) would be more appropriate for a larger environment.
This chapter included an overview of VoIP in the home. Several service providers have equipment in telephone COs scattered across the country. If we have a broadband (for example, DSL or cable modem) connection in our home along with a router that allows us to connect phones, our phones can connect to the service provider's equipment over the Internet (that is, over our broadband connection) and from there connect to the PSTN. VoIP in the home has the potential to offer comparable features to our existing phone service. However, because VoIP in the home is an emerging technology, subscribers should understand exactly how their service provider handles 911 calls.