ble TV or VolP traffic telecom company's cabling VolP calls on computers or phones using providers such as Google, majicJack, and Skype Figure 5-1 Transmitting Voice over IP traffic (VoIP) "over the top" of carriers' infrastructure. OTT VoIP services are generally less costly than those offered by traditional phone and cable TV companies. Their costs are offset by the other sources of revenue and the fact that they don't have the expense of providing their own cabling infrastructure. In addition, OTT VoIP providers generally bill through credit cards and thus don't have the expense of fee collections, and bill generation. These services are usually paid in advance; customers provide credit card information and service providers bill them monthly, in advance. VolP Services for Commercial Organizations Many large and small business and nonprofit organizations now have VoIP telephone systems for internal calls. However, because of its consistent quality, they often use the PSTN for calls to people outside of their organizations. Organizations that use VoIP for internal calls and the PSTN for external calls are required to purchase extra hard- ware, or gateways, to convert between VoIP and PSTN formats. Moreover, converting between these formats can somewhat lower quality on these calls. In addition, calls on the PSTN, particularly those to international locations, might be more costly than those carried as VoIP traffic on data links or over the Internet. Newer VoIP systems now often come equipped with interfaces compatible with VoIP networks. These interfaces, which are based on Session Initiation Protocol (SIP), are compatible with the VoIP trunk interfaces of most carriers. Thus, connections between the customer telephone system and VoIP networks do not require gateways. This simplifies hardware requirements on these telephone systems. Carriers send Broadband and Wide Area Network Services VoIP-formatted calls to the PSTN. Interfaces at the PSTN convert the VoIP signals to those with which it is compatible (see Fi