during backup. Port Cost Traditional analog or T1/E1 lines require extra hardware (for example, FXO port, PRI port, and SO on) in the gateway chassis that can cost you from $600 to $3500 per card. In the case of SIP trunking, you don't need that card. If you can do everything over an Ethernet interface, there is substantial savings in using a SIP trunk. You also might need to buy a software license to run SIP features. We discuss that in the next section. Digital Signal Processor (DSP) Cost In theory, you can find a lot of references on SIP trunks. You don't need any DSP in an SBC. This might not be true for all the vendors because different vendors support differ- ent features in their SBCs. The cost of the DSP is one of the major components in your PSTN gateway. Traditionally, DSPs are expensive. If you can design your network with a single negotiated codec (such as G.711 or G.729) end-to-end, you might avoid using DSPs. However, if you need the codec translation and conferencing feature in the same router, you need DSPs. Moreover, Cisco IOS Software might use DSPs to improve your voice quality even though it is IP end-to-end. So, when you calculate your cost for DSPs, check the features with your ven- dors to determine whether they need DSPs. If you can avoid the codec complexity, you can avoid the cost of DSPs. Chapter 4: Cost Analysis 49 Software License Cost Although you don't need extra hardware to run SIP trunking, you might need a software license to enable all the SIP features. In general, the price of a software license is deter- mined by the number of ports or concurrent calls. You might have enough bandwidth to run hundreds of simultaneous calls, but your software might enable only the number of calls your license buys. Licenses can be incremented by a single port. So, if you need to add more concurrent calls during peak hours, you can expand accordingly. When you sign up with service providers, you might not need to pay for a software license separately but it can be built into your package. If the 