es of CPE that are provided when SP offers a SIP trunk service vary widely based on the SP and the type of features offered on the SIP trunk. A common deployment for SIP Trunk services is when SP offers SIP trunks for PSTN access over its own physical infrastructure. In this case, the typical components of a CPE are Physical connection to the SP, generally Layer 1-based, such as T1 or DS3, but could be Wireless (for example, 3G) or other connectivity methods Ethernet (either FE or GigE) connection to the enterprise LAN Chapter 5: Components of SIP Trunks 65 Control and monitoring software on the devices that ensure that the SP can trou- bleshoot issues The software in the CPE is often in the form of a B2BUA. In this case, the CPE device would physically terminate the SIP messages and regenerate a new SIP message toward the SP for messages from the enterprise to the SP. For messages from the SP to the enter- prise, the CPE would terminate the message from the SP and regenerate a new message to the enterprise. Generally, the role of the CPE is to take SIP messages (or in some cases H.323 messages) from the IP-PBX on the customer premise, terminate them, extract the relevant information for routing (for example, calling number, called number, codec, pri- vacy indicators, and SO on), and reconstruct a new SIP message that would come from the CPE directed to the SP. This ensures that the SP core network needs only to route and respond to messages presented in a manner that is understood, from a device that is con- trolled by the SP. Similarly, because messages in a B2BUA can be "tweaked" to conform with the customer equipment that it is sending messages to, the SP can ensure that there is compatibility with a wide range of customer equipment. Another form that the software in the CPE can take exclusively is a Network Address Translator (NAT) and security device. In this case, the SIP messages are not terminated the CPE equipment; they are instead validated to ensure that they conform to a predeter- mined set of standa