rds. These include things such as length of message, contents of Session Descriptor Protocol (SDP), and SO on. After the validation, the messages are passed through a SIP Firewall and Application Layer Gateway (SIP ALG). This is a com- ponent of a firewall that utilizes NAT, that takes the addresses of the outside network exposed to the SP, and translates them into IP addresses valid for the internal network of the enterprise and vice versa. The other function of the firewall is to open a hole in the firewall to let RTP traffic flow between the enterprise and SP based on the information within the signaling packets. This NAT and SIP ALG function is usually coupled with some type of specialized troubleshooting or network testing software. The troubleshoot- ing software included in the CPE can often simulate VoIP packets and calls and act as an independent termination point for call signaling to aid in troubleshooting. This helps to complete the role of a CPE as an aid in the troubleshooting functionality of the SP. The IP Multimedia Subsystem (IMS) is an architectural framework for delivering many servic- es over an IP network, including SIP Trunking. In the IMS architecture, the CPE equip- ment is known as a Border Gateway (BGW). The features and roles of SP CPE are shown in Figure 5-4. 66 SIP Trunking SP CPE Customer IP-PBX A Terminates customer IP-PBX traffic on inside address Terminates SP SIP traffic on outside address Provides point of troubleshooting and Ethernet demarcation Network Edge SBC Terminates and interworks Physical IP address layer from SP to Enterprise Validated SIP Traffic on SP Routeable Customer address-Targeted IP-PBX B toward SP Edge Customer can have multiple IP- PBXs connected to the same SP CPE equipment. Each SP would have their own CPE equipment. Generally the connection to the customer is via Ethernet. Enterprise Control Service Provider Control Figure 5-4 Features and Roles of SP CPE SP Network-Media Gateways (Voice and Video) As SPs look to offer service on SIP trunks, the first and