redictable patterns of codec use by calls (that is, typically G.711 for calls within a site on the LAN and G.729A for calls that traverse the WAN between sites). The bandwidth consumption of inbound SIP trunk calls is partly based on the service provider's configuration, but an enterprise can use a CUBE to influence codec selection (also called codec filtering or stripping) or to transcode streams in the codec selections the enterprise prefers to use. Call Admission Control (CAC) Gateways connecting to the PSTN through a TDM interface provide an implicit form of CAC in both directions (inbound and outbound) by virtue of the limited number of chan- nels (or timeslots) physically available on the analog, BRI, T1, or E1 interface. No more calls can simultaneously arrive from the PSTN into the enterprise than there are timeslots available on the gateway TDM trunks, providing implicit call admission control. With a SIP trunk entering your network on a physical GE connection (possibly fiber or OC3 transport within the service provider's network before hand-off to your network), nothing physical limits the number of calls that could enter or exit your network at any one time. Top-tier service providers exert CAC control in their networks, and how much protection this offers your enterprise network depends on who your service provider is and how well 126 SIP Trunking the controls are implemented. But there is virtually no physical limit, and it is strongly recommended that you protect your own network with your own CAC controls at your Border Element (especially if you are considering a SIP trunk offering without an explicit SLA). This protects against occasional unplanned bursts or surges in legitimate traffic and against potential malicious Dos attack traffic. Lack of CAC control could overrun band- width on your network and adversely impact network operations. One general problem with CAC implementations is that many policies are often based on simple call-counting mechanisms (such as the CUCM Locations CAC feature) a