SCF) handles emergency services, such as 911 calls. BGCF/TF-breakout gateway control function/transit function; provides optimal routing to the TDM network. The transit function is also included in this element, which supports IP interoperation with other carriers. HSS-home subscriber server; master database with subscriber profiles, performs authentication. DNS-domain name system; allows network elements to address one another using sym- bolic names. ENUM-E.164 Number to Uniform Resource Identifier (URI) mapping; translates tele- phone numbers into Internet addresses. DRA-diameter routing agent; finds the HSS that corresponds to a particular subscriber since the subscriber's data are not common to all HSSs. CCF-charging collection function; receives accounting request (ACR) information from the network elements and assembles them into billing records, or CDRs. MRF-Media resource function; provides tones and announcements. SCC-AS-service centralization and continuity AS; accommodates call continuity when a UE leaves LTE coverage, such as moving into a 3G-only area. LIG-lawful intercept gateway; interface for communications assistance for law enforce- ment act (CALEA) intercept requests. The access layer contains the border controllers and gateways: A-SBC-access session border controller; serves as a firewall, handles IPv4-to-v6 inter- working and other services. Implements the proxy CSCF. I-SBC-interworking session border controller; similar to the above but used between untrusted systems. DBE-data border element; provides a stateful firewall, deep packet inspection, and other services. IWSF-IMS web security function; provides a secure interface for a variety of endpoints. MGCF-media gateway control function; handles signaling for the corresponding MGW. MGW-media gateway; converts RTP to TDM. The IMS service platform is a fixed set of components (I/S-CSCF, CCF, HSS, DRA, DNS, ENUM, CALEA, etc.). It is typically deployed in georedundant configurations at a few centralized core sites. The placement of the core elem