1. Switch a/k/a 5E CO End Office Line Side Card Station Ports Trunk Side Cards Interoffice trunking Local Service Trunk Side Cards Interoffice trunking To LATA Tandem for routing per PIC or LPIC DECISIONS* Incoming call – Look at the digits. Where do I need to connect this? (What port?) Available or busy? Forward? Voice mail? Outgoing call – Look at the digits. Does it stay here or leave? Is it local BellSouth? Is it local CLEC or toll? Do I have an empty channel? Overflow or fast busy? ……………………etc etc etc CENTRAL OFFICE To / From other BellSouth local end offices CENTREX* CUSTOMER PREMISES CENTREX* Does this number (port) belong to this CTX group? Features - Is it allowed to do what it’s asking to do? Calling out – How many NARs? How many are active now? Process the call or return busy? Analog or Digital CENTREX GENERIC NARSNARS Network Access Registers (NARs) are the gateway between the private Centrex and the public switched network. They are the soft ware equivalent of lines or trunks, providing inward, outward or two-way access. Internal calls do not use NARs.

2. Switch a/k/a 5E CO End Office Line Side Card Station Ports Trunk Side Cards Interoffice trunking DECISIONS* CENTRAL OFFICE CENTREX* CUSTOMER PREMISES NARs: Network Access Registers Trunk equivalents: How many simultaneous calls are allowed to/from “outside” Line Side Card Station Ports Trunk Side Cards CENTREX & CENTREX PRI with Business Communications Manager A BCM is a small version of a very intelligent switch. Centrex numbers for the stations are carried across a PRI used as 23 tie lines. The BCM also has features, voice mail and many advanced options for call handling, but is still able to make on-net calls to other Centrex stations outside the BCM. NARSNARS INCOMING & OUTGOING CALLS CENTREX PRI

3. CO CENTREX NARs POLICE CITY HALL Parks FIRE STATION “OUTSIDE LINES” FOUR DIGIT NUMBER CALL DIAL 9 “OUTSIDE” CALL CENTREX & CENTREX PRI Connecting to a PRI ISDN Compatible Switch CITY Fiber 1 Centrex PRI