 problem lies, the telephone company can test its trunk to the RJ21x jack, and the equipment vendor can test service between the equipment or router and the interface. The RJ21x jack is the demarcation point between the telephone company line and the inside wir- ing. Figure 2-6 illustrates a demarcation point. IP Private Branch Exchange Architecture RJ21x demarcation Telephone company central office Internet In-house wiring frame Switch III ######## ******** ######## HE! Router Gateway in router Server LAN = Local Area Network Figure 2-6 The demarcation point to which outside trunks are wired. Individual T1 lines terminate at data jacks (RJ48 jacks), and T3 lines terminate on 75 ohm connectors. These are both at the customer premises, and they are the points at which telephone companies test the T1 and T3 lines if there is an outage. T1 is a high-speed line capable of carrying 24 switched voice lines or a mix of voice and data traffic. A T3 line is equivalent to 48 T1 trunks: 672 voice channels or a mix of voice and data. Direct Inward Dialing Direct Inward Dialing (DID) is a feature that routes incoming calls directly to a tele- phone system extension without operator intervention. DID traffic can be carried on existing T1 trunks used for incoming and outgoing voice calls and on SIP-type trunks carrying voice and data traffic. As Figure 2-7 illustrates, DID service is made up of "software" telephone num- bers. Each number is not assigned a specific trunk; rather, a few hundred DID numbers Data Centers and IP Private Branch Exchanges T1 trunk with 24 paths Central office telephone for incoming DID and numbers 555-6200 to outgoing calls 555-6299 belong to XYZ Company Gateway to convert Layer 2 switch Media server PSTN signals to IP, and vice versa Ext. 6261 xxx-555-6261 dialed DID = Direct Inward Dial PSTN = Public Switched Telephone Network Figure 2-7 DID soft numbers pointed to an organization's circuits. are able to share 24 T1 slots (paths). When DID calls reach a specific slot, the carrier's switching equipme