zations with IP telephone systems need to support two different sets of trunks, one set for voice and another set for data. Or they can use a gateway to translate their IP non-SIP calls to SIP. (Trunks are used to carry voice and data on public networks.) Without SIP there are extra costs and long- distance fees for gateways, and voice quality is impaired. The signal's quality is impaired when converting VoIP to formats compatible with SIP data networks when they are sent, and converting them back to VoIP when they are received. Impairment results because compression does not re-create voice in exactly the same format when it's decompressed, and vice versa. Thus, its quality is degraded every time voice data is converted. Direct Inward Dialing Direct Inward Dialing (DID) is a feature that routes incoming calls directly to a telephone system extension without operator intervention. DID traffic can be carried on existing digital trunks used for incoming and outgoing voice calls and on SIP-type trunks carrying voice and data traffic. www.TechnicalPDF.com As Figure 2-9 illustrates, DID service is made up of "software" telephone numbers. Each number is not assigned a specific trunk; rather, a few hundred DID numbers are able to trunk share paths. When DID calls reach a specific slot in the telephone company's switch, the carrier's switching equipment looks at the digits dialed and identifies the call as belonging to a particular organization. The telephone company equipment passes the last three or four digits of the dialed number to the organization's telephone system. The onsite telephone system reads the digits and sends the call directly to the correct telephone. www.TechnicalPDF.com Telephone company sends digits 1327 to Company X Server with IP PBX Switch software Xxx-555- 1327 dialed Extension 1327 receives call Figure 2-9 DID soft numbers pointed to an organization's broadband circuits. (Photo by Chun- www.TechnicalPDF.com Tso Lin/123RF) UNIFIED COMMUNICATIONS, CONTACT CENTERS, AND VIDEO CONFERENCING Value-add