- D. The channels do not have to be grouped, and may be mapped between links in any way, provided there are available channels on the links. Demarc Demarc (pronounced dee-mark) is a slang abbreviation for demarcation point. The demarcation point is the point where the telecom provider's responsibilities end and yours begin. Demarcs are often telecom closets or similar locations that can be secured to allow access for the telecom provider's engineers. If you need your T1s to terminate somewhere in your building other than the demarc, the telecom provider may install an extended demark to accommodate your needs. Telecom Glossary T1 - B (24 Channels) T1 - A (24 Channels) T1 - C (24 Channels) T1 - D (24 Channels) Digital Access Cross Connect System (DACCS) Figure 20-3. DACCS Digital signal hierarchy The digital signal (DS) hierarchy describes the signaling rates of trunk links. These links are the physical links on which logical T-carriers are placed. The carrier numbers grow larger as the number of multiplexed DSOs increases. DSO is the smallest designation, and is the rate required for a single phone line. The hierarchy is shown in Table 20-1. Table 20-1. Digital signal hierarchy Designator Carrier Transmission rate Voice channels 64 Kbps 1.544 Mbps 6.312 Mbps 44.736 Mbps 274.176 Mbps 4,032 E-carrier The E-carrier hierarchy is similar to the US T-carrier hierarchy (described later), though the speeds are slightly different, as is the signaling. The European E-carrier hierarchy is shown in Table 20-2. Table 20-2. European E-carrier hierarchy Designator Transmission rate Voice channels 64 Kbps 2.048 Mbps 8.448 Mbps 34.368 Mbps Chapter 20: Telecom Nomenclature Designator Transmission rate Voice channels 139.268 Mbps 1,920 565.148 Mbps 7,680 ISDN stands for Integrated Services Digital Network. ISDN is a form of digital transmission for voice and data. Unlike normal POTS lines or channelized T1 services-which use the voice path for signaling-ISDN uses a separate channel called the data channel for signaling, SO the rema