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2: PPP use none none integration tocreate none with none 2d qrcode For more information on TDM, refer to Statistical Time Division Multiplexing In another ana none none logy, compare TDM to a train with 32 railroad cars. Each car is owned by a different freight company, and every day the train leaves with the 32 cars attached. If one of the companies has cargo to send, that car is loaded.

If the company has nothing to send, the car remains empty but stays on the train. Shipping empty containers is not very efficient. TDM shares this inefficiency when traffic is intermittent, because the time slot is still allocated even when the channel has no data to transmit.

Statistical time-division multiplexing (STDM) is a variation of TDM that was developed to overcome this inefficiency. STDM uses a variable time slot length, allowing channels to compete for any free slot space, as shown in Figure 2-7..

Figure 2-7. Statistical TDM D D STDM MUX STDM MUX D D D E E F F D E E F D E 8 Bits per Timeslot TS16 TS30 TS31 In the TDM exa mple, signals D, E, and F were always sent in sequential order. Notice that in Figure 2-7, the signal transmissions are no longer sent sequentially. Instead, STDM embeds the signals as required into any available time slot.

To do so, it employs buffer memory that temporarily stores the data during periods of peak traffic. STDM does not waste high-speed line time with inactive channels using this scheme. However, STDM requires each transmission to carry identification information (a channel identifier).

. TDM Examples: ISDN and SONET An example of a technology that uses synchronous TDM is ISDN. ISDN basic rate interface (BRI) has three channels: two 64-kbps B channels (B1 and B2) and a 16-kbps D channel. The TDM has ten time slots, which are repeated in the sequence shown in Figure 2-8.

. Accessing the WAN, CCNA Exploration Companion Guide Figure 2-8. TDM Example: ISDN 64 kbps ISDN NT1 Control TDM 64 kbps B1 B2 B1 B2 B1 B2 B1 B2 B1 B2 D Bit-Interleaved On a larger sc none for none ale, the telecommunications industry uses the SONET or SDH standard for optical transport of TDM data. SONET, used in North America, and SDH, used elsewhere, are two closely related standards that specify interface parameters, rates, framing formats, multiplexing methods, and management for synchronous TDM over fiber. Figure 2-9 shows an example of statistical TDM.

SONET/SDH takes n bit streams, multiplexes them, and optically modulates the signal, sending it out using a light-emitting device over fiber with a bit rate equal to (incoming bit rate) * n. Thus, traffic arriving at the SONET multiplexer from four places at 2.5 Gbps goes out as a single stream at 4 * 2.

5 Gbps, or 10 Gbps. This principle is illustrated in the figure, which shows an increase in the bit rate by a factor of 4 in time slot T..

Figure 2-9 STDM Example: SONET Bit Rate = 2.5 Gbps Link Rate = (2.5 Gbps x 4 = 10 Gbps) 10 Gbps Incoming Stream (T). The original u none none nit used in multiplexing telephone calls is 64 kbps, which represents one phone call. This is referred to as a DS0 (digital signal level zero). In North America, 24 DS0 units are multiplexed using TDM into a higher bit-rate signal with an aggregate speed of.

2: PPP 1.544 Mbps for none none transmission over T1 lines. Outside North America, 32 DS0 units are multiplexed for E1 transmission at 2.

048 Mbps. Table 2-1 shows the signal level hierarchy for multiplexing telephone calls. As an aside, although it is common to refer to a 1.

544-Mbps transmission as a T1, it is more correct to call it DS1.. Table 2-1 Signal Bit DS0 Units Rate Voice Slots DS0 DS1 DS2 DS3 64 kbps 1.544 Mbps 6.312 Mbps 44.736 Mbps 1 DS0 24 DS0s 96 DS0s 672 DS0s or 28 DS1s T-carrier refe rs to the bundling of DS0s. For example, a T1 equals 24 DSOs, a T1C equals 48 DSOs (or two T1s), and so on. Figure 2-10 shows a sample T-carrier infrastructure hierarchy.

The E-carrier hierarchy is similar.. Note For more information, refer to Figure 2-10. T-Carrier Hierarchy DSO DSO DSO T2 T3 T3 T4 T3 T3 T3 T3 T2 T1 T2 T2 T2 T2 T2 T1Cs T1 T1Cs DSO DSO DSO DSO DSO DSO DSO DSO DSO DSO DSO DSO DSO DSO T4 = 6 T3s 274 Mbps 5 T3 = 7 T2s 45 Mbps 5 T2 = 2 T1Cs 6.312 Mbps 5 T1C = 2 T1s 3.152 Mbps 5 DSO DSO DSO DSO T1 = 24 Voice Channels 1.544 Mbps 5 DSO DSO DSO Accessing the WAN, CCNA Exploration Companion Guide Demarcation Point Before deregul ation in North America and other countries, telephone companies owned the local loop, including the wiring and equipment on the premises of the customers. Deregulation forced telephone companies to unbundle their local loop infrastructure to allow other suppliers to provide equipment and services. This led to a need to delineate which part of the network the telephone company owned and which part the customer owned.

This point of delineation is the demarcation point, or demarc..
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