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Transport Capacity

Frequency Wire Size Distance
1.544 Mbps 24 AWG 18,000 ft
2.048 Mbps 24 AWG 16,000 ft
2.048 Mbps 26 AWG 15,000 ft
6.312 Mbps 24 AWG 12,000 ft
6.312 Mbps 26 AWG 9,000 ft
8.448 Mbps 24 AWG 9,000 ft
12.960 Mbps 24 AWG 4,500 ft
25.920 Mbps 24 AWG 3,000 ft
51.84 Mbps 24 AWG 1,000 ft

Table 9-5. Data rates

The data rate is dependent upon the wire size and distance from the CO to the customer premises. Table 9-5 compares multiple frequencies and the maximum distance the signal can be transmitted, assuming a near perfect transmission line. Evident from the table is the relationship between wire size and distance for any given frequency. As the wire size decreases, the maximum distance decreases.

Channel Frequency
1x1.536 Mbps 1.536 Mbps
2x1.536 Mbps 3.072 Mbps
3x1.536 Mbps 4.608 Mbps
4x1.536 Mbps 6.144 Mbps
1x2.046 Mbps 2.048 Mbps
2x2.046 Mbps 4.096 Mbps

Table 9-6. Bearer channels


Figure 9-14.   ATM over ADSL

Various ADSL data rates, called bearer channels, are available for service providers to utilize for service offerings. See Table 9-6. Perhaps the consumer can choose the quality of picture and only pay for the bandwidth, and quality of picture, desired. Data rates depend upon several factors including the length of the transmission path, the size of the wire, even the age of the wire, and the number of taps and splices between the CO and customer premises. Table 9-5 lists some of the data rate choices available to service providers and the maximum length of the transmission path assuming near perfect conditions.

Framing

The ADSL channels are synchronized to the 4 KHz ADSL DMT frequency (also called symbol rate). The data and service information passing over each channel has a specific format called framing. ADSL first transmits service information, then a large number of individual data frames before finally transmitting a sync frame. The modem combines all the data frames and service information frames into one distinct data unit called a superframe. Figure 9-13 shows the arrangement of the individual frames within the superframe.

Figure 9-13.ADSL superframe

ADSL framing consists of 68 data frames combined with an 8-bit CRC frame and a 24-bit Operations Administration and Management (OAM) frame to produce the ADSL superframe. Each data frame period is 250 microsec, yielding a 4 Kbaud rate for that particular data frame. The sync frame is added on the end of the superframe to ensure the receiving circuitry is properly timed to the signal.

ATM Over ADSL

ATM over ADSL is capable of delivering every conceivable video product to the home without any changes to the current customer premises (my house) infrastructure (wiring, equipment, etc.) other than connecting the incoming telephone line (the common twisted pair copper line) to an ADSL modem. Just plug it in like it was another telephone, only plug the telephone into it.

Getting a video signal from a video service provider to your home over ATM is not very difficult. Figure 9-14 shows a simplified diagram of the transmission path from the video service provider to a home. The video service provider analog video signal is digitized, then cellutized, that is, assembled into ATM cells. The ATM converter shown in Figure 9-14 is the digitizing and cellutizing function. In the real world, the conversion of an analog video signal to ATM cells could be performed by several pieces of equipment. The ATM converter sends the cells out to the service providers ATM network source node for transport over the ATM network from the source location to the destination location. This portion of the trip is represented by the "cloud." Ever wonder why they use a cloud to represent the service providers network? Ever seen "inside" a "cloud"?

At the destination location in the ATM network cloud, the ATM cell stream is converted to an ADSL signal suitable for transmission over the humble pair to your home, represented by the CPE. The ADSL modem converts the signal to first an ATM cell stream, then to the original video signal. Thats all there is to it...more or less. The upstream channel can be used for service requests, such as requesting access to a particular channel.

Figure 9-14 shows a single home receiving video from a single source. From an enterprise perspective, the really beautiful aspect of ATM over ADSL is the ability to provide video products to the whole country from one single location.


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