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The typical real-time adaptation rate is 32 Kbps. Just pray your modem doesnt decide to adapt in the middle of the TV program you have waited all year to watch. Actually, to maintain a minimum Quality of Service level acceptable to consumers, sufficient bandwidth margin is engineered into the system to ensure that under average conditions, your viewing pleasure will not be interrupted nor will the picture be degraded as the modem adapts to the usually changing conditions of the line. If your viewing conditions fall outside the spectrum defined as average, then you may not be a good candidate to receive ADSL service. By the way, just how often does a UFO pass near your home?


Figure 9-11.   ADSL modem

Application Architecture

The ADSL CPE set-top box is called the ADSL Network Termination (ANT). Figure 9-10 shows a simplified diagram of an ANT. The ADSL Line Interface Module transmits signals to and receives signals from the ADSL Access Adapter located at the CO. The ADSL Line Interface Module conditions (adjusts the level and ensures proper timing intervals) the ADSL signals and converts the ATM cell stream to/from the ADSL signal. Also, the ANT includes the POTS splitter/combiner.

The ATM bus, operating at 10 Mbps, multiplexes/demultiplexes the cells received from or sent to the individual service modules. The output of the service modules is the native (original) electrical signal necessary for the service to work properly. So, the output of the LAN service module is Ethernet packet data and it is sent to/from the business computer. The output of the video service module to the television is the analog video signal and the output is ATM video cells to the ATM bus.

The video, audio, Internet, and LAN boxes shown in Figure 9-10 comprise the ADSL modem portion of the ANT. The functions of the ADSL modem are shown in Figure 9-11. The direction of the arrows indicates this figure depicts a CPE modem. A CO modem is the reverse image of this figure.

Let’s follow a signal that is sent from the CO to the CPE. The downstream signal (from CO to customer premises) comes into the receiver where the signal is converted from a high frequency analog ADSL signal to a digital signal. The receiver applies the digital bit stream to the channel separator. The channel separator separates the individual channel signals into their respective bit streams. The native digital signals are sent out to their respective terminal equipment (computer). The non-native digital signals (originally analog) are applied to the D/A converter which, strangely enough, converts each digital bit stream into its native analog signal. The output of the D/A converter is an analog reproduction of the original transmitted signal. The analog signals are routed to the appropriate outputs with the analog telephone signal passed through the POTS splitter.

The transmit path from CPE to CO is as follows. The analog telephone signal is sent to the A/D converter through the POTS splitter. Also, video and audio signals are sent to the A/D converter. The A/D converter, strangely enough, converts the native analog signals into digital bit streams. The digital bit streams are combined with the native digital bit streams from the native digital sources (computers) and are applied to the multiplexer. The multiplexed digital signal is now sent to the transmitter. The transmitter converts the multiplexed digital bit stream into the analog ADSL signal and sends it on its way to the CO.


Figure 9-12.   ADSL access adapter

System Architecture

The central office includes ADSL access adapters for interfacing the ATM network to the ADSL modems. Figure 9-12 shows a simplified diagram of a typical access adapter. The number of ADSL line termination boards varies from CO to CO depending upon the traffic patterns of the service area.

The ADSL access adapter shown in Figure 9-12 consists of a network interface, a 155 Mbps ATM bus, four ADSL line termination boards, and POTS access. Data is received from and transmitted to the ATM network through the network interface. The network interface performs signal conditioning (signal level) and terminates the line, such as a fiber termination. The ATM bus multiplexes the transmit data going out to the ATM network and demultiplexes data received from the ATM network. The ATM bus multiplexes/demultiplexes the ATM VC/VPs, which can be various bit rates. This feature allows the ADSL line termination boards to operate at different bit rates, if desired. The ADSL line termination boards support a wide range of access speeds from 1.5 Mbps all the way up to the ATM bus speed of 155 Mbps.

The ADSL line termination boards terminate four ADSL lines. The ATM bus side of the boards receives/transmits ATM cells from/to the ATM bus. The CPE side of the boards transmits/receives ADSL traffic to/ from the CPE. The POTS splitter at the CO is physically located on the ADSL line termination board. At this point the narrowband voice signal is combined with (going to the CPE) or split from (coming from the CPE) the ADSL signal.

Local telephone companies install local access nodes remotely from the CO to reduce the length of the transmission path from CO to customer premises. The area serviced by the remote nodes is called a distribution area (DA) and is intended to service an average of 1,500 customers. However, in urban areas the number of customer premises could double to 3,000. Some remote access nodes are fed with DS-1 (T1) lines using HDSL. The particular system architecture required for any given site is dependent upon the distance from the CO and the type of services desired. All things must be considered before a site can be connected to DSL technology and have an acceptable level of performance. Not all the capabilities of DSL technology are available to everyone. However, the service of most interest is available to almost everyone, video-on-demand.

POTS Splitter

A CPE filter is necessary to split the narrowband voice channel (POTS channel) from the broadband data channel. The filter can be either active or passive. An active filter uses integrated circuits and the passive filter uses reactive components (inductors, capacitors) to perform the filtering function. Both types of filters are designed so if they fail, the user will always receive POTs and not broadband data (video/ Internet) service. The filter will most likely be physically located in the set-top box.

A POTS splitter is really a combiner/splitter depending upon the direction the POTS signal is traveling. A POTS signal that is added to an ADSL signal is really combined with the ADSL signal. A POTS signal that is removed from an ADSL signal is really split from it. So, when the literature refers to a "POTs splitter," we are really referring to a "combiner/splitter."


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