Broadband Access technologies

1. Broadband Access technologies • As the multimedia content grows, we need faster internet connectivity for SOHO users. Broadband access technologies shown below are considered to provide integrated voice, data, and video services to homes (broadband pipes to home): • ADSL (asymmetrical digital subscriber loops) • HDSL (high-speed digital subscriber loop) • RADSL (rate-adaptive digital subscriber loop) • SDSL (symmetrical digital subscriber loop) • VDSL (very high-speed digital subscriber loop) • IDSL (ISDN digital subscriber loop) • We shall concentrate on ADSL. Others are some variation of this technology • ADSL is an asymmetric system that makes it suitable for internet access. The term asymmetric is used because the data rates in the two directions are not equal. The ADSL has fast downstream channel from the central office or ISP to the home occupies the band from 138 kHz to 1.104 MHz, while the slow upstream channel occupies the band from 4 to 138 kHz.[Figure 1]. Multiplexed application for backbone network is shown in Figure 2.

2. ADSL Operation • ADSL uses BW above the 4‑KHz voice band (up to 1.104 MHz) and allows a phone call and a high­speed data connection to co-exist. A splitter which is a high­pass/low-pass filter (HPF/LPF) combination, is used to separate the voice and data signals at each end. ATU-R is the ADSL transceiver on the remote side, and ATU-C is at CO (Figures 3). Discuss HP/LP filter requirements. • In theory, ADSL can provide up to 8 Mbps (Max. More like 6 Mbps; 64-QAM @ [6bits/Hz x (1.104 –0.138)]  6.0 Mbps) of data from CO downstream to users, and up to 800 Kbps [ 138.0-4 = 134 x 6 = 804 Kbps]from subscriber to CO. Along with these directional data is voice communication [Fig. 6] • Channel performance over a twisted pair can vary greatly depending uponthe length and condition of the wire. A pair is unsuitable for ADSL if it includes a loading coil, or if it terminates in a 4-kHz band. Perhaps one-third of all pairs are unsuitable for this reason. The performance of the remaining pairs is highly variable depending again upon their length and the extent of far ‑end and near ‑end cross ‑talk. The concept and configuration of FEXT and NEXT is shown in Figure 4. Thus, only a subset (limited %) of all local loops qualify for ASDL

3. 0 4 138 1104 Fig. 1 KHz Fig. 2

4. [Figure 3]

5. [Fig. 4]

6. ADSL Design considerations • Figure 3 shows the system layout. Filters (HP/LP) of right type is included in ATU-C and ATU-R so that the digital signal passes through and voice band energy is blocked. Noise and linearity are the biggest design consideration. • As in Figure 3 the total line attenuation in dB integrated over the up to l.l04 MHz band as a function of line length in kft for two types of wire gauge AWG24 and AWG26. (AWG26 is a thinner gauge and accordingly exhibits higher loss.) • From the graph it is evident that a total attenuation up to 50 or 60 dB is easily possible on long lines. • Thus, the received signal at the end of a long line can have a very small amplitude and thus the receiver noise performance is critical. Therefore, noise is a huge concern during the development of the ADSL front end. • To counteract degradation due to noise, ADSL employs a discrete multi tone modulation scheme. Non-linearity in the analog signal path cause harmonic and inter-modulation distortion.

7. ADSL Design considerations • The total band of 1.104 MHZ is divided into 256 frequency bands (bins) that are 4.3125 KHz apart [Fig. 5]. Those channels having more noise carry less data. Channels having less noise (high S/N ratio) carry more data. • Before transmitting data there is a period of initialization (training) where channel quality is measured.Test signal consists of 248 tones over the entire 1.104 MHz band with eight missing tones or holes where noise (IMD noise) are measured. Estimation of noise power along with signal plus noise power measured in each bin provides S/N ratio [ 1 + S/N] • ANSI T1E1.413 standard specifies the maximum transmitted power density to be - 40 dBm/Hz over a telephone line. Thus, the maximum power level in 1.104 MHz band is  - 40 +10log10(1.104 x 106) = 60.4-40 = 20.4 dBm. Line driver at the central office must deliver this much power.

8. Fig. 5 Fig. 6

9. ADSL Design considerations • Crest factor: Variable modulation in different bins cause a very high peak voltages over telephone lines. Independent frequency and phase of 256 carriers, create a statistical distribution of composite signal amplitude over the transmission line. Thus, there will be times when the phases of multiple tones will align in a manner that causes the peak amplitudes of the individual tones to add to each other so that the composite waveform has a large peak amplitude. The ratio of this peak amplitude to the RMS value is called the "crest factor" and for ADSL it is typically chosen to be 5.6, or 15 dB (20 log10(5.6), shown in Figure below 7). At this crest factor value, the probability of exceeding 5.6 volts is reduced to approximately 1 x 10-7. Given this crest factor, the maximum voltage at the client end can be determined. Assuming a maximum power level of 8 dBm (SQRT(10-2.2/100.0), the resulting RMS voltage level on a 100 ‑ ohm line will be 0.8 V. Converting from RMS to peak by using a 5.6 crest factor yields a peak voltage of 4.48, or 8.96 V, peak -to‑peak.

10. ADSL Design considerations • If the A D converter is implemented using 3.3V logic, as is often the case, then this maximum incoming signal at the receiver must be attenuated from 8.89V down to approximately 2V. At 2V, the converter is able to digitize the signal. This means that for maximum level signals, the receiver circuitry must be capable of attenuating the signal by about 13 dB [Fig. 8].

11. Fig. 7

12. Fig. 8