## 2 Broadband System
### Fixed-Line Broadband System
#### 2.1.3 Digital Subscriber Line  
DSL technology offers means of delivering digital broadband services through the local telephone network [129]. There has been notable competition between the DSL service providers and the cable companies to offer the triple play services--the Internet, Internet protocol TVs (IPTVs), and VoIPs. Consequently, DSL is one of the dominating broadband access technologies in the network [129]. For efficient support of data-intensive and \begin{table}
\begin{tabular}{p{42.7 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline
**DOCSIS Version** & **Downstream Capacity** & **Upstream Capacity** & **Production Date** & **Features** & **Reference** \\ \hline
1.0 & 40 Mbps & 10 Mbps & 1996 & Initial release with high-speed internet access & [114; 115; 116; 117; 118; 119; 120] \\ \hline
1.1 & 40 Mbps & 10 Mbps & 1999 & Added voice over IP service, streaming, and gaming capabilities & [114; 115; 116; 117; 118; 119; 120] \\ \hline
2.0 & 40 Mbps & 30 Mbps & 2001 & Improved upstream speed and symmetric service capability & [114; 115; 116; 117; 118; 119; 120] \\ \hline
3.0 & 1 Gbps & 100 Mbps & 2006 & Offers increased capacity (both downstream and upstream). Also supports IPve and channel bonding & [114; 115; 116; 117; 118; 119; 120] \\ \hline
3.1 & 10 Gbps & 1–2 Gbps & 2013 & Considerable efficiency and capacity advancement, wideband channel, OFDM & [114; 115; 116; 117; 118; 119; 120] \\ \hline (3.1) \({}^{\mathrm{a}}\) & 10 Gbps & 10 Gbps & 2017 & Enhanced upload speeds and symmetrical streaming & [111; 114; 115; 117; 118; 119; 120] \\ \hline \end{tabular}  
* Full duplex.  
\end{table} 
Table 1: DOCSIS evolution. Figure 3: A typical BPL architecture. multimedia services, DSL providers are striving for higher data rates to ensure effective competition through several innovative technologies with different features [129] as summarized in Table 2. One notable approach of realizing higher speed access being adopted by DSL service providers is bandwidth expansion. On the other hand, the approach may result in crosstalk that can subsequently cause interference in the system [129; 130]. Therefore, in DSL networks, the major impediment to performance improvement is the electromagnetic interference between different lines of a cable bundle being produced by the electrical energy carried by the individual line [129; 131; 132]. Moreover, Far-End-Crosstalk (FEXT) and Near-End-Crosstalk (NEXT) are the crosstalk types normally observed in the DSL networks. The observed crosstalk by neighboring lines at the transmitter (Tx) side is known as NEXT while the one being observed by neighboring lines at the receiver (Rx) side is FEXT. In DSL, Frequency-Division Duplexing (FDD) is normally employed to eliminate the NEXT interference. Although, with FDD implementation, the FEXT signal is considerably attenuated, however FEXT interference is still present. Therefore, improved data rates can be realized by eliminating the FEXT interference [131; 7; 132]. In general, the crosstalk can be effectively addressed through proper Spectrum Management (SM) [129; 130; 7; 131]. Static SM (SSM) is the basic form of SM that can be employed; however, its implementation is based on the assumption of a worst-case scenario for all users, which is not ideal. Consequently, the SSM approach may cause inefficient spectrum use, subsequently resulting in sub-optimal performance. To improve the DSL system performance, dynamic SM (DSM) can be employed. Basically, DSM is based on transmission coordinate over multiple DSL lines through adaptive application of different spectral masks for each user, in order to maximize system throughput [7; 129]. Three levels of DSM have been presented for performance improvement. The spectrum balancing is implemented independently from line to line to alleviate crosstalk in the DSM Level 1. On the other hand, in DSM Level 2, the implementation is done cooperatively through multiple lines. Moreover, signal-level coordination is done on Level 3 to remove the crosstalk [7; 140].