G. xDSL — DIGITAL SUBSCRIBER LINE

The term DSL or xDSL stands for Digital Subscriber Line and encompasses all the technologies used for the digital transmission of data over a standard telephone connection line (twisted-pair copper cable). xDSL technologies are divided into two main categories: those using symmetrical transmission (equal upload and download speeds) and those using asymmetrical transmission.

The rapid development of information technology has given rise to new services that require high transmission capacity. The idea of using existing twisted-pair cabling seems the most suitable solution, as there were already more than 800 million such connections in place worldwide. All that is required is to add equipment to the telephone exchange (DSLAM) and install equipment at the user’s premises (modem/router) to access xDSL services.

1. Common features

These technologies differ in terms of transmission speed, maximum transmission distance, the difference in data rate between upstream and downstream traffic, and whether the connection is symmetrical or not. The point-to-point connection is established via a telephone line between the NT (Network Termination) installed at the user’s premises and the LT (Line Termination) installed in the exchange.

2. Symmetrical solutions

HDSL (High bit rate DSL): the first DSL-based technology, which emerged in the early 1990s. Enables speeds of up to 2 Mbps in both directions over three twisted pairs (European E1 standard) or 1.5 Mbps over two pairs (American T1 standard). Distance: 3 to 7 km depending on the wire diameter. No telephone channel available.

SDSL (Single-pair DSL / Symmetric DSL): designed for shorter distances than HDSL, using a single twisted pair. Symmetric data rates ranging from 128 kbps to 2 Mbps, depending on the distance.

Tableau — Distances et débits d'une liaison SDSL

3. Asymmetric solutions

After examining various scenarios, we realised that it was possible to transmit data more quickly from a central office to a user. The idea is therefore to use an asymmetric system, imposing a lower data rate from the subscriber to the exchange (upstream). This imbalance is well suited to residential internet usage (downloading >> uploading).

ADSL (Asymmetric DSL): a system enabling a high-speed downstream channel (up to 8 Mbps), a medium-speed upstream channel (up to 1 Mbps) and a telephone channel (POTS) to coexist on the same line. Maximum distance: 5.5 km from the DSLAM.

RADSL (Rate Adaptive DSL): based on ADSL. The transmission speed is set automatically and dynamically by identifying the maximum possible speed on the line and continuously readjusting it without interruption. Upload speeds range from 128 kbps to 1 Mbps and download speeds from 600 kbps to 7 Mbps.

VDSL (Very High bit rate DSL): capable of supporting data rates of 13 to 55.2 Mbps downstream and 1.5 to 6 Mbps upstream over a single twisted pair cable over short distances (up to 1.5 km). Originally developed to carry high-speed ATM.

VDSL2 (Very High bit rate DSL 2): the successor to VDSL, achieving speeds of up to 300 Mbps in full-duplex mode (Profile 35b). Maximum distance between the subscriber and the DSLAM: 3,500 metres. This is the technology on which the FTTN/FTTB roll-out in France is based (VDSL2 has been active on short lines since 2013).

4. xDSL equipment

DSLAM (Digital Subscriber Line Access Multiplexer): equipment typically installed in telephone exchanges that multiplexes ATM traffic to the transport network. It can accommodate various DSL cards (ADSL, SDSL, VDSL) and consolidates the connections of several hundred or thousands of subscribers.

ADSL modem/router (ATU-R): equipment at the subscriber’s premises that decodes ADSL data and manages connection sharing on the local network (NAT, Wi-Fi, VoIP).

Splitter and microfilter: The splitter is a crossover filter that separates the bandwidth reserved for the telephone service from the bandwidth used for DSL transmission. The microfilter is a low-pass filter installed on analogue connections (telephone sockets) to prevent interference.

5. Summary table of DSL technologies