With 750 million of them worldwide, the telephone “home loop” is the most widespread communications environment. The loop consists of a telephone twisted pair (or TTP), a pair of thin, insulated copper wires twisted together, which radially connects end users to a distribution point (a “telephone exchange” or “central office”). Employing the existing TTP infrastructure for broadband digital transmission offers the obvious advantage of its ubiquity, where feasible. We will examine how DSL technology overcomes the problems of TTP as a broadband medium, beginning with a brief history of using the telephone system for transmitting digital signals.
From Analog to Digital: A Brief History from Telegraphy to ISDN Technology.
Long-distance digital communication over copper wire predates the telephone. Both the telegraph and telex used a very simple form of digital encoding. In the late 1950s, the armed forces, airlines, and banks used "standard" telephone wire, along with telegraph systems and codes, to connect their terminals to a remote central computer. These systems typically had very low transfer speeds (usually between 100 and 150 bps) and employed very simple "on/off" signaling, but they worked over distances of up to approximately 30 km.
During the 1960s, to provide higher speeds, "MODEMs" (MOdulator-DEmodulator devices) began to appear, which converted the simple digital "on/off" signal into "audio tones" that could be carried over a standard telephone channel. Over time, modems achieved increasingly higher speeds (up to 56 kbps). In the early 1970s, however, operators began using digital techniques for switching within the exchange equipment, and it became clear that a fully digital connection to the subscriber could create a substantially better system. Integrated Services Digital Networks (ISDN), standardized in the early 1980s, provided digital access to the telephone subscriber and could deliver two 64 kbps digital "voice" channels and one 16 kbps data/signaling channel over the standard TTP wire at distances of up to 5 km and, with repeaters, perhaps up to 20 km.
In the late 1980s, operators developed "T1" (1.544 Mbps) and "E1" (2 Mbps) services, which required four-wire connections and repeaters located approximately every 1.5 km. These were the forerunners of HDSL, the first of several DSL solutions.
High-Speed Digital Subscriber Line (HDSL)
HDSL speeds are equal to those of T1 and E1 services—and even exceed them if transmission is primarily unidirectional over distances of up to 6 km, depending on line quality. HDSL is useful in small businesses for traditional applications such as connecting to a PBX and transmitting data. It is much more useful than ADSL for small websites, as it offers significantly higher upload speeds than the 640 kbps offered by ADSL.
Other standards have succeeded HDSL. SHDSL uses the same DMT technology as ADSL (see below), but only one wire pair. SDSL (Symmetric Digital Subscriber Line) is a protocol very similar to HDSL that operates at 784 kbps in both directions over a single twisted pair of copper telephone wire (TTP) over distances of up to 6 km. IDSL (ISDN Digital Subscriber Line) is similar in structure to SDSL, but uses ISDN chipsets and infrastructure (basic rate) to provide a single 128 kbps access. IDSL offers a maximum transmission speed of 128 kbps, but allows the use of ISDN repeaters to reach distances of up to 20 km.
Asymmetric Digital Subscriber Line (ADSL)
ADSL technology was developed around 1990 for television transmission over telephone lines, in response to US cable television operators who offered telephone connections through their coaxial cable. The ADSL protocol is “asymmetric” because the downstream speed (to the user) is considerably higher than the upstream speed, making it ideal for watching television or browsing the web. Consequently, although ADSL technology doesn't appear to have been widely used for television viewing, it was well-suited to providing high-speed connections during the internet boom a few years later. The maximum data transmission speeds specified in the standard are 6 Mbps downstream and 640 kbps upstream, but in reality, they vary depending on the connection and contract: the provider may offer 1.5, 3, or 6 Mbps services at different prices.
ADSL-Lite and RADSL:
In homes with multiple telephone jacks, a technician can install a passive splitter to separate voice from broadband data, but this comes at a cost. The ADSL-Lite protocol (also known as G.lite) allows for the direct connection of simple distributed filters, saving on both technician and cabling costs, but limits the maximum transmission speed to 1.5 Mbps download and 512 kbps upload. RADSL (Adaptive Rate DSL) uses a different physical layer protocol and can dynamically adjust the transmission speed depending on the quality of the available circuit.
Overcoming TTP's Significant Bandwidth Limitations:
The key to the success of ADSL and other DSL protocols has been the "multicarrier transmission" used to compensate for TTP's weakness in carrying digital signals. The original designers of TTP never envisioned the system carrying high-bandwidth digital signals, and many of the characteristics of the TTP connection, while perfectly adequate for analog telephone signals, significantly limit the quality of high-bandwidth transmission. Some of these are:
Attenuation, which increases dramatically with increasing frequency.
Noise and crosstalk. Coupling effects from other pairs on the same cable or other environmental sources (such as radio transmitters) can cause noise, as can crosstalk between pairs and, more commonly, between the transmitter and receiver at each end of the cable.
Tap bridging. When telephone lines are disconnected, the twisted pairs are not physically removed but left unterminated. When new users are connected to the circuit, these unterminated pairs can also carry part of the signal, causing reflections back towards the transmitter. In some operator networks, up to 30% of all user connections include at least one bridged jack, which, while not preventing analog connections, seriously affects the DSL connection.
DMT Technology: The Key to ADSL Speed.
The specific multi-carrier system used in ADSL and VDSL, called “DMT” (Discrete Multi-Tone), has proven particularly effective at overcoming these limitations. The principle behind DMT is that limitations occur in different ways in different parts of the frequency spectrum. Noise only affects a few frequencies; attenuation varies with frequency, and the effects of reflection and crosstalk also depend on frequency. If a small part of the band is limited, this affects the entire signal. For this reason, instead of sending a single high-speed broadband signal, DMT uses a large number of low-speed, narrowband signals (or channels). Noise and distortion occurring in one sub-channel cannot affect any of the others.
DMT can take many forms. Here it is described in its “normal” ADSL version, which uses 249 independent channels. All channels are sent and received together in the same operation (transmission uses an inverse Fast Fourier Transform, while reception uses a Fast Fourier Transform) through digital signal processing. Although this requires enormous computing power, these functions are currently performed by adapted VLSI chips at a relatively low cost.
In ADSL, each individual subchannel is treated as a separate entity and carries between 0 and 15 bits of information per signal period (baud). Technically, it uses QAM (Quadrature Amplitude Modulation) with Trellis coding. A "baud" is one change of state of the line (signal period or symbol) per second. With 4000 data symbols per second, the baud rate in DMT is, of course, very low. During initialization, the ADSL transceiver analyzes each subchannel to determine its quality and characteristics. It then assigns a number of bits (between 0 and 15) to each one, according to its quality.
In ADSL, the rates described above for the channels result in the following maximum data transmission speeds:
- Upload: 25 channels * 15 bits/symbol/
channel * 4 Kbaud = 1.5 Mbps
- Download: 249 channels * 15 bits/symbol/
channel * 4 Kbaud = 14.9 Mbps
Viable ADSL systems offer a maximum download speed of between 6 and 9 Mbps.
Very High Speed DSL (VDSL):
As explained in the previous article, having a cabinet containing active DSL equipment located along the existing cable route significantly increases speed by reducing the distance between the headend and the user. The uplink copper cable (multipair) is replaced with a fiber optic pair, leaving the user's copper wire connection intact. The shorter cable can support much higher frequencies and is less vulnerable to interference.
Having a higher frequency bandwidth than ADSL means that VDSL uses more subchannels and also gains speed by increasing the symbol rate (VDSL-2 uses 8 kbaud). The required, faster signal processing is now available at a relatively low cost. VDSL-2, currently the most advanced technology, operates over relatively short distances, between 350 meters and 3 km, at speeds of up to 100 Mbps in both directions. Furthermore, VDSL-2 is compatible with users' existing ADSL and ADSL-2 equipment (an essential requirement). This allows operators to upgrade their ADSL equipment to VDSL-2 without requiring users to make any changes. Of course, users cannot benefit from the new speeds unless they upgrade their equipment.
Next steps: VDSL or PON?
Most developed countries are experiencing ever-increasing demand for very high-speed internet access in homes and businesses, and meeting this demand is crucial for their economic development. Consumer demand is likely to skyrocket with the introduction of television distribution during a period of revolutionary change. At the same time, and unsurprisingly, the demand for reliable access to traditional telephone service remains as strong as ever. Although ADSL networks and other copper wire connections have achieved enormous speed increases, they are no longer adequate to support the evolution of these services. The use of fiber optics in one form or another is another good technical solution to the problem, although the cost of fiber optics must be taken into account. In areas where copper TTP connections to the customer still exist, an FTTN (fiber to the node) solution using VDSL technology for the last 350-1000 meters is generally the most economical solution. Where new infrastructure needs to be installed, PON (Passive Optical Network) systems, and especially GPON, offer an immediate, complete, and cost-effective solution, with open capacity for future development. The next article in this series will focus on PON systems.
Author: Patrick Gahwiller. Partner Account Manager, R&M.
