xDSL: Broadband made Easy

Md. Abdul Mutaleb
CEO, Emem Systems Ltd.

Introduction:

That familiar eeeeeeh … du, du, du, du du ….. screech that your dial-up modem makes when you connect to the Internet is rapidly becoming a sound of the past, not because of the annoying screech but because of the demand for greater bandwidth and faster Internet connections.

Most mid-level home and small office computers are capable of more than 1GHz processing and are only seriously hampered when they log into a dial-up Internet connection. The analogue telephone frequency range has a limited bandwidth, so if data communication equipment is limited to only use that range, then only small amounts of information can be sent and received over the phone line at one time. In the same way that more lanes on a highway enable more cars to travel on it at one time, broadband telecommunication enables a wide band of frequencies or channels to transmit information concurrently on a wire, which means more information, can be sent in a given amount of time.

There are number of broadband access platforms, but the one with the most rapid uptake is DSL. Other broadband platforms include leased line, fibre optics, fixed wireless access (FWA), mobile wireless etc. A data from recent past have shown that the DSL users account for 65.70% of the total worldwide broadband market. DSL is also accounts for 88% of all broadband users in Middle East and Africa – the fastest growing broadband region. In EU the market share is 81%. China has the most DSL subscribers in the world which is approx 100 millions in the year 2010. DSL is the default choice for broadband users in India which is more than 10 million.

What is DSL?

Digital Subscriber Line (DSL) is a broadband connection that uses the existing telephone line. DSL provides high-speed data transmissions over the twisted copper wire, the so-called “lastmile” or “local loop”, that connects a customer’s home or office to their local telephone company Central Offices (COs).There are a number of different types of DSL and they are referred to collectively as xDSL.

How does DSL work?

The traditional phone service (also known as the Plain Old Telephone Service or POTS) was created to exchange voice information using an analogue signal. Computers, however, use digital signals to communicate, so in order for this signal to travel over the telephone network, a modem is needed to convert the digital data to analogue and back again.

The telephone analogue frequency uses only a small proportion of the bandwidth on a line (under 4kHz).The maximum amount of data that conventional dial-up modems can transmit through a POTS system is about 56Kbps. Using this method to send data, the transmission through the telephone company is a bandwidth bottleneck. Typical telephone cabling is capable of supporting a greater range of frequencies (around 1MHz). With DSL modems, the digital signal is not limited to 4kHz of voice frequencies, as it does not need to travel through the telephone switching system. DSL modems enable up to 1MHz of bandwidth to be used for transmitting digital (data) alongside analogue (voice) signals on the same wire by separating the signals, thereby preventing the signals from interfering with each other. Figure 1 shows how the analogue and digital frequencies are split.

Figure 1: Splitting the frequencies

DSL modems establish a connection from one end of a copper wire to the other end of that copper wire, as shown in Figure 2. Although DSL stands for Digital Subscriber Line, DSL actually signifies the pair of modems at each end of a line and not the line at all. So when customers buy a DSL connection they actually buy the modems or the use of the modems at each end of a line. In contrast, dial-up modems establish a data stream between two arbitrary points using the entire telephone system—from the sender’s local loop, through the telephone switching system and then to the receiver’s local loop. These dial-up modem connections can span continents, with one end being thousands of kilometers from the other

Figure 2: DSL connection setup

What are the variants of DSL?

There are numerous different DSL technologies. The range of DSL types reflects the numerous different applications requiring different data rates. Generally speaking, the variations of DSL technology have been implemented to meet the needs of different users, such as home users, small to medium sized businesses, schools and colleges. The following table gives comparison of the data rates and their applications.

Comparison of DSL types

DSL type	Maximum Downstream Data rate	Maximum Upstream Data rate	Maximum Wire Length	Customer Application
Symmetric
HDSL High data rate Digital Subscriber Line	2.048 Mbps (E1)	2.048 Mbps (E1)	3.7 km with three lines for E1	HDSL is for data only. It does not allow telephone connection over the same line.
SDSL Symmetric Digital Subscriber Line	2.3 Mbps	2.3 Mbps	3 km	Individual customer premises with a single telephone line
SHDSL Symmetric High bit rate Digital Subscriber Line	2.3 Mbps (single wire pair) 4.6 Mbps (two wire pairs)	2.3 Mbps (single wire pair) 4.6 Mbps (two wire pairs)	3 km at 2.3 Mbps 5 km at 2.3 Mbps	Business applications requiring greater bandwidth in both directions
Asymmetric
ADSL Asymmetric Digital Subscriber Line	10 Mbps	1 Mbps	5.5 km	Home and small office users who require faster download rates for video on demand, home shopping, Internet access, remote LAN access, multimedia access
ADSL lite Asymmetric Digital Subscriber Line lite	1.5 Mbps	384 kbps	5.5 km	Residential and small office users wanting a simple plug and play setup
ADSL 2 Asymmetric Digital Subscriber Line two	12 Mbps	1 Mbps	5.5 km	ADSL 2 addresses the growing demand for more bandwidth to support services such as video.
ADSL 2+ Asymmetric Digital Subscriber Line two plus	24 Mbps	1 Mbps	5.5 km	ADSL 2+ enables even greater downstream data rates for subscribers who are relatively near to the telephone exchange
ADSL 2++ Asymmetric Digital Subscriber Line two plus plus	52 Mbps			ADSL 2++ proposes to further increase downstream data rates for customers who are relatively near the telephone exchange.
Symmetric and Asymmetric
VDSL Very high rate Digital Subscriber Line	Symmetric 10 Mbps Asymmetric – 52 Mbps over short distance	10 Mbps 1.5 Mbps	1.3 km 0.3 km	Campus environment where there is short distance to a neighborhood cabinet that is linked by fiber optic to the exchange. VDSL handles a number of high bandwidth applications such as VPNs, file down/up loading, VOD, HDTV, broadcast TV, tele-medicine, tele-conferencing, surveillance systems.

What are the benefits of DSL?

DSL is a more cost effective option than many other broadband connections, such as leased lines, terrestrial broadcast, cable and cable modem, satellite and fibre optic connections, because it is able to take advantage of the existing telephone infrastructure for both voice and data traffic. Only the user’s modem and the telecommunications equipment need to be upgraded when moving to a DSL connection because it utilizes the existing cable infrastructure. DSL is always on, always fast and always reliable. DSL connections are point-to-point dedicated circuits that are always connected, so there is no time lost dialing up. In a nutshell DSL leverages the investment on copper network by providing multi service like internet, video, e-learning, IPTV etc through the same copper infrastructure which was meant for voice only.

Focus on ADSL

Introduction

Asymmetric Digital Subscriber Line (ADSL), an important variant of the DSL family, has become very popular. With ADSL, most of the data bandwidth is devoted to sending data downstream towards the user and a smaller proportion of the bandwidth is available for sending data upstream towards the service provider. This scenario suits Internet browsing applications, which typically involve much more downstream than upstream dataflow.

How is an ADSL network setup?

When digital data is sent from a customer’s premises, it travels from their computer through a DSL modem and a splitter. When analogue voice signals are sent from a customer’s telephone they are also sent through the splitter, which combines the analogue voice and digital data signals, enabling them to be sent over the same line.

At the other end of the line, the local loop goes into a splitter at the local phone company’s COs, which splits the digital data frequencies from the analogue voice frequencies. The voice frequencies are sent to the local telephone exchange and the digital data is sent to a Digital Subscriber Line Access Multiplexer (DSLAM) before being sent on to the Internet Service Provider (ISP).The digital data never enters the standard telephone switching system.

Voice and data frequencies going in the opposite direction—to the customer’s premises—follow the reverse route from the ISP through a DSLAM, then a splitter at the CO, and are then sent over the copper wires to the customer’s site before being split again.

The DSLAM is the equipment that really allows DSL to happen. The DSLAM handles the high speed digital data stream coming from numerous customers and aggregates it onto a single high-capacity connection (ATM or Gigabit Ethernet line) to the Internet Service Provider and vice versa. DSLAMs are generally flexible and can support a number of different DSL connections as well as different protocol and modulation technologies in the same type of DSL. Figure 3 shows how an ADSL network is setup.

Figure 3: ADSL Network Setup

How does DSL get more bandwidth from the same old copper wires?

The DSL transmission technology exploits the fact that all telephone signals are below 4 kHz in frequency and makes use of the rest of the 1MHz that a typical copper pair line can support. ADSL modems use specialised modulation technology to divide the available bandwidth on a copper pair line and create multiple channels for sending and receiving signals.

Modulation technologies

Modulation is the overlaying of information (or the signal) onto an electronic or optical carrier waveform. There are two competing and incompatible standards for modulating the ADSL signal, known as Discrete Multi-Tone (DMT) and Carrier less Amplitude Phase (CAP). CAP was the original technology used for DSL deployments, but the most widely used method now is DMT.

Discrete Multi-tone (DMT)

Discrete Multi-Tone (DMT), the most widely used modulation method, separates the DSL signal so that the usable frequency range is separated into 256 channels of 4.3125 kHz each. DMT has 224 downstream frequency bins (or carriers) and 32 upstream frequency bins. Up to 15 bits per signal can be encoded on each frequency bin on a good quality line.

Figure 4: Discrete Multi-Tone (DMT)

Each of the 256 channels is monitored separately to ensure the data traveling along it is not impaired. DMT constantly shifts signals between different channels to ensure that the best channels are used for transmission and reception. DMT can take advantage of all usable tones in the spectrum and works around areas where interference is present. Some of the lower channels can be used as bi-directional channels for both upstream and downstream information. DMT is more complex than CAP because it monitors and sorts out the information on the bidirectional channels and maintains the quality on all of the 256 channels. DMT also provides more flexibility on lines of differing quality than CAP.

What are the main factors affecting the performance of DSL?

There are a number of factors that can affect DSL performance. The most common factors are attenuation, bridge taps, load coils and cross talk. Other factors affecting performance include return loss, longitudinal balance, noise, split pairs, gauge changes, and interruptions. Environmental factors can also have adverse affects on the transmission lines used for telecommunications.

Attenuation: With all DSL technologies there is a trade off between the data rate and cable distance, so as the distance between the customer’s premises and Telephone Company’s local office increases the data rate drops. On network cables, the degradation of the digital signal or a loss of amplitude of an electric signal during transmission is known as attenuation. As the line length increases, the downstream frequencies suffer from attenuation, which also increases with higher temperatures.

Bridge Taps: A bridge tap is an extra length of wire with an unterminated cable end, which is connected to the local loop. Bridge taps are usually left over when a new subscriber is connected to existing pair of copper wires, and the original subscriber at the end of another pair is disconnected, leaving an open lead at that end of the wire. When the DSL signal is sent across the wire, it reflects through the bridge tap cable pair towards the open lead and then bounces back along the wire. This signal is then mixed with the original signal and can confuse the modem. However, most modems will only listen to the stronger original signal and ignore the weaker, reflected signal. Most cable pairs in the world have bridge taps that should be removed before installing a DSL connection in order to ensure a clear signal.

Crosstalk: Crosstalk is a disturbance in a circuit that is caused by the electric or magnetic fields of another telecommunication signal. This disturbance can cause a severe degradation in transmission. The two main types of crosstalk are Near End Crosstalk (NEXT) and Far End Crosstalk (FEXT). NEXT occurs when a strong local transmitter interferes with another collocated receiver and is the most problematic. FEXT is when there is interference at the receiving end. Other DSL lines in the same bundle of wires may cause crosstalk with each other, depending on their frequency ranges. For example, if a E1 line is running in the same bundle with a DSL line, some of the frequencies may not be useable. The AM radio frequency, which goes up from around 600kHz, may also interfere with DSL lines.

Return Loss: Return loss is a measure of the ratio of signal transmitted into a system to the amount of the signal that is reflected back to the source. Return loss is caused by an incorrectly terminated line and can reveal line faults that are caused by mismatching.

Longitudinal Balance: It is important for xDSL services that the resistance to the flow of a current in a circuit (or impedance) to earth, for each conductor, is as equal as possible. This balance is known as longitudinal balance. When the line is balanced, there is no difference between the signals on the two conductors.

Noise: The total power in a transmission line is made up of the signal and noise. Noise is always present but it should always be kept as low as possible in relationship to the signal. Factors that introduce noise are: radio and television transmitters, power distribution systems, electrical machinery and mechanical vibration.

Impulse Noise Brief, but large spikes of electrical interference (typically caused by power switching equipment). DMT modulation (which has a relatively long cycle time) is quite tolerant of this sort of disturbance.

Split Pairs Split pairs occur when one conductor in a pair becomes separated from the other conductor. Split pairs result in noise, crosstalk and radiation, and seriously degrade xDSL services.

Gauge Changes While conductors with different gauges are suitable for xDSL connections, mixing the gauges in cable runs can cause impairments that affect the line’s ability to carry xDSL services.

Recommendation:

Worldwide the telcos have embraced the DSL technology which helped them to earn substantial amount of revenue by introducing assorted education and entertainment broadband services. BTCL is uniquely positioned in Bangladesh who is the only operator who can provide DSL service. DSL technology can help cease the declining trend of subscriber acquisition of BTCL and turn the huge copper infrastructure as a gold mine leveraging the investment which was meant for voice service. The prospect of this field proven service seems very bright if the following recommendations are taken into consideration:

• Improvement of existing age old copper infrastructure
• Deployment of ONUs (Optical Network Unit) in places far away from exchanges and where copper cable is not at par to the standard of ADSL service
• Introducing FTTC (Fibre to the Cabinet) where the cabinet will host the DSLAMs thus bringing down the distance from DSLAM to subscriber site to a minimum. This will enable to provide highest possible bandwidth to the subscriber’s end.
• Introducing video services like IPTV, VOD(Video On Demand) will popularize this technology.
• Deployment of ADSL service to Upazilla level exchanges which will help government to materialize ‘Digital Bangladesh’.