xDSL

Chapter 16: xDSL




What is xDSL?


 The term DSL is an acronym for Digital Subscriber Lines and the term xDSL is the name of the family that provide technology for digital data transmission through the network over the wires of the local telephone network, which are usually made up of copper wires. 

• Asymmetrical digital subscriber line (ADSL)
• ISDN (like) digital subscriber line (IDSL)
• High bit−rate Digital Subscriber Line (HDSL)
• Consumer Digital Subscriber Line (CDSL)
• Single High Speed DSL (SHDSL)
• Rate−adaptive digital subscriber line (RADSL)
• Very high−bit rate digital subscriber line (VDSL)
• Single or symmetric digital subscriber line (SDSL)

The two main categories of this xDSL are ADSL or Asymmetric Digital Subscriber Line and SDSL or Symmetric digital subscriber line. Apart from these two major categories, there are two other types of categories present in the market as well, named HDSL or High Data Rate DSL and VDSL or Very High DSL. These two types of the DSL are quite expensive and are installed only in the large sized enterprise, as they installation and the operational costs are both high. The most common type of the DSL that is used by many people throughout the world is ADSL, as it is cheaper and the operational costs are also low.

This DSL technology basically pack the data on to the copper wire by using a sophisticated modulation technique and schemes and this technology is only used for connection from the telephone lines to homes and offices but not for providing connections between the switching stations. This xDSL technology is quite similar to the ISDN technology as they both operate on the copper wires and both require the short run to the telephone office. However, the major difference between these two lies in the matter of speed.

The xDSL technology is capable of giving speed up to 32 Mbps for upstream traffic and max speed of 1 Mbps for the downstream traffic. This technology is currently undergoing revision and it is quite possible that the speed that this technology has to offer to the users might even increase drastically, after all the main thing is loading the cables with the data packets with the help of modulation schemes.

What is a Modem?

 A modem modulates outgoing digital signals from a computer or other digital device to analog signals for a conventional copper twisted pair telephone line and demodulates the incoming analog signal and converts it to a digital signal for the digital device.

Why do the DSL technology needs modem technology?

A modem is a communications device that can be either internal or external to your computer. It allows one computer to connect another computer and transfer data over telephone lines. The original dial-up modems are becoming obsolete because of their slow speeds and are being replaced by the much faster cable and DSL modems. The modem acts as the Data Circuit terminating Equipment (DCE) for the link.The modem then converts the data from a computer terminal into a voice−equivalent analog signal.Newer technologies will produce much higher compressed speeds of up to 230 to 300 Kbps on a modem, but these are now in their infancy. They are not a major factor in our communications networks yet.

The figure above are the modems that are installed at the customer's location and use the existing telephone wires to transmit data across the voice network.

There are types of Modems 

1. Analog dial-up modem, came with all laptops until 2-3 years ago. Used for dial-in connections ONLY. Not needed much anymore with Wireless and Ethernet connections. If you have dial-in and NO modem you can always buy a USB modem.

2. Cable/DSL Modem. By far far the most used today Required for Cable or DSL service. It is an EXTERNAL device not part of the computer. It will convert the signal to either a USB output, Ethernet output or a wireless output or a combination of all three (depending on the modem you get.)
All you need to connect a Cable/DSL modem to your computer is an Ethernet port. (Wide Phone jack) or a USB connection. Use Ethernet port if available (Greater Speed)
You can also connect wireless if you have a wireless modem
and a wireless card in the computer. 

This is HDSL is impervious to the bridge and splices. The T1 is split onto two pairs.

As already mentioned, HDSL runs at 1.544 Mbps (T1 speeds) in North America and at 2.048 Mbps

(E1 speeds) in other parts of the world. Both speeds are symmetric (simultaneous in both

directions). Originally, HDSL used two wire pairs at distances of up to 15K. HDSL at 2.048 Mbps

uses three pairs of wire for the same distances, but no longer. The most recent version of HDSL

uses only one pair of wire and is expected to be more accepted by the providers. Nearly all the

providers today deliver T1 capabilities on some form of HDSL.

xDSL Coding Techniques

There are a lots of approaches that the xDSL has in encoding the data. One of that is the Carrierless Amplitude Phase Modulation (CAP) and the Discreet multitone (DMT) modulation. another technique that the xDSL has is the Quadrature with Phase Modulation (QAM).  The industry, as a rule, selected DMT, but several developers and providers have used CAP. It is, therefore, appropriate to summarize both of these techniques. The SHDSL technology uses a trellis−coded pulse amplitude modulation (TCPAM) technique to gain the benefits of the single−pair services or two−pair service.