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New Zealand Engineering 1999 June IT & Networks The Age of Steam is Dead, Long Live the Age of DSL What is happening in New Zealand? OK, I will admit there have been some advances in the telephone system since Bell signed the patent on 20 January, 1876, but I bet he wouldn’t find it too difficult to see his original invention lurking inside a modern handset. Unfortunately, in a digital world the phone has remained stubbornly analogue, but just as Alexander Graham Bell witnessed the end of the steam age, and the birth of Rudolf Diesel’s engine, we are witnessing the birth of the DSL age. Sorry, was there a question at the back? What is DSL? DSL stands for Digital Subscriber Line, and it is finally starting to make its way in real world. It has been driven by the huge demand for bandwidth, and we all know what has driven that - the internet. DSL is a "last mile" technology. It is designed to connect you to a network, read the internet for most people, using your already installed standard copper telephone cable. Everyone who uses the internet complains about the WWW or world-wide-wait. Everything seems to take forever, file down-loading times are exacerbated by the bloat factor, you know, those 600 kb word documents with two pictures in them. Whereas once the internet was used by the scientific community to send text messages to each other, with message sizes averaging 1 kb, the net is now full of all singing and dancing multimedia, large video files and 22 Mb monster packages like Microsoft internet Explorer 5. The plain old telephone system, (POTS), was never designed to cope with the demands placed on it by analogue modems. The fastest speed possible for analogue technology on POTS is theoretically 64 kb, with the current generation of 56 k modems effectively already at the limit. Cable companies have eyed the potential market for getting fast internet connections to consumers, allowing them to provide services like Video-on-Demand and video conferencing which are seen as having huge commercial potential. Unfortunately, running a cable to each and every consumer is very, very expensive. That’s where Telecom has a real advantage. They already have two copper wires going into almost every home in New Zealand. The problem is that consumers would like to keep the telephone AND have these new services. DSL achieves this by leaving the analogue phones 4 kHz bandwidth alone and utilising the huge amounts of untouched bandwidth available in the upper frequency ranges. Figure 1 shows the spectrum diagram for Asymmetric Digital Subscriber Line (ADSL), the front runner for a number of competing DSL technologies, grouped under the generic name xDSL. Note that the downstream component, what you receive, is much larger than the upstream component, what you send. This suits video-on-demand services and the internet, which is why asymmetric services are more popular than symmetrical Single Line Digital Subscriber Line SDSL. Also note that your existing telephone bandwidth is untouched, meaning you can simultaneously have internet access and use the phone. In Table 1 the most common forms of DSL are listed, along with some limitations. (Click here to see Table 1 - DSL Systems) What is happening in New Zealand? Telecom has started ADSL trials in Wellington and Auckland. Clear Communications, relying on Telecom for the last-mile connection, is waiting to get details on ADSL interconnection and line costs. Some internet Service Providers have bypassed ADSL already and have set up their own broad-band services. The Internet Group, also known as IHUG, operates Direct Broadcast Satellite, which although slower at 400 kbps, is here now and available across most of the country. Why is taking so long? The RADSL system provides asymmetric data transmission with rates of up to 6 Mbps downstream (to the customer) and up to 800 kbps upstream. The actual rate for each line will vary and is dependent on line length and condition. This is substantially faster than competing technologies. The New Zealand Herald reported in the 23 September 1998 Business Section that in their own tests an 18.8 Mb file was transferred in just 55 seconds. Try that on a modem and re-read War and Peace while you wait. Telecom is running trials in Wellington and Auckland, but only for customers attached to a few exchanges. It will surprise no one that these exchanges happen to be in the wealthier suburbs and the CBD. A POTS splitter is used at each end of the line, (see Figure 2 - Standard connection diagram from home to exchange for xDSL), to separate your normal telephone voice service so that only the data is transferred to the ADSL receiving modem at the Telecom end. Telecom then uses its own broad-band links into the internet, or, in the future, to your preferred ISP. What about business links to the internet? Video-on-Demand (VoD) Due to error correction requirements for video handling and the dynamic speed range involved with ADSL, the industry also develops Video DSL (VDSL), for faster speeds within a lower dynamic range, allowing better design compromises and promising simplicity that will be reflected on prices. VDSL may later evolve to symmetrical service capabilities. Comparing xDSL with competing technologies
The alternatives : ISDN - integrated services digital network
Cable Modems
56 k Modems: Singles or Pairs
Direct Broadcast Satellite
What
does it cost?
There are two prices: the first depends on having a clear line of sight to the Auckland Skytower; otherwise a satellite covers the rest of New Zealand.
The roll-out of broad-band services to home users is being pushed hard by the needs of the internet. If telephone companies do not move with RADSL fast enough, competing technologies such as IHUG’s satellite system will fill the gap. Once consumers have invested money in one system or another, it will take a considerable incentive to change. I for one would welcome a straightforward fixed rate charge for fast access to the internet, and I know my family would appreciate being able to use the phone in the evenings again! ADSL Asymmetric Digital Subscriber Line: Consists of modems installed in the exchange and customer premises attached by twisted pair copper wiring that can transmit from 1.5 Mbps to 6.0 Mbps downstream (to the subscriber) and from 16 kbps to 800 kbps upstream, depending on line distance. ISDN Integrated Services Digital Network: A digital network with circuit and packet switching for voice and data communications at data rates up to 1.544 or 2.048 Mbps. Basic Rate Access (BRA) provides two B channels at 64 kbps each and a D channel at 16 kbps. Downstream/Upstream Downstream: refers to data flowing from the source such as a corporate host or internet service provider (ISP) to the end user. Upstream: refers to data flowing from the end user back to the corporate host or ISP. DSLAM Digital Subscriber Line Access Multiplexer: specifically, a device that takes a number of DSL subscriber lines and concentrates these to a single ATM line DSL Modem Short for MOdulator-DEModular, this hardware device converts ATM cells to Ethernet packets and vice versa in the use of DSL. Ethernet Connection A high-speed, direct connection to a network where an Ethernet network interface card (NIC) is installed so that the user can access any host connected to the network. POTS Plain Old Telephone Service: the only name recognised around the world for basic analogue telephone service. In New Zealand also referred to as PSTN (Public Service Telephone Network). POTS takes the lowest 4 kHz of bandwidth on twisted pair wiring. Any service sharing a line with POTS must either use frequencies above POTS or convert POTS to digital and interleave with other data signals. Splitter Filters which separate high frequency (DSL) and low frequency (POTS - basic analogue telephone service) signals at both the end user and central office end points. ADSL Asymmetric Digital Subscriber Line: uses the frequency spectrum between 55 kHz and 1.1 MHz to carry data over a twisted pair copper line, usually a telephone line. This leaves the frequency spectrum between 300 Hz and 4 kHz free for the existing telephone service. The ADSL system provides asymmetric data transmission with rates of up to 6 Mbps downstream (to the customer) and up to 800 kbps upstream RADSL Rate Adaptive ADSL Simon Bunn, is an associate with Beca Carter Hollings & Ferner |
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