Blog: DOCSIS, ADSL, Fibre, Mobile Broadband
I wrote the blog at the bottom of this page some 8 years ago, so it is well overdue for an update. There have of course been many changes in telecommunications over the last few years so there is plenty to write about.
There have been interesting evolutions in terminology. The word 'wireless' today as an adjective has had a renaissance following its earlier adoption well into the twentieth century, both as an adjective and as a noun. Some of us will remember a smart, veneered wooden box in the corner of the room, designed to look like a piece of furniture and using valves (vacuum tubes), the radio technology of that era, which we called 'The Wireless'.
'Wireless' of course means 'no wires' and today is still accurately used as an adjective, but for relatively short range communications perhaps serving just a handful of nodes and typically under the control of the telecommunications consumer like you or I. We probably would not describe mobile (digital cellular) or satellite as wireless, but they are of course. These represent big infrastructure investments and employ technologies, the results of international collaborative efforts by large telecommunications operators, manufacturers and regulatory authorities. Probably the most popular of the wireless technologies today would be WiFi, an abbreviated name for 'Wireless Fidelity'. Perhaps the idea came from HiFi (High Fidelity), which audiophiles will be familiar with. WiFi, which started life as IEEE 802.11 in about 1985, has been updated many times. WiFi has been designed as a high capacity wireless networking standard as a potential replacement for the wired alternative, Ethernet, derived from IEEE 802.3. Ethernet itself has been updated many times over the years and has copper and optical fiber versions.
The wireless standard Bluetooth, in this case starting as IEEE 802.15.1, is now managed by the Bluetooth Special Interest Group and includes smaller capacity options than WiFi or Ethernet. This is one of the 'low rate wireless personal area networks', together with IEEE 802.15.4. If the high data capacity of something like WiFi is not actually required, the Shannon Nyquist capacity theorem allows the bandwidth and/or power to be reduced thus enabling more channels and potentially reliable communications over greater distances.
All of these wireless technologies are unlicensed and use frequency bands allocated to unlicensed equipment, also described as Industrial, Scientific and Medical (ISM). Here unlicensed means that the user or operator does not need to get a licence to use it, but nevertheless the hardware has to meet the necessary electromagnetic compatibility (EMC) requirements specified by the regulatory authorities. Those of us who like hacking things unfortunately have to refrain from this with the ISM equipment and perhaps study for an amateur radio licence where we are licensed to hack within the rules.
Digital cellular (mobile) communication technologies have now evolved up to what is generally considered to be fourth generation (4G) and also referred to long term evolution (LTE)., LTE itself has evolved to LTE advanced (LTE-A) and no doubt there will be further evolutions in the future years.
I can confirm that the client I visited 8 years ago can now get a 3G mobile broadband but, better than that, he now has a DOCSIS (broadband) cable connection. This was part of an investment initiative to bring broadband to remoter parts of the UK. He is located about 6 km from the (former) telephone exchange which was a very long distance for ADSL, effectively making it unusable. It appears that the telecommunications operator has installed fibre optic lines from the telephone exchange, with a considerable distance overhead. (That is a guess on my part because when I was in the area recently I saw notices on the poles to the effect that they were not carrying copper conductors, perhaps directed at potential scrap metal thieves). Even before fiber optics most of the nearby cables in and out of these little exchanges were underground so there was probably duct capacity available for fibre optics.
I need to sidetrack a little here. When I talk about 'telephone exchanges', that name is probably out of date now. The small rural telephone exchanges that I am referring to were typically about the size of a small garage and located around small population centres. They were used originally to provide the telephone circuit switched routing (exchange) service for local subscribers, part of the national PSTN (public switched telephone network). Today, few countries still use circuit switching, it being replaced in the digital era by packet routing technology. In the UK, many of these small telephone exchanges were built by 'The Post Office' around the mid-twentieth century. The Post Office was nationalised and, despite its name, was also responsible for the telecommunications of the era, very much a minor offshoot of its postal services empire which generated much more revenue. When I was young lad out on my bicycle in a rural part of Cornwall sometimes I would find these garage-like structures and wonder what they were all about. I thought they were something to do with telephones but, looking around, there were usually few, if any, telegraph poles and wires visible (we used to call them telegraph poles but they were more correctly 'telephone poles'). They were of course underground. Because of the large number of telephone wire connections required close to the exchange, the necessary dense clusters of telegraph poles and associated wire pairs would have been an eyesore against the countryside.
In those days relatively few people could afford the cost of renting a telephone line, not to mention the call charges. There were many more public telephone kiosks than today and the location of the closest kiosk was often a major factor in the decision: the thought of more than half an hour's walk on a wet and windy winter's night to the nearest kiosk, perhaps to then need to wait in a queue, was as effective as many Post Office advertisements. Strangely, the need to receive incoming calls was perceived to be less important.FCL recently supported a client to look at his best options for a broadband internet connection to his small business in a rather rural part of the UK. It was not a big entity with the resources to negotiate special terms with a service provider.
For those of us who live in or near urban centres we may have a broadband internet connection provided by broadband cable, usually just referred to as 'cable' or colloquially as cable television (CATV). Alternatively we might have digital subscriber line (DSL), usually asymmetric DSL (ADSL).
By cable we usually mean an evolution of CATV in which a cable service provider makes a cable connection to the customer's premises. To provide the broadband data service, the cable actually carries digitally modulated information over some of the bandwidth as well as the original (analog) television service, if it is still required. (In fact with most cable providers the value of the TV channels is so small compared to all but the slowest broadband that customers are 'forced' to take it). The specifications which regulate broadband cable are known as the 'data over cable service interface specifications' (DOCSIS) and were originally drawn up by Cable Television Laboratories (Cable Labs) in the USA where CATV was pioneered.
ADSL uses the same pair of wires that was originally used for the analog telephone connections to the local telephone exchange but modified as far as possible to carry digital modulation. Whilst ADSL refers to the generic service, it is often available in the various forms of enhanced services. For example, in the UK these are known as ADSL2 and ADSL2+.
CATV was originally designed to provide a reliable distribution of analog television channels to neighbourhoods via cables using frequency division multiplex (FDM) and therefore not involving any potentially unreliable (or even impossible) radio wave propagation. The CATV infrastructure provided appreciable 'downstream' bandwidth ranging from around 50 MHz to an upper band edge of between 300 MHz and 870 MHz, depending on the level of service provided. DOCSIS services use bandwidth sections of these cables to carry broadband data streams using various types of digital modulation. They also provide for data transmission in both directions: downstream and upstream; their proportions determined by the degree of asymmetry of the connection. Data is carried over DOCSIS broadband cables using digitally modulated carriers as opposed to any form of 'baseband' services. The provision of a digital baseband over a cable designed for CATV would not be successful because:
- -the baseband may contain many octaves and the useable bandwidth is not equalised down to any particularly low frequency as would normally be necessary with baseband services;
- -digital modulation provides more flexible bandwidth allocations;
- -the percentage bandwidth of a digitally modulated service is normally a small fraction of that of a baseband service;
- -unlike with the original CATV function, it is not a broadcast or point to point service but a point to multipoint service and all customers could not be supplied with the same baseband.
To provide the digital broadband services in the downstream direction, from the cable head to the customer's premises, the available cable bandwidth is split up into channels each with a bandwidth of around 6 MHz to 8 MHz. Each channel is then used to carry a digitally modulated carrier using, typically, 64 or 256 quadrature amplitude modulation (64QAM or 256QAM). Every cable broadband customer is supplied with a cable modem to interface the local network , usually via Ethernet or universal serial bus (USB) on the customer side, an RF coaxial connection on the cable company's side.
In the UK and many other countries, the regional broadband cable infrastructures are owned by individual companies, so at any one location there usually will not be another competing broadband cable provider. However, for commercial reasons broadband cable services are usually installed in fairly urban and suburban areas where there may well also already be a relatively short, but analog, connection from the customer's premises to the local telephone exchange. The original analog telephone service built up by PSTNs over many years across the World since the invention of the telephone is often rather accurately called the plain old telephone service (POTS).
A POTS connection may be adequate to support an ADSL service, but the length of the connection must be very short, ideally less than about one to two miles to compete effectively with the DOCSIS service. To support any reasonable speed broadband connection with today's technology, say 10 Mbit/s to 20 Mbit/s. ADSL achieves this by widening the bandwidth of the POTS line. Unfortunately, as POTS was not designed originally to be a wideband connection - as we now understand the expression wideband or broadband, it only had to support about 3 kHz of analog bandwidth.
When the original telephone line was installed, the natural capacitance between the wires was corrected for by periodically adding series inductances, known as loading coils, along the line. However, these too were only designed for very low frequencies and so quickly became very lossy at higher frequencies, so they are removed if the line requires conversion to ADSL. ADSL exploits the available bandwidth of the remaining line itself, without loading coils. The original audio section of the bandwidth is separated out using filters at both ends of the line and retained for a POTS telephone connection, usually supplied as a package 'bundled' with the broadband service. The upper part of the frequency band is split up into 4312.5 Hz segments, the greater the available bandwidth, the more segments will be created. The ADSL modem is designed to detect the segments that are workable and use digital modulation over them. The available bandwidth for ADSL is the aggregate of all of the workable segments after making some allowances for overheads. As the connection was not originally designed to be wideband, the line usually becomes heavily absorptive as its length increases
As expected, at the premises owned by FCL's client there was no broadband cable available. ADSL was available in theory but the distance to the telephone exchange was about 6 km thus limiting the broadband speed to a very pedestrian 500 kbit/s. However, a visit to the client's premises revealed some interesting facts. It was a very rural area on some quite high ground above a tidal estuary. There were two fishing ports and some small towns and villages in the area. Despite their low lying position, FCL's representative found that the ports received a very good mobile cellular coverage. Some research and driving around the local area showed that the coverage to most of the estuary area was excellent and was provided by three base stations that were within less than 2 miles of the client's premises. After researching the service providers' information for mobile services and technologies available in the area, FCL found that a much better alternative than ADSL: mobile broadband.
In the UK and many other countries the cellular service providers offer 'mobile broadband' services which provide data services to laptop computers and other mobile devices based on enhancements of the second generation and third generation (2G and 3G) mobile cellular communication services. These are generically termed high speed download packet access (HSDPA) for the 3G service and general packet radio access (GPRS) for the 2G service. Provided one has purchased the necessary contract and receives a sufficiently good signal, up to 14 Mbit/s download are available from HSDPA but only 114 kbit/s from GPRS. The necessary wireless modems themselves serve the dual purpose of also being a USB memory stick and are often called USB 'dongles'. The dongle itself carries the subscriber identity module (SIM) card. Competition is quite strong in the UK with prices ranging from about GBP 7.50 to GBP 25.00 per month and still dropping, depending on the speed and total volume of data that may be downloaded per month. Although FCL's client was at a fixed location, that did not preclude the use of mobile services and the location was found to receive very good coverage from one of the 3G service providers in the area. To support mobile services at fixed locations, dongle driver software may of course be installed in a desktop PC if preferred instead of a laptop and the dongle may be positioned at an optimum location using a USB extension cable. Some dongles even have a very small connector which may be fed from a suitable antenna. Alternatively, dedicated antenna modules are available, equipped to take the SIM card. Unfortunately the official regulations do not allow the connection of any ad-hoc type of antenna, only a properly approved one. The client in fact opted for a prepayment dongle supporting a 3.6 Mbit/s service. This worked very reliably with good signal strength so it may be possible to upgrade to 7 Mbit/s or even 14 Mbit/s, after of course upgrading the contract accordingly.
The prices of mobile broadband services are now getting competitive with fixed broadband so it may be a very real option to provide a better broadband internet connection for many who live in rural locations