The term `broadband' covers a multiplicity of technological sins. This month Dr Carlo Kopp explores the depth and breath of broadband technologies -- and takes a peek at some possible futures.
The bandwidth bottleneckThe sad reality of the early 21st century is ageing infrastructure in developed nations like Australia. While trains coming off railroads grabs the headlines, crumbling and corroded low speed telephone wiring to homes and business sites does not.
The only truly ubiquitous data connection we have in this country is the plain old telephone line (POTS) and the voiceband modem -- and it presents a true `bandwidth bottleneck'.
While many voiceband modems achieve 115Kb/s on paper, this performance is contingent on the quality of the connection. With many lines to end users of dubious electrical quality, and often over larger distances than seen in the US and EU nations, the unfortunate reality is that most modem users have to be satisfied with 33Kb/s -- like it or not.
This problem is often described as `the last mile of connection bandwidth problem' -- with optical fibres between telephone exchanges the bottleneck is well and truly in the last mile -- or more often in Australia, three miles.
The robust long term fix to this problem is to run single mode optical fibre to every household. Given the investment costs involved, and shareholder expectations of short-term payback in the Telco industry, this is unlikely to happen any time soon.
Other lower performance and less elegant solutions have cropped up in recent years and become established in the market, with varying degrees of success, as many readers will know from experience.
Digital subscriber line technologiesThe starting point for any discussion of DSL technologies is DSL/ISDN, or Integrated Services Digital Network. When initially conceived, ISDN was envisaged to be the digital replacement for the established POTS service. Every household was to have its analog service replaced with ISDN, which would provide a pair of 64Kb/s circuit switched synchronous channels (ie: the 2B in 2B+D) for telephone and data services, and a single 32 or 16 Kb/s packet mode channel (the D in 2B+D) for the purposes of link management and data services. The D-channel uses the LAP-D protocol which is closely related to the established X.25 LAP-B protocol (LAP-A having been a failure by design).
While ISDN struggled to receive customer acceptance, voiceband modems on POTS lines plugged the gap with line bit rates up to and beyond 28.8Kb/s. Fast voiceband modems competed directly against a single 64Kb/s ISDN B channel. The attraction of a more expensive, in terms of ongoing rental costs, ISDN service was therefore debatable.
The explosive growth in the Internet we have seen since the emergence of W3 service a decade ago produced a huge demand for services with much higher throughput than standard ISDN. ISDN simply missed the boat.
ADSL The basis of the recently introduced `broadband' services in many parts of Australia, Asymmetric Digital Subscriber Line (ADSL) service was designed to provide high data rate services directly to customer premises. The asymmetry in ADSL reflects two factors (the same as in cable modem technology). The first is that interference and crosstalk problems in large bundles of multipair cables make it much more difficult to send high data rates upstream. This is because the upstream signal is at its weakest the closer it gets to the exchange, where it must share a large cable bundle with a large number of downstream signals which coming straight out of the exchange, are at full power levels.
The second factor is that a large proportion of customer premises applications are inherently asymmetrical in behaviour, where downstream traffic is much larger than upstream traffic.
The ADSL service is designed to provide a range of transmission speeds, with speed traded off against distance. Nominal downstream rates are T.1 to 6,000 metres, E.1 to 5,000 metres, 6.312Mb/s (DS2) to 4,000 metres and 8.448Mb/s to 3,000 metres. Upstream rates range between 64Kb/s and 640Kb/s.
The intent with ADSL is to support compressed video (MPEG or H series protocol) downstream, as well as circuit switched traffic and packet switched traffic, such as IP.
Because all of these traffic types require different quality of service and hence error correction or detection protocols, the ADSL protocol is complex and this will reflect in complex modem designs. A typical ADSL modem in the market today combines modem functions with an embedded IP router, and typically an Ethernet/802.3 network connection.
VDSL The VDSL (Very high data rate Digital Subscriber Line) service is the fastest and most sophisticated of the DSL family of protocols. As such it is the most immature and at the time of writing was yet to be deployed.
VDSL is intended to provide comparable performance to cable modems, out to customer premises, using a combination of optical fibre and twisted pair. While optical fibre is by all means the best possible medium for data transfer, most Telcos consider the economics of running fibre to customer sites unsupportable by the short-term revenues it can generate.
The compromise solution is similar to that being used by cable modem vendors, where optical fibre is used to distribute the service to a suburb, and copper twisted pair then to the customer's premises.
VDSL is conceptually similar to ADSL but much faster.It is envisaged that VDSL will use integer fractions of the ATM transmission rates. The nominal 155Mb/s stream splits into three 51.84Mb/s, six 25.92Mb/s and twelve 12.96Mb/s streams. VDSL is intended to support 12.96-13.8Mb/s up to a distance of 1,500 metres, 25.96-27.6Mb/s to 1,000 metres and 51.84-55.2Mb/s to 330 metres.
Asymmetrical upstream rates of 1.6-2.3Mb/s and 19.2Mb/s are envisaged, or a symmetrical rate equal to the downstream rate. It is expected that the higher asymmetrical upstream rate, and the symmetrical upstream rate will only be available for short cable lengths. Early implementations of the VDSL protocols are expected to support only the lowest upstream rates.
A large part of VDSL's use will be the transmission of video services, and to this effect a forward error control (FEC) scheme will be used. Useful bandwidth will thus be somewhat lower than the nominal bandwidth. It is expected that Reed-Solomon coding similar to that in ADSL will be used.
VDSL is intended to support passive taps much like cable modems do, with active taps as an option. As a result, VDSL systems using the passive option will share a similar topology to CATV while also using similar strategies for multiplexing upstream traffic (this is indeed a generic problem with branched segmented topologies).
Cable modem technologiesCable TV technology has been around for many decades, and is a well developed and mature service in the US. The central idea in this model is to distribute TV carriers to household sockets through a coaxial cable. A single cable can be threaded through multiple sites, with a high impedance tap delivering the signal to the user's wall socket.
The classical CATV topology is a tree like structure, with branches fanning out from linear segments. Because a coaxial cable can provide a bandwidth of about a GHz, cca 150 television channels can be carried on the single cable. With about 6MHz bandwidth allocated to a channel, and typically only a fraction of the channels used, it was only a matter of time before somebody would think of exploiting the unused capacity.
Interestingly, the connection between Ethernet and cable TV technology is historically strong. When Metcalf and Boggs originally conceived the Ethernet LAN, they merged two then quite well established ideas -- packet radio networking and CATV cabling.
To translate the model into a cabled, baseband
Issue: 133 | February, 2012