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A method of image to image communications that was made easy to use through simple terminal devices that in turn made use of the ubiquitous telephone network (PSTN). The underlying principle of facsimile is the synchronisation of scanning and printing processes, a technique that pre-dates the telephone and originates from a patent grated to Alexander Bain in 1843. The first international standard was not achieved until 1968 but, although it was improved in the meantime, the big breakthrough came in 1980 with the definition of a "Group 3" standard that allowed the transmission of an A4 page in about 1 minute. Group 3 introduced digital and compression techniques to make best use of the PSTN. Subsequently a Group 4 standard was produced for use over ISDN but the commercial success remained with Group 3. The advent of e-mail and other communications from PC to PC, as well as between facsimile machine and PC, meant that the use of fax was in decline by the end of the 20th century.
Fax provides a classic example of a product lifecycle in telecommunications and shows how a latent principle can become successful years later (in this case more than a century) through the availabilities of other technologies (e.g. plastics and semiconductors) allied to ease of use by the customer and a price point that coincides with a customer need.
The strength of a sky-wave signal varies with time because of fluctuations in the properties of the ionosphere. The relatively short-period variations which have durations of the order of a minute and less are generally called fading. In multi-hop transmissions frequencies as little as 100 Hz apart can fade independently. This can cause distortion in a modulated signal referred to as selective fading. Sky waves arriving by different numbers of hops, by reflections from different layers of the ionosphere, or in combination with a ground wave will all suffer fading. Fading can be minimised through the use of diversity systems.
A version of Ethernet that operates at 100 Mbit/s.
A small icon which can be displayed by most browsers in the address bar and in the favourites list provided that the web-site developer has included a favicon.ico file. The display size of a favicon is 16 bit by 16 bit. A favicon is typically the logo of a company.
Fax - see facsimile
Determining whether or not something is achievable before embarking on an attempt of the full realisation of it. One of the ten phases of the project lifecycle.
Fibre/fibre optic - see optical fibre
Fibre to the premises/home (FTTP/FTTH)
The use of optical fibre in the access network has typically only proved cost effective for supplying the high-bandwidth demands of large corporate companies. For new and established network operators, however, the increasing demand for bandwidth to deliver bearer, interactive and bundled services to business and residential customers is requiring them seriously to consider the high volume roll-out of optical-fibre-based systems. See Fibre to the home - infrastructure deployment issues, BTTJ, Vol 20, No 4.
A filter is a device, normally inserted between a source and a load, whose function is to separate some sinusoidal frequencies from others. There are many analogue systems where it is necessary to attenuate sinusoidal signals whose frequencies lie outside a desired band. Telephone, television and radar in the pre-digital era would have been impossible without a means of separating desired frequencies from undesired ones. There are four basic types of filter: low-pass, high pass, band-pass and band-stop.
Firewalls are regarded as the primary defence mechanism for connecting private IP networks to the Internet. For more information see Firewalls - evolve or die. BT Technology Journal, Vol 19, No 3.
This refers to 99.999 which has become a watchword for the telecommunications industry indicating its high availability and extreme resilience, especially compared to conventional industrial computing systems and applications. It has its origins in Strowger switching design and originally applied to the up-time of switches and major network fabric but did not include the local loop. The stated requirement for the availability of electromechanical (Strowger) systems was that they should not be out of service for more than 2 - 4 hours in 20 - 40 years (the exact figures depending on the administration and the system). 2 hours in 20 years is 6 mins a year. And Five Nines is 315 seconds or 5.25 mins a year.
For a period the worlds of fixed and mobile networks were regarded as different and indeed there were, and are, a number of differences. However, as most terminals move to using wireless for their connection to a network, as all networks allow for mobility and as cellular networks install more and more cells thereby relying on larger fixed core networks the technologies they all use have more and more in common such that the distinction is no longer clear cut. A common division of the network is into the access network and the core network.
A packet-oriented network access protocol that is defined in ITU-T recommendations I.122 and Q.922 and was designed as a cost-effective service for enterprises needing dedicated connectivity between offices without the expense of dedicated leased lines. Frame relay is aimed at bursty data and is characterised by high transmission speeds, low network delay (latency), high connectivity and efficient bandwidth use, including bandwidth sharing and bandwidth on demand. It is used as an access protocol for data only, to feed into ATM core transport networks which have efficient cell relay switches carrying data, voice and video.
The popularity and pricing of frame relay services encouraged the retention and development of an ATM/frame relay network alongside IP and voice networks. Frame relay is similar to X25, in that it is a data only protocol, but whilst X25 uses the bottom 3 OSI layers during transmission frame relay uses only the bottom two. This means that frame relay performs error detection only (i.e. no error correction and retransmission). This takes advantage of modern low error-rate transmission links and intelligent devices at the ends of the network and results in less processing at the network nodes and thus less overall delay across the network.
Frame relay supports
- frame delineation (using HDLC flags)
- zero bit insertion
- CRC checking
- multiplexing and de-multiplexing of many virtual circuits on to a single physical link
- indications for forward and backward congestion
- an indication for low priority frames
The format of a frame is shown in the figure below:
Frame relay is commonly used to transport IP traffic between LANs. IP packets up to a maximum size of several thousand bytes are encapsulated into frames which are than carried, typically by a leased line, into the local frame relay point of presence.
One of the three basic forms of modulation - the others being amplitude modulation and phase modulation. In frequency modulation the amplitude is kept constant but the frequency is varied in relationship to the modulating signal. The digital form is called frequency shift keying (FSK) in which the two binary states can be represented by two different frequencies. As with all modulation systems complex variations exist.
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