This is the second part of my course study in telecommunication technology (see Development of telecommunication, part 1 - History). In this write-up I will try to clarify where the development of telecommunication technology stands currently
(note: with "currently" I mean around the turn of the millennium). I will cover various fixed and mobile connection types with some information about their specific features.
Modulation and multiplexing
Since the capacity of cabled links is limited and the amount of traffic vast, the data is fed to the cables modulated and/or multiplexed in various ways, in order to maximize the throughput. Modulation is a process by which voice, music, data or other "intelligence" is added to a transmission - or carrier - signal by a transmitter (radio, computer, megaphone etc). There
are several ways to modulate the signal including, but not limited to: frequency, phase and amplitude modulation. Multiplexing means combining several data streams to a single, larger stream, which is then transmitted along a single cable. There are also several ways for multiplexing, for example by frequency, time or wavelength-division (two signals varying enough from each other can be transmitted in a single cable without them interfering with each other and can, at the receiving end, be separated from each other again.)
Time-divided multiplexing (each data stream is allocated specific timeframes, during which only one stream sends data along the cable) is widely used in concurrent networking techniques. Normal phone network (PDH, or POTS "Plain Old Telephone System") uses time-division, as does its newer, packet-switched version SDH, or SONET. Of these two, the multiplexing
of PDH is complicated and cumbersome, due to the poor synchronization of the streams. SDH is easier to handle, mainly because the streams are accurately synchronized and any single stream can be separated from the multiplexed stream without a need to demultiplex all data in the stream. Partly thanks to this, the maximum bandwidth of concurrent SDH networks is almost 10 Gbit/s (STM-64).
Wavelength-divided multiplexing is used on optical fibres and it is listed as a technique of its own, eventhough physically it is exactly the same as frequency-divided multiplexing. The idea of wavelength-division is to divide light to different wavelengths (colors) and send them along the fibre. Concurrent fibre-optic cables can carry 160 (thanks to Teiresias for this update, my text originally said 40) OC-192 wavelengths at a time (DWDM, Dense Wavelength Division Multiplexing), for a total capacity of 1.6Tbit/s. This makes SDH + DWDM a specifically good combination, since SDH offers flexibility and error tolerance, while DWDM offers a very broad bandwidth. As a conclusion it can be said that the bandwidth is no longer going to be the limiting factor in long-distance data transmission. SDH + DWDM combination is used, for example, in TAT-14 and FA-1, which both are capable of transfer rate of 2.4Tbit/s (approx. 10 million simultaneous phonecalls) at maximum bandwidth utilization.
Viable fixed solutions for the home-user
In urban areas, home-users already have a wide selection of network connection alternatives to choose from. Cablemodem, ISDN, ADSL all use already existing television/phonecables. In some areas, optic-fibre is also available but, it is often not needed by a normal home-user.
A cablemodem connection is always on but, the transmission media itself is divided between an arbitrary number of users. Thus, the actual transfer rates may vary heavily.
ISDN uses normal phoneline and is available practically anywhere. ISDN is slower than cablemodem (~100kbit/s vs. ~10Mbit/s) and, the connection has to be established separately each time the user wants to connect to the net but, the connection is established faster than when using a regular phone. Also, ISDN data connection does not prevent normal telephone operation, like a normal modem would.
ADSL also uses the phonecable but, offers a fixed connection and a faster transfer rate than ISDN. ADSL is asymmetric, meaning that the transfer rate is faster in one direction (usually from ISP to home-user) than the other. This is a way to avoid heavy network loads at the ISP's end of the cable, while offering the user a fast network connection. ADSL doesn't interfere
with normal phone operations either since it uses different frequencies. The maximum transfer rate over an ADSL connection is 8Mbit/s but (in Europe), typical connection speeds offered for home-users are below 1Mbit/s.
Wireless connection types
Wireless connections can basically be divided into three main categories, based on the required range of operation: long distance (GSM), short distance (WLAN), and very short distance (Bluetooth).
At first, GSM was an acronym for "Groupe Speciale Mobile", which was an investigation started by CEPT. The goal was to build an ISDN-compatible wireless network across all Europe, with roaming. After the publication of the GSM Phase 2, there were already approximately 5.5 million GSM users in about 60 countries. By 1996, the amount of GSM users had risen to over 270 million - in Europe alone.
In addition to normal phone services, GSM network offers the option for data and fax calls, SMS service and several other features (call waiting etc). GSM networks in Europe also offer a crudely WWW-resembling WAP (Wireless Application Protocol) service and, in Japan, its highly more sophisticated counterpart i-Mode.
GSM network is a radio network utilizing frequencies of 900MHz and 1800MHz elsewhere in the world except in Japan and North-America (where a frequency of 1900MHz is used).
2.5G, a transition phase between the second and third generation GSM networks offers noticeable improvements, in the form of high-speed data transmission: HSCSD, GPRS and EDGE. HSCSD is capable of 57.6kbit/s transfer rate, GPRS of 171.2kbit/s transfer rate and EDGE, at maximum, of 384kbit/s transfer rate. HSCSD requires normal connection establishment, while GPRS/EDGE are always on.
3G - IMT-2000 by its official name - or, the third generation GSM network offers yet more improvements, compared to the older versions but, is likely to be run over and left in the shadow of other networking techniques, unless it is taken into use very fast, and probably even then (prediction made by the course lecturer Rob Parker, CERN-IT).
3G supports both circuit- and packet-switched data transfer at over 100kbit/s transfer rates in any case, 384kbit/s transfer rates while the user is relatively still (in urban areas) and up to 2Mbit/s transfer rates in pico cells (eg. indoor office). There are several standards, of which the couple most important ones are: UMTS (Europe), CDMA2000 (Japan) and TD-CDMA (the rest of the world). Frequencies utilized by 3G are 1885-2025MHz and 2110-2200MHz.
WLAN, or Wireless Lan (ETSI standard EN 300 652) offers a maximum of 20Mbit/s wireless data transfer rate, and uses frequencies of 5.15-5.30GHz.
HIPERLAN 2 (High-Performance Radio LAN) is also standardized by ETSI but, has only minor differences with the IEEE 802.11a -standard. The frequencies used are the same as with WLAN while the data transfer capacities range from 6-54Mbit/s. HIPERLAN 2 has a compulsory buffering error correction (compare: CD-player's read buffer).
IEEE 802.11b is a wireless network connection type standardized by IEEE, which operates on the unlicensed 2.4GHz frequency still widely used by industrial, scientific and medical equipments. IEEE 802.11b is an improved model of IEEE 802.11a and offers improved security options in addition to regular challenge-response (password/passphrase) authentication (configurable MAC allowance lists and 40-bit RSA encryption).
in the basic infrastructure of IEEE 802.11b (BSS), at least one of the link-points has a physical network connection so that the mobile devices can have access to the fixed cable network. In addition, there are independent link-points (IBSS) where the wireless devices and the link-points communicate with each other only.
WLL, or Wireless Local Loop is a viable solution in areas where - for some reason - an ADSL connection can not be established.
Bluetooth is a very short distance network connection, with a usable range of only few meters, approx. 10m at absolute maximum. Bluetooth is at its best in preventing a cable mess between computer and printer, computer and mobile phone/PDA or, for instance, stereoset and headphones -pairs. Bluetooth also utilizes the unlicensed 2.4GHz frequency and offers a maximum data transfer rate of 1Mbit/s. The advantages of Bluetooth are affordable price, low power use, simplicity and
error tolerance in difficult situations (Bluetooth uses frequency hopping, 1600 hops/sec). Also, Bluetooth devices are auto-configurable, meaning that they will "sniff" available services within their range, automatically. The obvious drawback is the very limited usable
For a normal home-user, probably the best connection type to choose is either ADSL or cable-modem. ADSL can cover even the more demanding needs for a fixed connection. For mobile connectivity, the options are a bit thin currently but, probably the best option to choose from is a GSM with HSCSD.
My personal prediction is that, within the next 3 to 5 years, the best mobile connection type is going to be a PDA utilising GPRS/EDGE and that, the wireless GPRS/EDGE and WLAN solutions will also - partly - be replacing the currently fixed home connections; high-speed wireless Internet connectivity is a very strong trend currently, and the sales of such devices and services are likely to be rocketing skyhigh within a couple of years.
This text is a part of a course-study in telecommunications technology.