ISDN (Integrated Services Digital Network)

As all modern telephone exchanges transmit data digitally anyway, why do we need modems at all? This is because the connection between the home or office and the local exchange is still typically by analogue signals along copper wires. This is called the 'subscriber loop'. ISDN (Integrated Services Digital Network) removes the need for a modem by replacing this last analogue link by a digital one.

Conventional telephone equipment will not work with an ISDN line without a special terminal adapter (TA).

ISDN telephones provide a much clearer voice signal and dialling is much faster. PCs equipped with a TA card can connect to the Internet, or another PC, in a fraction of a second and data transmission is faster than the best modems (64 or 128 Kbits/s). ISDN is available as basic rate (equivalent to the capacity of two phone lines or 128 Kbits/s) for domestic or small office use, or primary rate, which gives an aggregate bandwidth of over 2 Mbits/s in Europe. North American ISDN offers slightly different configurations and bandwidth.

ISDN has all sorts of applications in the world of eCommerce - its use really is dependent on the type of solution being looked at. A real-life example is its use to connect a remote (perhaps home) office to a main office computer network so that an executive can work remotely but have all the benefits of being in the main office. However, one of the drawbacks to using ISDN is that it is still essentially a dial-up service, and each channel is charged at normal telephone rates. So using both channels becomes quite expensive.



Broadband

The explosive growth in the numbers of business people working remotely from their head office (often in offices at home) coupled with the slower than expected take-up of ISDN has resulted in the demand for new services to provide faster Internet connections at more competitive rates.

Broadband services are based on a variety of transmission technologies:

• DSL (Digital Subscriber Line) services, that make use of existing telephony infrastructure;
• Cable Modem services, that make use of cable TV infrastructure;
• Metropolitan Area Ethernet services, that generally require the installation of new cable infrastructure, often based on fibre optic cabling, although some variants of this technology make use of DSL technology.


ADSL

The most common DSL service available is the ADSL service. The 'A' stands for 'Asymmetric', which means that the data transfer rates are not the same in both directions. The transmission rate from the subscriber to the Internet is lower than in the reverse direction; the idea here is that downloading large web pages will account for a high proportion of the use, whereas the volume of data that needs to be transmitted from the user to the network is generally smaller.

The data rates available with ADSL, and in fact the actual availability of ADSL, depend critically on the distance between the subscriber and the nearest digital telephone exchange, and also on the number of subscribers that are using the service. Typically the service offers up to 500 Kilobits/second for downloads, and up to 250 Kilobits/second for uploads. So, unlike leased lines, ADSL gives no guarantees as to the actual transmission rates that will be made available.



Cable Modems

The Cable Modem services offered by the cable TV companies are only available in those areas already served by a cable TV network. The data transmission rates available with these services are similar to ADSL, and as with ADSL, the number of subscribers sharing the infrastructure affects the transmission rates that are actually delivered. With both ADSL and Cable Modem services, it is a simple matter to connect the cable modem or the ADSL filter into an existing LAN, allowing the external Internet connection to be accessed by all of the PCs connected to the LAN. A router is used to achieve this. One side of the router connects to the LAN, and the other side to the cable modem or the ADSL filter.

Metropolitan Area Ethernet services
Where significantly higher data transmission rates are needed, Metropolitan Area Ethernet services are emerging that will allow data rates of up to 10 Gigabits/second to be achieved between the customer premises and the service provider, if suitable fibre optic cabling is available (or can be installed) in order to make this possible. This kind of approach might be used in order to allow a large company that was spread across a number of sites to interconnect their LANs, making them look like one single LAN from the point of view of the users.


Leased Lines

Telecoms operators offer a variety of options for permanent connections for data transmission. These include ISDN, but also higher capacity dedicated links and variable bandwidth links, where capacity is shared with other users. Connections are also available from 'network providers', companies which have private network capacity who lease some or all of their capacity from telecoms companies. Leased lines may be used to connect LANs into a WAN (e.g. head office to a branch) or for teleconferencing.

Wireless Networking

The majority of networking installations to date have been based on fixed cabling of one kind or another, from coaxial cables to twisted pair cabling and optical fibre. The problem with cable-based networking infrastructures is just that - they are cable-based, and so every device that needs to connect to the LAN has to have a cable trailing from it. Two relatively new networking technologies aim to change the way we think about constructing and using LAN installations:

• 802.11 Wireless LANs, sometimes known as 'Wi-Fi' (an adaption of the popular home entertainment term 'Hi-Fi');
• Bluetooth.

Wireless LAN
A wireless LAN, as the name implies, offers a wireless connection between a computer system and the LAN, via a wireless access point. The data transmission rates that can be achieved are relatively low when compared to the performance of the early 10 Megabits/second Ethernet LANs, but are more than adequate for connecting the average PC or laptop to a LAN, unless applications that demand high data rates are being used.
Wireless access points generally have an Ethernet port that allows them to be connected to a conventional wired LAN backbone, but for small business use, it is possible to network a number of PCs together using only a wireless access point.

Connecting a PC or laptop to the wireless LAN involves the use of a wireless LAN interface that will slot into the PCMCIA socket of a laptop, or plugs into a USB socket of a desktop PC. Increasingly, modern laptop computers are being manufactured with inbuilt wireless LAN capability, in anticipation that they will be used in this way. Assuming that the security features of the technology have been correctly set up, the wireless 'connection' between the computer and the wireless access point is established automatically when the two devices are within radio range. The range of transmission is variable, depending on the construction of the building that it is used in, but can cover distances of up to 100 metres under the right conditions. If it is necessary to use more than one wireless access point to achieve the necessary coverage within a building, a wireless-enabled PC will automatically connect to the access point that offers the best signal.


Bluetooth

Bluetooth is a technology that was originally conceived as a means of replacing the cables between a computer and its peripherals - for example, printers, scanners, and so on. The data transfer rates that it can achieve are adequate for this kind of use, but not for connecting a PC to a LAN. It is now starting to appear in conjunction with other kinds of devices as well - for example, to connect a palmtop computer to a host PC, or to connect a hands free kit to a mobile phone. The transmission range for Bluetooth is significantly shorter - tens rather than hundreds of metres, and as less power is needed to drive the radios in Bluetooth devices, this technology is more appropriate than Wi-Fi for use in smaller, low power devices.

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Thompson said...

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