Telecommunications *

Contents *

Advantages and Disadvantages of Networks *

Advantages and Disadvantages of Fibre Optics over Copper *

Unit 7 Topic 4: - Radio Transmitters and Receivers *

Other Technologies *

Resource Sharing
The sharing of expensive peripheral devices such as printers and scanners and of services and data which may be held on a dedicated machine and made available to many users. This is especially important with applications such as multi-user databases where the data is held centrally and accessed from many remote workstations.

Improved Communications
Data sharing applications and e-mail facilities allow information to be passed quickly between users. Files can be passed electronically rather than being printed out and sent by the mail.

Increased Reliability, Availability and Survivability
The computing power is distributed so that the loss of any one system should not affect the whole of the network. Key systems may be configured as a redundant pair so that non-stop working is possible even if one of the pair should fail.

Centralised Administration and Support
Networking computers can also simplify support tasks. It is far more efficient for technical personnel to support one version of one operating system or application and to set-up all computers in the same manner than to support many individual and unique systems and set-ups.

System Evolution

Disadvantages

Reduced Security
With a LAN it is likely that the data will be distributed, or at least, that access to the data may come from multiple sources. This raises questions of integrity (e.g. two users trying to update the database simultaneously), security, and privacy.

Creeping Escalation
This happens with the dispersal of computer equipment and the ease of incrementally adding equipment, where it becomes easier for managers of sub organisations to justify equipment procurement for their department. Although each procurement may be individually justified, the sum of all procurements may well exceed the organisation’s requirements.

Proprietary Systems
A LAN does not guarantee that two devices can be used co-operatively. This concept is known as inter-operability. For example two word processors from different vendors can be attached to a LAN and can perhaps exchange data; however, they will probably use different file formats and control characters. It may not be possible to directly take a file from one and begin editing it on the other. Some form of form conversion software is needed.

Loss of Control

Explain the legal requirements for transmitter operation.

Regulatory authorities
International Telecommunications Union (ITU)
Radiocommunications Agency (RA) in UK www.radio.gov.uk

Frequency of operation

Stability tolerance

Spurious emissions

Signal to Noise Ratio in FM Systems

The signal to noise ratio at the output of a FM system is directly proportional to the deviation ratio and hence to the bandwidth.
This means that the signal to noise ratio can be improved at the expense of greater bandwidth and for this reason FM transmissions use the VHF and UHF bands. With narrow band FM there is little or no improvement in S/N ratio over AM.
During transmission the FM signal will be subject to noise and interference voltages. Unfortunately the demodulation stage (discriminator) responds to both amplitude and frequency variations so that "limiting" is necessary.
A limiter is an amplifier stage, which "saturates" and "cuts off" with a small input signal. Once the input exceeds a small "threshold" voltage no amplitude variations can occur.

Other Technologies

DWDM is a technology that puts data from different sources together on an optical fibre, with each signal carried at the same time on its own separate light wavelength. Using DWDM, up to 80 (and theoretically more) separate wavelengths or channels of data can be multiplexed into a lightstream transmitted on a single optical fibre. Each channel carries a time division multiplexed (TDM) signal. In a system with each channel carrying 2.5 Gbps (billion bits per second), up to 200 billion bits can be delivered a second by the optical fibre. DWDM is also sometimes called wave division multiplexing (WDM).

Since each channel is demultiplexed at the end of the transmission back into the original source, different data formats being transmitted at different data rates can be transmitted together. Specifically, Internet (IP) data, Synchronous Optical Network data (SONET), and asynchronous transfer mode (ATM) data can all be travelling at the same time within the optical fibre.

DWDM promises to solve the "fibre exhaust" problem and is expected to be the central technology in the all-optical networks of the future.

Asynchronous Transfer Mode

ATM is a dedicated-connection switching technology that organises digital data into 53-byte cell units and transmits them over a physical medium using digital signal technology. Individually, a cell is processed asynchronously relative to other related cells and is queued before being multiplexed over the transmission path.

Because ATM is designed to be easily implemented by hardware (rather than software), faster processing and switch speeds are possible. The pre-specified bit rates are either 155.520 Mbps or 622.080 Mbps. Speeds on ATM networks can reach 10 Gbps. Along with Synchronous Optical Network (SONET) and several other technologies, ATM is a key component of broadband ISDN (BISDN).

Synchronous Optical Network

SONET is the American National Standards Institute standard for synchronous data transmission on optical media. The international equivalent of SONET is synchronous digital hierarchy (SDH). Together, they ensure standards so that digital networks can interconnect internationally and, that existing conventional transmission systems can take advantage of optical media through tributary attachments.
SONET provides standards for a number of line rates up to the maximum line rate of 9.953 gigabits per second (Gbps). Actual line rates approaching 20 gigabits per second are possible. SONET is considered to be the foundation for the physical layer of the broadband ISDN (BISDN).
Asynchronous transfer mode runs as a layer on top of SONET as well as on top of other technologies.
SONET defines a base rate of 51.84 Mbps and a set of multiples of the base rate known as "Optical Carrier levels (OCx)."
By 1998, collaboration between ANSI and CCITT produced an international standard, a superset of SONET, called Synchronous Digital Hierarchy (SDH).

BISDN

BISDN is the broadband counterpart to Integrated Services Digital Network, which provides digital transmission over ordinary telephone company copper wires on the narrowband local loop.