Abstract:

There are numerous telecommunication services and students often feel confused about them while they find it difficult to realize the challenges related to integrated telecom networks capable of delivering different types of telecom services through the same physical infrastructure.

To dissolve the associated misconceptions, the first 2-hour lecture of the introductory “Telecommunication System” course (offered at the Electrical & Electronic Engineering Dept, School of Pedagogical & Technological Education (ASPETE), Athens, Greece) includes an overall presentation and a classification of telecom services according to basic parameters such as bit-rate, bit-error rate (BER) and transmission delay.

In the context above, the following comments are made:
- Services such as fixed or mobile telephony (generally called dial-up services) require a rather low bit-rate (a digitized telephone signal has a bit-rate of 64 kb/s) are generally tolerant regarding bit-error rate (even a BER equal to 10−4, that is 1 erroneous bit out of 10000, may be sufficient) but it is rather strict regarding transmission delay (which has to be kept well under 100 ms). In conventional telephone networks, the above requirements lead to the use of circuit switching (the communicating parts are assigned a dedicated channel for the whole time of their communication) since this is the obvious way of keeping bits reaching their destination in the correct order and with an acceptable delay.
- Data services on the other hand (e.g. computers communicating to each other) usually require high bit-rates (of the order of Mb/s or Gb/s) and very low bit-error rates (BER is expected to be under 10−9 or even 10−12), however, except for services such as gaming, they are rather tolerant regarding transmission delay. The above requirements allow for the use of packet switching (information is divided into packets each of which may follow a different route to reach its destination) which results in a much more reasonable use of network resources.
- Finally, audiovisual services (such as TV) require a high bit-rate (an uncompressed digitized video signal requires a bit-rate of 270 Mb/s) and a BER (typically lower that 10−9. Audiovisual services are rather loose regarding delay (it may be a few seconds and mainly caused by signal processing) however they are strict regarding differential delay (pixels must reach at the same time together with the associated sound). Signals are broadcasted so switching is not necessary (to reach the end user).

The above classification is also compared to that of modern multimedia services according to the ITU I.211 recommendation. It is easy to see that dialogue services, message-exchanging as well as information-retrieval and distributed-transmission services are roughly equivalent to dial-up, message exchanging and broadcasting services respectively.

Through the above considerations, the students can create a perspective on the telecom subject and can be better prepared to evaluate the information they will obtain through the telecom courses to follow. They also realize why there used to be separate networks for telephony, data and audiovisual services and appreciate the difficulties and challenges regarding the development of modern integrated networks.

The presentation may be combined with the use of gaming tool (such as Kahoot) through which the students will be asked to classify certain services such as video-calls, emails or video-on-demand.

Keywords:

Engineering Education, Electronic Engineering Education, Telecommunications.