 a normal case noise has a negative impact on QoS. However, if there is no background noise at all, you feel uncomfortable, because you do not know if the connection is broken or not. To take care of this, a so-called 'comfort noise' is added at the receiver's end in applications where everything is coded as silence below a certain threshold. This goes for both mobile telephony and IP telephony. 7.3.3 DATA COMMUNICATION So far we have discussed the quality of telephony. Some of this is applicable for other services as well, some of it is not. The ability to successfully connect to other subscribers is important for voice services, since this is the basis for charging. For Internet service providers (ISP) often a flat rate applies. Anything that makes the subscribers select you instead of your competitor as their ISP, will feel like 'good quality'. So what affects the perceived quality when we for example connect to the Internet from home? Short set-up time Bandwidth Fast data delivery without excessive queuing Low bit error rate (BER) Accessibility to many fellow subscribers Differentiated priority Some services, or some users, need a higher priority and are prepared to pay for it. This is true for real-time services. Or you can imagine a business user at an airport who wants to download mail with attachments before entering the aircraft. Differentiated service classes, differentiated billing and resource reservation are methods used to improve this aspect of quality. Security Business information, money transactions and data of a private nature all demand some kind of protection against 'eavesdropping'. This can be solved with encryption, or by using tunnelling. There are also methods for putting an electronic signature on a data message, to ensure the receiver that the sender is who he claims to be (compare to the desire to hear who is talking in a telephone conversation). Public key ciphering is a method that solves both encryption and the signature. More security issues are included in Chapter 6. 'Future-proo