ltimedia is explained and defined in Chapter 5 and media gateway is allocated a chapter of its own. Media stream is the user information (such as speech, tones, announcements, MPEG video, JPEG pictures, XML data etc.) carried on a bearer. In multimedia calls, a number of media streams are carried on a (possibly different) number of bearers. Another example is the interpretation of the word 'service provider'. Since a service is a very general word, you would expect also 'service provider' to be quite general. However, it often has a narrow meaning, for example compare Internet service provider, or 'service provider' as opposed to 'network provider' or 'network operator'. In this book we will use 'network operator' quite often. Therefore 'service provider' will normally be used in its narrow sense, and does not include 'network operator'. There are other problems with the word 'service' as well. Supplier units offer profes- sional services to network operators/service providers. Often the word 'professional' is deleted and just 'services' remains. 'Service' is also used to indicate technical resources. In the telephony world the word has got a flavour of IN (intelligent network) services. This is normally a kind of network intelligence supporting fairly dumb terminals, such as ordinary telephones. This network intelligence mainly offers advanced routing, charging and number conversion tasks. Unfortunately the term 'application' is also sometimes used Introduction for such network-based services. In another context 'service' could refer to frame relay, ATM (asynchronous transfer mode) or IP (Internet protocol) communication facilities. A better expression is 'network services', as opposed to 'applications' located in servers outside the network. The expression 'multi-service gateways' used in this book is network service oriented. Applications normally consist of a terminal component and one or several cooperating application components in servers. Applications are not involved in the media transfer. See Figure 1.1