ions on the user's behalf. As we have seen, protocols in the PSTN do not meet this criterion. Some systems using SS7 such as GSM resemble master/slave architectures, where a node in the network-the Mobile Switching Center (MSC)-sends commands to the GSM terminal, which obeys unconditionally. This architecture leads to a network that makes assumptions about what the user wants, instead of letting the user dictate what he wants. When end-to-end services are implemented in such a network, unintended effects sometimes occur. Consider, for instance, a call between two GSM terminals (see Fig- ure 1-19). GSM terminals use GSM codecs for encoding the voice. The MSC that handles the caller's terminal decides, on behalf of the terminal, that transcoding from the GSM codec to PCM-the codec used in the fixed PSTN-is needed. When the call reaches the MSC handling the callee's ter- minal, it performs the opposite transcoding, from PCM to GSM, before at last sending the voice to the terminal. Unhappily transcoding notably reduces the voice quality, and when it is performed twice, as in this sce- nario, its effect is amplified. This example shows clearly that end-to-end services are better imple- mented using end-to-end protocols, which would enable both terminals to negotiate the codec to be used, avoiding the situation we've just examined. Reliability Because the PSTN concentrates system intelligence in some net- work nodes, a failure in one of the nodes means disruption of service for the user. Consequently, network nodes must be highly reliable. Techniques such as processor redundancy ensure that a certain network node does not just Figure 1-19 A call between GSM mobiles. Signalling in the Circuit-Switched Network fail. However, implementing nodes that (almost) never fail results in a sig- nificant increase in equipment costs. We will see that on the Internet, since the intelligence is pushed to the terminals, failure in a network node such as a router is usually not critical. Other nodes can take over its task. Here reliabi