G with information about a certain precedence handling for a context. The MGC can also use the priority to control autonomously the traffic precedence in the MG in a smooth way in certain situations (e.g., restart), when a lot of context must be handled simultaneously. An indicator for an emergency call is also provided to allow a preference handling in the MG. Megaco uses a series of commands to manipulate terminations, contexts, events, and signals. For example, the add command adds a termination to a context and may be used to create a new context at the same time. Of course we would expect the subtract VOICE-OVER PACKETS IN A PACKET NETWORK command to remove a termination from a context and may result in the context being released if no terminations remain. There is also the modify command used to modify the description of a termination (e.g., the type of voice compression in use). Notify is used to inform the gateway controller if an event occurs on a termination such as a telephone in an off-hook condition, or digits being dialed. There is also a service change command. Terminations are referenced by a TerminationID, which is an arbitrary schema selected by the MG. TerminationIDs of physical terminations are provisioned by the media gate- way. The TerminationIDs may be chosen to have structure. For example, a TerminationID may consist of a trunk group and a trunk within the group (Ref. 9). REVIEW EXERCISES 1. How many bits are in a conventional PCM voice sample? 2. How is a telephone call on the PSTN usually terminated? 3. Give the two basic components of a data message? 4. Every data frame or packet carries two components (at least) in the header. What are they? 5. In exercise 4, how does this differ from a PCM frame? 6. What is a CRC sequence used for? Give at least two different ways the PSTN may set up a voice circuit. 8. List at list five drawbacks or challenges designers faced in making VoIP a reality. 9. List at least four tasks the gateway may do. 10. List at least four transmission media and format