7 Emergency and Operator Services (part 5)

The PSTN supports extensive Emergency and Operator Services. Subscribers can dial 911 or the local equivalent and reach Emergency Services under almost any conditions. A Next-Generation VoIP Network needs to provide similar support leading to the following requirements:  
• Support for legacy Emergency and Operator Services Interfaces, for example MF and SS7.

• Support for lifeline support where this is a regulatory requirement.

• Provision of location information so that a caller’s physical location can be determined.

Call Routing and Number Plans
The PSTN is able to route calls between telephones anywhere in the world, for example a user can call Australia from Canada. This is achieved by having a well-defined number plan both nationally and internationally. Routing tables can be built using this numbering plan to provide end-to-end connectivity.  A Next-Generation VoIP Network must provide the same capability, which requires the following: 
• International and National numbering /addressing plans, for example ENUM implementations • Interconnection to the PSTN and E.164 numbers • SIP endpoint addressing schemes • Allocation of numbers/addresses and number portability issues • Call routing between numbers/addresses

DTMF and Other Tones and Telephony Events
When using VoIP there is an issue in transporting DTMF and Other Tones and Telephony Events. These can flow transparently using a full rate code such as G.711 but can’t be transported using lower-bit codecs such as G.729.   There are several solutions used for transporting these tones and events but the most widespread are  
• use RTP packets as specified by RFC 2833 
• transport the DTMF tones out of band using the signaling protocol, e.g. SIP or H.248.