Reliability / Availability (part 5)

The PSTN achieves five-nines reliability, equivalent to fewer than five minutes per year downtime, and it handles millions of simultaneous calls. A VoIP network needs to achieve similar levels of reliability and scalability.  The required reliability and scalability can be achieved in a VoIP network by using redundant and loadsharing equipment and networks. The call agent, access gateway, trunk gateway, signaling gateway and media server need to be fault tolerant. The types of functionality often used to achieve fault tolerance include: 
• Redundant hardware 

• Redundant network connections

• Hot-swap capability
 • No single point of failure 

• Software and firmware that can be upgraded without loss of service.

Lawful Interception
Historically, lawful interception (wiretapping) of telephone conversations has been a relatively well-defined and straightforward process. Typically, a law enforcement agency applied to a court for an order to tap a particular phone number. Once the agency had the order, it served that order on the provider of the telephone service for the number to be tapped. The service provider then put a tap on the circuit, extracted all the necessary information and passed it to the law enforcement agency. The introduction of VoIP complicates this process considerably.   The law varies according to location (in the United States, the relevant legislation is the Communications Assistance for Law Enforcement Act - CALEA). The following requirements are typical for any network including VoIP networks and the PSTN. 

•  No wiretap is permitted without a court order. 
• Wiretaps apply to phone numbers, not particular suspects.
•  Wiretaps fall into two categories.

- Call detail – a tap in which the details of the calls made and received by a subscriber are passed to the law enforcement agency. (Referred to as pen register and trap and trace in the U.S.).
- Call content – a tap in which the actual contents of a call are passed to the law enforcement agency.

• The suspect must not detect the tap, so the tap must occur within the network and not at the subscriber gateway. Also, the tap may not be detectable by any change in timing, feature availability or operation.
• A suspect may be tapped by more than one agency. The taps are separate, and the various agencies are not aware of each other’s taps.  The taps do not have to be of the same category.
• It is the responsibility of the telecommunications carrier that originates or terminates calls to provide lawful interception.

As described in section 3, VoIP networks typically contain separate call agents and media gateways. The call agent is responsible for all call control and is the element that collects all the details of the calls required in a call detail tap. However the call agent does not see the call content, so call content must be collected elsewhere in the network.  The requirement to be able to tap the content of calls without the subscriber being able to detect any change leads to the conclusion that all calls, whether they remain within the carrier’s IP network or access another network (e.g. PSTN) must be routed by the Call Agent via a device capable of duplicating the content and passing it to law enforcement.