DoS (Denial of Service) is a type of cyber-attack intended to overwhelm the resources of the target PSAP and deny the ability of legitimate users of the target to use the the normal service the target provides.
DDoS (Distributed Denial of Service Attack), Denial of Service Attack
A form of DoS in which the attack source is more than one, often thousands of, unique IP addresses. A DDoS attack occurs when multiple systems flood the bandwidth or resources of a targeted system. Such an attack is often the result of multiple compromised systems (for example a botnet) flooding the targeted system with traffic.
TDoS (Telephone Denial of Service)
A form of DoS directed at a telephony interface which generates numerous phone calls, tying up the network and preventing the destination from receiving legitimate calls. Occasionally the "T" in TDoS may be shown as Telephone or Telecommunications.
Relevant NENA Documents