of vocoders. The standard requires, as a minimum, support for the mixed-excitation linear prediction (MELP) coder, an enhanced MELP algorithm known as MELPe, with additional preprocessing, analyzer and synthesizer capabilities for improved intelligibility and noise robustness. The old MELP and the new MELPe are interoperable and both operate at 2400 bit/s, sending a 54 bit data frame every 22.5 milliseconds but the MELPe has optional additional rates of 1200 bit/s and 600 bit/s. 2400 bit/s MELPe is the only mandatory voice coder required for SCIP. Other voice coders can be supported in terminals. These can be used if all terminals involved in the call support the same coder (agreed during the negotiation stage of call setup) and the network can support the required throughput. G.729D is the most widely supported non-mandatory voice coder in SCIP terminals as it offers a good compromise between higher voice quality without dramatically increasing the required throughput. Encryption (SCIP 23x) The security used by the multinational and national modes of SCIP is defined by the SCIP 23x family of documents. SCIP 231 defines AES based cryptography which can be used multinationally. SCIP 232 defines an alternate multinational cryptographic solution. Several nations have defined, or are defining, their own national security modes for SCIP. US National Mode (SCIP 230) SCIP 230 defines the cryptography of the US national mode of SCIP. The rest of this section refers to SCIP 230. For security, SCIP uses a block cipher operating in counter mode. A new Traffic Encryption Key (TEK) is negotiated for each call. The block cipher is fed a 64-bit state vector (SV) as input. If the cipher's block size is longer than 64 bits, a fixed filler is added. The output from the block cipher is xored with the MELP data frames to create the cipher text that is then transmitted. The low-order two bits of the state vector are reserved for applications where the data frame is longer than the block cipher output. The next 42 bits are the counter. Four bits are used to represent the transmission mode. This allows more than one mode, e.g. voice and data, to operate at the same time with the same TEK. The high-order 16 bits are a sender ID. This allows multiple senders on a single channel to all use the same TEK. Note that since overall SCIP encryption is effectively a stream cipher, it is essential