Download

336 SIGNALING IN CELLULAR MOBILE TELECOMMUNICATIONS12.2 AMPS TONE SIGNALS AND MESSAGE WORDSThis section begins the description of the AMPS signaling protocol. The protocol isbased on the original development by Bell Laboratories [1,2] but includes a numberof additions and modifications specified by the EIA/TIA (Electronics IndustriesAlliance/Telecommunications Industry Association)—see [6].The signaling between mobile and base station is a combination ofcommon-channel and channel-associated digital signaling messages, and a singlefrequencysignal.The forward and reverse control channels (FOCC and RECC) carry signalingmessages only. They are “common” channels, used for signaling between a basestation and all mobiles in the cell that are active, but not involved in a call. The messageson these channels include a mobile identification number (MIN) that identifies aparticular mobile.A voice channel is allocated to a MS at the start of a call. Its forward and reversevoice channels (FVC and RVC) carry speech and channel-associated signalingmessages (which do not include MIN).12.2.1 Supervision ToneThis 10-kHz tone is sent on RVC only and represents the on-hook (tone on) andoff-hook (tone off) states of the mobile. When the mobile is in a conversation,on-hooks of 400 ms and 1.8 s indicate “flashes” and “disconnects,” respectively.12.2.2 Transmission of MessagesMessages consist of one or more words, transmitted at 10 kb/s. On forward channels(FOCC, FVC), the word length is 40 bits (28 information bits, followed by 12 paritybits for error checking). Words on reverse channels (RECC, RVC) have 48 bits (36information bits, and 12 parity bits). Figure 12.2-1(a), (b), and (c) show the generalstructure of transmitted word blocks for messages with two words on, respectively,RVC, FVC, and RECC. In the figure, the fields in the message streams are denotedby acronyms, and the corresponding field lengths (number of bits) are shown belowthe acronyms.Repeated Transmission. Each message word is repeated several times. Thisgreatly reduces the probability that a word is completely missed by a receiverbecause of fading. Figure 12.2-1(a) shows that message words W1 and W2 on aRVC are repeated five times (W1-1, ..., W1-5; W2-1, ..., W2-5). Words onFOCC and RECC (Fig. 12.2-1(c), (d)) are also repeated five times, but words onFVC (Fig. 12.2-1(b)) are repeated 11 times, for reasons to be discussed later.Error Checking. Error correction by retransmission (used on SS7 signalinglinks—see Sections 8.4 and 8.5) is not practical in mobile signaling. Errors inreceived messages are minimized by taking advantage of the repeat transmissions