an initial channel access message such as a random access channel preamble is used to mean that not only that a channel access has been requested by a mobile station but also that a particular type of service has been requested . by associating a type of service with the particular signature that is used to form the rach preamble it speeds up the service negotiation process . both the communication system and the ms know what service is associated with which signature , so as soon as a rach preamble is received by the system , it can determine if it has the resources available to accommodate the request .