● SMS: Given its limited length of 160 characters per short message, the overhead of the WAP that would be required to be transmitted in an SMS message would mean that even for the simplest of transactions several SMS messages may have to be sent.
● CSD: Most of the trial-based services use CSD as the underlying bearer. CSD lacks immediacy — a dial-up connection taking about 10 sec is required to connect the WAP client to the WAP gateway; and this is the best case scenario when there is a complete end-to-end digital call.
● USSD: It is a means of transmitting information or instructions over a GSM network. In USSD a session is established and the radio connection stays open until the user, application, or time-out releases it. USSD text messages can be up to 182 characters in length. USSD can be an ideal bearer of WAP on GSM networks. USSD is preferable due to the following advantages:
1. Turnaround response times for interactive applications are shorter for USSD.
2. Users need not access any particular phone menu to access services with USSD but they can enter the command directly from the initial mobile phone screen.
● GPRS: GPRS is a new bearer because it is immediate, relatively fast, and supports virtual connectivity, allowing relevant information to be sent from the network as and when it is generated. There are two effi cient means of delivering proactively sending (pushing) content to a mobile phone: by SMS (which is, of course, one of the WAP bearers) or by the user maintaining more or less a permanent GPRS session with the content server. WAP incorporates two different connection modes — WSP connection mode or WSP connection protocol. This is similar to the two GPRS point-to-point services — connection-oriented and connectionless. For the interactive menu-based information exchanges that WAP anticipates, GPRS and WAP can be ideal bearers for each other.
Source of Information : Elsevier Wireless Networking Complete