Description
The Wireless Datagram Protocol (WDP) operates as the transport layer within the Wireless Application Protocol (WAP) stack, defined to offer connectionless, unreliable datagram services similar to UDP but tailored for wireless networks. Its primary function is to provide a consistent data transport interface to the upper layers (WTP, WSP) regardless of the underlying bearer network's characteristics. WDP achieves this through bearer adaptation: it includes specific adaptations for different bearer services like GSM Circuit Switched Data (CSD), SMS, USSD, GPRS, CDMA, and others. Each adaptation maps the native bearer's addressing, maximum transmission unit (MTU), and reliability features into a common WDP service primitives.
Architecturally, WDP sits above the data link layer of the bearer network and below the Wireless Transaction Protocol (WTP). It is responsible for port-based multiplexing and demultiplexing, allowing multiple WAP applications to coexist on a single device. A WDP address consists of a bearer-specific address (e.g., a phone number for SMS) and a port number. The protocol handles segmentation and reassembly if the WDP datagram exceeds the bearer's MTU, breaking it into bearer-specific packets for transmission and reconstructing it at the destination. WDP also provides optional error detection via checksums, depending on the bearer's inherent capabilities.
In operation, when a WAP application sends data, the WDP layer in the device receives the payload and destination port from the upper layer. It consults its bearer adaptation layer to format the datagram according to the active network's requirements, adds the necessary headers (including source and destination ports), and passes it to the bearer for transmission. On the receiving end, the process is reversed. Within 3GPP, WDP is referenced in specifications like TS 23.057 for MExE, ensuring that WAP services can be delivered over standardized 3GPP bearers. Its design emphasizes minimal overhead and efficiency, crucial for the constrained wireless links of early mobile data networks.
Purpose & Motivation
WDP was created as part of the WAP suite to solve the problem of application interoperability across the highly fragmented landscape of early wireless networks. Different network technologies (GSM, CDMA, iDEN, etc.) offered vastly different data transport services with unique addressing, message sizes, and reliability characteristics. Developing applications directly for each bearer was impractical. WDP provided an abstraction layer that hid these differences, offering a uniform datagram service to upper-layer WAP protocols. This allowed developers to write applications once for the WAP environment, with confidence they would work across multiple bearer types.
It addressed the limitations of using standard Internet protocols like UDP directly over wireless bearers, which often had small MTUs, high error rates, and non-IP addressing. By incorporating bearer-specific adaptations, WDP could optimally utilize each network's capabilities, such as packing data into SMS messages or using CSD connections. Its inclusion in 3GPP standards, starting from Release 4, ensured that UMTS and evolved networks supported a standardized method for WAP transport, facilitating the deployment of mobile internet services like browsing and messaging across global operator networks. WDP was instrumental in enabling the first wave of mobile data services before the full IP convergence of later 3GPP releases.
Key Features
- Bearer adaptation layer providing uniform datagram service over diverse wireless networks
- Port-based multiplexing to support multiple applications on a single device
- Segmentation and reassembly to handle bearer-specific MTU limitations
- Support for both connection-oriented and connectionless underlying bearers
- Optional error detection via checksums where bearer lacks inherent protection
- Lightweight header structure to minimize protocol overhead on constrained links
Evolution Across Releases
WDP was first incorporated into 3GPP specifications in Release 4, primarily within the context of MExE and WAP interoperability. This adoption standardized its use over 3GPP-defined bearers like GSM CSD, SMS, and GPRS, providing a consistent transport mechanism for WAP-based services in UMTS networks.
Defining Specifications
| Specification | Title |
|---|---|
| TS 21.905 | 3GPP TS 21.905 |
| TS 23.057 | 3GPP TS 23.057 |