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Protocols and Interfaces

Protocols and interfaces are related topics, and are a key consideration when evaluating wireless networks.

Protocols

In many cases, the mobile computer functions as a remote node of the network you are accessing -- the Internet or a corporate intranet. If the wireless network uses the same networking protocols as the fixed-end network, connections are more streamlined, and routers can interconnect the wireless network to the fixed-end network. Otherwise, some form of gateway will be required to translate between different network protocols. For circuit-switched connections this is less of a concern because the wireless connection is essentially a Layer 1 (physical) connection, much like an existing landline modem connection. Except for wireless LANs, which are not discussed here, it is uncommon to use a bridge between a wireless network and a fixed-en d network.

Our profiles of specific networks further below explain what specific protocols are used. But the general pattern for the wireless link is as follows.

• Physical layer An RF carrier signal that is digitally modulated to create a bit stream. This bit stream incorporates forward error correction, interleaving and other techniques to mitigate the effects of interference and weak signals that can produce high-bit-error rates.

• Link layer Usually a specialized radio protocol that employs a form of medium access optimized for the radio environment. Most link protocols involve interactions between the wireless modem and a base station, and mobile units do not communicate directly with each other.

• Network layer Some wireless WANs, such as RAM Mobile Data and ARDIS, use network-layer protocols designed specifically for that network, however the trend is toward using IP. This is the case with CDPD, as well as the packet services being developed for PCS networks (GSM, CDMA, TDMA).

• Transport and higher layers These layers usually are not part of the wireless network, but implemented as part of the application solution. Some transports have been designed specifically for wireless networks. But it is also possible to use tried and proven transports such as TCP, though some optimization of TCP's timing parameters and algorithms tends to yield better results.

Interfaces

Where protocols determine the packaging of data for communication across a network, interfaces determine access points to the network, both at the mobile computer and at the fixed-end locations. At the mobile computer, the interfaces of interest are between the application and the protocol stack and between the mobile computer and the wireless modem. At the fixed-end, we are interested in the interfaces the wireless network presents.

Most wireless modems use a serial interface even when imp lemented as a PC Card device. Many use AT commands for configuration and control, similar to landline modems. If the network supports standard networking protocols, such as CDPD, which supports IP, then the mobile computer uses a TCP/IP stack which presents a WinSock interface to applications assuming a Windows environment. CDPD modems then use the Serial Line Interface Protocol (SLIP) or Point-to-Point (PPP) to communicate between the computer and the modem.

If the network uses unconventional networking protocols, the software interfaces are more likely to be unique or proprietary to that network. See Profile of Wireless Networks for the specific details of each network. One new approach wireless modem vendors are taking is to develop NDIS drivers with wireless extensions for their wireless modems. These wireless extensions have been defined by the Portable Computing and Communications Association (www.outlook.com/pcca) an organization that develops standards for wireless networking.

Wireless middleware solutions typically present a standard interface such as MAPI or a virtual file system, or use proprietary interfaces. Narrowband Sockets (NBS) is middleware designed for wireless networks and presents a WinSock interface.

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