PCCA Standards for Wireless Networks

Since its formation in 1992, the Portable Computing and Communication Association (PCCA) has played an important role in promoting and enabling interoperability between products in the wireless-data industry. With broad representation including wireless service providers, modem vendors, computer manufacturers, software companies and application developers, it has provided a forum for companies to work together constructively to advance the industry. Some well-known companies participating include 3Com, Aironet, American Mobile, AT&T Wireless Services, BellSouth Wireless Data, Ericsson, Intel, Microsoft, Motorola, Nokia, Telxon, WRQ and Xircom.

The standards committee of the PCCA has developed key interface standards that would not otherwise have been addressed by other standards bodies. The standards committee has also developed implementation guidelines for various aspects of mobile computing. Since the PCCA is not a government accredited standards body, the standards committee sometimes submits its standards to accredited standards organizations such as the Telecommunications Industry Association (TIA).

A brief, but not exhaustive, history of projects undertaken by the standards committee includes the following.

PCCA STD-101 - Wireless AT Commands

This standard, the first major achievement of the standards committee, addresses how the AT command set (normally used for controlling modems) can be extended into wireless environments. One key aspect of this standard is its concept that the wireless modem may have a connection to one (or more) of many different types of wireless data networks, and provides a mechanism (the +WS46 parameter) for the attached computer to query or set the type of wireless network used.

Using STD-101, developers of communications applications can use a consistent set of commands to control wireless modems, irrespective of the modem manufacturer and the type of network being used.

STD-101 was submitted to the TR30.2 subcommittee of the TIA as a contribution and has subsequently been published as TIA standard 678. Key portions of this standard have been incorporated into Microsoft’s modem implementation guidelines, GSM data standards from the European Telecommunications Standards Institute as well as standards of the International Telecommunications Union (ITU) which is the leading world body for communication standards. PCCA STD-101 has been implemented in a number of wireless modems being sold today.

A number of annexes have been developed for STD-101. One of these covers miscellaneous commands related to wireless networks, such as for signal strength, registration status, battery level, and antenna selection. Another covers the use of packet assembler/disassembler functions in a wireless modem, and the most recent annex addresses AT commands for Cellular Digital Packet Data (CDPD) modems. These annexes are also being incorporated into TIA-678.

STD-201 - Wireless Extensions to NDIS

The PCCA worked closely with Microsoft to develop a standard that specifies a set of wireless extensions to NDIS (network driver interface specification). NDIS defines an interface between protocol stacks (or network management software) and underlying networking systems. See figure one. With NDIS, a protocol stack can use the same interface in the same way to communicate across any network for which an NDIS driver is available. The NDIS driver is supplied by the network hardware vendor, e.g., Ethernet card vendor or wireless modem vendor. In effect, the NDIS driver translates between standardized networking functions and the proprietary hardware interfaces of the network adapter.

The goal of the wireless extensions was to add useful information regarding wireless networks. Using the extensions, applications and protocol stacks can know what wireless connections are available, can choose between them, and can monitor network conditions. For example, using the NDIS extensions a mobile computer could determine that it is no longer in coverage of a wireless local-area network and could automatically switch to a wide-area network connection. Applications could then automatically reconfigure themselves for the lower bandwidth. For instance, an e-mail application might no longer download large attachments unless explicitly directed by the user.

Similar in scope to STD-101, the wireless extensions specify wireless objects for items such as base station information, signal strength, data throughput and battery level. There are also objects for specific networks such as DataTAC, Mobitex and CDPD. STD-201 has been incorporated into Microsoft Windows NT version 4.0. Work is currently underway to develop objects for wireless local area networks as well as objects for General Packet Radio Service, a wireless-data service being developed for GSM and IS-136 networks.

Figure One: Wireless extensions to the NDIS interface

The PCCA concentrated initially on the NDIS interface because Windows represents a large installed base of computers. But realizing the importance of other emerging platforms, the PCCA intends to work with other companies and organizations to apply wireless-specific information and control to platforms such as Java.

TCP/IP Recommendations for Wireless Networking

TCP/IP has evolved over the years to operate reliably over many different types of networks. But wireless networks present a new set of challenges that can affect the reliability and performance of TCP algorithms. For example, variable latency is common with wireless-data communications, and can occur when a weak radio signal or heavy interference causes packets to be retransmitted multiple times before they are received without error. TCP incorrectly interprets such delays as network congestion, and throttles back the rate at which it transmits data. The result is significantly lower throughput than the channel itself provides.

Recognizing these types of problems, the PCCA developed a set of recommendations for how TCP/IP should be implemented to handle wireless connections. These recommendations encompassed experience gained by member companies using Mobitex, CDPD and the Metricom Ricochet Networks. A technical discussion of the most important recommendations is as follows:

Adjustable parameters. Stack parameters should be adjustable. Wireless-aware applications can then make the adjustments they need for optimal performance. Different wireless networks may need values adjusted differently. Some specific parameters are discussed next.
Window size. TCP/IP connections over different wireless networks work better with different window sizes. Metricom found that a 16 Kbyte window size gives the best performance for their Ricochet network. For other networks, a smaller size may be better.
Slow start. How "slow start", the way TCP gradually increases the rate at which it transmits data, is implemented in a stack can make a large difference in performance over wireless connections. What can help slow start is to set the initial congestion window to allow for at least two packets to be sent. Since many stacks only acknowledge every other packet, this will prevent a delayed acknowledgement, which could otherwise result in a retransmission. One way to accomplish this is to set the initial congestion window to at least twice the maximum segment size.
Round trip time. Due to higher latencies in wireless networks compared to LANs and wireline WANs, too low an initial setting will result in retransmissions. Higher values such as four seconds work better for some wireless networks. It can also be set by TCP based on knowledge of the link speed.
Quick recovery. The Van Jacobsen Quick Recovery algorithm should be implemented. Three identical acknowledgements constitute a reasonable trigger value.
Maximum segment size (MSS). Maximum size should be set through a process of Maximum Transmission Unit (MTU) discovery. Starting with a large value (e.g. Ethernet frame size), TCP can send data with the "don't fragment" bit set. If it receives an ICMP (Internet Control Message Protocol) message from a router indicating an error, it can reduce the size and try again.
Dynamic link recovery. It is common with wireless connections to lose the radio link for short intervals of time. If the layer-two link can be quickly reestablished without dropping the layer-four TCP connection, users will find the connection to be more reliable and more convenient. Such a capability though it is not a trivial undertaking. At the minimum the stack should prevent the link from being easily dropped.

Microsoft adopted many of these recommendations in the most recent versions of their TCP/IP protocol stacks.

Mobile MIB

The standards committee is currently working on the definition of a Management Information Base (MIB) that defines management information unique to mobile computers and communications devices. MIBs are the databases used by the Simple Network Management Protocol (SNMP), an Internet standard defined by the Internet Engineering Task Force (IETF). Parameters for the Mobile MIB will not only include wireless-related information (e.g. wireless performance statistics) but also other information such as the type of network adapters and modems installed, and version of firmware. With a Mobile MIB implemented, a network manager located at a central location using could use wireless network connections to track mobile-computer inventory and query the operational status of mobile equipment.

Computer and network management standards are evolving beyond SNMP, and so the standards committee is in the process of coordinating its efforts with other standards groups. One such group is an industry consortium called the Desktop Management Taskforce (DMTF) which is developing management architectures referred to as Web Based Enterprise Management and the Common Information Model. The PCCA will work to ensure that these emerging management standards take into account the unique requirements of mobile computers.

IP Modem

Another standard in development is a packet interface between computers and wireless modems. The intent is to make it possible to support multiple data streams simultaneously, allowing a user for example to have simultaneous voice, data, fax and messaging connections with future multimedia wireless phones. Current interfaces only allow for one active-data type. The standards committee has decided to base this interface on Internet Protocols. The advantage of IP is that it readily supports multiplexing of multiple data streams. Another advantage is that nearly all computers already have IP protocol stacks, so no specialized software or drivers will be required to take advantage of next generation wireless modems that implement this standard.

The way that IP modem will work is that once in an online mode, data will be transferred between the mobile computer and the IP Modem using IP over a suitable link protocol such as PPP. Within the IP Modem, an internal IP router will then directs datagrams to appropriate "service agents" within the modem. Some of these service agents will provide local services such as a directory. Others will provide connectivity to external networks, which may or may not be based on IP communications.

Return to home page