Software architecture for the PC

SWAP devices will be supported in Microsoft Windows using the network driver interface specification (NDIS) driver library. The NDIS library performs many of the functions common to all networking device drivers, such as synchronization, and also provides a standard interface for higher-level applications to access. Network adapter manufacturers are only required to produce a miniport driver that provides functionality specific to their hardware. Miniports of a given media type can be used with higher-level protocols knowledgeable about that media type with no further modifications as shown in Figures 1a and 1b , where the shaded blocks are provided by the OS.

NDIS exports two distinct interfaces – a connectionless interface (used by broadcast media such as Ethernet) and a connection-oriented interface (used by media that have explicit connections between endpoints, such as ATM). Hardware manufacturers producing asynchronous devices should write a connectionless miniport that declares itself a member of the Ethernet media type. To higher-level protocols, SWAP will be indistinguishable from regular Ethernet adapters, allowing Ethernet-knowledgeable applications to immediately function with SWAP devices.

Hardware manufacturers producing isochronous devices should write a miniport that declares itself as a member of the Ethernet media type, but that exposes a connection-oriented interface, not the connectionless interface traditionally used. Connection-oriented NDIS is present in Windows 2000 and Windows 98, but not in legacy OSes. Thus, isochronous nodes are only supported in the aforementioned Windows OS.

In addition to miniport functions, isochronous drivers must also provide call management functions such as those required for setting up and terminating calls. Call control of SWAP adapters should be done via the telephony application programming interface (TAPI). TAPI is a simple, generic set of objects, interfaces, and methods for establishing connections between devices. TAPI communicates with NDIS through a TAPI service provider. In Windows 2000, the service provider is furnished; this module is the TAPI proxy. TAPI applications will be able to set up, control, and take down calls on SWAP devices via the TAPI proxy. In Windows 98, the TAPI proxy is not included, but manufacturers can easily write a custom service provider to provide this functionality (see Figure 2 ).

Some device designers may wish to stream voice conversations between the SWAP adapter and another adapter within the PC in real time. An example scenario would be that of a voice conversation between the SWAP adapter (a user with a SWAP handset communicating with the SWAP adapter) and another adapter in the PC (such as a modem attached to a phone line or a sound card attached to speakers and a microphone). In Windows 2000 and Windows 98, voice data can be streamed between adapters via the DirectShow streaming architecture. A DirectShow filter graph is plumbed from the data source (in this case, the SWAP adapter) to the data sink (the modem or sound card). In Windows 2000, voice streams coming in via NDIS will be redirected to the raw channel access filter (RCA), which will send them into DirectShow. The RCA filter ships with the OS, so connection-oriented miniports are automatically voice stream-enabled. In Windows 98, no standard architecture exists for redirecting voice streams to DirectShow, but a Winsock application achieving this goal can easily be written.

PHY and components

The PHY specification for SWAP was largely adapted from the IEEE 802.11FH and OpenAir standards with significant modifications to reduce cost, while maintaining more than adequate performance for home usage scenarios. The PHY specification takes tremendous advantage of the frequency hopping nature of the 2.4-GHz band. Significant interference sources are easier to hop away from or to momentarily defer to, rather than to filter them out. Some of the key SWAP physical layer specifications include:

• Transmit power. Up to +24 dBm (or nominally 100 to 250mW)
• Receiver sensitivity in 2FSK or 0.8 Mbps mode. 80 dBm
• Optional low transmit power mode. 0 to +4 dBm (for portable devices with limited peak-current capability)
• Hopping time. 300 ýs (to allow conventional synthesizers to be used)
• Transceiver turnaround time. 142 ýs (very easy to achieve with existing synthesizers)
• Adjacent and alternate channel filtering. No requirement (substantial relaxation from 802.11).

The combination of the transmit power and the receiver sensitivity represent a typical range that should easily exceed 50 meters in most home environments. In the optional low-power mode, reliable indoor range is expected to be 10 to 20 meters (which covers the bulk of the interior of most homes). As with OpenAir and IEEE 802.11FH, a 4FSK or 1.6 Mbps mode is available. However, in SWAP the requirements impose substantially lower cost constraints for three reasons. First, the required sensitivity limit is relaxed by about 10 dB. Second, the greatly relaxed channel filtering specification causes dramatically less intersymbol interference due to filter group delay variations in the passband. And third, the SWAP packet headers for 4FSK add a special training sequence to allow optimum slicing threshold values to be determined for the changing propagation environment. Thus for usage within most homes, the 1.6-Mbps data rate is really available with SWAP and adds virtually no cost to the 0.8-Mbps solution.

In fact, the entire SWAP PHY-layer specification has been written specifically to accommodate very low cost, single-chip implementation in CMOS technology. A typical system partitioning is shown in Figure 3 . For many of the digital devices envisioned by HomeRF, the digital MAC baseband portion of the component solution can be integrated into a large ASIC already in the device. At about 30k gates for the SWAP data core, this is an extremely low-cost option in the sub-0.25ý CMOS era. The modem functionality can interface with the digital baseband using a simple serial interface (with no analog quantities). The modem and RF functionality can all be integrated into a single mixed-signal CMOS IC because of the specific technical requirements on filtering and modulation chosen by HomeRF. Note that it does not make sense to integrate the RF front-end functionality, such as the low-noise amplifier, the power amplifier (if present), the antenna switches, and the band-select filter, onto the CMOS IC even though it is technically feasible. The semiconductor die area for the front-end functions is typically much less than 5% of the rest of the modem (hence it is already low cost) and the overall power consumption performance is driven largely by optimizing these functions in detail.

With such high levels of integration and optimized front-end, the overall RF modem section cost in a multimillion unit volume should be well below $10 (similar to the situation today with DECT), while the digital multiply and accumulate (MAC) section approaches zero (not including IP royalties). While this seems extraordinarily low compared to today's $100-per-node RF data, it is still expensive when compared to the low cost of IrDA transceivers or USB controllers.

Thus, cost remains a significant issue in making HomeRF a throwaway item for every electronic device. But consumers have consistently shown with voice that they will pay extra for personal mobility. Even today, cordless phones are significantly more expensive than corded phones yet much more popular. If consumers begin to value mobility within the home for Internet-based content the way they do today for PSTN-based content, then the present cost projections for HomeRF should not be a serious barrier.

Future HomeRF derivatives

The HomeRF organization is already discussing a variety of future derivatives for the initial SWAP specification. One possible derivative is simply to increase the data rate within the existing 2.4-GHz band, while retaining full backward compatibility with the initial specification. The group is presently considering options in this regard that would scale SWAP to 10 Mbps in the 2.4-GHz band. In addition, HomeRF is also developing a major new market requirements document. Called SWAP-MM (for multimedia), it is looking at true video applications within the home enabled by wireless networking. This work will likely proceed to a formal technical proposal for the 5 GHz band. It is unclear at this point whether the SWAP-MM specification can ever be as near global as the 2.4-GHz SWAP case. As with SWAP, achieving consumer price points with a SWAP-MM solution will be critical.