HomeRF SWAP basics

- information, article tutorial about the basics of HomeRF, a wireless networking for the home

The Home RF Shared Wireless Access Protocol (SWAP) was designed primarily as a wireless networking solution for home applications and aimed at a broad range of consumer devices. SWAP was an open industry standard for computers, their peripherals, cordless phones and a variety of other devices to communicate with each other around the home without the cost and inconvenience of having to install a wired network. To achieve this it was able to carry both voice and data. Unlike Bluetooth that is aimed primarily for use with cellular technology, or IEEE802.11 that is intended for business use, SWAP was intended for home use where the developers see a vast and growing market.

Unfortunately HomeRF did not catch a sufficiently large market and as a result the HomeRF working group was disbanded in January 2003. Nevertheless a short summary is included here for reference.

HomeRF Capability

The system can support data rates of between 10 and 20 Mbps combined with sufficient range for most residential applications. If operating over slightly greater distances the speed can back off to 5 Mbps. Like 802.11b and Bluetooth, HomeRF operates in the 2.4 GHz ISM band. It uses frequency hopping with a hop rate of between 50 and 100 hops per second.

The system can be set up in either ad-hoc, peer to peer mode, or as a network with a Control Point (CP) giving access to a wired network within the house. In this respect it has many similarities to 802.11b.

When requiring to send data the CSMA/CS protocol derived from 802.11 is used. Here the unit wanting to send data listens to see if the frequencies are clear to send data. If it is clear then the data is sent, otherwise it waits for a random amount of time before trying to send the data again.

Data structure

he data within a HomeRF transmission is contained within repeating frames that are transmitted. Each one of these frames is either 10 or 20 mS long dependent upon the number of active voice calls at any given time. Within the frame, the bulk of the available capacity is used for asynchronous data transmission. However within the capacity reserved for this, priority is given to the streaming media sessions. Up to eight of these sessions are allowed, but when fewer than this number are present, the available capacity remaining can be used for ordinary data transmission.

The last part of the data frame structure is reserved for providing voice quality communications. These full duplex pairs of phone quality slots use the Digital Enhanced Cordless Telecommunications(DECT) system. The HomeRF protocols map directly onto this allowing an already well established system to be incorporated into the HomeRF technology. The full specification for HomeRF allows for a total eight of landline quality links to be supported.