By Richard Nesin, president, HomePNA
So by now you must be dieing to know more about HomePNA’s guaranteed QoS. First off – HomePNA also supports priority QoS (actually it has since HomePNA 2). Unlike some of other home networking technologies, HomePNA 3 and 3.1 use a “synchronous MAC” which eliminates collisions on the network and guarantees glitch-free delivery of each real-time packet with the required latency, jitter, and error rate parameters.
Like most other home network technologies, HomePNA is built of two components; a MAC and a PHY. The MAC part, short for medium access control, manages the communication between devices on the home network. Every HomePNA device includes a MAC and PHY however one device, the “master,” is special. The master is what makes the HomePNA MAC synchronous and enables its best-in-class QoS. The HomePNA Master breaks up the transmission time into small parts the way a train is broken up into cars (HomePNA calls them Transmission Opportunities or TXOPs if you really want to know). HomePNA devices with data to send contact the master and ask it for reservations to send a data “stream” to another device (the required bandwidth, maximum latency, jitter and error rate it can tolerate).
The master determines which train cars are available, how many are needed, and how far apart they can be spaced. It puts that information into a schedule which, for the sake of simplicity, HomePNA calls a Media Access Plan or MAP. The master sends the schedule to every device on the network at the same time, synchronizing every device on the network together. Every device then knows which train cars (or TXOPs if you want to get technical) are reserved for its sole use.
Devices sending best effort data are welcome to use any unassigned train cars however they must be prepared to vacate at a moments notice if the master assigns the car to a stream requiring QoS. The master regularly updates and sends the schedule to all of the devices on the network allowing it to add and remove streams from the network whenever required. The QoS parameter information itself is determined in a number of ways. It can be extracted from the 802.1p priority bits, gotten from a “TSpec” sent by the stream originator, or by some other means.