Hamnet70: IP-based networking in the 70 cm amateur radio band

A packet encoded by the PHY consists of the following blocks:

Packets are transmitted in bursts which can combine multiple packets in a contiguous transmission.

A burst consists of the following elements:

The ramp-up, ramp-down and idle sequences consist of alternating +1/-1 BPSK symbols.

During ramp-up, their amplitude \$A_u(k)\$ is scaled up across \$L_u\$ symbols, following the curve \$A_u(k) = sin(π/2 * k / L_u)\$. Ramp-down follows a similar sinusoidal for \$L_d\$ symbols: \$A_d(k) = cos(π/2 * k / L_d)\$. The purpose of these sequences is to avoid splatter while turning on and off the transmitted signal.

During the pre-packet idle sequence, the alternating BPSK symbols are continued with constant amplitude 1.0. This sequence may help synchronizers acquire the signal better (especially the gain and timing regulation can benefit), but is not needed if they are fast enough. If and how this part is used is still to be determined.

Each packet in a burst includes a preamble. Packets are directly concatenated, i.e. after the last data symbol follows the first preamble symbol.

Pulse forming is applied to limit the bandwidth of the transmitted signal and to reduce inter-symbol interference at the receiver.

This system uses Root Raised Cosine (RRC) filters with a roll-off factor of \$alpha=0.2\$. Therefore the bandwidth is 1.2 times the symbol rate.

The filter is applied to the whole symbol sequence of the burst. The same filter must be applied at the receiver to minimize inter-symbol interference.

The link layer frame consist of three elements, as follows:

The CRC16 is calculated using the generator polynomial \$x^15 + x^2 + 1\$.

Node addresses use the HAM-64 address format. This format allows to encode callsigns of up to 12 characters in a number of 16, 32, 48 or 64 bits. Additionally, it allows to replace the full version of an address with a temporary short address with only 16 bits that is used during a connection. Multicast and broadcast addresses are also specified.

Each layer 2 packet contains a header that identifies how the message should be interpreted. It is of variable length and composed as follows:

The total Link Layer Header length is therefore at least 6 byte and at most 18 byte.

The length of the source and destination addresses is encoded as follows: 00 = 16 bit, 01 = 32 bit, 10 = 48 bit, 11 = 64 bit.

Lost packets are automatically repeated in Hamnet70. As the system is also designed for high throughput, it uses the Go-Back-N algorithm to handle this. The algorithm is described in this section.

Both the digipeater and the client send two sequence numbers with each packet.

The TX sequence number is the number of the current packet. It uniquely identifies that packet at the time it is transmitted. The TX sequence number is incremented (with overflow) with each new packet that is sent.

The RX sequence number is the packet number the sending station expects to see next from the other side. It is incremented when a packet with this expected sequence number is decoded successfully. Received packets with other sequence numbers than the expected one must be dropped.

Example: The two stations A and B just connected and expect to see sequence number 0 from each other. Station A transmits a burst with three packets: P0, P1, P2. Station B decodes packet P0 and increases the expected number to 1. P1 is not decoded due to noise and therefore lost. P2 is again decoded, but is dropped because its sequence number is 2 and not the expected 1. In its transmission cycle, station B tells station A that it expects P1 next. Station A therefore goes back to P1 in its transmission queue and starts transmitting all packets after P1 again.

If a station does not have anything to transmit, it shall transmit an Empty Frame to acknowledge the data from the other side.

As the sequence numbers in Hamnet70 have 4 bits, up to 15 packets can be transmitted in a burst before the numbers become ambiguous.

Due to the long possible bursts, this system can achieve high throughput if there is low packet loss. However, it can also become very slow if many packets are lost because all packets after the first error have to be repeated. Therefore it might be a good idea to adjust the burst length depending on the packet loss and send smaller bursts if only few packets go through.

This section documents state diagrams for the digipeater and the clients. They are only informational; different implementations are possible and allowed as long as the procedures defined by the message sequence charts are obeyed. However, the state diagrams shown here reflect the reference implementation.

In binary phase shift keying, a 1 bit is sent as -1.0 and a 0 bit is sent as +1.0.

tbd.

tbd.