doc: start defining the layer 2

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Thomas Kolb 2024-04-27 23:35:45 +02:00
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@ -106,6 +106,86 @@ The filter is applied to the whole symbol sequence of the burst. The same filter
== Layer 2: Link Layer
The Hamnet70 link layer is designed to ensure reliable communication between a central station (digipeater) and several clients, while making efficient use of the available airtime. To accomplish this, it uses several technologies, including variable-length addressing and Go-Back-N retransmission.
=== Node Addresses
Node addresses use the https://github.com/arngll/arnce-spec/blob/main/n6drc-arnce.md#introduction[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.
=== Link Layer Header
Each layer 2 packet contains a header that identifies how the message should be interpreted. It is of variable length and composed as follows:
[cols="2,1,5", options="header"]
.Link layer header layout
|===
| Field | Length | Description
| Message type
| 3 bit
| The type of the message. See below for a list of values.
| TX request
| 1 bit
| After this message, the burst ends and the destination may transmit.
| Source address length
| 2 bit
| Length of the source address.
| Destination address length
| 2 bit
| Length of the destination address.
| TX sequence number
| 4 bit
| Sequence number of the currently transmitted packet.
| RX sequence number
| 4 bit
| Sequence number of the packet expected next.
| Source address
| 16, 32, 48 or 64 bit
| HAM-64 address identifying the sending station.
| Destination address length
| 16, 32, 48 or 64 bit
| HAM-64 address identifying the target station.
|===
The total Link Layer Header length is therefore at least 6 byte and at most 18 byte.
[cols="1,2,5", options="header"]
.Message types
|===
| Value | Name | Description
| `000`
| Data frame
| A regular data packet.
| `100`
| Connection management
| Includes functions such as establishing (or denying) new connections or closing open connections.
| _other_
| _reserved_
| All values not explicitly listed are reserved and shall be ignored by receivers.
|===
The length of the source and destination addresses is encoded as follows: `00` = 16 bit, `01` = 32 bit, `10` = 48 bit, `11` = 64 bit.
=== Ideas
To be defined:
- connection establishment procedure (request, response)
- when are new clients allowed to connect?
- handling of packets from unknown clients that are not connection requests
- signal quality handling
- go-back-N algorithm (maybe dynamic burst length depending on previous packet loss?)
== Higher Layer Protocols
[appendix]