doc: start defining the layer 2
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@ -106,6 +106,86 @@ The filter is applied to the whole symbol sequence of the burst. The same filter
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== Layer 2: Link Layer
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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.
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=== Node Addresses
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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.
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=== Link Layer Header
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Each layer 2 packet contains a header that identifies how the message should be interpreted. It is of variable length and composed as follows:
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[cols="2,1,5", options="header"]
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.Link layer header layout
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|===
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| Field | Length | Description
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| Message type
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| 3 bit
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| The type of the message. See below for a list of values.
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| TX request
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| 1 bit
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| After this message, the burst ends and the destination may transmit.
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| Source address length
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| 2 bit
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| Length of the source address.
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| Destination address length
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| 2 bit
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| Length of the destination address.
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| TX sequence number
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| 4 bit
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| Sequence number of the currently transmitted packet.
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| RX sequence number
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| 4 bit
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| Sequence number of the packet expected next.
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| Source address
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| 16, 32, 48 or 64 bit
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| HAM-64 address identifying the sending station.
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| Destination address length
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| 16, 32, 48 or 64 bit
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| HAM-64 address identifying the target station.
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|===
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The total Link Layer Header length is therefore at least 6 byte and at most 18 byte.
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[cols="1,2,5", options="header"]
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.Message types
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|===
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| Value | Name | Description
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| `000`
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| Data frame
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| A regular data packet.
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| `100`
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| Connection management
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| Includes functions such as establishing (or denying) new connections or closing open connections.
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| _other_
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| _reserved_
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| All values not explicitly listed are reserved and shall be ignored by receivers.
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|===
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The length of the source and destination addresses is encoded as follows: `00` = 16 bit, `01` = 32 bit, `10` = 48 bit, `11` = 64 bit.
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=== Ideas
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To be defined:
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- connection establishment procedure (request, response)
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- when are new clients allowed to connect?
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- handling of packets from unknown clients that are not connection requests
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- signal quality handling
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- go-back-N algorithm (maybe dynamic burst length depending on previous packet loss?)
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== Higher Layer Protocols
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[appendix]
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