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AL0Y > TECH 21.12.20 03:35l 29 Lines 2312 Bytes #999 (0) @ WW
BID : 620_AL0Y
Read: GUEST
Subj: Packet layers
Path: IW8PGT<IZ3LSV<IK5FKA<IK5FKA<PE1RRR<AL0Y
Sent: 201221/0134Z @:AL0Y.#NNJ.NJ.USA.NOAM #:620 [WAYNE] $:620_AL0Y
From: AL0Y@AL0Y.#NNJ.NJ.USA.NOAM
To : TECH@WW
There are several "layers" which go into moving data across packet networks. From what I understand, we normally see the following 4 layers.
Layer 1 - Audio Tones
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Layer 1 is the actual audio tones you hear over the air. For 1200 Baud, it's 1200 and 2200hz. When using 300 Baud on HF, the tones will vary, but generally are 1600hz and 1800hz. Both 300 and 1200 Baud have center tones (midpoint between mark and space) of 1700hz.
Layer 2 - AX.25
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Layer 2 is the basis of the packet connection (AX.25). It's headers contain a source and destination call (or destination address if unproto, like an ID, Beacon Nodes Broadcast or Axxnnn in APRS). In it's simplest form, a Layer 2 connection can be between two people and have a keyboard to keyboard conversation. Likewise, a user to your BBS is a layer 2 connection. It starts with a [C]onnection or SABM frame. The response to this would be an unnumbered acknowledgment (UA). At this point, packet sequencing and checksums are active, which ensures all the frames arrive error free and in order. If there's an issue with the frame a [REJ]ect frame is sent and the packet is resent, etc. Good technical documentation on Layer 2 exist here: http://www.tapr.org/pub_ax25.html
Layer 3 - Net/Rom peer to peer
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Layer 3 is the introduction of Net/Rom on top of (AX.25). If you watch a connection from one Net/Rom node to another, you will see the calling node establish a Layer 2 connection to the destination node. Once this Layer 2 connection is active, you will then see Net/Rom traffic similar to AX.25 (Connection requests), but this connection request actually travels along a data packet, not a [C]onnection or SABM Layer 2 frame. The entire Layer 3 session will ride "on top of" the established Layer 2 session. Just like you can have multiple Layer 2 connections, you can have parallel Layer 3 connections within a single Layer 2 connection.
Layer 4 - Net/Rom pass through
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Layer 4 is the "routing" portion of Net/Rom. You will have a Layer 4 link when there is one or more intermediate nodes between the calling node and destination node.
Original from:
http://ohiopacket.org/index.php/Packet_Layers
73 de AL0Y
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