| |
EA2CMN > AX25 29.09.94 06:27l 187 Lines 6758 Bytes #-11689 (0) @ EU
BID : 41773_EA2CMN
Read: DJ9SS DB9CP DL2NAT DK9OS DG6MAY DG3SFT DG2MNF DG1MTG DG6MHZ DH1NFJ GUEST
Subj: AX.25 Doc : Part 2/9
Path: DB0KCP<HB9OS<HB9EAS<DB0GE<F6KVE<F6KLZ<F5KSF<F8KLY<F6BIG<F6KDC<F1HAQ<
F6FBB<F5JGK<EB2CYQ<EA2CMO<EA2CMN
Sent: 940927/1433Z @:EA2CMN.EAZ.ESP.EU #:41773 [Zaragoza] Bid:41773_EA2CMN FBB5
De : EA2CMN@EA2CMN.EAZ.ESP.EU
A : AX25@EU
AX25.DOC PART 2/9
AX.25 Amateur Packet-Radio Link-Layer Protocol
Version 2.0 October 1984
2.2.10 Frame Abort
If a frame must be prematurely aborted, at least fifteen
contiguous ones shall be sent with no bit stuffing added.
2.2.11 Interframe Time Fill
Whenever it is necessary for a DXE to keep its
transmitter on while not actually sending frames, the time
between frames should be filled with contiguous flags.
2.2.12 Link Channel States
Not applicable.
2.2.13 Address-Field Encoding
The address field of all frames shall be encoded with
both the destination and source amateur call signs for the frame.
Except for the Secondary Station Identifier (SSID), the address
field should be made up of upper-case alpha and numeric ASCII
characters only. If level 2 amateur "repeaters" are to be used,
their call signs shall also be in the address field.
The HDLC address field is extended beyond one octet by
assigning the least-significant bit of each octet to be an
"extension bit". The extension bit of each octet is set to zero,
to indicate the next octet contains more address information, or
one, to indicate this is the last octet of the HDLC address
field. To make room for this extension bit, the amateur Radio
call sign information is shifted one bit left.
2.2.13.1 Nonrepeater Address-Field Encoding
If level 2 repeaters are not being used, the address
field is encoded as shown in Fig. 2. The destination address is
the call sign and SSID of the amateur radio station to which the
frame is addressed, while the source address contains the amateur
call sign and SSID of the station that sent the frame. These
call signs are the call signs of the two ends of a level 2 AX.25
link only.
First
Octet Sent
Address Field of Frame
Destination Address Source Address
A1 A2 A3 A4 A5 A6 A7 A8 A9 A10 A11 A12 A13 A14
Fig. 2 -- Nonrepeater Address-Field Encoding
A1 through A14 are the fourteen octets that make up the
two address subfields of the address field. The destination
subaddress is seven octets long (A1 thru A7), and is sent first.
This address sequence provides the receivers of frames time to
check the destination address subfield to see if the frame is
addressed to them while the rest of the frame is being received.
The source address subfield is then sent in octets A8 through
A14. Both of these subfields are encoded in the same manner,
except that the last octet of the address field has the HDLC
address extension bit set.
There is an octet at the end of each address subfield
that contains the Secondary Station Identifier (SSID). The SSID
subfield allows an Amateur Radio operator to have more than one
packet-radio station operating under the same call sign. This is
useful when an amateur wants to put up a repeater in addition to
a regular station, for example. The C bits (see 2.4.1.2, below)
and H bit (see 2.2.13.2, below) are also contained in this octet,
along with two bits which are reserved for future use.
Fig. 3A shows a typical AX.25 frame in the nonrepeater
mode of operation.
Octet ASCII Bin.Data Hex Data
Flag 01111110 7E
A1 K 10010110 96
A2 8 01110000 70
A3 M 10011010 9A
A4 M 10011010 9A
A5 O 10011110 9E
A6 space 01000000 40
A7 SSID 11100000 E0
A8 W 10101110 AE
A9 B 10000100 84
A10 4 01100100 68
A11 J 10010100 94
A12 F 10001100 8C
A13 I 10010010 92
A14 SSID 01100001 61
Control I 00111110 3E
PID none 11110000 F0
FCS part 1 XXXXXXXX HH
FCS part 2 XXXXXXXX HH
Flag 01111110 7E
Bit position 76543210
Fig. 3A -- Nonrepeater AX.25 frame
The frame shown is an I frame, not going through a level
2 repeater, from WB4JFI (SSID=0) to K8MMO (SSID=0), with no level
3 protocol. The P/F bit is set; the receive sequence number
(N(R)) is 1; the send sequence number (N(S)) is 7.
2.2.13.1.1 Destination Subfield Encoding
Fig. 3 shows how an amateur call sign is placed in the
destination address subfield, occupying octets A1 thru A7.
Octet ASCII Bin.Data Hex Data
A1 W 10101110 AE
A2 B 10000100 84
A3 4 01101000 68
A4 J 10010100 94
A5 F 10001100 8C
A6 I 10010010 92
A7 SSID CRRSSID0
Bit Position--> 76543210
Fig. 3 -- Destination Field Encoding
Where:
1. The top octet (A1) is the first octet sent, with bit
0 of each octet being the first bit sent, and bit 7
being the last bit sent.
2. The first (low-order or bit 0) bit of each octet is
the HDLC address extension bit, which is set to zero
on all but the last octet in the address field, where
it is set to one.
3. The bits marked "r" are reserved bits. They may be
used in an agreed-upon manner in individual networks.
When not implemented, they should be set to one.
4. The bit marked "C" is used as the command/response
bit of an AX.25 frame, as outlined in 2.4.1.2 below.
5. The characters of the call sign should be standard
seven-bit ASCII (upper case only) placed in the
leftmost seven bits of the octet to make room for the
address extension bit. If the call sign contains
fewer than six characters, it should be padded with
ASCII spaces between the last call sign character and
the SSID octet.
6. The 0000 SSID is reserved for the first personal
AX.25 station. This establishes one standard SSID for
"normal" stations to use for the first station.
END AX25.DOC PART 2/9
----------------------
73's Nacho EA2CMN @EA2CMN.EAZ.ESP.EU
QTH: Zaragoza (Spain)
[27-Sep-1994 15:31]
Read previous mail | Read next mail
| |
