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EA2CMN > AX25 29.09.94 06:27l 156 Lines 6122 Bytes #-11689 (0) @ EU
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Subj: AX.25 Doc : Part 3/9
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AX25.DOC PART 3/9
AX.25 Amateur Packet-Radio Link-Layer Protocol
Version 2.0 October 1984
2.2.13.2 Level 2 Repeater-Address Encoding
If a frame is to go through level 2 amateur packet
repeater(s), there is an additional address subfield appended to
the end of the address field. This additional subfield contains
the call sign(s) of the repeater(s) to be used. This allows more
than one repeater to share the same RF channel. If this subfield
exists, the last octet of the source subfield has its address
extension bit set to zero, indicating that more address-field
data follows. The repeater-address subfield is encoded in the
same manner as the destination and source address subfields,
except for the most-significant bit in the last octet, called the
"H" bit. The H bit is used to indicate whether a frame has been
repeated or not.
In order to provide some method of indicating when a
frame has been repeated, the H bit is set to zero on frames going
to a repeater. The repeater will set the H bit to one when the
frame is retransmitted. Stations should monitor the H bit, and
discard any frames going to the repeater (uplink frames), while
operating through a repeater. Fig. 4 shows how the repeater-
address subfield is encoded. Fig. 4A is an example of a complete
frame after being repeated.
Octet ASCII Bin.Data Hex Data
A15 W 10101110 AE
A16 B 10000100 84
A17 4 01101000 68
A18 J 10010100 94
A19 F 10001100 8C
A20 I 10010010 92
A21 SSID HRRSSID1
Bit Order --> 76543210
Fig. 4 -- Repeater-address encoding
Where:
1. The top octet is the first octet sent, with bit 0 being
sent first and bit 7 sent last of each octet.
2. As with the source and destination address subfields
discussed above, bit 0 of each octet is the HDLC address
extension bit, which is set to zero on all but the last
address octet, where it is set to one.
3. The "R" bits are reserved in the same manner as in the
source and destination subfields.
4. The "H" bit is the has-been-repeated bit. It is set to
zero whenever a frame has not been repeated, and set to
one by the repeater when the frame has been repeated.
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 01101000 68
A11 J 10010100 94
A12 F 10001100 8C
A13 I 10010010 92
A14 SSID 01100000 60
A15 W 10101110 AE
A16 B 10000100 84
A17 4 01101000 68
A18 J 10010100 94
A19 F 10001100 8C
A20 I 10010010 92
A21 SSID 11100011 E3
Control I 00111110 3F
PID none 11110000 F0
FCS part 1 XXXXXXXX HH
FCS part 2 XXXXXXXX HH
Flag 01111110 7E
Bit position 76543210
Fig. 4A -- AX.25 frame in repeater mode
The above frame is the same as Fig. 3A, except for the
addition of a repeater-address subfield (WB4JFI, SSID=1). The H
bit is set, indicating this is from the output of the repeater.
2.2.13.3 Multiple Repeater Operation
The link-layer AX.25 protocol allows operation through
more than one repeater, creating a primitive frame routing
mechanism. Up to eight repeaters may be used by extending the
repeater-address subfield. When there is more than one repeater
address, the repeater address immediately following the source
address subfield will be considered the address of the first
repeater of a multiple-repeater chain. As a frame progresses
through a chain of repeaters, each successive repeater will set
the H bit (has-been-repeated bit) in its SSID octet, indicating
that the frame has been successfully repeated through it. No
other changes to the frame are made (except for the necessary
recalculation of the FCS). The destination station can determine
the route the frame took to each it by examining the address
field.
The number of repeater addresses is variable. All but
the last repeater address will have the address extension bits of
all octets set to zero, as will all but the last octet (SSID
octet) of the last repeater address. The last octet of the last
repeater address will have the address extension bit set to one,
indicating the end of the address field.
It should be noted that various timers (see 2.4.7, below)
may have to be adjusted to accommodate the additional delays
encountered when a frame must pass through a multiple-repeater
chain, and the return acknowledgement must travel through the
same path before reaching the source device.
It is anticipated that multiple-repeater operation is a
temporary method of interconnecting stations over large distances
until such time that a layer 3 protocol is in use. Once this
layer 3 protocol becomes operational, repeater chaining should be
phased out.
END AX25.DOC PART 3/9
73's Nacho EA2CMN @EA2CMN.EAZ.ESP.EU
QTH: Zaragoza (Spain)
[27-Sep-1994 15:37]
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