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[aprssig] SSID Standardization

Lynn W. Deffenbaugh (Mr) ldeffenb at homeside.to
Wed Jun 9 23:07:32 UTC 2010


 From the AX.25 spec (all page numbers relative to the ax25.2.2.PDF):

Page 19: 3.12 The SSID is a four-bit integer that uniquely identifies 
multiple stations using the same amateur callsign.

Page 20: The SSID octet at the end of each address subfield (A7 and A14) 
contains the SSID and the “C” bit. The C
bits identify command and response frames (see Section 6.1.2). The SSID 
octet at the end of each optional
Layer 2 repeater address subfield (A21 and A28) contains the SSID and 
the “H” bit (“[H]as-been-repeated”).
The H bits indicate that the Layer 2 repeater station has repeated the 
frame (see Section 3.12.3). Each SSID octet
contains two bits that are reserved for future use.

So, the SSID is an octet (byte) but is only allocated 4 bits (00-15) out 
of the 8 bits. 2 bits are reserved, one is the C and one is the H bit.

Yes, the AX.25 header is the reason for the 00-15 "restriction" on APRS 
SSIDs.

Lynn (D) - KJ4ERJ - Going to the source...

Rudy Benner wrote:
> One BYTE gets you from zero to 255.
>
> --------------------------------------------------
> From: "Jim WU3V" <james at wu3v.net>
> Sent: Wednesday, June 09, 2010 6:28 PM
> To: "TAPR APRS Mailing List" <aprssig at tapr.org>
> Subject: Re: [aprssig] SSID Standardization
>
>> Actually one byte duh!
>>
>> ----- Original Message ----- From: "Jim WU3V" <james at wu3v.net>
>> To: "TAPR APRS Mailing List" <aprssig at tapr.org>
>> Sent: Wednesday, June 09, 2010 22:12
>> Subject: Re: [aprssig] SSID Standardization
>>
>>
>>> The ax.25 protocole had but one bit to assign to an ssid since it is 
>>> a hex
>>> numeral 0-9 and a-f thus 16 ssid'd
>>>
>>> Jim NH0E
>>> ----- Original Message ----- From: "Tim N9PUZ" <tim.n9puz at gmail.com>
>>> To: <aprssig at tapr.org>
>>> Sent: Wednesday, June 09, 2010 22:09
>>> Subject: Re: [aprssig] SSID Standardization
>>>
>>>
>>>>> Standardizing on SSIDs has its merits, but
>>>>> the standards need to be very broad because
>>>>> we only have 16 SSIDs available to us.
>>>>
>>>> Is there a licensing/legal or technical reason that there are only 16?
>>>> Just curious.
>>>>
>>>> Tim, N9PUZ
>>>> (N9PUZ-10 Winlink Gateway)
>>>>




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