RE: [ASN.1] extend bit_mask

[PD20 Ka Fong Tang]

Title: extend bit_mask

Hi,

 

Since nobody answered my question, let me state my question more specifically.  Here are 2 bitstream generated from OSS software for H.245 commands:

    02400106 00088175 00075800 00010001 003C8400 002E0001 800180

and,

    02400106 00088175 00075800 00010001 003C05C0 01800100

 

The first bitstream, if we interpret it bit-by-bit, we have:

02 0         MultimediaSystemControlMessage, no extension
   00        MultimediaSystemControlMessage, choice = 1
   0         RequestMessage, no extension
   0010      RequestMessage, choice = 3
40 0         TerminalCapabilitySet, no extension
   100       TerminalCapabilitySet, option = 0x80
   0000      byte-alignment
01 0000 0001 Sequence Number, value = 1
06 0000 0110 ProtocolIdentifier, length = 6 bytes
00 0000 0000
08 0000 1000
81 1000 0001
75 0111 0101
00 0000 0000
07 0000 0111 ProtocolIdentifier, Id = 0x000881750007
58 0         MultiplexCapability, no extension
   10        MultiplexCapability, choice = 3
   1         H223Capability, with extension
   1         TransportWithI_frames, TRUE
   0         VideoWithAL1, FALSE
   0         VideoWithAL2, FALSE
   0         VideoWithAL3, FALSE
00 0         AudioWithAL1, FALSE
   0         AudioWithAL2, FALSE
   0         AudioWithAL3, FALSE
   0         DataWithAL1, FALSE
   0         DataWithAL2, FALSE
   0         DataWithAL3, FALSE
   00        byte-alignment
00 0000 0000
01 0000 0001 MaximumAl2SDUSize, value = 1
00 0000 0000
01 0000 0001 MaximumAl3SDUSize, value = 1
00 0000 0000
3C 0011 1100 MaximumDelayJitter, value = 60
84 1         H223MultiplexTableCapability, choice = 2
   0         Enhanced, no extension
   0001      MaximumNestingDepth, value = 2
   00        byte-alignment
00 0000 0000 MaximumElementListSize, value = 2
00 0000 0000 MaximumSubElementListSize, value = 2
2E 0010 1    Indicates 6 extension elements <-- 5 bits long***
   110
00 000       Extension option = 0xC0
   00000     byte-alignment
01 0000 0001 1 byte for the next extension element
80 1         MaxMUXPDUSizeCapablility, TRUE
   000 0000  byte-alignment
01 0000 0001 1 byte for the next extension element
80 1         NsrpSupport, TRUE
   000 0000  byte-alignment

 

For the second bitstream, the first 16 bytes are identical to the first one.  We may look at the last 8 bytes and we have:
00 0000 0000
3C 0011 1100 MaximumDelayJitter, value = 60
05 0         H223MultiplexTableCapability, choice = 1
             basic
   000 0101  Indicates 6 extension elements <-- 7 bits long***
C0 110000    Extension option = 0xC0
   00        byte-alignment
01 0000 0001 1 byte for the next extension element
80 1         MaxMUXPDUSizeCapablility, TRUE
   000 0000  byte-alignment
01 0000 0001 1 byte for the next extension element
00 0         NsrpSupport, FALSE
   000 0000  byte-alignment

Compare those bits before extension option or bit_mask (which I mark *** up there).  Does anybody know how to determine how many bits should we use while encoding?  5 bits or 7 bits?  Why?

Ka-Fong Tang

-----Original Message-----
From: PD20 Ka Fong Tang
Sent: Tuesday, May 08, 2001 10:31 AM
To: [EMAIL PROTECTED]
Subject: [ASN.1] extend bit_mask

Hi,

I have a question about extensible bit_mask of Sequence Type when extend bit is present.  In the encoded ASN.1 bitstream, when a Sequence Type has an extend bit = 1, there will be several (usually, 5 or 7) bits before extensible bit_mask indicate the number of extensible elements in the current structure.  How do we determine if it is 5 or 7 or any other number?  Does it have a rule?

                                        Ka-Fong Tang