Dynamically Assigning PSM Values to Support Multiple Upper Protocol
Implementations
Original Post: L2CAP - PSM Value (eGroups Msg. )
Date: 2000-08-24
The L2CAP spec (page. 278) say "PSM values are
separated into two ranges. Values in the first range are assigned by the
Bluetooth SIG and indicate protocols. The second range of values [0x1001 -
0xffff] are dynamically allocated and used in conjunction with the Service
Discovery Protocol (SDP). The dynamically assigned values may be used to
support multiple implementations of a particular protocol, e.g., RFCOMM,
residing on top of L2CAP or for prototyping an experimental
protocol."
Question
So the question is how would you go about deciding
what PSM value to assign to RFCOMM residing on top of L2CAP. For example,
if the connection requires SDP and RFCOMM channel, how can this PSM value
be assigned.
Answer
Normally you would assign the default PSM-value to
the protocols that you have on top of L2CAP (e.g. RFCOMM is 0x0003).
However, you can assign any dynamically assigned number to any protocol.
this is required if you have multiple implementations of RFCOMM for
example, as you have already used up your default PSM, so you must assign
the RFCOMM implementations different (dynamic) PSM values
These PSM values would be handled by the SDP, as you
have to write into your SDP that the protocol with PSM 0xXXXX behaves as a
protocol of type 0xYYYY where 0xYYYY is one of the reserved PSM values.
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