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Short technical overview of isdn4bsd
====================================
Copyright (c) 1998 Hellmuth Michaelis. All rights reserved.
$Id: Overview,v 1.3 1998/10/27 10:28:54 hm Exp $
Last edit-date: [Tue Oct 27 11:26:03 1998]
-hm starting an overview ...
Contents:
---------
Functional block diagram
Layer 1
Layer 2
Layer 3
Debugging control
Layer 4
ISDN protocol trace
Functional block diagram
========================
isdndebug isdnd isdntrace
+-------+ +----------------------------------------------------+ +--------+
| | | | | |
| | | | | |
+-------+ +----------------------------------------------------+ +--------+
| | |
| | |
| /dev/i4bctl Userland | /dev/i4b /dev/i4btrc<n> |
===============================================================================
| Kernel | |
| | |
+-------+ +----------------------------------------------------+ +--------+
| | | | | |
|i4bctl | | i4b | | i4btrc |
| (6) | | (7) | | (8) |
| debug | | Layer 4 - common call control interface | | ISDN |
|control| | | | trace |
+:-:-:--+ +----------------------------------------------------+ +--------+
: : : ^ ^ ^
: : : Call | various ptr arrays | Call %
. . . Control | in i4b_l3l4.h | Control %
V V %
+----------------------+ +----------------------+ %
| | | | %
| i4bq931 | ISDN | active card | %
| (5) | ##### | %
| Layer 3 (Q.931) | # | driver | %
| | # | | %
+----------------------+ # +----------------------+ %
^ # B + %
| i4b_l2l3_func function # | + +------------+ %
| ptr array in i4b_l2l3.h # C +++++ isp |----> %
V # h + +------------+ IP %
+----------------------+ # a + Subsys %
| | # n + +------------+ %
| i4bq921 | # n +++++ ipr |----> %
| (4) | # e + +------------+ IP %
| Layer 2 (Q.921) | # l + Subsys %
| | # + +------------+ %
+----------------------+ # D +++++ tel/rbch |----> %
^ # a + +------------+ to %
| i4b_l1l2_func function # t + /dev/i4btel<n> %
| ptr array in i4b_l1l2.h # a + or /dev/i4brbch<n>%
V # + %
+----------------------+ # +---------------------+ %
| | # | | %
| isic (ISAC part) | D-ch trace # | isic (HSCX part) |B-ch%
| (2) |%%%%%%%%%%%% # | (3) |%%%%%
| Layer 1 (I.430) | % # | non-HDLC / HDLC |trc %
| | % # | | %
+----------------------+ % # +---------------------+ %
^ % # ^ %
D-channel | % # B-channels | %
+-----------------------------------------------+ %
| function ptr in % # %
| in isic_softc in %%%%%%%%#%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
V i4b_l1.h #
+----------------------+ #
| | #
| isic | #
| (1) | #
| Card hardware driver | #
| for Teles, AVM, USR | #
| | #
+----------#-----------+ #
# #
# #
# #
# #
|=========#===============================#============================|
ISDN S0 bus
Layer 1
=======
(1) A driver for a special card hardware consists of a
- probe routine
- attach routine
- FIFO read
- FIFO write
- register read
- register write
routines. These routines handle the card/manufacturer specific stuff
required to talk to a particular card.
This addresses of the read/write routines are put into a arrays found
in struct isic_softc and they are later called by the macros:
ISAC_READ(r), ISAC_WRITE(r,v), ISAC_RDFIFO(b,s), ISAC_WRFIFO(b,s),
HSCX_READ(n,r), HSCX_WRITE(n,r,v), HSCX_RDFIFO(n,b,s), HSCX_WRFIFO(n,b,s)
(see file layer1/i4b_l1.h)
Files currently used for this purpose are
- i4b_avm_a1.c AVM A1 and AVM Fritz!Card drivers
- i4b_ctx_s0P.c Creatix S0 PnP (experimental!)
- i4b_tel_s016.c Teles S0/16 and clones
- i4b_tel_s0163.c Teles S0/16.3
- i4b_tel_s08.c Teles S0/8 and clones
- i4b_tel_s0P.c Teles S0/16 PnP (experimental!)
- i4b_usr_sti.c 3Com USRobotics Sportster
(2) The files i4b_isac.c and i4b_isac.h contain the code to control the
ISAC chip by using the above mentioned macros.
Files i4b_l1.c and i4b_l1.h handle stuff used to access layer 1
functions from layer 2.
Layer 1 and layer 2 access functionality of each other by using
a well known function pointer array, which contains addresses of
"primitives" functions which are defined in I.430 and Q.921. The
function pointer array for layer 1/2 communication is defined in
file include/i4b_l1l2.h and is initialized i4b_l1.c at the very
beginning.
File i4b_isic.c contains the main code for the "isic" device driver.
i4b_l1fsm.c is the heart of layer 1 containing the state machine which
implements the protocol described in I.430 and the ISAC data book.
(3) All above code is used for handling of the D channel, the files
i4b_bchan.c, i4b_hscx.c and i4b_hscx.h contain the code for handling
the B-channel, the HSCX is used to interface the userland drivers
isp, ipr, tel and rbch to one of the B-channels and i4b_hscx.c and
i4b_hscx.h contain the code to handle it (also by using the above
mentioned macros). i4b_bchan.c contains various maintenance code for
interfacing to the upper layers.
Layer 2
=======
(4) Layer 2 implements the LAPD protocol described in Q.920/Q.921. Layer 2
interfaces to layer 1 by the above described function pointer array,
where layer 1 calls layer 2 functions to provide input to layer 2 and
layer 2 calls layer 1 functions to feed data to layer 1.
The same mechanism is used for layer 2 / layer 3 communication, the
pointer array interface is defined in include/i4b_l2l3.h ad the array
is initialized at the very beginning of i4b_l2.c which also contains
some layer 1 and some layer 3 interface routines. As with l1/l2, the
l2/l3 array also contains addresses for "primitives" functions which
are specified in Q.920/Q.921 and Q.931.
i4b_l2.h contains the definition of l2_softc_t, which describes the
complete state of a layer 2 link between the exchange and the local
terminal equipment.
i4b_l2.c contains the entrance of data from layer 1 into the system,
which is split up in i4b_ph_data_ind() into the 3 classes of layer 2
frames called S-frame, I-frame and U-frame. They are handled in files
i4b_sframe.c, i4b_iframe.c and i4b_uframe.c together with the respective
routines to send data with each ones frame type.
i4b_l2timer.c implements the timers required by Q.921.
i4b_tei.c contains the TEI handling routines.
i4b_lme.c implements a rudimentary layer management entity.
i4b_util.c implements the many utility functions specified
in Q.921 together wit some misc routines required for overall
functionality.
i4b_mbuf.c handles all (!) requests for mbufs and frees all mbufs used
by the whole isdn4bsd kernel part. It should probably be moved else-
where.
i4b_l2fsm.c and i4b_l2fsm.h contain the heart of layer 2, the state-
machine implementing the protocol as specified in Q.921.
Layer 3
=======
(5) i4b_l2if.c and i4b_l4if.c contain the interface routines to communicate
to layer 2 and layer 4 respectively.
i4b_l3timer.c implements the timers required by layer 3.
i4b_q931.c and i4b_q931.h implement the message and information element
decoding of the Q.931 protocol.
i4b_q932fac.c and i4b_q932fac.h implement a partial decoding of facility
messages and/or information elements; the only decoding done here is
the decoding of AOCD and AOCE, advice of charge during and at end of
call.
As usual, i4b_l3fsm.c and i4b_l3fsm.h contain the state machine required
to handle the protocol as specified in Q.931.
Layer 3 uses a structure defined in include/i4b_l3l4.h to store and
request information about one particular isdncontroller, it is called
ctrl_desc_t (controller descriptor). It contains information on the
state of a controller (controller ready/down and which B channels are
used or idle) as well as a pointer array used for communication of
layer 4 with layer 3: layer 3 "knows" the routines to call within
layer 4 by name, but in case layer 4 has to call layer 3, several
possibilities exist (i.e. active / passive cards) so it has to call
the routines which the ISDN controller had put into the the function
pointer array (N_CONNECT_REQUEST, N_CONNECT_RESPONSE etc) at init time.
Layer 3 shares a structure called call_desc_t (call descriptor) with
layer 4. This structure is used to describe the state of one call. The
reference to layer 3 is the Q.931 call reference value, the reference to
layer 4 (and the isdn daemon, isdnd) is the cdid, an unique integer
value uniquely describing one call, the call descriptor id.
This structure is used to build an array of this structures
(call_desc[N_CALL_DESC]), which must be large enough to hold as many
calls as there are B channels in the system PLUS a reserve to be able
to handle incoming SETUP messages although all channels are in use.
More, this structure contains the so called "link table pointers"
(isdn_link_t *ilt and drvr_link_t *dlt) which contain function pointers
to "link" a B-channel (better the addresses of functions each participant
needs to access each others functionality) after a successful call setup
to a userland driver (such as isp, ipr, rbch or tel) to exchange user
data in the desired protocol and format.
Debugging control
=================
(6) the device driver for /dev/i4bctl in conjunction with the userland
program isdndebug(8) is used to set the debug level for each of the
layers and several other parts of the system, information how to use
this is contained in machine/i4b_debug.h and all parts of the kernel
sources. It is only usable for passive cards.
Layer 4
=======
(7) Layer 4 is "just" an abstraction layer used to shield the differences
of the various possible Layer 3 interfaces (passive cards based on
Siemens chip-sets, passive cards based on other chip-sets, active cards
from different manufacturers using manufacturer-specific interfaces)
and to provide a uniform interface to the isdnd userland daemon, which
is used to handle all the required actions to setup and close calls
and to the necessary retry handling and management functionality.
Layer 4 communicates with the userland by using a well defined protocol
consisting of "messages" sent to userland and which are read(2) by the
isdnd. The isdnd in turn sends "messages" to the kernel by using the
ioctl(2) call. This protocol and the required messages for both
directions are documented in the machine/i4b_ioctl.h file and are
implemented in files i4b_i4bdrv.c and i4b_l4.c, the latter also
containing much of the Layer 4 interface to the lower layers.
i4b_l4mgmt.c contains all the required routines to manage the above
mentioned call descriptor id (cdid) in conjunction with the call
descriptor (array) and the call reference seen from layer 3.
i4b_l4timer.c implements a timeout timer for Layer 4.
ISDN protocol trace
===================
(8) ISDN D-channel protocol trace for layers 2 and 3 is possible by using
hooks in the ISAC handling routines.
In case D-channel trace is enabled, every frame is prepended with a
header containing further data such as a time stamp and sent via the
i4btrc driver found in driver/i4b_trace.c to one of the /dev/i4btrc<n>
devices, where <n> corresponds to a passive controller unit number.
If desired, B-channel data can be made available using the same
mechanism - hooks in the HSCX handler send data up to the i4btrc
device.
The raw data is then read by the isdntrace userland program which
decodes the layer 2 and/or layer 3 protocol and formats it to be
easily readable by the user.
B-channel data is not interpreted but dumped as a hex-dump.
/* EOF */
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