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Based on kernel version 3.16. Page generated on 2014-08-06 21:40 EST.

1	Linux CAIF
2	===========
3	copyright (C) ST-Ericsson AB 2010
4	Author: Sjur Brendeland/ sjur.brandeland@stericsson.com
5	License terms: GNU General Public License (GPL) version 2
6	
7	
8	Introduction
9	------------
10	CAIF is a MUX protocol used by ST-Ericsson cellular modems for
11	communication between Modem and host. The host processes can open virtual AT
12	channels, initiate GPRS Data connections, Video channels and Utility Channels.
13	The Utility Channels are general purpose pipes between modem and host.
14	
15	ST-Ericsson modems support a number of transports between modem
16	and host. Currently, UART and Loopback are available for Linux.
17	
18	
19	Architecture:
20	------------
21	The implementation of CAIF is divided into:
22	* CAIF Socket Layer and GPRS IP Interface.
23	* CAIF Core Protocol Implementation
24	* CAIF Link Layer, implemented as NET devices.
25	
26	
27	  RTNL
28	   !
29	   !	      +------+	 +------+
30	   !	     +------+!	+------+!
31	   !	     !	IP  !!	!Socket!!
32	   +-------> !interf!+	! API  !+	<- CAIF Client APIs
33	   !	     +------+	+------!
34	   !		!	    !
35	   !		+-----------+
36	   !		      !
37	   !		   +------+		<- CAIF Core Protocol
38	   !		   ! CAIF !
39	   !		   ! Core !
40	   !		   +------+
41	   !	   +----------!---------+
42	   !	   !	      !		!
43	   !	+------+   +-----+   +------+
44	   +--> ! HSI  !   ! TTY !   ! USB  !	<- Link Layer (Net Devices)
45		+------+   +-----+   +------+
46	
47	
48	
49	I M P L E M E N T A T I O N
50	===========================
51	
52	
53	CAIF Core Protocol Layer
54	=========================================
55	
56	CAIF Core layer implements the CAIF protocol as defined by ST-Ericsson.
57	It implements the CAIF protocol stack in a layered approach, where
58	each layer described in the specification is implemented as a separate layer.
59	The architecture is inspired by the design patterns "Protocol Layer" and
60	"Protocol Packet".
61	
62	== CAIF structure ==
63	The Core CAIF implementation contains:
64	      -	Simple implementation of CAIF.
65	      -	Layered architecture (a la Streams), each layer in the CAIF
66		specification is implemented in a separate c-file.
67	      -	Clients must call configuration function to add PHY layer.
68	      -	Clients must implement CAIF layer to consume/produce
69		CAIF payload with receive and transmit functions.
70	      -	Clients must call configuration function to add and connect the
71		Client layer.
72	      - When receiving / transmitting CAIF Packets (cfpkt), ownership is passed
73		to the called function (except for framing layers' receive function)
74	
75	Layered Architecture
76	--------------------
77	The CAIF protocol can be divided into two parts: Support functions and Protocol
78	Implementation. The support functions include:
79	
80	      - CFPKT CAIF Packet. Implementation of CAIF Protocol Packet. The
81		CAIF Packet has functions for creating, destroying and adding content
82		and for adding/extracting header and trailers to protocol packets.
83	
84	The CAIF Protocol implementation contains:
85	
86	      - CFCNFG CAIF Configuration layer. Configures the CAIF Protocol
87		Stack and provides a Client interface for adding Link-Layer and
88		Driver interfaces on top of the CAIF Stack.
89	
90	      - CFCTRL CAIF Control layer. Encodes and Decodes control messages
91		such as enumeration and channel setup. Also matches request and
92		response messages.
93	
94	      - CFSERVL General CAIF Service Layer functionality; handles flow
95		control and remote shutdown requests.
96	
97	      - CFVEI CAIF VEI layer. Handles CAIF AT Channels on VEI (Virtual
98		External Interface). This layer encodes/decodes VEI frames.
99	
100	      - CFDGML CAIF Datagram layer. Handles CAIF Datagram layer (IP
101		traffic), encodes/decodes Datagram frames.
102	
103	      - CFMUX CAIF Mux layer. Handles multiplexing between multiple
104		physical bearers and multiple channels such as VEI, Datagram, etc.
105		The MUX keeps track of the existing CAIF Channels and
106		Physical Instances and selects the appropriate instance based
107		on Channel-Id and Physical-ID.
108	
109	      - CFFRML CAIF Framing layer. Handles Framing i.e. Frame length
110		and frame checksum.
111	
112	      - CFSERL CAIF Serial layer. Handles concatenation/split of frames
113		into CAIF Frames with correct length.
114	
115	
116	
117			    +---------+
118			    | Config  |
119			    | CFCNFG  |
120			    +---------+
121				 !
122	    +---------+	    +---------+	    +---------+
123	    |	AT    |	    | Control |	    | Datagram|
124	    | CFVEIL  |	    | CFCTRL  |	    | CFDGML  |
125	    +---------+	    +---------+	    +---------+
126		   \_____________!______________/
127				 !
128			    +---------+
129			    |	MUX   |
130			    |	      |
131			    +---------+
132			    _____!_____
133			   /	       \
134		    +---------+	    +---------+
135		    | CFFRML  |	    | CFFRML  |
136		    | Framing |	    | Framing |
137		    +---------+	    +---------+
138			 !		!
139		    +---------+	    +---------+
140		    |	      |	    | Serial  |
141		    |	      |	    | CFSERL  |
142		    +---------+	    +---------+
143	
144	
145	In this layered approach the following "rules" apply.
146	      - All layers embed the same structure "struct cflayer"
147	      - A layer does not depend on any other layer's private data.
148	      - Layers are stacked by setting the pointers
149			  layer->up , layer->dn
150	      -	In order to send data upwards, each layer should do
151			 layer->up->receive(layer->up, packet);
152	      - In order to send data downwards, each layer should do
153			 layer->dn->transmit(layer->dn, packet);
154	
155	
156	CAIF Socket and IP interface
157	===========================
158	
159	The IP interface and CAIF socket API are implemented on top of the
160	CAIF Core protocol. The IP Interface and CAIF socket have an instance of
161	'struct cflayer', just like the CAIF Core protocol stack.
162	Net device and Socket implement the 'receive()' function defined by
163	'struct cflayer', just like the rest of the CAIF stack. In this way, transmit and
164	receive of packets is handled as by the rest of the layers: the 'dn->transmit()'
165	function is called in order to transmit data.
166	
167	Configuration of Link Layer
168	---------------------------
169	The Link Layer is implemented as Linux network devices (struct net_device).
170	Payload handling and registration is done using standard Linux mechanisms.
171	
172	The CAIF Protocol relies on a loss-less link layer without implementing
173	retransmission. This implies that packet drops must not happen.
174	Therefore a flow-control mechanism is implemented where the physical
175	interface can initiate flow stop for all CAIF Channels.
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