Introduction¶

CAIF is a MUX protocol used by ST-Ericsson cellular modems forcommunication between Modem and host. The host processes can open virtual ATchannels, initiate GPRS Data connections, Video channels and Utility Channels.The Utility Channels are general purpose pipes between modem and host.

ST-Ericsson modems support a number of transports between modemand host. Currently, UART and Loopback are available for Linux.

Architecture¶

The implementation of CAIF is divided into:

• CAIF Socket Layer and GPRS IP Interface.
• CAIF Core Protocol Implementation
• CAIF Link Layer, implemented as NET devices.

RTNL ! !          +------+   +------+ !         +------+!  +------+! !         !  IP  !!  !Socket!! +-------> !interf!+  ! API  !+       <- CAIF Client APIs !         +------+   +------! !            !           ! !            +-----------+ !                  ! !               +------+             <- CAIF Core Protocol !               ! CAIF ! !               ! Core ! !               +------+ !       +----------!---------+ !       !          !         ! !    +------+   +-----+   +------+ +--> ! HSI  !   ! TTY !   ! USB  !   <- Link Layer (Net Devices)      +------+   +-----+   +------+

Implementation¶

Layered Architecture¶

The CAIF protocol can be divided into two parts: Support functions and ProtocolImplementation. The support functions include:

• CFPKT CAIF Packet. Implementation of CAIF Protocol Packet. TheCAIF Packet has functions for creating, destroying and adding contentand for adding/extracting header and trailers to protocol packets.

The CAIF Protocol implementation contains:

• CFCNFG CAIF Configuration layer. Configures the CAIF ProtocolStack and provides a Client interface for adding Link-Layer andDriver interfaces on top of the CAIF Stack.
• CFCTRL CAIF Control layer. Encodes and Decodes control messagessuch as enumeration and channel setup. Also matches request andresponse messages.
• CFSERVL General CAIF Service Layer functionality; handles flowcontrol and remote shutdown requests.
• CFVEI CAIF VEI layer. Handles CAIF AT Channels on VEI (VirtualExternal Interface). This layer encodes/decodes VEI frames.
• CFDGML CAIF Datagram layer. Handles CAIF Datagram layer (IPtraffic), encodes/decodes Datagram frames.
• CFMUX CAIF Mux layer. Handles multiplexing between multiplephysical bearers and multiple channels such as VEI, Datagram, etc.The MUX keeps track of the existing CAIF Channels andPhysical Instances and selects the appropriate instance basedon Channel-Id and Physical-ID.
• CFFRML CAIF Framing layer. Handles Framing i.e. Frame lengthand frame checksum.
• CFSERL CAIF Serial layer. Handles concatenation/split of framesinto CAIF Frames with correct length.

                +---------+                | Config  |                | CFCNFG  |                +---------+                     !+---------+     +---------+     +---------+|   AT    |     | Control |     | Datagram|| CFVEIL  |     | CFCTRL  |     | CFDGML  |+---------+     +---------+     +---------+       \_____________!______________/                     !                +---------+                |   MUX   |                |         |                +---------+                _____!_____               /           \        +---------+     +---------+        | CFFRML  |     | CFFRML  |        | Framing |     | Framing |        +---------+     +---------+             !              !        +---------+     +---------+        |         |     | Serial  |        |         |     | CFSERL  |        +---------+     +---------+

In this layered approach the following “rules” apply.

• All layers embed the same structure “struct cflayer”

• A layer does not depend on any other layer’s private data.

• Layers are stacked by setting the pointers:

  layer->up , layer->dn

• In order to send data upwards, each layer should do:

  layer->up->receive(layer->up, packet);

• In order to send data downwards, each layer should do:

  layer->dn->transmit(layer->dn, packet);

CAIF Socket and IP interface¶

The IP interface and CAIF socket API are implemented on top of theCAIF Core protocol. The IP Interface and CAIF socket have an instance of‘struct cflayer’, just like the CAIF Core protocol stack.Net device and Socket implement the ‘receive()’ function defined by‘struct cflayer’, just like the rest of the CAIF stack. In this way, transmit andreceive of packets is handled as by the rest of the layers: the ‘dn->transmit()’function is called in order to transmit data.