Appendix A
Addressed Serial Protocol Specification
Call Direct Cellular Solutions have implemented the following protocol to
Allow a master station to communicate with a number of remote stations
via GPRS.
In particular the master station must be able to identify individual
remote stations sending data to the master and in turn be able to direct
transmissions to a specific remote station.
In order to provide this addressability the following protocol is
defined.
All data will be sent encapsulated in HDLC like frames with the IP
address of the remote station (in standard network byte order) as the
first 4 information bytes of each frame, application data starts at the
5th byte of the information field of each frame.
Frame Format
A summary of the HDLC-like frame structure is shown below
.
This
figure does not include bits inserted for synchronization (such as
start and stop bits for asynchronous links), nor any bits or octets
inserted for transparency. The fields are transmitted from left to
right.
+----------+----------+-------------+----------+
| Flag | Protocol | Information | Flag |
| 01111110 | 8/16 bits| * | 01111110 |
+----------+----------+-------------+----------+
NB The standard HDLC Address, Control and FCS fields are not used
or present.
Flag Field
The Flag Sequence indicates the beginning or end of a frame, and is
used for frame synchronization. The bit stream is examined for the
binary sequence 01111110 (hexadecimal 0x7e).
Transparency
An octet stuffing procedure is used. The Control Escape octet is
defined as binary 01111101 (hexadecimal 0x7d), most significant bit
first.
Each Flag Sequence and Control Escape octet, is replaced by a two
octet sequence consisting of the Control Escape octet followed by
the original octet exclusive-or'd with hexadecimal 0x20.
Escaped data is transmitted on the link as follows:
0x7e is encoded as 0x7d, 0x5e. (Flag Sequence)
0x7d is encoded as 0x7d, 0x5d. (Control Escape)
Protocol Field
The Protocol field is consistent with the ISO 3309 (HDLC) extension
mechanism for Address fields. All Protocols are assigned such that
the least significant bit of the most significant octet equals "0",
CDM800 Series V.250 Manual
Page 32 of 33
To suit software version 1.52.2
4/4/2008
