Codec
Introduction
A codec is a device or computer program for encoding or decoding a digital data stream or signal. Codec is a portmanteau of coder-decoder. A codec encodes a data stream or a signal for transmission and storage, possibly in encrypted form, and the decoder function reverses the encoding for playback or editing.
Codec ID table
| Codec 8 | Codec 8 extended | Codec 12 | Codec 13 | Codec 16 |
|---|---|---|---|---|
| 0x08 | 0x8E | 0x0C | 0x0D | 0x10 |
Codec 8
- AVL data packet
Because the smallest information amount that can be written is one bit, there can be some bits left unused when result is byte array. Any unused bits should be left blank.
Below table represents AVL data packet structure.
| 4 zeros | Data field length | Codec ID | Number of Data 1 | AVL Data | Number of Data 2 | CRC-16 |
|---|---|---|---|---|---|---|
| 4 bytes | 4 bytes | 1 byte | 1 byte | 30-147 bytes | 1 byte | 4 bytes |
Number of data – number of encoded data (number of records). Codec ID is constant 08.
Data field length is the length of bytes [codec id, number of data 2]. Number of data 1 should always be equal to number of data 2 byte. CRC-16 is 4 bytes, but first two are zeroes and last two are CRC-16 calculated for [codec id, number of data 2] Minimum AVL packet size is 45 bytes (all IO elements disabled). Maximum AVL packet size for one record is 783 bytes.
- AVL data
| Timestamp | Priority | GPS Element | IO element |
|---|---|---|---|
| 8 bytes | 1 byte | 15 bytes | 6-123 |
Timestamp – difference, in milliseconds, between the current time and midnight, January 1, 1970 UTC
- Priority
| 0 | Low |
| 1 | High |
| 2 | Panic |
- GPS Element
| Longtitude | Latitude | Alitude | Angle | Stellites | Speed |
|---|---|---|---|---|---|
| 4 bytes | 4 bytes | 2 bytes | 2 bytes | 1 bytes | 2 bytes |
X Longitude
Y Latitude1
Altitude In meters above sea level1
Angle In degrees, 0 is north, increasing clock-wise 1
Satellites Number of visible satellites1
Speed Speed in km/h. 0x0000 if GPS data is invalid1
Longitude and latitude are integer values built from degrees, minutes, seconds and milliseconds by formula
d - Degrees
m - Minutes
s - Seconds
ms - Milliseconds
p - Precision (10000000)
If longitude is in west or latitude in south, multiply result by –1. To determine if the coordinate is negative, convert it to binary format and check the very first bit. If it is 0, coordinate is positive, if it is 1, coordinate is negative.
Example:
Received value: 20 9c ca 80 Converted to BIN: 00100000 10011100 11001010 10000000 first bit is 0, which means coordinate is positive Convered to DEC: 547146368 For more information see two‘s compliment arithmetics.
- IO Element
| Event IO ID | 1 byte |
| N of Total IO | 1 byte |
| N1 of One Byte IO | 1 byte |
| 1’st IO ID | 1 byte |
| 1’st IO Value | 1 byte |
| ... | |
| N1’th IO ID | 1 byte |
| N1’th IO Value | 1 byte |
| N2 of Two Bytes | 1 byte |
| 1’st IO ID | 1 byte |
| 1’st IO Value | 2 bytes |
| ... | |
| N2’th IO ID | 1 byte |
| N2’th IO Value | 2 bytes |
| N4 of Four Bytes | 1 byte |
| 1’st IO ID | 1 byte |
| 1’st IO Value | 4 byte |
| ... | |
| N4’th IO ID | 1 byte |
| N4’th IO Value | 4 bytes |
| N8 of Eight Bytes | 1 byte |
| 1’st IO ID | 1 byte |
| 1’st IO Value | 8 bytes |
| ... | |
| N8’th IO ID | 1 byte |
| N8’th IO Value | 8 bytes |
Event IO ID – if data is acquired on event – this field defines which IO property has changed and generated an event. If data cause is not event – the value is 0.
N total number of properties coming with record (N=N1+N2+N4+N8)
N1 number of properties, which length is 1 byte
N2 number of properties, which length is 2 bytes
N4 number of properties, which length is 4 bytes
N8 number of properties, which length is 8 bytes
Example
Received data:
00000000 4 zeroes, 4 bytes
0000008c data length, 4 bytes
08 – Codec ID
0- Number of Data (1 record)
1’st record data
0000013feb55ff74 – Timestamp in milliseconds (1374042849140)
GMT: Wed, 17 Jul 2013 06:34:09 GMT
00 – Priority
GPS Element
0f0ea850 – Longitude 252618832 = 25,2618832º N
209a6900 – Latitude 546990336 = 54,6990336 º E
0094 – Altitude 148 meters
0 – Angle 214º
0 – 12 Visible sattelites
0 – 0 km/h speed
IO Element
00 – IO element ID of Event generated (in this case when 00 – data generated not on event)
1e – 30 IO elements in record (total)
09 – 9 IO elements, which length is 1 Byte
0 – IO element ID = 01
0 – IO element’s value = 0
02 – IO element ID = 02
0 – IO element’s value = 0
03 – IO element ID = 03
0 – IO element’s value = 0
4 – IO element ID = 04
0 – IO element’s value = 0
16 – IO element ID = 22 (dec)
0 – IO element’s value = 1
47 – IO element ID = 71 (dec)
03 – IO element’s value = 3
F0 – IO element ID = 240 (dec)
0 – IO element’s value = 0
15 – IO element ID = 21 (dec)
04 – IO element’s value = 0
C8 – IO element ID = 200 (dec)
0 – IO element’s value = 0
0C – 12 IO elements, which value length is 2 Bytes
09 – IO element ID = 9 (dec)
0073 – IO element’s value
0a – IO element ID = 10 (dec)
0046 – IO element’s value
0b – IO element ID = 11 (dec)
0050 – IO element’s value
13 – IO element ID = 19 (dec)
0046 – IO element’s value
43 – IO element ID = 67 (dec)
06d7 – IO element’s value
1 – IO element ID = 68 (dec)
0 – IO element’s value
B5 – IO element ID = 181 (dec)
000b – IO element’s value
B6 – IO element ID = 182 (dec)
0007 – IO element’s value
42 – IO element ID = 66 (dec)
2e9f – IO element’s value
2 – IO element ID = 24 (dec)
0 – IO element’s value
cd – IO element ID = 205 (dec)
3 – IO element’s value
CE – IO element ID = 206 (dec)
0 – IO element’s value
07 – 7 IO elements, which value length is 4 Bytes
C7 – IO element ID = 199 (dec)
0 – IO element’s value
f1 – IO element ID = 241 (dec)
0000601a – IO element’s value
46 – IO element ID = 70 (dec)
00000134 – IO element’s value
48 – IO element ID = 72 (dec)
00000bb8 – IO element’s value
4 – IO element ID = 73 (dec)
00000bb8 – IO element’s value
4a – IO element ID = 74 (dec)
00000bb8 – IO element’s value
4c – IO element ID = 76 (dec)
1 – IO element’s value
02 – 2 IO elements, which value length is 8 Bytes
4e – IO element ID = 78 (dec)
0 – IO element’s value
cf – IO element ID = 207 (dec)
0 – IO element’s value
0 – Number of Data (1 record)
00003fca - CRC-16, 4 Bytes (first 2 are always zeros)
- SENDING DATA OVER TCP/IP
First when module connects to server, module sends its IMEI. First comes short identifying number of bytes written and then goes IMEI as text (bytes).
For example IMEI 356307042441013 would be sent as 000f333536333037303432343431303133
First two bytes denote IMEI length. In this case 000F means, that imei is 15 bytes long.
After receiving IMEI, server should determine if it would accept data from this module. If yes server will reply to module 01 if not 00. Note that confirmation should be sent as binary packet. I.e. 1 byte 0x01 or 0x00.
Then module starts to send first AVL data packet. After server receives packet and parses it, server must report to module number of data received as integer (four bytes).
If sent data number and reported by server doesn’t match module resends sent data.
Example:
Module connects to server and sends IMEI:
000f333536333037303432343431303133
Server accepts the module:
01
Module sends data packet:
| AVL data packet header | AVL data array | CRC |
|---|---|---|
| Four zero bytes,
‘AVL data array’ length – 254 |
CodecId – 08,
NumberOfData – 2. (Encoded using continuous bit stream. Last byte padded to align to byte boundary) |
CRC of ‘AVL data array’ |
| 00000000000000FE | 0802...(data elements)...02 | 00008612 |
Server acknowledges data reception (2 data elements): 00000002
- SENDING DATA OVER UDP/IP
- UDP channel protocol
UDP channel is a transport layer protocol above UDP/IP to add reliability to plain UDP/IP using acknowledgment packets. The packet structure is as follows:
| UDP datagram | ||
| Example | 2 bytes | Packet length (excluding this field) in big ending byte order |
| Packet Id | 2 bytes | Packet id unique for this channel |
| Packet Type | 1 byte | Type of this packet |
| Packet payload | m bytes | Data payload |
| Header text | Header text | Header text |
|---|---|---|
| Example | Example | Example |
| Example | Example | Example |
| Example | Example | Example |