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
UDP channel packet x N	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

Packet Type
1	Data packet requiring acknowledgment

Acknowledgment packet should have the same packet id as acknowledged data packet and empty data payload. Acknowledgement should be sent in binary format

Acknowledgment packet
Packet length	2 bytes	0x0003
Packet id	2 bytes	same as in acknowledged packet
Packet type	1 byte	0x02

• Sending AVL data using UDP channel

AVL data are sent encapsulated in UDP channel packets (Data payload field).

AVL data encapsulated in UDP channel packet
AVL packet id (1 byte)	Module IMEI	AVL data array