X.25 can be monitored in the VHF and UHF bands. This mode uses indirect FM (sub carrier) modulation.
|
Parameter |
Value |
|
Frequency range |
VHF/UHF |
|
Operation modes |
|
|
Modulation |
FM, SUB FSK |
|
Symbol rate |
300, 600 and 1200 Bd |
|
Center frequency |
1700 Hz |
|
Shift |
1000 Hz |
|
Receiver settings |
FM BW = 12 kHz |
|
Input format(s) |
AF, IF |
|
Additional Info |
ITA-5 with block coding |
To start X.25, select 1200 Bd, 600 Bd or 300 Bd. The correct shift or center may be set in the Demodulator menu. Standard values for the center frequency and the shift are 1700 Hz and 1000 Hz.
X.25 uses a HDLC computer network protocol. X.25 is a synchronous system in which data is transmitted in packets of 8 bit (octets).
X.25 provides three frame formats, the difference being the size of the sequence number.
The Basic frame has a frame sequence number size of 3 bits, so that the increment of the sequence number is a modulo 8 operation. In case of the Extended frame, the increment is a modulo 128 operation and in case of the Super frame a modulo 32768 operation.
Packet structure
A X.25 packet constitutes the data field of a LAPB (HDLC) frame with a size of 64 to 4096 bytes.
X.25 PLP encapsulation in a LAPB frame (Cisco “Internetworking Handbook”).
The GFI (General Format Identifier) field contains general information of the packet format, the LCI (Logical Channel Identifier) field contains information on the logical channel and the PTI (Packet Type Identifier) field contains information on the PLP packet type.
The WAVECOM implementation of the X.25 decoder features some options that can be selected:
Options | Frame Format… offers the decoding of Basic, Extended or Super frames.
Options | Display Mode… offers the display of All Frames or Error free Frames.
Options | Display… selects the kind (RAW, FULLFRAME, DATA ONLY) of data which ar to be displayed
Ø RAW: The content of the frame is not decoded, only the byte values, starting with the address-field, are displayed (hex being the best choice). In this way signals can be displayed which only to some degree conform to the standard.
Ø FULL FRAME: This is the general case in
which header data as well as user data are displayed.
User data ar displayed
in the text window and frame and packet headers are displayed in the status
lines as well as in the text window.
Ø DATA ONLY: Frame and packet headers are displayed in the status lines. In the text window only user data are displayed.