Wavecom W61PC V7.5.0 - Manual (Page 248)

238  Transmission Modes WAVECOM Decoder W61PC/LAN Manual V7.5
Parameter Value
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 Demodula-
tor 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).
Frame Format
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 num-
ber 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 (Logi-
cal Channel Identifier) field contains information on the logical channel and the PTI (Packet Type Identifi-
er) 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
 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.