In 802.11 DSSS systems, information is transmitted in frames. The total frame length is variable. The N7617C Signal Studio for WLAN 2024 Update 1.0 software supports framing for both the long and short preambles, as detailed in the following figures.
Figure 1. PPDU frame with long PLCP preamble
the user can turn scrambling On, Off, or On for the Preamble only.
the user cannot modify the SFD field.
the user cannot modify the signal fields.
Bit 0 is set to zero to indicate that the data is not CCK-OFDM, and cannot be modified by the user.
Bits 1 and 4 are reserved bits as indicated by the 802.11 standard, and cannot be modified by the user.
Bit 3 is set to zero if CCK is the modulation format, and is set to one if PBCC is the modulation format. This is automatically set by the software, and the user cannot modify this bit.
Bits 5, 6, and 7 are the length extension fields and are used by the PBCC modes. This field is set automatically by the software and cannot be modified by the user.
Bit 7 is the length extension bit. If the payload data rate is over 8 Mbps, then this bit will be set to one automatically by the software. The user cannot modify this bit.
the user cannot modify the length fields.
automatically calculated by the software, and cannot be modified by the user.
Figure 2. PPDU frame with short PLCP preamble
The payload field is contained within the PPDU frame. A MAC header and FCS can be added to the data payload. The following table provides a summary of the payload characteristics for the 802.11b DSSS mode.
Table 1. DSSS mode summary
|
Mbps |
Chip Rate Mcps |
Spreading/Coding Scheme |
Modulation |
|---|---|---|---|
|
1 |
11 |
11 Chip Barker Code |
DBPSK |
|
2 |
11 |
11 Chip Barker Code |
DQPSK |
|
5.5 |
11 |
CCK |
DQPSK |
|
11 |
11 |
CCK |
DQPSK |
|
5.5 |
11 |
PBCC |
BPSK |
|
11 |
11 |
PBCC |
QPSK |
The following figure is an illustration of the MAC header structure. The description and details of each of these fields can be found in the IEEE 802.11 standard. The user has control over all of these fields.
Figure 3. MAC header structure