Service set identifier (SSID)
A BSSID is an address that is defined for each unique BSS and IBSS. In the former context
a 48-bit address is used to uniquely identify a BSS, such as an AP; whereas, a 46-bit random number is generated to uniquely identify an IBSS. A Service Set Identifier
(SSID) is a unique field that is included within the MAC management frame and is used
to identify a packet. The identifier is made up of thirty-two alphanumeric octets, which
is typically identified by a user when connecting to an AP; some administrators choose
to turn this identifier off in an ineffective attempt to hamper hackers.
The next field in the general MAC frame format is the sequence control field,
which contains two further fields, as shown in Figure 13.18. The Fragment Number
field is a 4-bit entity that identifies the number of the fragment of a MSDU. The 12-bit
Sequence Number field represents the sequence reference of the MSDU which is
assigned when an STA transmits its payload. The remaining two fields in our MAC
frame format are the frame body and FCS fields. The FCS field is a 32-bit CRC,
which is appended to the end of the MAC frame. Finally, the frame body is of variable
length and essentially contains the message, which may be one of the frame types and
sub-types, as we discussed earlier.