744
21 Wireless Local Area Networks
The
sequence control fi eld
is a 16-bit fi eld that consists of two subfi elds.
The subfi elds are a 4-bit fragment number and a 12-bit sequence number. This
fi eld is used to allow a receiving station to eliminate duplicate received frames.
The
sequence number subfi eld
contains numbers assigned sequentially by the
sending station to each MSDU. This sequence number is incremented after each
assignment and wraps back to zero when incremented from 4095. The sequence
number for a particular MSDU is transmitted in every data frame associated with
the MSDU. It is constant over all transmissions and retransmissions of the MSDU.
If the MSDU is frangmented, the sequence number of the MSDU is sent with
each frame containing a fragment of the MSDU. The
fragment number subfi eld
contains a 4-bit number assigned to each fragment of an MSDU. The fi rst, or
only, fragment of an MSDU is assigned a fragment number of zero. Each succes-
sive fragment is assigned a sequentially incremented fragment number. The frag-
ment number is constant in all transmissions or retransmissions of a particular
fragment.
The
frame body fi eld
contains the information specifi c to the particu-
lar data or management frames. This fi eld is variable in length. It may be as
long as 2034 bytes without encryption, or 2312 bytes when the frame body is
encrypted. The value of 2304 bytes as the maximum length of this fi eld was cho-
sen to allow an application to send 2048-byte pieces of information, which can be
encapsulated by as many as 256 bytes of upper layer protocol headers and
trailers.
The
frame check sequence (FCS) fi eld
is 32 bits in length. It contains the
result of applying the C-32 polynomial to the MAC header and frame body.
The original 802.11 standard suffers from some serious limitations which
prevent it from becoming a leading technology and a serious alternative to wired
LAN. The following are some of the problems:
Low data rate: The 802.11 protocol imposes very high overhead to all packets
that reduce real data rate signifi cantly
No QoS guarantees
Several extensions to the basic 802.11 standard have been introduced by
IEEE to provide higher data rates or QoS guarantees. 802.11a, 802.11b, and
802.11g focus on higher data rates whereas 802.11e is aimed at providing QoS
guarantees.
