Wireles Networks The Definitive Guide.pdf - Csbdu.in

3.5.6 Frame Check Sequence
As with Ethernet, the 802.11 frame closes with a frame check sequence (FCS). The FCS is often
referred to as the cyclic redundancy check (CRC) because of the underlying mathematical operations.
The FCS allows stations to check the integrity of received frames. All fields in the MAC header and
the body of the frame are included in the FCS. Although 802.3 and 802.11 use the same method to
calculate the FCS, the MAC header used in 802.11 is different from the header used in 802.3, so the
FCS must be recalculated by access points.
When frames are sent to the wireless interface, the FCS is calculated before those frames are sent out
over the RF or IR link. Receivers can then calculate the FCS from the received frame and compare it
to the received FCS. If the two match, there is a high probability that the frame was not damaged in
transit.
On Ethernets, frames with a bad FCS are simply discarded, and frames with a good FCS are passed up
the protocol stack. On 802.11 networks, frames that pass the integrity check may also require the
receiver to send an acknowledgment. For example, data frames that are received correctly must be
positively acknowledged, or they are retransmitted. 802.11 does not have a negative acknowledgment
for frames that fail the FCS; stations must wait for the acknowledgment timeout before retransmitting.
3.6 Encapsulation of Higher-Layer Protocols Within 802.11
Like all other 802 link layers, 802.11 can transport any network-layer protocol. Unlike Ethernet,
802.11 relies on 802.2 logical-link control (LLC) encapsulation to carry higher-level protocols. Figure
3-13 shows how 802.2 LLC encapsulation is used to carry an IP packet. In the figure, the "MAC
headers" for 802.1h and RFC 1042 might be the 12 bytes of source and destination MAC address
information on Ethernet or the long 802.11 MAC header from the previous section.
Figure 3-13. IP encapsulation in 802.11
Two different methods can be used to encapsulate LLC data for transmission. One is described in RFC
1042, and the other in 802.1h. As you can see in Figure 3-13, though, the two methods are quite
similar. An Ethernet frame is shown in the top line of Figure 3-13. It has a MAC header composed of
source and destination MAC addresses, a type code, the embedded packet, and a frame check field. In
the IP world, the Type code is either 0x0800 (2048 decimal) for IP itself, or 0x0806 (2054 decimal) for
the Address Resolution Protocol (ARP).
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