User`s guide

Aironet Wireless Communications, Inc. 7-59 Confidential and Proprietary
Encapsulation Transformations
This section is only applicable to hosts that require ethernet style payloads. If the host has configured the
card for LLC payloads, ie. unmodified payloads, (see OperatingMode in the General Configuration) then
all payloads will pass through as is. 802.11 networks require 802.3 payload encapsulations. The
PC4500/4800 will translate transmitted payloads and received payloads as required for ethernet hosts.
(NOTE: when registering to an Aironet AP with Aironet extensions enabled, the PC4500/4800 will
"adopt" the encapsulation rules in use by the AP -- this is done to prevent tables from being out of sync.)
Two methods of encapsulating ethertype packets on an 802.3 network are available:
• RFC 1042
• 802.1H
RFC1042 place the following LLC/SNAP in front of the payload: 0xAA 0xAA 0x03 0x00 0x00 0x00
802.1H places the following LLC/SNAP in front of the payload: 0xAA 0xAA 0x03 0x00 0x00 0xF8
When transmitting an ethernet packet onto the wireless medium:
1. if the ethertype field is a length (less than or equal to 0x5DC), then the ethertype field is
stripped and the remaining payload (LLC) is used.
2. otherwise, translation is performed by scanning the list of ethertypes in the encapsulation
RID. If a match is found, then the actions specify RFC1042 or 802.1H encapsulation. A zero
ethertype ends the list and specifies the default actions when a match is not found.
When receiving a wireless packet:
1. if the packet is 802.1H, then the 802.1H header is stripped (back to ethertype)
2. if the packet is RFC1042, then the RFC1042 header is stripped only if the matching ethertype
action or default action so specifies
3. if the packet is neither RFC1042 nor 802.1H, a length field is prepended to the packet
Offset Name Type
Factory
default Description
+0x0000 u16RidLen u16 read-only
0x22
Length of this RID including the length field
+0x0002 EtherType u16 0 Note, the ethertype values are in network transmission
order. So IP (0x800) is actually (0x0008).
Zero ends the list and selects the default action.
+0x0004 Action u16 0 This field is bit encoded as follows:
bit 0 (0x0001)
1=RFC1042 is kept for receive packets.
bit 1 (0x0002)
0=RFC1042 is used for transmit encapsulation.
1=802.1H is used for transmit encapsulation.
… Repeat above two
fields
0
The Encapsulation RID allows for 7 ethertypes to be entered plus a default action (ethertype = 0).