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AIR INTERFACE FOR FIXED BROADBAND WIRELESS ACCESS SYSTEMS

IEEE Std 802.16-2001

PDU. If the MAC SDU size is n bytes, the receiving side can unpack simply by knowing that the length field in the MAC header will be n*k+j, where k is the number of MAC SDUs packed into the MAC PDU and j is the size of the MAC header and any prepended MAC subheaders. A MAC PDU containing a packed sequence of fixed-length MAC SDUs would be constructed as in Figure 27. Note that there is no added overhead due to packing in the fixed-length MAC SDU case, and a single MAC SDU is simply a packed sequence of length 1.

Header

= n*k+j

MAC

LEN

 

 

 

k

MAC SDUs

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Fixed-length

Fixed-length

Fixed-length

 

 

 

 

 

 

 

 

 

Fixed-length

 

 

MAC SDU

MAC SDU

MAC SDU

 

 

 

 

 

 

 

 

 

MAC SDU

 

 

 

 

 

 

 

length = n

length = n

length = n

 

 

 

 

 

 

 

 

 

length = n

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 27—Packing fixed-length MAC SDUs into a single MAC PDU

6.2.3.4.2 Packing variable-length MAC SDUs

When packing variable-length MAC SDU connections, such as Ethernet, the n*k+j relationship between the MAC header’s length field and the higher-layer MAC SDUs no longer holds. This necessitates indication of where one MAC SDU ends and another begins. In the variable-length MAC SDU case, the MAC attaches a Packing subheader (PSH) to each MAC SDU. This subheader is described in 6.2.2.2.3.

A MAC PDU containing a packed sequence of variable-length MAC SDUs is constructed as shown in Figure 28. If more than one MAC SDU is packed into the MAC PDU, the type field in the MAC header indicates the presence of Packing subheaders. Note that unfragmented MAC SDUs and MAC SDU fragments may both be present in the same MAC PDU, as described in 6.2.3.4.2.1.

 

 

 

 

 

k MAC SDUs

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

HeaderMAC =LENj 00001Xb=Type

PSH Length=a+2

variable-length

PSH Length=b+2

variable-length

 

 

 

 

 

 

 

 

 

 

PSH Length=c+2

variable-length

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

MAC SDU

 

MAC SDU

 

 

 

 

 

 

 

 

 

 

 

MAC SDU

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

length = a

 

length = b

 

 

 

 

 

 

 

 

 

 

 

length = c

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Note: If Type=0x03, a GM subheader (not shown in figure) is also present immediately following the Generic MAC header.

Figure 28—Packing variable-length MAC SDUs into a single MAC PDU

If only one MAC SDU (or fragment thereof) is present in a MAC PDU, the type field in MAC header indicates the absence of Packing subheaders. This is shown in Figure 29.

MAC Header LEN = j+6

Fixed-length MAC SDU length = j

Figure 29—Packing a single variable-length MAC SDU into a single MAC PDU

Copyright © 2002 IEEE. All rights reserved.

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