8.3 Fragmentation, Packing and Concatenation


8.3 Fragmentation, Packing and Concatenation

As in almost all other recent wireless systems, it may be interesting to fragment a MAC SDU in many MAC PDUs or, inversely, to pack more than one MSDU in many PDUs. The advantage of fragmentation is to lower the risk of losing a whole MSDU to the risk of losing part of it, a fragment. The inconvenient is to have more header information. This is interesting when the radio channel is relatively bad or packets too long. Conversely, packing allows less headers to be needed at the risk of losing all the packed packets. This is interesting when the radio channel is relatively good. Concatenation is the fact of transmitting many PDUs in a single transmission opportunity. Fragmentation, packing and concatenation are included in the 802.16 standard.

8.3.1 Fragmentation

Fragmentation is the process by which a MAC SDU is divided in two or more MAC PDUs. When the radio channel is relatively bad, this process allows efficient use of available bandwidth while taking into account the QoS requirements of a connection service flow. The presence of fragmentation is indicated by bit 2 of the Type field (see Section 8.2) of a generic MAC frame. Usually, fragmentation concerns relatively long packets (such as IP packets). Fragmentation of a packet is shown in Figure 8.5.

image from book
Figure 8.5: Illustration of the fragmentation of an MAC SDU giving three MAC PDUs (or MAC frames)

The three MPDUs obtained in the example shown each contain a Fragment subheader. Thus bit 2 of the Type field in the generic MAC header will be set to 1 (see Section 8.2.3). The Fragment subheader will contain information such as if the fragment is the first, middle or last, etc.

The capabilities of fragmentation and reassembly are mandatory.

8.3.2 Packing

When packing is turned on for a connection, the MAC layer may pack multiple MAC SDUs into one single MAC PDU. When the radio channel is relatively good, this allows a better use of available resources. The transmitting side has the full decision of whether or not to pack a group of MAC SDUs in a single MAC PDU. The presence of packing is indicated by bit 1 of the Type field of the generic MAC frame (see Section 8.2.3).

Packing is especially efficient for relatively short packets. A packed packet is shown in Figure 8.6. The payload of the frame will contain many packing subheaders, and each one will be followed by its MAC SDU. The sum of packed headers is smaller than the sum of headers of normal SDUs. This is why packing saves bandwidth resources. On the other hand, if the packed PDU is lost, all component SDUs are lost (while possibly only one would have been lost if packing was not done).

Generic MAC
Header
(6 bytes)

Packing
Sub-Header
(2 or 3 bytes)

MAC
SDU

Packing
Sub-Header
(2 or 3 bytes)

MAC
SDU

––––––––––––––

Optional CRC (4 bytes)


Figure 8.6: Illustration of the packing of MAC SDUs in one MAC PDU

If the ARQ mechanism is turned on, subheaders of fragmentation and packing are extended. For example, the subheaders of a packed packet are made of 3 bytes instead of 2.

The capability of unpacking is mandatory.

8.3.3 Concatenation

Concatenation is the procedure of concatening multiple MAC PDUs into a single transmission (see Figure 8.7). Concatenation is possible in both the uplink and downlink. Since each MAC PDU is identified by a unique CID, the receiving MAC entity is able to present the MAC SDU to the correct instance of the MAC SAP. It is then possible to send MPDUs of different CIDs on the same physical burst. Then, MAC management messages, user data and bandwidth request MAC PDUs may be concatenated into the same transmission. Evidently, in the uplink all the MPDUs are transmitted by the same SS.

image from book
Figure 8.7: Illustration of the concatenation for an uplink burst transmission. (From IEEE Std 802.16-2004 [1]. Copyright IEEE 2004, IEEE. All rights reserved.)




WiMAX. Technology for Broadband Wireless Access
WiMAX: Technology for Broadband Wireless Access
ISBN: 0470028084
EAN: 2147483647
Year: 2007
Pages: 124

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