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مشاهدة النسخة كاملة : How does AAL5 work?



A7med Baraka
04-15-2009, 08:47 PM
Here is is a very simplified view of AAL5 and AALs in general.
AAL5 is a mechanism for segmentation and reassembly of packets. That is,
it is a rulebook which sender and receiver agree upon for taking a long
packet and dividing it up into cells. The sender's job is to segment the
packet and build the set of cells to be sent. The receiver's job is to
verify that the packet has been received intact without errors and to
put it back together again.

AAL5 (like any other AAL) is composed of a common part (CPCS) and a service
specific part (SSCS). The common part is further composed of a convergence
sublayer (CS) and a segmentation and reassembly (SAR) sublayer.

+--------------------+
| | SSCS
+--------------------+
| CS |
| ------------------ | CPCS
| SAR |
+--------------------+

SAR segments higher a layer PDU into 48 byte chunks that are fed into
the ATM layer to generate 53 byte cells (carried on the same VCI). The
payload type in the last cell (i.e., wherever the AAL5 trailer is) is marked
to indicate that this is the last cell in a packet. (The receiver may
assume that the next cell received on that VCI is the beginning of a
new packet.)

CS provides services such as padding and CRC checking. It takes an SSCS
PDU, adds padding if needed, and then adds an 8-byte trailer such that
the total length of the resultant PDU is a multiple of 48. The trailer
consist of a 2 bytes reserved, 2 bytes of packet length, and 4 bytes of CRC.

SSCS is service dependent and may provide services such as assured
data transmission based on retransmissions. One example is the SAAL
developed for signalling. This consists of the following:

+--------------------+
| SSCF |
| ------------------ | SSCS
| SSCOP |
+--------------------+
| CS |
| ------------------ | CPCS
| SAR |
+--------------------+

SSCOP is a general purpose data transfer layer providing, among other
things, assured data transfer.

SSCF is a coordination function that maps SSCOP services into those
primitives needed specifically for signalling (by Q.2931). Different
SSCFs may be prescribed for different services using the same SSCOP.

The SSCS may be null as well (e.g. IP-over-ATM or LAN Emulation).

There are two problems that can happen during transit. First, a
cell could be lost. In that case, the receiver can detect the problem
either because the length does not correspond with the number of cells
received, or because the CRC does not match what is calculated. Second,
a bit error can occur within the payload. Since cells do not have any
explicit error correction/detection mechanism, this cannot be detected
except through the CRC mismatch.

Note that it is up to higher layer protocols to deal with lost and
corrupted packets. This can be done by using a SSCS which supports
assured data transfer, as discussed above.