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SAR PDU payload contains 47 octets (376 bits) of TDM data without regard to frame alignment or timeslot byte
alignment. All AAL1 SAR PDUs are non-P format for unstructured bundles.
Structured-without-CAS bundles, for E1/T1 interfaces, support rates of N
64 kbps, where N is the number of
timeslots configured to be assigned to a bundle. For this format, the N timeslots from one E1/T1 frame are
sequentially mapped into an N-octet structure. This N-octet structure is then mapped into the AAL1 SAR PDU
payloads, octet-aligned. This process is repeated until all octets of the AAL1 SAR PDU payload are filled. The last
octet of the payload may contain a timeslot other than the last timeslot of the structure. The remaining timeslots of
the structure are mapped into the next AAL1 SAR PDU payload in the same manner and the process continues.
Figure 10-37. AAL1 Mapping, Structured-Without-CAS Bundles
With this mapping each AAL1 SAR PDU can start with a different timeslot. To enable the far end TDMoP function
to identify the start of a structure, a pointer to it is sent periodically in one of the even-numbered AAL1 SAR PDUs
of every SN cycle. When this pointer is sent, a P-format AAL1 SAR PDU is used. In a P-format AAL1 SAR PDU the
first byte of the AAL1 SAR PDU payload contains the pointer, and the last 46 bytes contain payload.
Structured-with-CAS bundles, for E1/T1 interfaces, support rates of N
64 kbps, where N is the number of
timeslots configured to be assigned to a bundle. This mapping is similar to the structured-without-CAS mapping
described above except that the structure is an entire E1/T1 multiframe of the N timeslots assigned to the bundle,
and a CAS signaling substructure is appended to the end of the structure. The addition of CAS only affects the
structure arrangement and contents. CAS data from one timeslot is 4 bits long, meaning one octet can contain CAS
data of 2 timeslots. Bundles containing an odd number of timeslots need a padding of 4 zeroes in the last CAS
octet. For example, a 3-timeslot bundle of an E1 frame with CAS yields the following structure octet sequence:
TS1, TS2, TS3 repeated 16 times (a whole E1 multiframe) and then CAS1+CAS2, CAS3+padding.
10.6.6.2 Ethernet-to-TDM Direction
In the Ethernet-to-TDM direction, AAL1 SAR PDUs of a bundle are being received only after the synchronization
process. The synchronization process includes packet SN synchronization, AAL1 SAR PDU SN synchronization,
and pointer synchronization. AAL1 SAR PDUs with CRC or parity errors in their header are discarded. Pointer
mismatch imposes jitter buffer under-run and bundle resynchronization. AAL1 SAR PDU header errors or pointer
errors may be ignored depending on per-bundle configuration. Missing AAL1 SAR PDUs are detected and restored
in the jitter buffer.