Large MSUs

As shown in Figure C-3, a general purpose implementation of the ATM high-speed signaling link protocol stack would allow for large MSUs to be transferred across an ANSI or E1 ATM high-speed signaling link. The SSCOP layer can handle user data from SSCF that is up to 4096 bytes long. Since the SSCF layer does not add a trailer to MTP3 data, the ATM protocol stack is able to transfer MTP3 data packets up to 4096 bytes. When the SSCOP trailer is added, 4100 bytes of user data are handed to the AAL5CP layer.

These MSU sizes are much larger than the MTP3 data size currently supported by a low-speed signaling link. Currently, the largest MSU transferred on a low-speed signaling link is 279 bytes (2 flag bytes, 3 level 2 header bytes, 2 FCS bytes, and 272 bytes of MTP3 data). A large MSU is any MSU that contains 273 to 4096 bytes of MTP data.

The ATM high-speed signaling link (ANSI or E1) does not support large MSUs. Either UIM 1172 or 1173 is generated when a large MSU is received. See the UIMs section for more information on these UIMs.