Optical Networks - _J_3 Quality of Service_173

Optical Networks - _J_3 Quality of Service_173 - 806...

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806 Asynchronous Transfer Mode is protected by a CRC (4 bits of SN are protected by a 3-bit CRC and a 1-bit parity check), the 47-byte payload is unprotected. This is considered adequate for the circuit emulation and voice applications that AAL-1 is designed to support. J.2.2 AAL-5 AAL-5 is designed to transport variable-sized packets, up to 2 16 = 65 , 536 bytes in length, over an ATM network. Its most significant use is for the transport of IP packets over an ATM network. AAL-5 segments the user packets into cells but does not add any overhead (AAL header or trailer) in every cell. Instead, it uses the Payload Type field in the ATM header to indicate whether a cell is the last cell of a segmented IP packet or not. If a cell is the last cell of a segmented IP packet, the last 2 bytes of the cell constitute the AAL-5 trailer and contain the length of the IP packet and a CRC covering the entire IP packet. Thus, in all but one cell, the AAL-5 payload is equal to the 48-byte ATM payload, and AAL-5 has lower overhead compared to
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