MIL-STD-188-176
5.2.3.1 AAL 3/4 functions and structure . The functions and structures of AAL 3/4 shall be in accordance with ANSI T1.629, section 9. The protocol accepts variable-length frames/ packets
(up to 64 kbytes) and segments them into cells. It provides for the transparent and sequential transfer of PDUs between corresponding upper-layer entities with an agreed quality of service (QOS). Each cell carries 44 payload octets, plus a
2-octet header and a 2-octet trailer. The payload header and trailer provide protection against misordering of cells, and a
10-bit CRC for error detection. AAL 3/4 provides optional multiplexing of multiple CPCS connections.
5.2.3.2 AAL 3/4 procedures . The AAL 3/4 protocol shall operate in accordance with ANSI T1.629, section 10.
5.2.4 AAL 5. AAL 5 supports VBR services and provides a
48-octet payload. AAL 5 common part protocol shall comply with
ITU-T I.363, section 6. No error detection is provided in the
cell payload. A 32-bit CRC for error detection is provided in
the CPCS PDU, in accordance with ITU-T I.363, section 6. For
assured service, the SSCS provides retransmission of erroneous
CPCS PDUs. Optional error discard allows corrupted PDUs to be
delivered to the user. For non-assured service, the SSCS is
null.
5.3 ATM wide area network (WAN) . The interface between ATM WAN switches is referred to as the network-node interface (NNI)
(see Figure 8). In the user plane the NNI consists of a physical layer and an ATM layer. The control plane also includes a
network layer.
5.3.1 Physical layer . The Physical layer shall use synchronous optical network (SONET) specifications. All electrical interface rates and formats shall conform to ANSI T1.105, sections 6 through 12. A compatible version, referred to as synchronous digital hierarchy (SDH) was published as ITU-T recommendations. The basic modular signal is called the synchronous transport signal level 1 (STS-1) and the optical counterpart is the optical carrier level 1 signal (OC-1). The
standard multiplexed rates are OC-N, where N = 3, 12 or 48 (see Table I). The payload rate is the SONET payload, which includes path overhead. User payload is reduced by path overhead. A
single-mode fiber-optic transmission interface shall be in accordance with ANSI T1.106.
5.3.2 ATM layer . The ATM layer at the NNI is the same as the ATM layer at the UNI (see 5.1.2) except for the first 4 bits of the cell header. These bits are reserved for generic flow control (GFC) at the UNI. At the NNI, the first 4 bits are used to extend the VPI field.
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