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TCP/IP Tutorial and Technical Overview - IBM Redbooks

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To be exact, the <strong>TCP</strong>/<strong>IP</strong> PDU is encapsulated in an IEEE 802.2 LLC header<br />

followed by an IEEE 802.1a SubNetwork Attachment Point (SNAP) header <strong>and</strong><br />

carried within the payload field of an AAL5 CPCS-PDU (Common Part<br />

Convergence Sublayer). The following figure shows the AAL5 CPCS-PDU<br />

format (Figure 2-7).<br />

Figure 2-7 AAL5 CPCS-PDU format<br />

Where:<br />

CPCS-PDU Payload The CPCS-PDU payload is shown in Figure 2-8 on<br />

page 55.<br />

Pad The Pad field pads out the CDCS-PDU to fit exactly into<br />

the ATM cells.<br />

CPCS-UU The CPCS-UU (User-to-User identification) field is used<br />

to transparently transfer CPCS user-to-user information.<br />

This field has no function for the encapsulation <strong>and</strong> can<br />

be set to any value.<br />

CPI The Common Part Indicator (CPI) field aligns the<br />

CPCS-PDU trailer with 64 bits.<br />

Length The Length field indicates the length, in bytes, of the<br />

payload field. The maximum value is 65535, which is<br />

64 KB - 1.<br />

CRC The CRC field protects the entire CPCS-PDU, except the<br />

CRC field itself.<br />

54 <strong>TCP</strong>/<strong>IP</strong> <strong>Tutorial</strong> <strong>and</strong> <strong>Technical</strong> <strong>Overview</strong>

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