field and supports up to 1,024 logical connection numbers (LCNs). Command/Response (C/R) bit The C/R is a standard bit used in HDLC framing (an OSI standard). This is not used in Frame Relay. Extended Address (EA) bit When set to 0, this extends the DLCI address. When set to 1, this indicates that the address is only carried in the first byte (6 bits). Forward Explicit Congestion Notification (FECN) This is set in the frames going out into the network toward the destination address. Backward Explicit Congestion Notification (BECN) This is set in frames returning from the network to the source address. Discard Eligibility (DE) bit This bit is used by the source equipment to denote whether the frame is eligible to be discarded by the network if the network gets congested. When set to 1, this indicates that the frame is eligible to be discarded during congestion period. EA bit When set to 1, this is used to end the DLCI. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PCUSN-to-SGSN Interface (Gb) Chapter 9 1 byte 2 bytes Variable up to 1610 2 bytes 1 byte Figure 9-4 The frame PDU. Header I field Where: Backward Explicit Congestion Notification Command/Responses Discard Eligibility Data Link Connection Identifier Extended Address Forward Explicit Congestion Notification Protocol Data Unit The FECN, BECN, and DE parts of the addresses are designed to alert the end-user equipment on the status of the network's capability to process frames. Higher-level protocols use a window size control procedure or other form of control to handle traffic flow. These flow control mechanisms are designed to alert the network components and the end-user equipment to slow the delivery of frames to the network. If discarding is taking place while a router or other piece of equipment is in burst mode, the router or other equipment will begin to buffer the frames until