 transparency. Where the DLCI indicates a particular channel or channel group in the multiplex aggregate for PVC operation. 12.5 SPEEDING UP THE NETWORK: FRAME RELAY Inspection of the frame to ensure that it is not too long or too short. Detection of (but not recovery from) transmission errors. Congestion control functions. In other words, ANSI has selected certain features from the LAPD structure/protocol, rejected others, and added some new features. For instance, the control field was removed, but certain control functions have been incorporated as single bits in the address field. These are the C/R bit (command/response), DE (discard eligibility), FECN bit (forward explicit congestion notification), and BECN bit (backward explicit congestion notification). 12.5.4 Frame Structure User traffic passed to a FRAD (frame relay access device) is segmented into frames with a maximum length information field or with a default length of 262 octets. The minimum information field length is one octet. Figure 12.8 illustrates the frame relay frame structure. LAPD (see Section 10.10.3) uses HDLC flags (01111110) as opening and closing flags. A closing flag may also serve as the opening flag of the next frame. Address field. This consists of two octets, but may be extended to three or four octets. However, there is no control field as there is in HDLC, LAPB, and ISDN LAPD. In its most reduced version, there are just 10 bits allocated to the address field in two octets (the remainder of the bits serve as control functions) supporting up to 1024 logical connections. It should be noted that the number of addressable logical connections is multiplied because they can be reused at each nodal (switch) interface. That is, an address in the form of a data-link connection identifier (DLCI) has meaning only on one trunk between adjacent nodes. The switch (node) that receives a frame is free to change the DLCI before sending the frame onwards over the next link. Thus, the limit of 1024 DLCIs applies to the link, not the network. Inform