 The overhead of continuous polling is thus eliminated. Asynchronous balanced mode (ABM) is a balanced mode that provides symmetric data transfer capability between combined stations. Each station operates as if it were a primary station, can initiate data transfer, and is responsible for error recovery. One application of this mode is hub polling, where a secondary station needs to initiate transmission. 10.10.3.1 HDLC Frame. Figure 10.25 shows the HDLC frame format. Note the sim- ilarity to the generic data-link frame illustrated in Figure 10.8. Moving from left to right in the figure, we have the flag field (F), which delimits the frame at both ends with the unique pattern 01111110. This unique field or flag was described in Section 10.7.2. The address field (A) immediately follows the opening flag of a frame and precedes the control field (C). Each station in the network normally has an individual address and a group address. A group address identifies a family of stations. It is used when data messages must be accepted from or destined to more than one user. Normally the address is 8 bits long, providing 256 bit combinations or addresses (28 = 256). In HDLC (and ADCCP) the address field can be extended in increments of 8 bits. When this is implemented, the least significant bit is used as an extension indicator. When that bit is Frame Opening Address Control Information Frame check field Closing field field field 8 Bits 8 Bits 8 Bits Variable 16 Bits 8 Bits (expandable (expandable (32 bits optional) in 8-bit in 8-bit increments) increments) Span of frame check sequence Figure 10.25 The HDLC frame format (Ref. 25). DATA COMMUNICATIONS 0, the following octet is an extension of the address field. The address field is terminated when the least significant bit of an octet is 1. Thus we can see that the address field can be extended indefinitely. The control field (C) immediately follows the address field (A) and precedes the infor- mation field (I). The control field conveys commands, responses, and sequence numb