mitted. These elements reduce throughput as compared to the bit-oriented protocols, which operate in full-duplex mode and permit the transmission of several consecutive frames without requiring an intervening acknowledgment. Also, bisync is cramped by a char- acter-based control format that makes it sensitive to code structure. Since the new protocols, such as HDLC, use bit sequences to convey control commands and information, they provide transparency to code structure. Fig. 3-5A. Character oriented protocols. PROTOCOLS AND CODES 41 3.6 HDLC PROTOCOL Internationally, the adopted level 2 standard is HDLC, which is sim- ilar to IBM's synchronous data link control procedure (SDLC), and ANSI's advanced data communication control procedure (ADCCP). HDLC is actually a family of link control procedures defined by a number of modes and options. For instance, HDLC has two basic modes of operation-normal response mode, designed for central- ized systems in which a primary station polls the secondary station; and the asynchronous response mode designed for situations in which either station can transmit at any time. Also, HDLC is continuously undergoing revision, and there are now other modes such as a balanced mode of operation in which neither station is designated as primary or secondary. The HDLC frame format allows for data transparency by means of bit stuffing. Eight-bit address sequences are also used to control the flow of in- formation (see Fig. 3-5B). For its data link control protocol, the X.25 standard uses a subset of HDLC known as the link access procedure (LAP). 3.7 SDLC OVERVIEW SDLC is one of the most popular bit oriented protocols. It can be viewed as an envelope in which data are transferred from one station to another across a data communication link. The link can be either multipoint or point to point and involves at least two participating stations. SDLC data links assign two roles to these stations: primary (commanding) and secondary (responding). The primary station is Want to Transmitter Receiver r