emoves "0" Inserts "0" After fifth 1 in data field 01111101 011111001 01111101 Fig. 3-5B. To achieve data transparency, protocols employ bit stuffing. In bit-oriented pro- tocols (BOP's) such as HDLC, the transmitter "stuffs" an extra "0" after each sequence of five 1 Bits appearing in the data; the receiver deletes any such "0" bit to avoid confusion. 42 TELECOMMUNICATIONS Table 3-2. Common Protocol Characteristics FEATURE BISYNC DDCMP Full-duplex Half-duplex Message format Variable Fixed Fixed Fixed Link control Control Header Control Control character (fixed) field field Station addressing Header Header Address Address field field Error detection VRC/LRC CRC-CCITT CRC-CCITT Control characters Numerous SOH, DLE, Idle flag Idle flag Character codes ASCII, EBCDIC ASCII responsible for the data link and issues commands to which all other participating stations respond. The basic unit of information on an SDLC link is a "frame." Each frame is enclosed in "flags," which are used for frame syn- chronization and have the binary configuration 01111110. The open- ing flag serves as a reference for the position of the address and control fields. The closing flag terminates the check for transmission errors. Table 3-2 presents a comparison of protocol characteristics. 3.8 SYSTEM NETWORK ARCHITECTURE (SNA) Advances in computing, processing and communications technolo- gies have prompted increased interconnection of terminals and com- munications facilities into networks for distributed access to processing and data-base resources. A variety of networking applications have been developed for air- line reservations, banking, store checkout, process control, and of- fice systems. The network environment requires a master interconnection strategy so that diverse products and applications can share computational and communications facilities while inter- acting compatibly. Since its introduction in 1974, IBM's systems network architecture (SNA) has provided the blueprint by which the capabilities of IBM networking products have