The High-Level Data-Link Control protocol (HDLC) is a popular ISOstandard, bit-oriented Data Link layer protocol. It specifies an encapsulation method for data on synchronous serial data links using frame characters and checksums. HDLC is a point-to-point protocol used on leased lines. No authentication can be used with HDLC.
In byte-oriented protocols, control information is encoded using entire bytes. Bit-oriented protocols, on the other hand, may use single bits to represent control information. Bit-oriented protocols include SDLC, LLC, HDLC, TCP, IP, etc. HDLC is the default encapsulation used by Cisco routers over synchronous serial links. Cisco’s HDLC is proprietary—it won’t communicate with any other vendor’s HDLC implementation—but don’t give Cisco grief for it; everyone’s HDLC implementation is proprietary. Figure 10.2 shows the Cisco HDLC format.
In byte-oriented protocols, control information is encoded using entire bytes. Bit-oriented protocols, on the other hand, may use single bits to represent control information. Bit-oriented protocols include SDLC, LLC, HDLC, TCP, IP, etc. HDLC is the default encapsulation used by Cisco routers over synchronous serial links. Cisco’s HDLC is proprietary—it won’t communicate with any other vendor’s HDLC implementation—but don’t give Cisco grief for it; everyone’s HDLC implementation is proprietary. Figure 10.2 shows the Cisco HDLC format.
As shown in the figure, the reason that every vendor has a proprietary HDLC encapsulation method is that each vendor has a different way for the HDLC protocol to communicate with the Network layer protocols. If the vendors didn’t have a way for HDLC to communicate with the different layer-3 protocols, then HDLC would only be able to carry one protocol. This propriety header is placed in the data field of the HDLC encapsulation. If you had only one Cisco router and you needed to connect to, say, a Bay router because you had your other Cisco router on order, then you couldn’t use the default HDLC serial encapsulation. You would use something like PPP, which is an ISO standard way of identifying the upper-layer protocols.
