The High-Level Data-Link Control (HDLC) protocol is a popular ISO-standard, 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. On the otherhand, bit-oriented protocols use single bits to represent the control information. Some common bit-oriented protocols include SDLC, LLC, HDLC, TCP, and IP.
HDLC is the default encapsulation used by Cisco routers over synchronous serial links. And 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 1 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 encapsulate multiple Network layer protocols. If the vendors didn’t have a way for HDLC to communicate the different layer 3 protocols, then HDLC would only be able to carry one protocol. This proprietary header is placed in the data field of the HDLC encapsulation.
So let’s say you only have one Cisco router, and you need to connect to a non-Cisco router because your other Cisco router is on order. What would you do? You couldn’t use the default HDLC serial encapsulation because it wouldn’t work. Instead, you would use something like PPP, an ISO-standard way of identifying the upper-layer protocols. You can check out RFC 1661 for more information on the origins and standards of PPP. Let’s discuss PPP in more detail and how to connect to routers using the PPP encapsulation
In byte-oriented protocols, control information is encoded using entire bytes. On the otherhand, bit-oriented protocols use single bits to represent the control information. Some common bit-oriented protocols include SDLC, LLC, HDLC, TCP, and IP.
HDLC is the default encapsulation used by Cisco routers over synchronous serial links. And 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 1 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 encapsulate multiple Network layer protocols. If the vendors didn’t have a way for HDLC to communicate the different layer 3 protocols, then HDLC would only be able to carry one protocol. This proprietary header is placed in the data field of the HDLC encapsulation.
So let’s say you only have one Cisco router, and you need to connect to a non-Cisco router because your other Cisco router is on order. What would you do? You couldn’t use the default HDLC serial encapsulation because it wouldn’t work. Instead, you would use something like PPP, an ISO-standard way of identifying the upper-layer protocols. You can check out RFC 1661 for more information on the origins and standards of PPP. Let’s discuss PPP in more detail and how to connect to routers using the PPP encapsulation
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