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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 other hand, bit-oriented protocols may use single bits to represent control information. Bit-oriented protocols include SDLC, LLC, HDLC, TCP, IP, and others.

 

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 10.4 shows the Cisco HDLC format.


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CCENT Practice Exam Questions
A receiving host has failed to receive all of the segments that it should acknowledge. What can the host do to improve the reliability of this communication session?
Send a different source port number.
Restart the virtual circuit.
Decrease the sequence number.
Decrease the window size.
A receiving host can control the transmitter by using flow control (TCP uses Windowing by default). By decreasing the window size, the receiving host can slow down the transmitting host so the receiving host does not overflow its buffers.
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