CCNA Cisco Certified Network Associate Study Guide - FTP Server

482 Chapter 10 � Wide Area Networking Protocols
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.
FIGURE 10.2 Cisco HDLC frame format
Flag
Address
Cisco HDLC
• Each vendor’s HDLC has a proprietary data field to support multiprotocol environments.
Flag Address Control Data FCS Flag
• Supports only single-protocol environments.
Control Proprietary Data FCS Flag
HDLC
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.