HDLC is a data link-layer protocol derived
from the Synchronous Data Link Control (SDLC) encapsulation protocol.
HDLC is Cisco's default encapsulation for serial lines. This
implementation is very streamlined; there is no windowing or flow
control, and only point-to-point connections are allowed. The address
field is always set to all ones. Furthermore, a 2-byte proprietary
type code is inserted after the control field, which means that HDLC
framing is not interoperable with other vendors' equipment.
If both ends of a dedicated-line
connection are routers or access servers running Cisco Internetwork
Operating System (IOS) software, HDLC encapsulation typically is used.
Because HDLC encapsulation methods may vary, you should use PPP with
devices that are not running Cisco IOS software.
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Threaded
Case
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Washington
Project: PPP
Encapsulation
Although
both PPP and HDLC are appropriate frame types for
point-to-point connections, you should use PPP on
point-to-point links in the Washington School
District network. PPP offers the following
advantages:
- Interoperability
between networking vendors
- LCP
for negotiating basic line interoperability
- A
family of network control protocols for
negotiating individual Layer 3 protocols
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