|  | 2.4 | WAN Encapsulation Formats
| 2.4.3 | HDLC

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.
| Threaded Case Study
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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