Data Link Details of Leased Lines
A leased line provides a Layer 1 service. In other words, it promises to deliver bits between the devices connected to the leased line. However, the leased line itself does not define a data link layer protocol to be used on the leased line.
Because leased lines define only the Layer 1 transmission service, many companies and standards organizations have created data link protocols to control and use leased lines. Today, the two most popular data link layer protocols used for leased lines between two routers are High-Level Data Link Control (HDLC) and Point-to-Point Protocol (PPP). This next topic takes a brief look at HDLC, just to show one example, plus a few comments about how routers use WAN data link protocols. ...
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