Data-Link Details of Leased Lines
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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.
All data-link protocols perform a similar role: to control the correct delivery of data over a
physical link of a particular type. For example, the Ethernet data-link protocol uses a destination
address field to identify the correct device that should receive the data, and an FCS
field that allows the receiving device to determine whether the data arrived correctly. HDLC
provides similar functions.
HDLC has less work to do because of the simple point-to-point topology of a point-topoint
leased line. When one router sends an HDLC frame, it can go only one place: to the
other end of the link. So, while HDLC has an address field, the destination is implied. The
idea is sort of like when I have lunch with my friend Gary, and only Gary. I do not need to
start every sentence with “Hey Gary”—he knows I am talking to him.
NOTE In case you wonder why HDLC has an address field at all, in years past, the telcos
offered multidrop circuits. These circuits included more than two devices, so there was more
than one possible destination, requiring an address field to identify the correct destination.
HDLC has other fields and functions similar to Ethernet as well. Table 3-3 lists the HDLC
fields, with the similar Ethernet header/trailer field, just for the sake of learning HDLC
based on something you have already learned about (Ethernet ).
HDLC exists today as a standard of the International Organization for Standardization
(ISO), the same organization that brought us the OSI model. However, ISO standard HDLC
does not have a Type field, and routers need to know the type of packet inside the frame.
So, Cisco routers use a Cisco-proprietary variation of HDLC that adds a Type field, as
shown in Figure 3-6.
Figure 3-6 HDLC Framing