The two most common point-to-point WAN
encapsulations are HDLC and PPP. All the serial line encapsulations
share a common frame format, which has the following fields, as shown
in the Figure:
- Flag-Indicates the beginning of the
frame and is set to the hexadecimal (base 16) pattern 7E.
- Address-A
1- or 2-byte field to address the end station in multidrop
environments.
- Control-Indicates whether the frame is an
information, a supervisory, or an unnumbered type frame. It also
contains specific function codes.
- Data-The encapsulated data.
- FCS-The frame check sequence (FCS).
- Flag-The trailing 7E flag
identifier.
Each WAN connection type uses a Layer 2
protocol to encapsulate traffic while it is crossing the WAN link. To
ensure that the correct encapsulation protocol is used, you need to
configure the Layer 2 encapsulation type to use for each serial
interface on a router. The choice of encapsulation protocol depends on
the WAN technology and the communicating equipment. Encapsulation
protocols that can be used with the WAN connection types covered in
this chapter are PPP and HDLC.
|